The Spinal Cord I
The spinal cord is a thin, white, cylindrical structure that consists of nerve fibers and other tissue. It consists of millions of nerve fibers that transmit information to and from the extremities, trunk and organs and connects nearly all parts of the body to the brain. The spinal cord processes sensory information from the limbs, trunk, and internal organs, directly controls body movements, and regulates many visceral functions. The spinal cord serves as a conduit for the transmission of both sensory information that ascend to the brain and motor information that descends from the brain in tracts.
It is approximately 45 centimeters/18 inches in length in males and 2 centimeters less in females, with a diameter of 8mm (pencil thick). It transitions superiorly/rostrally, in adults, from the caudal margin of the medulla, exits the foramen magnum of the skull and travels inferiorly/caudally toward the sacrum. It is contained within the vertebral column and ends at the level of the first or second lumbar vertebra as the conus medullaris. The Cauda Equina (horse's tail) extends caudally below the level of the conus medullaris and consists of spinal nerves that extend downward into the spinal canal and exits the foramen of the vertebrae. It comprises the distal/caudal two-thirds of the central nervous system. Denticular ligaments provide mechanical isolation so that the spinal cord is not subject to physical stress.
The spinal cord has a modular organization, in which every segment has a similar basic structure. It is subdivided into levels corresponding to the vertebral regions surrounding it. Within each level, the cord is further subdivided into segments, corresponding to the segmental appearance of the spinal nerves. The spinal cord is the only part of the central nervous system that has an external segmental organization. Just as the cerebral cortex and brain stem, the spinal cord is structurally composed of gray and white matter. The central cord is composed of gray matter composed of nerve cell bodies and is surrounded by a peripheral white matter region containing myelinated axons.
The spinal cord is covered in meninges; dura, arachnoid, pia similar to those that surround the brain, the dura mater surrounding the brain continues downward and completely envelopes the spinal cord. The spinal cord itself is closely covered by a thin layer of arachnoid and an inner layer of pia mater. The outer or periosteal layer of dura is closely adherent to the walls of the vertebral (spinal) canal, serving as its periosteum, a dense layer of vascular connective tissue that envelopes bones. The inner or meningeal layer of dura forms a closed tube around the cord and continues caudally as far as the second sacral vertebra, at which it encloses the filum terminale (plate 9-2). Extensions of the dura pass laterally like sleeves along the spinal roots to become continuous with the epineurial sheaths of the spinal nerves. Between dural sleeves and along the length of the spinal cord, the pia mater thickens and extends laterally across the subarachnoid space to the meningeal layer of dura mater. The spinal cord itself is surrounded by cerebral spinal fluid which provides hydraulic support and descends caudally through the vertebral canal of the vertebral column.
The subarachnoid space contains most of the blood vessels to the cord. It is this same space that becomes the dural/thecal sac caudal to the termination of the spinal cord. The limited space between the inner and outer dural layers is called the epidural space. It contains aggregates of adipose tissue and the internal vertebral venous plexus. This space is often used as a site for the administration of anesthetic agents in the lumbar or sacral regions (caudal block), especially in obstetrics. The spinal cord is ventrally protected by the posterior longitudinal ligament.
Each spinal cord segment contains a pair of nerve roots (and associated rootlets) called the dorsal and ventral roots. Dorsal roots contain only sensory axons, which transmit sensory information into the spinal cord. By contrast, ventral roots contain motor axons, which transmit motor commands to muscle and other body organs. Dorsal and ventral roots exemplify the separation of function in the nervous system,. These sensory and motor axons, which are part of the peripheral nervous system, become intermingled in the spinal nerves en route to their peripheral targets. There are dorsal and ventral spinal roots at every level for sensory input and motor output.
Thirty-one pairs of spinal nerves are attached to the spinal cord via dorsal (posterior) and ventral (anterior) nerve roots: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal spinal nerve on each side. Cervical nerves exit spinal cord above respective vertebra C1-C7. C8 exits below C7 (8 cervical nerves and only 7 cervical vertebrae). All other spinal nerves exit below respective vertebra. The denticulate ligaments suspend the cord within the canal - somewhat like the vertical support cables supporting the roadway of a suspension bridge - and separate the anterior and posterior roots. The collection of spinal nerve roots contained in the lumbosacral spinal canal is known as the cauda equina and ends at the filum terminale.
The cervical level consisting of eight segments (C1 - C8), is characterized by an enlargement or swelling (cervical enlargement) from C4-T1 that reflects the large amount of neurons and fibers associated with the innervation of the upper limbs. The cross-sectional appearance of the cervical cord shows the gray matter appearing as a butterfly circumscribed by a field of white matter. In comparing the section of the cervical cord with the other sections shown, note the differences in the configuration of the gray and white matter. Also observe the oval, relatively large size of the cervical section with its bulging anterior horns, indicative of the large number of anterior horn motor neurons dedicated to the upper limbs. The white matter in the cord here is extensive; it includes all the fibers ascending to the brain, collected from tall the nerve cells below, as well as all the axons descending from the brain, which terminate on neurons at the various levels of the spinal cord. Cervical nerves supply movement and sensation to the arms, neck, and upper trunk.
The thoracic segments (T1 - 12) are easy to distinguish because of the thin posterior and anterior horns, as well as the presence of the lateral horns, which are seen from the first thoracic segment to about the third lumbar segment. The white matter, of course, is still quite extensive, for the reason given previously. The thoracic level of the cord is considerably thinner than the cervical cord and contains a smaller amount of gray matter due to the lesser innervation density (number of receptors and effectors per unit area) of the thoracic and abdominal walls, and mid back. Thoracic nerves supply to the trunk and abdomen.
