The Brainstem
The brain stem consists of the portion of the brain that remains after removal of the cerebral and cerebellar hemispheres and is comprised of the midbrain, pons and medulla oblongata. Cranial nerves, nuclear groups and fiber tracts are accessories within the brain stem regulate and control autonomic body functions such as breathing, respiration, movements and generate subconscious neural patterns to free higher areas of the brain for more complex tasks. The brain stem controls behaviors that are necessary for survival; feeding (chewing, licking, swallowing, smiling), drinking, reproduction, sleep, arousal and emergency responses. Damage to the brain stem would affect unconscious control of regulated body functions but not higher brain functions as the frontal cortex is still intact but are almost always life threatening.
As with the spinal cord, the brain stem is a conduit for information flow because ascending sensory and descending motor tracts travel through it. It receives sensory information from cranial structures and controls the muscles of the head. The cranial nerves, the sensory and motor nerve roots that enter and exit the brain stem, are parts of the peripheral nervous system and are analogous to the spinal nerves. Nuclei in the brain stem integrate information from a variety of sources for arousal and other higher brain functions. The various divisions of the brain stem each subserve specific sensory and motor functions.
The pons is separated from the midbrain and medulla by a large bundle of prominent crossing fibers on the ventral and lateral surfaces of the brain stem that connect the brain stem to the cerebellum and the medulla is connected to the spinal cord caudally. Emerging from the brain stem are the remaining 10 pairs of cranial nerves.
On the dorsal brain stem surface are four major landmarks: dorsal columns, dorsal column tubercles, the fourth ventricle, and two sets of colliculi (superior and inferior). The dorsal columns and tubercles are part of the dorsal column-medial lemniscal system. With the cerebellum removed, the floor of the fourth ventricle can be identified by its rhomboid shape. The colliculi are four bumps located on the dorsal surface of the midbrain. The rostral pair, the superior colliculi, in controlling eye movements. The caudal pair, called the inferior colliculi, are involved in the processing of sounds.
Four major landmarks also can be identified on the ventral surface: pyramids, olives, the base of the pons, and bassis pedunculi. All four are key components of the motor system. In the medulla, the axons of the corticospinal tract are located in the pyramids and just lateral to them, the olives. Neurons in the olives together with those in the base of the pons, the large basal surface of the pons, are major sources of afferent information to the cerebellum which controls the accuracy of movement. Many of the axons immediately beneath the ventral midbrain surface in the basis pedunculi are part of the corticospinal tract. These axons descend through the base of the pons and emerge on the medullary surface in the pyramid.
The Midbrain lies between the diencephalon and the pons; initiation of involuntary motor responses; reflex actions of the eye from the superior colliculi and reflex actions of the ear from the inferior colliculi
The Pons is located between the midbrain and the medulla, transmits impulses between the brain and spinal cord, functions: motor control, control of sleep, relay information to the cerebellum and thalamus, sets respirator rate and depth.
The Medulla Oblongata is the "lowest," most caudal part of the brain stem which connects the spinal cord and the brain and transmits all ascending and descending impulses. It is the center for primitive life maintenance of heart rate, respiration and depth rate and blood pressure. There are three characteristic levels through the medulla which vary from rostral to caudal.
The rostral Medulla provides a clear view of the ventricular system. the central canal, which is a microscopic structure in the spinal cord, expands to form the fourth ventricle in the rostral medulla. Whereas the ventricular floor is formed by the dorsal medulla, the roof consists of a thin tissue formed by the apposition of two glial layers during development: pia and ependyma.
The mid-medulla, though the dorsal column nuclei which relay information about touch to the thalamus. At this level the dorsal column nuclei bulge to form the dorsal surface landmarks, the dorsal column tubercles. The second-order neurons of the dorsal column-medial lemniscal system originate in these nuclei. Their axons decussate and ascend to the thalamus in the medial lemniscus, in the medullary pyramid.
The third key medullary level is through the olive, a bulge located on the ventral medullary surface lateral to the medullary pyramid, which marks the position of the inferior olivary nucleus. this nucleus contains neurons whose axons project to the cerebellum, where they form one of the strongest excitatory synapses in the entire central nervous system.
The caudal-most level is at the junction with the spinal cord. The key feature of this level is the pyramidal (or motor) decussation which is where the corticospinal tract decussates. This decussation is also visible on the ventral brain stem surface. Because of this decussation one side of the brain controls muscles of the opposite side of the body.
