Functional Neuroanatomy: Spinal Somatic Sensory Systems
The somatic sensory systems mediate touch, and limb position senses, as well as pain, itch, and temperature senses. These systems are also critically involved in the maintenance of arousal and int eh sensory regulation of limb and trunk movements. To carry out these functions, the somatic sensory systems process stimuli both from the body surface and from within the body - from the muscles, joints, and viscera.
Spinal somatic sensory systems transmit information from the limbs, neck, and trunk to the brain stem and then to the thalamus and somatic sensory cortex. The trigeminal systems consist of separate pathways that begin in the brain stem and transmit information from the head to the thalamus and then the cortex. The spinal and trigeminal systems remain distinct as they travel to the cortex, contacting separate populations of neurons at each processing stage. However, even though the ascending spinal and trigeminal pathways are separate, their general organization is remarkably similar.
The general organization of the spinal sensory system is discussed first, giving an overview of the functional anatomy of the somatic sensory pathways. Then the regional anatomy is examined at different levels through the nervous system beginning with the morphology of the somatic sensory receptor neurons and continuing to the cerebral cortex
Spinal somatic sensory systems transmit information from the limbs, neck, and trunk to the brain stem and then to the thalamus and somatic sensory cortex. The trigeminal systems consist of separate pathways that begin in the brain stem and transmit information from the head to the thalamus and then the cortex. The spinal and trigeminal systems remain distinct as they travel to the cortex, contacting separate populations of neurons at each processing stage. However, even though the ascending spinal and trigeminal pathways are separate, their general organization is remarkably similar.
The general organization of the spinal sensory system is discussed first, giving an overview of the functional anatomy of the somatic sensory pathways. Then the regional anatomy is examined at different levels through the nervous system beginning with the morphology of the somatic sensory receptor neurons and continuing to the cerebral cortex
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The somatic or bodily senses consist of five distinct modalities: limb position sense, touch, temperature sense, pain and itch. The first two modalities comprise the meahnical sensations, whereas the later three complrise the protective sensations. Many of these modalites are engaged during routine activity. For example, in picking up a cup of hot coffee, you use limb position sense in identifying the shape of your hand as you grasp the handle; contact with the cup is detected by touch. Temperature sense provedes information about the temperature of the cup, and if the cup is too hot, you experience pain. Itch is triggered selectivey by chemical irritation of the skin, provoking the urge to scratch, thereby tending to remove the offending substance. Each modality can be further subdivided into submodalities, which adds to the richness of somatic sensations. For example, touch consists of several componnent mechanical sensations including senses of texture and pressure pain has distinctive pricking and burning sensations.
somatic sensations are clinically important. Pain typically brings a patient to visit a physician. Somatic sensations also can be importnat for diagnosing disease. For example, persistent itch can signal liver disease. Vibration sense, a touch submodality is routinely used to probe sensory function in numans suspected of having peripheral nerve or central nervous system damage.
The somatic or bodily senses consist of five distinct modalities: limb position sense, touch, temperature sense, pain and itch. The first two modalities comprise the meahnical sensations, whereas the later three complrise the protective sensations. Many of these modalites are engaged during routine activity. For example, in picking up a cup of hot coffee, you use limb position sense in identifying the shape of your hand as you grasp the handle; contact with the cup is detected by touch. Temperature sense provedes information about the temperature of the cup, and if the cup is too hot, you experience pain. Itch is triggered selectivey by chemical irritation of the skin, provoking the urge to scratch, thereby tending to remove the offending substance. Each modality can be further subdivided into submodalities, which adds to the richness of somatic sensations. For example, touch consists of several componnent mechanical sensations including senses of texture and pressure pain has distinctive pricking and burning sensations.
somatic sensations are clinically important. Pain typically brings a patient to visit a physician. Somatic sensations also can be importnat for diagnosing disease. For example, persistent itch can signal liver disease. Vibration sense, a touch submodality is routinely used to probe sensory function in numans suspected of having peripheral nerve or central nervous system damage.
The Dorsal Column Medial Lemniscal System and the Anterolateral System Mediate Different Somatic Sensations
Touch and limb position sense are mediated by the dorsal column-medial leniscal system named after its two principal compnents.
In contraxt, the anterolateral system subserves pain, itch and temperature senses and to a much lesser extend, touch. It is named for the location of its spinal axons, which ascend in the anterior portion of the lateral column. Each of these systems consists of a series of neurons from sensory receptor neurons in the periphery to the contralateral cerebral cortex.
The dorsal column medial lemniscal and anterolateral systems are examples of parallel sensory pathways, each of which connects the receptive sheet with the cortex but for distinciting functions.