The lumbar level of the cord consists of five segments (L1-5). The lumbar level characteristically has a massive numbers of neurons in the lumbar level of the cord involved in the lower limbs (ambulation, bipedal motion) is reflected in the lumbar enlargement and in the enlarged anterior and posterior horns, as seen in the cross section. The amount of white matter left in this region, committed only to the lower limbs and pelvis, is considerably reduced. Lumbar and Sacral nerves supply to the legs, bladder, bowel and sexual organs.
The sacral level consists of five segments (S1-5) and the coccygeal level one segment (C01 is concerned only with the area around the bony coccyx). The caudal part of the coccygeal level tapers to a point (conus medullaris) as the last of the gray and white matter disappears leaving the cauda equina or horses tail. The sacral and coccygeal levels, the most caudal/inferior levels of the cord are smaller than the levels superior/rostral to them. The posterior horns of the sacral segment are particularly large, due probably to the great influx of sensory fibers from the genitals and pelvis and the relatively small amount of white matter.
There are about a dozen features required to understand the essential organization of the spinal cord which displays the simplest organization of all seven major divisions of the nervous system but is still complex. The key in telling one level apart from another can be determined with 2-3 important features that can identify them unambiguously as the main features of spinal cord organization are strongly conserved. The criteria for classifying different spinal cord levels are absolute size, gray to white matter ratio, shape of the ventral horn, presence of the distinct cuneate fasciculus, and existence of the intermediolateral cell column.
A cross section of the spinal cord shows a symmetrical H/Butterfly Shaped pattern. Throughout much of the length of the cord, a spinal segment is not adjacent to its corresponding vertebral segment. The gray matter is divided into ten layers of functionally distinct nuclei/nerve cells in three major zones; the dorsal and ventral horns and the intermediate zone. The dorsal/posterior horns are composed of receptive, sensory nuclei from the peripheral nervous system and the ventral/anterior horn composed of transmissive, motor nuclei which project axons to skeletal muscle and organ tissue. The intermediate zone is composed of interneurons. The white matter contains afferent/ascending and efferent/descending axons organized into three rostrocaudally organized columns which function as distinct tracts; the dorsal, lateral and ventral columns. Between the gray matter on the two sides of the spinal cord is the central canal which is a component of the ventricular system filled with cerebrospinal fluid.
The segmental organization of the spinal cord is separated into the dorsal and ventral horns; the dorsal/posterior horn is the first common pathway for sensation, has a sensory role for pain and temperature and is a conduit for all somatic sensation and has an integrative role in descending (brain to spinal cord) control of pain. The ventral/anterior horn is the origin of motor outflow. The lamina of the dorsal and ventral horns of the spinal cord are intercalated with dorsal and ventral root ganglia (bundles of nervous tissue for synaptic transmission) bilaterally at each level which respectively act for nervous system somatic sensation, motor control and inhibition feedback loops.
For spinal circuitry, the dynamic function of nerve roots are classified into sensory and motor function respectively with the dorsal and ventral root ganglion. Sensory cell bodies are located in the dorsal root ganglia where afferent information is received in the cell body and then relayed to the spinal cord tracts where it either acts in the reflex arc or ascends to somatosensory cortex. Motor cell bodies are located in the ventral root ganglia where efferent information is ultimately transmitted from motor cortex where the signal descends to terminate at the neuromuscular junction. The terms afferent and efferent are often used in place of sensory and motor to describe the direction of information flow. For the dorsal root ganglion neurons, afferent information flow is from the periphery to the central nervous system, for the ventral root ganglion neurons, efferent information flow is from the central nervous system to the periphery.
Afferent axons carry impulses from skin receptors to cell bodies in the dorsal root ganglia. Spinal afferent axons come in different sizes and represent various modalities, each of the following groups is a class of axons in peripheral nerve; A, thickest fibers, large ganglion cell axons, intermediate size conveys information about pin-prick pain. Group B, small conduction velocity, myelinated, C slower conduction velocity unmyelinated; burning, grinding pain. Dorsal root ganglion neurons are often called primary afferent fibers. The dorsal root ganglion are the only route for entry into the central nervous system which serves as a highway for sensory information from afferent axons that carry impulses from skin receptors. The dorsal root ganglion and dorsal/posterior horn are conduits for somatic sensation and the only route into the central nervous system. It is first common pathway for sensation of pain and temperature.
The axons of the ganglion neurons enter the spinal cord through the dorsal root. Dorsal root ganglion neurons project their axons directly into the spinal gray and white matter. The ganglion neurons that synapse in the dorsal horn (not shown in the figure) are part of a circuit for protective senses: pain, temperature sense, and itch. The dorsal root ganglion neurons that branch into the white matter enter the dorsal column. These neurons transmit to the brain stem sensory information about touch and limb position sense and are part of the dorsal column-medial lemniscal system. Dorsal roots are effected by lesions which interrupt sensation, including anesthesia effects which can interrupt dynamic function.
Efferent motor neuron information flow is from the central nervous system to the motor end plate at neuromuscular junction fibers.
Ventral horn motoneurons are the sole source for motor output to reach muscle and alpha motoneurons are the force-developing, fast-firing cells that innervate skeletal muscle.
The motor end plate is the synaptic ending that depolarizes the muscle cell which causes it to contract. An adequate stimulus is the strength of a sensory cue required to elicit a motor output. Threshold is the level of sensitivity at which the cell discharges 50% of the time. Reciprocal facilitation of ipsilateral synergist motoneurons; the adaptive significance of reciprocal inhibition and facilitation.