The reticular formation/reticular activating system runs through the brain stem and is concerned with maintenance of consciousness (alertness vs sleep), controls respiration, cardiovascular function and digestion. It comprises the central core of the brain stem. Although the reticular formation begins in the caudal medulla, it is a large structure in the mid medulla as well as farther rostrally in other brain divisions. Neurons in this region regulate arousal by influencing the excitability of neurons throughout the central nervous system. Receiving input from all of the sensory modalities, neurons of the reticular formation can affect neuronal excitability directly through diffuse projections rostrally and caudally, or indirectly by contacting other neurons with diffuse projections. An important path by which the reticular formation affects the excitability of cerebral cortical neurons is through connections with thalamic nuclei that have diffuse cortical projections.
A significant characteristic of the brain stem is the presence of the cranial nerves and their associated nuclei. Knowledge of their locations helps develop a general understanding of brain stem anatomy. There are 12 pairs of cranial nerves, which, like the spinal nerves, mediate sensory and motor function but of cranial structures. Many of the nuclei of the brain stem are analogous in connections and functions to regions of spinal cord gray matter. For example, nuclei in the brain stem receive sensory input directly from the receptor neurons innervating cranial structures. These are the cranial nerve sensory nuclei, which have general functions similar to neurons of the dorsal horn. similarly, cranial nerve motor nuclei in the brain stem innervate cranial muscles and are similar to motor nuclei of the ventral horn.
As with the spinal cord, the brain stem is a conduit for information flow because ascending sensory and descending motor tracts travel through it. It receives sensory information from cranial structures and controls the muscles of the head. The cranial nerves, the sensory and motor nerve roots that enter and exit the brain stem, are parts of the peripheral nervous system and are analogous to the spinal nerves. Nuclei in the brain stem integrate information from a variety of sources for arousal and other higher brain functions. The various divisions of the brain stem each subserve specific sensory and motor functions.
The pons is separated from the midbrain and medulla by a large bundle of prominent crossing fibers on the ventral and lateral surfaces of the brain stem that connect the brain stem to the cerebellum and the medulla is connected to the spinal cord caudally. Emerging from the brain stem are the remaining 10 pairs of cranial nerves.
On the dorsal brain stem surface are four major landmarks: dorsal columns, dorsal column tubercles, the fourth ventricle, and two sets of colliculi (superior and inferior). The dorsal columns and tubercles are part of the dorsal column-medial lemniscal system. With the cerebellum removed, the floor of the fourth ventricle can be identified by its rhomboid shape. The colliculi are four bumps located on the dorsal surface of the midbrain. The rostral pair, the superior colliculi, in controlling eye movements. The caudal pair, called the inferior colliculi, are involved in the processing of sounds.
Four major landmarks also can be identified on the ventral surface: pyramids, olives, the base of the pons, and bassis pedunculi. All four are key components of the motor system. In the medulla, the axons of the corticospinal tract are located in the pyramids and just lateral to them, the olives. Neurons in the olives together with those in the base of the pons, the large basal surface of the pons, are major sources of afferent information to the cerebellum which controls the accuracy of movement. Many of the axons immediately beneath the ventral midbrain surface in the basis pedunculi are part of the corticospinal tract. These axons descend through the base of the pons and emerge on the medullary surface in the pyramid.
The Midbrain lies between the diencephalon and the pons; initiation of involuntary motor responses; reflex actions of the eye from the superior colliculi and reflex actions of the ear from the inferior colliculi
The Pons is located between the midbrain and the medulla, transmits impulses between the brain and spinal cord, functions: motor control, control of sleep, relay information to the cerebellum and thalamus, sets respirator rate and depth.
The Medulla Oblongata is the "lowest," most caudal part of the brain stem which connects the spinal cord and the brain and transmits all ascending and descending impulses. It is the center for primitive life maintenance of heart rate, respiration and depth rate and blood pressure. There are three characteristic levels through the medulla which vary from rostral to caudal.
The rostral Medulla provides a clear view of the ventricular system. the central canal, which is a microscopic structure in the spinal cord, expands to form the fourth ventricle in the rostral medulla. Whereas the ventricular floor is formed by the dorsal medulla, the roof consists of a thin tissue formed by the apposition of two glial layers during development: pia and ependyma.