The follwoing sections compare the corsal column-medial lemmniscal and anterolateral systems, emphasizing the four key differncs in their organization.
BREAK on page 109 and continued on dorsal column page
Touch and limb position sense are mediated by the dorsal column-medial leniscal system named after its two principal compnents.
In contraxt, the anterolateral system subserves pain, itch and temperature senses and to a much lesser extend, touch. It is named for the location of its spinal axons, which ascend in the anterior portion of the lateral column. Each of these systems consists of a series of neurons from sensory receptor neurons in the periphery to the contralateral cerebral cortex.
The dorsal column medial lemniscal and anterolateral systems are examples of parallel sensory pathways, each of which connects the receptive sheet with the cortex but for distinciting functions.
The follwoing sections compare the corsal column-medial lemmniscal and anterolateral systems, emphasizing the four key differncs in their organization.
BREAK on page 109 and continued on dorsal column page
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Descending Motor Pathways and the Motor Function of the Spinal Cord
The motor systems of the brain and spinal cord work together to control body movement. These systems must fulfill diverse tasks because the functions of the muslces of the body differe markedly. Consider, for instance the fine control required of skeletal muscles in the hand in graspina china cup in contrast to the gross strength reqired of back and leg muscles in lifting a box full of books. The muscles that move the eyes have an entirely different set of tasks, such as positioning the eyes to capture information from the visual world. The principal function of facial muscles is not movement but re=ather creating facial expressions as well as assisting in speech articulatin. These jobs are so varied that it is not surprising that the motor systems have specifi compnents devoted to their control.
The first three chapters covering the motor system examine the components of the brain and spinal cord that are essential for contracting muscle. Damage to these components can produce muscle weakness. This chapter focuses on the neuroanatomy of limb control and posture, first with an overview and then with an examinatio of the desending spinal motor pathways and the spinal cord motor nuclei to which they project. The pathways that control facial and other head muscles are discussed in Chapter 11. Because eye movement and balance share many neural circuits and a strong interrelationship with the vestibular system, these topics are covered jointly in Chapter 12
Descending Motor Pathways and the Motor Function of the Spinal Cord
The motor systems of the brain and spinal cord work together to control body movement. These systems must fulfill diverse tasks because the functions of the muslces of the body differe markedly. Consider, for instance the fine control required of skeletal muscles in the hand in graspina china cup in contrast to the gross strength reqired of back and leg muscles in lifting a box full of books. The muscles that move the eyes have an entirely different set of tasks, such as positioning the eyes to capture information from the visual world. The principal function of facial muscles is not movement but re=ather creating facial expressions as well as assisting in speech articulatin. These jobs are so varied that it is not surprising that the motor systems have specifi compnents devoted to their control.
The first three chapters covering the motor system examine the components of the brain and spinal cord that are essential for contracting muscle. Damage to these components can produce muscle weakness. This chapter focuses on the neuroanatomy of limb control and posture, first with an overview and then with an examinatio of the desending spinal motor pathways and the spinal cord motor nuclei to which they project. The pathways that control facial and other head muscles are discussed in Chapter 11. Because eye movement and balance share many neural circuits and a strong interrelationship with the vestibular system, these topics are covered jointly in Chapter 12
Mapping the Innervation of the Dorsal Roots - Sensory Pathway
Dermatomes are the area of skin innervated by a single dorsal root which are organized in a regular pattern, a clinically important diagnostic tool for localizing spinal cord injury due to distribution. (Kandel p. 445) The axons of a dorsal root originate from several different peripheral nerves, and peripheral nerves contribute axons to several adjacent dorsal roots, therefore there is an overlapping segmental distribution. Damage to a dorsal root can result in small sensory deficits throughout a larger area of root innervation. If a peripheral nerve is cut, this results in the complete sensory loss of a circumscribed area innervated by that nerve. There is no dorsal root at C1, only a ventral/anterior motor root. The facial skin is innervated by the three branches of the trigeminal nerve: the ophthalmic (I), maxillary (II), and mandibular (III) branches.
Dermatomes are the area of skin innervated by a single dorsal root which are organized in a regular pattern, a clinically important diagnostic tool for localizing spinal cord injury due to distribution. (Kandel p. 445) The axons of a dorsal root originate from several different peripheral nerves, and peripheral nerves contribute axons to several adjacent dorsal roots, therefore there is an overlapping segmental distribution. Damage to a dorsal root can result in small sensory deficits throughout a larger area of root innervation. If a peripheral nerve is cut, this results in the complete sensory loss of a circumscribed area innervated by that nerve. There is no dorsal root at C1, only a ventral/anterior motor root. The facial skin is innervated by the three branches of the trigeminal nerve: the ophthalmic (I), maxillary (II), and mandibular (III) branches.