Alpha motoneurons innervate extrafusal muscle and regulate the excitability of muscle receptors, gamma motoneurons innervate intrafusal muscle and regulate the excitability of muscle receptors and spinal interneurons mediate intraspinal interactions that coordinate motoneuron discharge.
Motor cell bodies are located in the ventral root ganglia; efferent information is transmitted from motor cortex, received in the cell body and then relayed to the spinal cord tracts. The ventral horn motoneurons are the sole source for motor output to reach muscle tissue. Alpha motoneurons are the force-developing, fast-firing cells that innervate skeletal muscle. The motor end plate is the synaptic ending that depolarizes the muscle cell, causing it to contract.The ventral (anterior) horn is the origin of motor outflow.
Alpha motoneurons innervate extrafusal muscle fibers and develop much force. Gamma motoneurons innervate intrafusal muscle and regulate the excitability of muscle receptors. Spinal interneurons mediate intraspinal interactions that coordinate motoneuron discharge. Ventral roots: effects of lesions include paralysis and loss of autonomic control.
Neurons of the ventral horn subserve limb and trunk movements. Motor neurons located here have axons that exit the spinal cord through the ventral root. Motor neurons also receive direct connections from the corticospinal tract, whose axons course in the lateral column of the white matter. Whereas in healthy tissue the location of the corticospinal tract can only be inferred, its location in tissue from a person who had a lesion involving the tract is clearly revealed as the lightly stained region in figure 2-4. The ventral column contains the axons of both ascending sensory and descending motor pathways.
The efferent (descending) system takes advantage of intercalated spinal interneurons and reciprocal inhibition to ensure precise and protective motor control.
CONNECTIONS
Monosynaptic connections occur between a certain type of dorsal root ganglion neuron that innervates stretch receptors in muscles and motor neurons. In certain segments of the spinal cord, this circuit mediates the knee-jerk reflex. A tap to the patella tendon stretches the quadriceps muscle, thereby stretching the receptors in the muscle. the central branches of dorsal root ganglion cells that innervate these receptors in the muscle. The central branches of dorsal root ganglion cells that innervate these receptors synapse on quadriceps motor neurons. because this synapse is excitatory, the quadriceps motor neurons are discharged and the muscle contracts. Many leg and arm muscles have similar stretch reflexes.
The dermatomes provide a segmental map of the body surface that is conserved in the brain.
Spinal roots; sacral, lumbar, thoracic, cervical, denticular ligaments (mechanical isolation), spinal dura, plexusus (brachial: C1-T1). Sympathetic chain ganglia promotes visceral output for digestion and urine production and parasympathetic ganglia are essential for voluntary control of micturition, ejaculation and milk ejection. The dermatomes provide a segmental map of the body surface that is conserved in the brain. Innervation density is proportional to sensitivity or fineness of movement. The intermediolateral cell column is the spinal basis for thoracicolumbar autonomic control. Autonomic motoneurons projecting to ganglia whose neurons innervate smooth muscle.
A recurring theme in the structure of the spinal cord and brain stem is that medial structures correspond to axial areas in the periphery, while lateral structures correspond to distal areas
reticulospinal and rubrospinal tracts; responsible for axial or postural control and proximal and distal muscles respectively. As you move more laterally shift moves to control more distal muscles
Central Axons of Dorsal Root Ganglia Neurons are somatotopically arranged; sacral near the mid-line, higher progressively more lateral.
Gracile and cuneate nuclei receive input from the trunk and limbs.
The cerebral cortex, the pons and the medulla all project strongly to the spinal cord and can affect spinal reflex output. Positive signs such as spasticity indicate release from normal inhibition, consequent hyperreflexia and increased muscle tone. Negative signs such as muscle weakness indicate pathological absence of normal facilitation, consequent hyporeflexia and decreased muscle tone.
The sacral spinal cord has less white matter than cervical, limb areas have lots of gray. Motor nuclei consists of motor neuron pools that form longitudinal columns spanning 1-4 spinal segments; medial-lateral, proximal-distal, dorsal-ventral, flexors-extensors
Understanding the framework of the spinal cord helps one to identify tracts and nuclei. The posterior median sulcus marks the posterior median septum which is a line of neuroglia that extends to the level of the gray commissure. The cervical and thoracic white matter is divided into two masses and the posterior and anterior lateral sulci mark the entrance and exit sites of the posterior and anterior roots of the spinal nerves. The anterior median fissure reaches the white commissure separating the anterior white matter into left and right halves.
The white matter of the cord is divided into three large divisions (funiculi); posterior lateral and inferior. The gray matter is arranged into posterior and anterior horns or columns. The columns are further divided into laminae.
Lamina I - The Posterior Marginal Nucleus - Outermost posterior horn. Primary afferent axons for pain and temperature. Spinothalamic tract
Lamina II - The Substantia Gelatinosa - Receives afferent fibers from Lissauer's fasiculus for pain, touch and temperature
Lamina III & IV - The Nuclei Propius - Interneurons that receive touch and pressure stimuli. Axons of certain neurons decussate to contralateral for the spinothalamic tract
Lamina V - Painful and non-painful stimuli. Interneurons decussate to contralateral for the spinothalamic tract
Lamina VI - Posterior horns in cervical and lumbar enlargements only, receives afferent input from primary sensory neurons
Lamina VII - The Intermediate Zone - Interneurons; dorsal nucleus of Clarke; Clarke's column for posterior spinocerebellar tract.