The mid-medulla, though the dorsal column nuclei which relay information about touch to the thalamus. At this level the dorsal column nuclei bulge to form the dorsal surface landmarks, the dorsal column tubercles. The second-order neurons of the dorsal column-medial lemniscal system originate in these nuclei. Their axons decussate and ascend to the thalamus in the medial lemniscus, in the medullary pyramid.
The third key medullary level is through the olive, a bulge located on the ventral medullary surface lateral to the medullary pyramid, which marks the position of the inferior olivary nucleus. this nucleus contains neurons whose axons project to the cerebellum, where they form one of the strongest excitatory synapses in the entire central nervous system.
The caudal-most level is at the junction with the spinal cord. The key feature of this level is the pyramidal (or motor) decussation which is where the corticospinal tract decussates. This decussation is also visible on the ventral brain stem surface. Because of this decussation one side of the brain controls muscles of the opposite side of the body.
The reticular formation/reticular activating system runs through the brain stem and is concerned with maintenance of consciousness (alertness vs sleep), controls respiration, cardiovascular function and digestion. It comprises the central core of the brain stem. Although the reticular formation begins in the caudal medulla, it is a large structure in the mid medulla as well as farther rostrally in other brain divisions. Neurons in this region regulate arousal by influencing the excitability of neurons throughout the central nervous system. Receiving input from all of the sensory modalities, neurons of the reticular formation can affect neuronal excitability directly through diffuse projections rostrally and caudally, or indirectly by contacting other neurons with diffuse projections. An important path by which the reticular formation affects the excitability of cerebral cortical neurons is through connections with thalamic nuclei that have diffuse cortical projections.
A significant characteristic of the brain stem is the presence of the cranial nerves and their associated nuclei. Knowledge of their locations helps develop a general understanding of brain stem anatomy. There are 12 pairs of cranial nerves, which, like the spinal nerves, mediate sensory and motor function but of cranial structures. Many of the nuclei of the brain stem are analogous in connections and functions to regions of spinal cord gray matter. For example, nuclei in the brain stem receive sensory input directly from the receptor neurons innervating cranial structures. These are the cranial nerve sensory nuclei, which have general functions similar to neurons of the dorsal horn. similarly, cranial nerve motor nuclei in the brain stem innervate cranial muscles and are similar to motor nuclei of the ventral horn.
Ventral inferior view of the surface of the brain stem, label the corticospinal tract.
At a certain point each hemispheres corticospinal tract decussates/sends its axons to the opposite (contralateral) side of the brainstem, locate the corticospinal decussation.
At the junction of the medulla and pons, two slender cranial nerve roots emerge.
What are they and what effect would damage to them have?
Just lateral and inferior to these nerves, lies the root of the Vagus (X; Roman numerals denote the number of critical nerves, proceeding from I [olfactory] to XII [hypoglossal, literally 'under the tongue']); these are complemented by 31 pairs of spinal nerves representing the body's sensory and motor functions)
Below cranial nerve XII is a bulge, the spino-olivary eminence.
What system is this structure related to? Think of the term olivo-cerebellar and what it might mean. Which ventricle is most closely associated with the medulla (see fig 419).
A massive expansion of the brain stem marks the pons, where is the corticospinal tracts at this level? Is it crossed or uncrossed?
On the dorsal and lateral surface of the brain stem above the pons are the cerebellar peduncles. These huge fiber bundles contain axons running to and from the cerebellum.
The mesencephalic-diencephalic junction lies rostral to the oculomotor (III) nucleus and superior colliculus.
The main diencephalic structures are the mammillary bodies, hypothalamus and thalamus of which only the first is visible in whole brains.
Rostral to the mammillary bodies lies the pituitary stalk, often obliterated during dissection and from which the pituitary gland is suspended.
The ponto-mesencephalic junction is marked by slender roots of the oculomotor nerves (III) medially and the still finer trochlear nerve (IV) roots emerging dorsolaterally. Just above the trochlear roots are the four bulges (two each) that form the roof (tectum) of the midbrain. The more prominent pair, the superior colliculi, are related to vision, especially to visual reflexes such as fixation and pursuit of objects in space.
The floor (tegmentum) of the midbrain is made up mainly of descending axons: corticospinal, corticopontine, corticoreticular, corticotrigeminal and so on. What midbrain cavity contains cerebrospinal fluid?