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Physiology III
Brainstem & Cerebellum
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(Haines, Ch. 10, 11, 12,13)


Open Medulla
Rostral portion of medulla; presence of the lower fourth ventricle.
Closed Medulla
Caudal portion of the medulla; absence of lower fourth ventricle.
Fourth Ventricle
Pyramid-shaped cavity bounded ventrally by the pons and medulla oblongata; its floor is also known as the rhomboid fossa. The lateral recess extends as a narrow, curved extension of the ventricle on the dorsal surface of the inferior cerebellar peduncle. The fourth ventricle extends under the obex into the central canal of the medulla.
The incomplete roof of the 4th ventricle is formed by the anterior and posterior medullary vela.
The point at which the fourth ventricle passes up into the cerebellum is called the apex or fastigium.
The lateral aperture (foramen of Luschka) is the opening of the lateral recess into the subarachnoid space near the flocculus of the cerebellum. A tuft of choroid plexus commonly present in aperture and partly obstructs the flow of CSF from the 4th ventricle to the subarachnoid space.
The medial aperture (foramen of Magendie) is an opening in the caudal portion of the roof of the ventricle. Most of the outflow of CSF from the 4th ventricle passes through this aperture, which varies in size.
The tela choroidea of the 4th ventricle is a layer of pia and ependyma that contains small vessels and lies in the posterior medullary velum. It forms the choroid plexus of the 4th ventricle and is supplied by branches of the posteroinferior cerebellar arteries.