Lamina VIII - Anterior horn for axons of descending tracts are known to synapse among the interneurons here
Lamina IX - Anterior horn; columns of large motor neurons axons of which form anterior roots of spinal nerves
Lamina X - Gray Commissure - gray matter surrounding the central canal consisting of small interneurons
It is approximately 45 centimeters/18 inches in length in males and 2 centimeters less in females, with a diameter of 8mm (pencil thick). It transitions superiorly/rostrally, in adults, from the caudal margin of the medulla, exits the foramen magnum of the skull and travels inferiorly/caudally toward the sacrum. It is contained within the vertebral column and ends at the level of the first or second lumbar vertebra as the conus medullaris. The Cauda Equina (horse's tail) extends caudally below the level of the conus medullaris and consists of spinal nerves that extend downward into the spinal canal and exits the foramen of the vertebrae. It comprises the distal/caudal two-thirds of the central nervous system. Denticular ligaments provide mechanical isolation so that the spinal cord is not subject to physical stress.
The spinal cord has a modular organization, in which every segment has a similar basic structure. It is subdivided into levels corresponding to the vertebral regions surrounding it. Within each level, the cord is further subdivided into segments, corresponding to the segmental appearance of the spinal nerves. The spinal cord is the only part of the central nervous system that has an external segmental organization. Just as the cerebral cortex and brain stem, the spinal cord is structurally composed of gray and white matter. The central cord is composed of gray matter composed of nerve cell bodies and is surrounded by a peripheral white matter region containing myelinated axons.
The spinal cord is covered in meninges; dura, arachnoid, pia similar to those that surround the brain, the dura mater surrounding the brain continues downward and completely envelopes the spinal cord. The spinal cord itself is closely covered by a thin layer of arachnoid and an inner layer of pia mater. The outer or periosteal layer of dura is closely adherent to the walls of the vertebral (spinal) canal, serving as its periosteum, a dense layer of vascular connective tissue that envelopes bones. The inner or meningeal layer of dura forms a closed tube around the cord and continues caudally as far as the second sacral vertebra, at which it encloses the filum terminale (plate 9-2). Extensions of the dura pass laterally like sleeves along the spinal roots to become continuous with the epineurial sheaths of the spinal nerves. Between dural sleeves and along the length of the spinal cord, the pia mater thickens and extends laterally across the subarachnoid space to the meningeal layer of dura mater. The spinal cord itself is surrounded by cerebral spinal fluid which provides hydraulic support and descends caudally through the vertebral canal of the vertebral column.
The subarachnoid space contains most of the blood vessels to the cord. It is this same space that becomes the dural/thecal sac caudal to the termination of the spinal cord. The limited space between the inner and outer dural layers is called the epidural space. It contains aggregates of adipose tissue and the internal vertebral venous plexus. This space is often used as a site for the administration of anesthetic agents in the lumbar or sacral regions (caudal block), especially in obstetrics. The spinal cord is ventrally protected by the posterior longitudinal ligament.
Each spinal cord segment contains a pair of nerve roots (and associated rootlets) called the dorsal and ventral roots. Dorsal roots contain only sensory axons, which transmit sensory information into the spinal cord. By contrast, ventral roots contain motor axons, which transmit motor commands to muscle and other body organs. Dorsal and ventral roots exemplify the separation of function in the nervous system,. These sensory and motor axons, which are part of the peripheral nervous system, become intermingled in the spinal nerves en route to their peripheral targets. There are dorsal and ventral spinal roots at every level for sensory input and motor output.
Thirty-one pairs of spinal nerves are attached to the spinal cord via dorsal (posterior) and ventral (anterior) nerve roots: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal spinal nerve on each side. Cervical nerves exit spinal cord above respective vertebra C1-C7. C8 exits below C7 (8 cervical nerves and only 7 cervical vertebrae). All other spinal nerves exit below respective vertebra. The denticulate ligaments suspend the cord within the canal - somewhat like the vertical support cables supporting the roadway of a suspension bridge - and separate the anterior and posterior roots. The collection of spinal nerve roots contained in the lumbosacral spinal canal is known as the cauda equina and ends at the filum terminale.
The cervical level consisting of eight segments (C1 - C8), is characterized by an enlargement or swelling (cervical enlargement) from C4-T1 that reflects the large amount of neurons and fibers associated with the innervation of the upper limbs. The cross-sectional appearance of the cervical cord shows the gray matter appearing as a butterfly circumscribed by a field of white matter. In comparing the section of the cervical cord with the other sections shown, note the differences in the configuration of the gray and white matter. Also observe the oval, relatively large size of the cervical section with its bulging anterior horns, indicative of the large number of anterior horn motor neurons dedicated to the upper limbs. The white matter in the cord here is extensive; it includes all the fibers ascending to the brain, collected from tall the nerve cells below, as well as all the axons descending from the brain, which terminate on neurons at the various levels of the spinal cord. Cervical nerves supply movement and sensation to the arms, neck, and upper trunk.
The thoracic segments (T1 - 12) are easy to distinguish because of the thin posterior and anterior horns, as well as the presence of the lateral horns, which are seen from the first thoracic segment to about the third lumbar segment. The white matter, of course, is still quite extensive, for the reason given previously. The thoracic level of the cord is considerably thinner than the cervical cord and contains a smaller amount of gray matter due to the lesser innervation density (number of receptors and effectors per unit area) of the thoracic and abdominal walls, and mid back. Thoracic nerves supply to the trunk and abdomen.