All the ventricles of the brain are continuous with each other and with the central canal of the spinal cord. The hollow ventricular chambers are filled with cerebrospinal fluid. The fourth ventricle is continuous with the third ventricle. The central spinal canal from the spinal cord comes up into the Medulla where it flares open to create the floor (along with the pons) and sides of the fourth ventricle. The ventricle closes up rostrally and creates a canal to the third ventricle called the "Cerebral Aqueduct" (Marieb pg. 408). It lies dorsal to the pons and superior to the medulla. It is somewhat pyramid shaped. (Haines pg. 91) The only openings between the ventricles of the brain and the sub arachnoid space surrounding the brain are the foramina of Luschka and Magendie in the fourth ventricle (Haines pg. 91).
Inferior Olive
Prominent at mid-medullary levels.
The cell groups receive input from a variety of CNS nuclei and project primarily to the contralateral cerebellum through the restiform body.
Reticular Formation
Consists of interconnected circuits of neurons in the tegmentum of the brainstem, the lateral hypothalmic area, and the medial, intralaminar, and reticular nuclei of the thalmus. Axons from nonspecific thalmic nuclei project to most of the cerebral cortex where they determine the level of activity of large number of neurons.
Characteristic appearance of loosely packed cells of varying sizes and shapes that are embedded in a dense meshwork of cell processes, including dendrites and axons.
It is not anatomically well defined because it includes neurons located in diverse parts of the brain.
Function – maintaining behavioral arousal and consciousness.
Some refer to it as the reticular activating system.
It also gives rise to descending axons, which pass to the spinal cord in the reticulospinal tract.Also carries axons that modulate autonomic activity in the spinal cord.
Visceral nociceptive information can reach cortex.
Receives spinoreticular inputs (mainly from lamina VII and VIII) and collaterals from the ALS.
Raphe Nuclei
Serotonin-containing neurons are present in raphe nuclei in pons and medulla.
Bilaterally symmetric cell groups in the brainstem that are located directly adjacent to the midline.
Structure of the ventral medulla, two longitudinal ridges formed by the large pyramidal (corticospinal) tracts descending from the motor cortex. Most of these fibers cross over to the opposite side before continuing their descent into the spinal cord.
Pyramidal Decussation
The crossover point of the pyramidal tracts in the medulla.
Medial Lemniscus
Crossed fiber bundle in caudal medulla. The lower part of the body is represented in the ventral portion of the lemniscus and the upper part of the body in the dorsal lemniscus.
In rostral medulla, the medial lemniscus begins to shift ventrolaterally and to rotate from the dorsoventral orientation more characteristic of the pons. By level of pons it is horizontal.
Cerebellar Peduncles
There are three large paired bundle of fibers – inferior, middle, and superior, that form connections with the cerebellum.
"Bridge", portion of brainstem lying between the midbrain rostrally and the medulla caudally.
Ventrally the pons consists of a massive bundle of transversely oriented fibers that enter the cerebellum as the middle cerebellar peduncle. The exit of the trigeminal nerve marks the transition from the basilar pons, which is ventral to the trigeminal root, to the middle cerebellar peduncle, which lies dorsal to the exit of the trigeminal nerve.
Cranial nerves that emerge from pons are the trigeminal (V), abducens (VI), facial (VII), and vestibulocochlear (VIII).
Pontine Nuclei
Located in the pons.
Relays for "conversation" between motor cortex and the cerebellum.
These cells receive from diverse regions of the neuroaxis. Most of axons cross the midline and enter the cerebellum via the middle cerebellar peduncle.
Crus Cerebri
Massive fiber bundle that includes corticospinal, corticobulbar, and corticopontine pathways in the base of the midbrain.
Substantia Nigra
Bandlike nucleus located deep to the cerebral peduncle; it is the largest nuclear mass in the midbrain. Its dark color reflects its high content of melanin pigment, a precursor of the neurotransmitter (dopamine) released by these neurons. It is functionally linked to the basal nuclei of the cerebral hemispheres and is considered part of the basal nuclear complex by some authorities.
It is related to motor function.
Functionally associated with the basal ganglia and is commonly divided into a compact part (pars compacta) and a reticular part (pars reticulata).
Cells contain neuromelanin, receives afferent fibers from the cerebral cortex and the striatum; it sends dopaminergic efferent fibers to the striatum.
Plays a key role in motor control.
Degeneration of the substantia nigra occurs in Parkinson’s disease.
Red Nucleus
A prominent structure in the midbrain tegmentum, is so named because in the unfixed brain the dense vascularity of the region gives it a pink color.
It is composed of caudal magnocellular and rostral parvocellular region.
Involved in motor function and has extensive connections throughout the neuraxis.
Its efferents include the rubrospinal tract, which travels to the contralateral spinal cord, and rubro-olivary fibers, which descend in the central tegmental tract to the ipsilateral inferior olivary complex.
Afferents to the red nucleus arise from the contralateral cerebellar nuclei and the ipsilateral cerebral cortex.
Oval shape, found between the substantia nigra and the cerebral aqueduct. Its reddish hue is due to its rich vascular supply and the presence of iron pigment in the cell bodies of its neurons. The red nuclei are relay nuclei in some descending motor pathways that effect limb flexion.
Inferior Colliculus
Located in midbrain. Part of the auditory relay from hearing receptors of the ear to the sensory cortex. Also act in the reflexive responses to sound (ex. startle reflex) which causes you to turn your head toward an unexpected sound.
Virtually all ascending auditory pathways terminate in the inferior colliculus.
Many cells in the inferior colliculus respond to input from either ear.
Involved inauditory reflexes and in determining the side on which a sound originates.
They project to the medial geniculate nucleus of the thalmus.
Caudal elevations on dorsal surface of adult midbrain (part of corpora quadrigemina).
Superior Colliculus
Contains neurons that receive visual as well as other input and serve ocular reflexes.
Rostral elevations on dorsal surface of adult midbrain (part of corpora quadrigemina).
Composed of alternating layers of gray matter (cells) and white matter (fibers).
Helps to control eye movements and mediates so-called visual reflexes.
Projects to the pulvinar in the thalmus.
Deep layers of the superior colliculus integrate auditory, visual, and somesthetic information and project to brainstem and cervical spinal cord nuclei via tectobulbospinal fibers, which are involved in controlling orientation of the head, eyes, and body to sound.
Layered structure found in the roof of the midbrain.
Periaqueductal Gray Matter
Oversee descending pain suppressor fibers that synapse in the dorsal horns.
Contains descending autonomic tracts as well as endorphin-producing cells that suppress pain.
Located behind the dorsal aspect of the pons and the medulla. It is separated from the occipital lobe by the tentorium and fills most of the posterior fossa.
Consists of the cerebella cortex and the underlying cerebellar white matter.
Divided into two symmetric hemispheres.
The main functions: coordinating skilled voluntary movements by influencing muscle activity and controlling equilibrium and muscle tone through connections with the vestibular systems and the spinal cord and its gamma motor neurons.
There is a somatotropic organization of body parts within the cerebellar cortex.
Cerebellum also receives collateral input from the sensory and special sensory systems.
Composed of a highly convoluted cerebellar cortex and a core of white matter containing cerebellar nuclei.
Anchored to brainstem via the cerebellar peduncles.
Located dorsal to the brainstem, inferior to the tentorium cerebelli, and internal to the occipital bone.
Receives input from many areas of neuraxis and influences motor performance through connections with the dorsal thalamus.
Accounts for about 11% of total brain mass, "small brain".
Located dorsal to the pons and medulla.
Protrudes under the occipital lobes of the cerebral hemispheres, from which it is separated by the transverse fissure. Rests in the posterior cranial fossa.
Processes input from the cerebral motor cortex, various brainstem nuclei, and sensory receptors to provide the precise timing and appropriate patterns of skeletal muscle contraction needed for the smooth, coordinated movements.
Activity occurs subconsciously.
Anterior and posterior lobes have completely overlapping sensory and motor maps of the entire body.
Cerebellar Hemispheres
Largest parts of the brain (2). Composed of three major subdivisions: cerebral cortex (layer of neuronal cell bodies that cover the entire surface of the hemisphere); subcortical white matter (made up of myelinated axons that carry information to or from the cerebral cortex); basal ganglia (prominent group of neuronal cell bodies).
Cortex is responsible for many higher brain functions, including manual dexterity; conscious, discriminative aspects of sensation, and cognitive activity (language, reasoning, aspects of learning and memory).
The paired hemispheres include the cerebral cortex which consists of six lobes on each side (frontal, parietal, temporal, occipital, insular, and limbic).
The surfaces of the cerebral hemispheres contain many fissures and sulci that separate the frontal, parietal, occipital, and temporal lobes from each other and the insula.
Most parts of the cerebral cortex are connected with their counterparts in the opposite hemisphere by axons that runin the corpus callosum (coordinates the activities of the two cerebral hemispheres).
Midline portion of the cerebellum that separates the two lateral lobes.
Connects medially the two cerebellar hemispheres – worm-like.
Connects the two bilateral cerebellar hemispheres. It is "worm-like". Its surface is heavily convoluted, but since the fissures are all transversely oriented, the cerebellar surface exhibits
Cerebellar Deep Nuclei
Four pairs embedded in the white matter of the cerebellum: fastigial, globose, emboliform, and dentate.
Neurons project out of the cerebellum, representing the major efferent pathway from the cerebellum.
Cells receive inhibitory input (GABA) from Purkinje cells. Also receive excitatory inputs from sites outside the cerebellum, including pontine nuclei, inferior olivary nucleus, reticular formation, locus ceruleus, and raphe nuclei. Cells fire tonically, at rates that reflect the balance between the opposing excitatory and inhibitory inputs that converge on them.


Last Updated 04/10/00 12:27:14 PM
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