The lumbar level of the cord consists of five segments (L1-5). The lumbar level characteristically has a massive numbers of neurons in the lumbar level of the cord involved in the lower limbs (ambulation, bipedal motion) is reflected in the lumbar enlargement and in the enlarged anterior and posterior horns, as seen in the cross section. The amount of white matter left in this region, committed only to the lower limbs and pelvis, is considerably reduced. Lumbar and Sacral nerves supply to the legs, bladder, bowel and sexual organs.
The sacral level consists of five segments (S1-5) and the coccygeal level one segment (C01 is concerned only with the area around the bony coccyx). The caudal part of the coccygeal level tapers to a point (conus medullaris) as the last of the gray and white matter disappears leaving the cauda equina or horses tail. The sacral and coccygeal levels, the most caudal/inferior levels of the cord are smaller than the levels superior/rostral to them. The posterior horns of the sacral segment are particularly large, due probably to the great influx of sensory fibers from the genitals and pelvis and the relatively small amount of white matter.
There are about a dozen features required to understand the essential organization of the spinal cord which displays the simplest organization of all seven major divisions of the nervous system but is still complex. The key in telling one level apart from another can be determined with 2-3 important features that can identify them unambiguously as the main features of spinal cord organization are strongly conserved. The criteria for classifying different spinal cord levels are absolute size, gray to white matter ratio, shape of the ventral horn, presence of the distinct cuneate fasciculus, and existence of the intermediolateral cell column.
A cross section of the spinal cord shows a symmetrical H/Butterfly Shaped pattern. Throughout much of the length of the cord, a spinal segment is not adjacent to its corresponding vertebral segment. The gray matter is divided into ten layers of functionally distinct nuclei/nerve cells in three major zones; the dorsal and ventral horns and the intermediate zone. The dorsal/posterior horns are composed of receptive, sensory nuclei from the peripheral nervous system and the ventral/anterior horn composed of transmissive, motor nuclei which project axons to skeletal muscle and organ tissue. The intermediate zone is composed of interneurons. The white matter contains afferent/ascending and efferent/descending axons organized into three rostrocaudally organized columns which function as distinct tracts; the dorsal, lateral and ventral columns. Between the gray matter on the two sides of the spinal cord is the central canal which is a component of the ventricular system filled with cerebrospinal fluid.
The segmental organization of the spinal cord is separated into the dorsal and ventral horns; the dorsal/posterior horn is the first common pathway for sensation, has a sensory role for pain and temperature and is a conduit for all somatic sensation and has an integrative role in descending (brain to spinal cord) control of pain. The ventral/anterior horn is the origin of motor outflow. The lamina of the dorsal and ventral horns of the spinal cord are intercalated with dorsal and ventral root ganglia (bundles of nervous tissue for synaptic transmission) bilaterally at each level which respectively act for nervous system somatic sensation, motor control and inhibition feedback loops.
For spinal circuitry, the dynamic function of nerve roots are classified into sensory and motor function respectively with the dorsal and ventral root ganglion. Sensory cell bodies are located in the dorsal root ganglia where afferent information is received in the cell body and then relayed to the spinal cord tracts where it either acts in the reflex arc or ascends to somatosensory cortex. Motor cell bodies are located in the ventral root ganglia where efferent information is ultimately transmitted from motor cortex where the signal descends to terminate at the neuromuscular junction. The terms afferent and efferent are often used in place of sensory and motor to describe the direction of information flow. For the dorsal root ganglion neurons, afferent information flow is from the periphery to the central nervous system, for the ventral root ganglion neurons, efferent information flow is from the central nervous system to the periphery.
Afferent axons carry impulses from skin receptors to cell bodies in the dorsal root ganglia. Spinal afferent axons come in different sizes and represent various modalities, each of the following groups is a class of axons in peripheral nerve; A, thickest fibers, large ganglion cell axons, intermediate size conveys information about pin-prick pain. Group B, small conduction velocity, myelinated, C slower conduction velocity unmyelinated; burning, grinding pain. Dorsal root ganglion neurons are often called primary afferent fibers. The dorsal root ganglion are the only route for entry into the central nervous system which serves as a highway for sensory information from afferent axons that carry impulses from skin receptors. The dorsal root ganglion and dorsal/posterior horn are conduits for somatic sensation and the only route into the central nervous system. It is first common pathway for sensation of pain and temperature.
The axons of the ganglion neurons enter the spinal cord through the dorsal root. Dorsal root ganglion neurons project their axons directly into the spinal gray and white matter. The ganglion neurons that synapse in the dorsal horn (not shown in the figure) are part of a circuit for protective senses: pain, temperature sense, and itch. The dorsal root ganglion neurons that branch into the white matter enter the dorsal column. These neurons transmit to the brain stem sensory information about touch and limb position sense and are part of the dorsal column-medial lemniscal system. Dorsal roots are effected by lesions which interrupt sensation, including anesthesia effects which can interrupt dynamic function.
Efferent motor neuron information flow is from the central nervous system to the motor end plate at neuromuscular junction fibers.
Ventral horn motoneurons are the sole source for motor output to reach muscle and alpha motoneurons are the force-developing, fast-firing cells that innervate skeletal muscle.
The motor end plate is the synaptic ending that depolarizes the muscle cell which causes it to contract. An adequate stimulus is the strength of a sensory cue required to elicit a motor output. Threshold is the level of sensitivity at which the cell discharges 50% of the time. Reciprocal facilitation of ipsilateral synergist motoneurons; the adaptive significance of reciprocal inhibition and facilitation.
Alpha motoneurons innervate extrafusal muscle and regulate the excitability of muscle receptors, gamma motoneurons innervate intrafusal muscle and regulate the excitability of muscle receptors and spinal interneurons mediate intraspinal interactions that coordinate motoneuron discharge.
Motor cell bodies are located in the ventral root ganglia; efferent information is transmitted from motor cortex, received in the cell body and then relayed to the spinal cord tracts. The ventral horn motoneurons are the sole source for motor output to reach muscle tissue. Alpha motoneurons are the force-developing, fast-firing cells that innervate skeletal muscle. The motor end plate is the synaptic ending that depolarizes the muscle cell, causing it to contract.The ventral (anterior) horn is the origin of motor outflow.
Alpha motoneurons innervate extrafusal muscle fibers and develop much force. Gamma motoneurons innervate intrafusal muscle and regulate the excitability of muscle receptors. Spinal interneurons mediate intraspinal interactions that coordinate motoneuron discharge. Ventral roots: effects of lesions include paralysis and loss of autonomic control.
Neurons of the ventral horn subserve limb and trunk movements. Motor neurons located here have axons that exit the spinal cord through the ventral root. Motor neurons also receive direct connections from the corticospinal tract, whose axons course in the lateral column of the white matter. Whereas in healthy tissue the location of the corticospinal tract can only be inferred, its location in tissue from a person who had a lesion involving the tract is clearly revealed as the lightly stained region in figure 2-4. The ventral column contains the axons of both ascending sensory and descending motor pathways.
The efferent (descending) system takes advantage of intercalated spinal interneurons and reciprocal inhibition to ensure precise and protective motor control.
CONNECTIONS
Monosynaptic connections occur between a certain type of dorsal root ganglion neuron that innervates stretch receptors in muscles and motor neurons. In certain segments of the spinal cord, this circuit mediates the knee-jerk reflex. A tap to the patella tendon stretches the quadriceps muscle, thereby stretching the receptors in the muscle. the central branches of dorsal root ganglion cells that innervate these receptors in the muscle. The central branches of dorsal root ganglion cells that innervate these receptors synapse on quadriceps motor neurons. because this synapse is excitatory, the quadriceps motor neurons are discharged and the muscle contracts. Many leg and arm muscles have similar stretch reflexes.
The dermatomes provide a segmental map of the body surface that is conserved in the brain.
Spinal roots; sacral, lumbar, thoracic, cervical, denticular ligaments (mechanical isolation), spinal dura, plexusus (brachial: C1-T1). Sympathetic chain ganglia promotes visceral output for digestion and urine production and parasympathetic ganglia are essential for voluntary control of micturition, ejaculation and milk ejection. The dermatomes provide a segmental map of the body surface that is conserved in the brain. Innervation density is proportional to sensitivity or fineness of movement. The intermediolateral cell column is the spinal basis for thoracicolumbar autonomic control. Autonomic motoneurons projecting to ganglia whose neurons innervate smooth muscle.
A recurring theme in the structure of the spinal cord and brain stem is that medial structures correspond to axial areas in the periphery, while lateral structures correspond to distal areas
reticulospinal and rubrospinal tracts; responsible for axial or postural control and proximal and distal muscles respectively. As you move more laterally shift moves to control more distal muscles
Central Axons of Dorsal Root Ganglia Neurons are somatotopically arranged; sacral near the mid-line, higher progressively more lateral.
Gracile and cuneate nuclei receive input from the trunk and limbs.
The cerebral cortex, the pons and the medulla all project strongly to the spinal cord and can affect spinal reflex output. Positive signs such as spasticity indicate release from normal inhibition, consequent hyperreflexia and increased muscle tone. Negative signs such as muscle weakness indicate pathological absence of normal facilitation, consequent hyporeflexia and decreased muscle tone.
The sacral spinal cord has less white matter than cervical, limb areas have lots of gray. Motor nuclei consists of motor neuron pools that form longitudinal columns spanning 1-4 spinal segments; medial-lateral, proximal-distal, dorsal-ventral, flexors-extensors
Understanding the framework of the spinal cord helps one to identify tracts and nuclei. The posterior median sulcus marks the posterior median septum which is a line of neuroglia that extends to the level of the gray commissure. The cervical and thoracic white matter is divided into two masses and the posterior and anterior lateral sulci mark the entrance and exit sites of the posterior and anterior roots of the spinal nerves. The anterior median fissure reaches the white commissure separating the anterior white matter into left and right halves.
The white matter of the cord is divided into three large divisions (funiculi); posterior lateral and inferior. The gray matter is arranged into posterior and anterior horns or columns. The columns are further divided into laminae.
Lamina I - The Posterior Marginal Nucleus - Outermost posterior horn. Primary afferent axons for pain and temperature. Spinothalamic tract
Lamina II - The Substantia Gelatinosa - Receives afferent fibers from Lissauer's fasiculus for pain, touch and temperature
Lamina III & IV - The Nuclei Propius - Interneurons that receive touch and pressure stimuli. Axons of certain neurons decussate to contralateral for the spinothalamic tract
Lamina V - Painful and non-painful stimuli. Interneurons decussate to contralateral for the spinothalamic tract
Lamina VI - Posterior horns in cervical and lumbar enlargements only, receives afferent input from primary sensory neurons
Lamina VII - The Intermediate Zone - Interneurons; dorsal nucleus of Clarke; Clarke's column for posterior spinocerebellar tract.
Lamina VIII - Anterior horn for axons of descending tracts are known to synapse among the interneurons here
Lamina IX - Anterior horn; columns of large motor neurons axons of which form anterior roots of spinal nerves
Lamina X - Gray Commissure - gray matter surrounding the central canal consisting of small interneurons
A nerve is a bundle of many nerve fibers that course along the same path in the nervous system. Most nerves include bundles of both sensory and motor fibers. A tract is a bundle of nerve fibers in the central nervous system. Nerves are made up of groupings of fibers that have different size and properties. Group A fibers are 1 - 20 um in diameter and have 3 -120 meter/second conduction velocity. The myelinated fibers are the thickest fibers that represent large ganglion cell axons innervating rapidly adapting corpuscular mechanoreceptors. Intermediate-sized group A axons convey information about pinprick-like pain. Group B fibers are 1 - 3 um in diameter and have 3-15 meters/second conduction velocity and are myelinated. Group C fibers are 0.3-1.3 um in diameter and have 0.6-2.0 meters/second conduction velocity and are unmyelinated. These axons arise from small dorsal root ganglion cells whose peripheral processes innervate free nerve endings that respond selectively to burning, grinding pain.
Peripheral Nervous System; spinal nerves; efferent axons extend to muscles and glands from the cell bodies in the ventral horns of the spinal cordThe Sensory Pathway ascends/is afferent to the brain and transmits sensory nerve impulses from receptors in the skin, sense organs, muscles, joints, and viscera toward the brain and spinal cord. The Motor Pathway descends/is efferent from the brain and transmits motor nerve impulses from the brain and spinal cord to effectors, muscles or glands.
Nerve Plexuses, brachial C1-T1 and Lumbar L1-4.Discuss the median, radial, ulnar nerves in addition to posterior tibial, popliteal fossa, access of the olecranon fossa and armpit, etcetera. conduction velocity in addition to stimulation sites and advantage of the most proximal location for stimulation and protection. *Insert bourgery image of hand here.Brachial Plexus, Lumbar Plexus. The major peripheral structures are the somatic nerves, the autonomic nerves and ganglia, the neuromuscular junction, the muscles and the peripheral sensory receptors. spinal nerves are formed by the joining of dorsal and ventral roots and thus contain motor and sensory nerve fibers. Fibers en route to the limbs come together and are rearranged in plexuses. The brachial plexus, located in the axillary region, redistributes the fibers to the major nerves of the upper arms: median, ulnar, radial, axillary and musculo-cutaneous.
The lumbosacral plexus located in the lower abdominal cavity and pelvis, redistributes the fibers to the major nerves int he lower legs, femoral, obturator, and sciatic, which divides into the tibial and peroneal nerves. The brachial plexus exiting from the cervical enlargement and most significantly from C1-T1 travel through the shoulders and move distally to the arm and hand attaching at muscles. The dermatomes provide a segmental map of the body surface that is conserved in the brain. Innervation density is proportional to sensitivity or fineness of movement. The denticular ligaments are triangular shaped. There are five (5) nerves that arise from the brachial plexus; axillary, musculocutaneous, radial nerve, median nerve, ulnar nerve. The brachial plexus is a network of nerve fibers formed by the ventral rami of C5-T1 spinal nerves; the nerves supply the muscles of the upper limb and shoulder. There are five (5) nerves that arise from the lumbar plexus; iliohypogastric, ilioinguinal, genitorfemoral, lateral femoral cutaneous, femoral obturator. The ventral rami of spinal nerves L4-5 and S1-4 form the sacral plexus. It is situated anterior to the sacrum and supplies the buttocks, perineum, and lower extremities. The largest nerve in the body, the sciatic nerve arises from the sacral plexus. The sciatic nerve (L4-S3) splits into two divisions at the knee; tibial and common peroneal nerve. Innervation density is proportional to sensitivity or fineness of movement. It contains motoneurons that innervate (have axons that are presynaptic to) skeletal or smooth muscles which convey descending information from the cortex to the spinal cord via the corticospinal tract and to serve as a highway for skin and joint afferent input ascending to the brain stem and cerebral cortex.
The spinal cord has afferent and efferent fiber tracts that interact with the matrix of nuclei located within the grey matter. Within these structures, intraspinal and supraspinal modulation of reflexes and propriospinal connections function. We will talk about fiber descriptions, reflex organization and tracts connecting muscle and nervous tissue.
Basic reflex organization; the neuromuscular spindle is a receptor embedded in the muscle that signals length, Ia afferent system conveys this information to the brain and spinal cord, the dorsal root ganglion is the only route for entry into the central nervous system.
The dorsal horn serves as a highway for entering ganglion cell fibers but receives no touch-related synaptic endings.
There is an integrative role in descending (brain to spinal cord) control of pain. Sensory nerve endings innervate peripheral tissue and transmit this sensory information to neurons in the central nervous system. Different classes of dorsal root ganglion neurons sense different kinds of stimuli.
The neuromuscular spindle is a receptor embedded in the muscle that signals length. The 1a afferent system conveys this information to (i) the spinal cord and (ii) the brain via the dorsal root ganglion.
The terms afferent and efferent are also commonly used to describe direction of information flow within the central nervous system in relation to a particular target. For example, with respect to the motor neuron, both dorsal root ganglion axons and axons in the corticospinal tract carry afferent information. There is a distinction, however, because only the former transmit sensory information.
Light touch receptors are in the gracile nucleus, located midline in the lumbar regions and central/medial in cervical levels in the dorsal column medial lemniscal tract. Light touch receptors from the pinky finger in cuneate nucleus which is lateral dorsal columns.
The intermediolateral cell column is the spinal basis for thoracicolumbar autonomic control. Autonomic motoneurons projecting to ganglia whose neurons innervate smooth muscle. Spinal afferent axons come in different sizes and represent various modalities; each of the following groups is a class of axons in peripheral nerve.
The cerebral cortex, the pons, and the medulla all project strongly to the spinal cord and can affect spinal reflex output. Positive signs such as spasticity indicate release from normal inhibition, with consequent hyper-reflexia and increased muscle tone. Negative signs such as muscle weakness indicate pathological absence of normal facilitation, with consequent hyper-reflexia and decreased muscle tone.
Peripheral Nervous System; spinal nerves; efferent axons extend to muscles and glands from the cell bodies in the ventral horns of the spinal cordThe Sensory Pathway ascends/is afferent to the brain and transmits sensory nerve impulses from receptors in the skin, sense organs, muscles, joints, and viscera toward the brain and spinal cord. The Motor Pathway descends/is efferent from the brain and transmits motor nerve impulses from the brain and spinal cord to effectors, muscles or glands.
Nerve Plexuses, brachial C1-T1 and Lumbar L1-4.Discuss the median, radial, ulnar nerves in addition to posterior tibial, popliteal fossa, access of the olecranon fossa and armpit, etcetera. conduction velocity in addition to stimulation sites and advantage of the most proximal location for stimulation and protection. *Insert bourgery image of hand here.Brachial Plexus, Lumbar Plexus. The major peripheral structures are the somatic nerves, the autonomic nerves and ganglia, the neuromuscular junction, the muscles and the peripheral sensory receptors. spinal nerves are formed by the joining of dorsal and ventral roots and thus contain motor and sensory nerve fibers. Fibers en route to the limbs come together and are rearranged in plexuses. The brachial plexus, located in the axillary region, redistributes the fibers to the major nerves of the upper arms: median, ulnar, radial, axillary and musculo-cutaneous.
The lumbosacral plexus located in the lower abdominal cavity and pelvis, redistributes the fibers to the major nerves int he lower legs, femoral, obturator, and sciatic, which divides into the tibial and peroneal nerves. The brachial plexus exiting from the cervical enlargement and most significantly from C1-T1 travel through the shoulders and move distally to the arm and hand attaching at muscles. The dermatomes provide a segmental map of the body surface that is conserved in the brain. Innervation density is proportional to sensitivity or fineness of movement. The denticular ligaments are triangular shaped. There are five (5) nerves that arise from the brachial plexus; axillary, musculocutaneous, radial nerve, median nerve, ulnar nerve. The brachial plexus is a network of nerve fibers formed by the ventral rami of C5-T1 spinal nerves; the nerves supply the muscles of the upper limb and shoulder. There are five (5) nerves that arise from the lumbar plexus; iliohypogastric, ilioinguinal, genitorfemoral, lateral femoral cutaneous, femoral obturator. The ventral rami of spinal nerves L4-5 and S1-4 form the sacral plexus. It is situated anterior to the sacrum and supplies the buttocks, perineum, and lower extremities. The largest nerve in the body, the sciatic nerve arises from the sacral plexus. The sciatic nerve (L4-S3) splits into two divisions at the knee; tibial and common peroneal nerve. Innervation density is proportional to sensitivity or fineness of movement. It contains motoneurons that innervate (have axons that are presynaptic to) skeletal or smooth muscles which convey descending information from the cortex to the spinal cord via the corticospinal tract and to serve as a highway for skin and joint afferent input ascending to the brain stem and cerebral cortex.
The spinal cord has afferent and efferent fiber tracts that interact with the matrix of nuclei located within the grey matter. Within these structures, intraspinal and supraspinal modulation of reflexes and propriospinal connections function. We will talk about fiber descriptions, reflex organization and tracts connecting muscle and nervous tissue.
Basic reflex organization; the neuromuscular spindle is a receptor embedded in the muscle that signals length, Ia afferent system conveys this information to the brain and spinal cord, the dorsal root ganglion is the only route for entry into the central nervous system.
The dorsal horn serves as a highway for entering ganglion cell fibers but receives no touch-related synaptic endings.
There is an integrative role in descending (brain to spinal cord) control of pain. Sensory nerve endings innervate peripheral tissue and transmit this sensory information to neurons in the central nervous system. Different classes of dorsal root ganglion neurons sense different kinds of stimuli.
The neuromuscular spindle is a receptor embedded in the muscle that signals length. The 1a afferent system conveys this information to (i) the spinal cord and (ii) the brain via the dorsal root ganglion.
The terms afferent and efferent are also commonly used to describe direction of information flow within the central nervous system in relation to a particular target. For example, with respect to the motor neuron, both dorsal root ganglion axons and axons in the corticospinal tract carry afferent information. There is a distinction, however, because only the former transmit sensory information.
Light touch receptors are in the gracile nucleus, located midline in the lumbar regions and central/medial in cervical levels in the dorsal column medial lemniscal tract. Light touch receptors from the pinky finger in cuneate nucleus which is lateral dorsal columns.
The intermediolateral cell column is the spinal basis for thoracicolumbar autonomic control. Autonomic motoneurons projecting to ganglia whose neurons innervate smooth muscle. Spinal afferent axons come in different sizes and represent various modalities; each of the following groups is a class of axons in peripheral nerve.
The cerebral cortex, the pons, and the medulla all project strongly to the spinal cord and can affect spinal reflex output. Positive signs such as spasticity indicate release from normal inhibition, with consequent hyper-reflexia and increased muscle tone. Negative signs such as muscle weakness indicate pathological absence of normal facilitation, with consequent hyper-reflexia and decreased muscle tone.