OPT113 Neuroanatomy Post-MT2 Study Guide Dec 2024 PDF

OPT113: Neuroanatomy Study Outline for Dr. Powell’s lectures Post-MT2 Final Exam: Wednesday, December 18, 2024 (10:00AM – Noon) Disclaimer: This is a general (not exhaustive) summary outline of the topics and items that you should know f...

OPT113: Neuroanatomy Study Outline for Dr. Powell’s lectures Post-MT2 Final Exam: Wednesday, December 18, 2024 (10:00AM – Noon) Disclaimer: This is a general (not exhaustive) summary outline of the topics and items that you should know for the class material covered after the second midterm (Limbic system, Basal nuclei/ganglia, diencephalon, basal nuclei, brainstem, cerebellum, and spinal cord). I strongly recommend that you refer to the lecture slides as well. Questions will be in multiple choice or multiple selection format. The final exam is comprehensive. It will contain 45-50 items. Approximately 1/3rd of the questions (~15) will come from the lecture material that was covered and tested on for the first two midterms (split roughly even, so ~7 items from the material covered for the first midterm and ~7 from the material covered for the second midterm). Refer to the study guides previously posted in Moodle as well as lecture slides and assigned readings to assist you in preparing for the material that was covered and tested on for the two midterms. LIMBIC SYSTEM Hippocampus is an extension of the cerebral cortex of the medial temporal lobe with axonal projections to the fornix via fimbria. o Functionally involved in transforming short-term to long-term memory. o Part of the limbic system of the brain. Fornix is a white matter pathway that takes hippocampal axons to other parts of the brain. Though it connects parts of the cerebrum, it is technically not considered to be a part of the cerebrum. o Axons from the hippocampus ascend in the crura (crus) of the fornix. o The crura bend anteriorly, becoming the body of the fornix. The right and left fornix bodies run side- by-side and have commissural attachments between them. ▪ The fornix bodies run directly inferior to the corpus callosum. o The fornix bodies begin to descend, splitting into anterior and posterior columns of white matter and are above the hypothalamus. The Papez circuit is involved in transforming short-term to long-term memory as well as controlling the interaction between the autonomic nervous system, endocrine system and emotions. Fear is mediated by the amygdala, which is connected to the olfactory tracts, septal nuclei, diencephalon and midbrain through various tracts. It is a part of the limbic system. o Phobias may be explained by early fearful amygdaloid reactions to stimuli before the hippocampus was fully developed. o Post-traumatic stress disorder is caused by strong inputs to the amygdala from cerebral cortex after intense sensation. Can affect hippocampus and long-term memory, allowing flashbacks to be triggered by certain stimuli. 1  DIENCEPHALON The diencephalon is the area of the brain surrounding the 3rd ventricle. There are right and left halves. Its boundaries are as follows: o Inferior surface is exposed externally, while all other surfaces are internal. It stretches from the optic chiasm region anteriorly to the caudal edge of the mammillary bodies posteriorly. Structures that are visible on the inferior surface are: ▪ Optic chiasm and optic tracts ▪ Tuber cinereum is a gray-matter protuberance which narrows into the hollow infundibulum, which widens again into the pituitary gland. ▪ Mammillary bodies are two protuberances composed of a gray matter core with a white matter capsule. o Superior surface is defined by the floors of the lateral ventricles. o Lateral surface is defined by the internal capsule. o Medial surface is defined by the lateral walls of the 3rd ventricle Major divisions of the diencephalon: Thalamus o Thalamus is an oval mass of gray matter. There is one on each side of the 3rd ventricle, and they communicate via an interthalamic adhesion. The thalamus is positioned superiorly and posteriorly in the diencephalon. ▪ Functions as a relay for all sensory systems (except for olfaction, which is integrated with taste before arriving at the thalamus via the mammillothalamic tract). ▪ Sensory information is organized here, then sent to the appropriate area of cerebral cortex. o Internal structure ▪ The thalamus is divided into three main portions by the Y-shaped internal medullary lamina; medial-posterior, lateral-posterior and anterior-superior divisions. These main parts are divided into thalamic nuclei, which are involved in different functional activities. ❖ The anterior-superior division contains the anterior thalamic nuclei, which receive the mammillothalamic tract, and project to the cingulate gyrus (part of the Papez circuit, which is involved in short-term memory and emotional state) ❖ The medial-posterior division contains many nuclei, involved in integration of somatic, olfactory, and visceral information and their relation to emotions. ❖ The lateral-posterior division is divided into dorsal and ventral tiers. o The ventral tier: ▪ Ventral anterior and ventral lateral nuclei ▪ Ventral posterior nucleus (commonly referred to as the VPN) receives sensation from the brainstem. Its medial portion receives trigeminal and gustatory signals, and its lateral portions receives spinal signals. The nucleus projects to the primary somatosensory cortex. o The dorsal tier: ▪ Lateral geniculate nucleus (LGN) is a protuberance on the inferior-lateral aspect of the pulvinar. It is important as part of the visual pathway, where retinal ganglion cell axons enter, to synapse. The new neurons send their axons, the optic radiations, through the parietal and temporal lobes to the primary visual cortex within the occipital lobe. ▪ Medial geniculate nucleus (MGN) is a protuberance on the inferior-medial aspect of the pulvinar. It receives auditory information from both ears (mainly the 2  contralateral ear) and sends efferent axons to the auditory cortex within the temporal lobe via the auditory radiations. ▪ The internal medullary lamina carries white matter fibers between the different thalamic nuclei. Major divisions of the diencephalon: Subthalamus o The subthalamus is inferior to the thalamus and superior to the midbrain. It is a very complex structure with many nerves and nuclei. The cranial ends of the red nucleus (involved in motor function) and substantia nigra (involved in basal nuclei function) are here, as well as other subthalamic nuclei. Major divisions of the diencephalon: Epithalamus o Epithalamus is located near the posterior thalamus. Some important areas include: ▪ Habenular nucleus, which is medial to posterior thalamus, is believed to be a center for olfactory, visceral, and somatic afferent integration. ❖ The habenular commissure is composed of fibers from the thalami that decussate to end up in the contralateral (opposite) habenular nucleus. ▪ Pineal gland is an endocrine gland that influences the endocrine system. It is posterior to the midbrain and connected to rest of diencephalon by a pineal stalk, which contains the habenular and posterior commissures. Major divisions of the diencephalon: Hypothalamus o The hypothalamus is anterior-inferior to the thalamus. There are many hypothalamic nuclei, which are gray matter centers of the hypothalamus. o Communicates with other parts of the body through nerves, bloodstream and CSF. o Functions: the hypothalamus is essential for life, as it controls homeostasis (via autonomic nervous system and endocrine system) and influences emotional behavior. Almost all bodily functions are influenced by the hypothalamus somehow: ▪ Temperature regulation via dilation of skin vessels and sweating from anterior hypothalamus; and constriction of skin vessels, inhibition of sweating, and shivering from posterior hypothalamus. ▪ Eating (hunger and satiety centers) and drink (thirst center) within the hypothalamic nuclei. ▪ Emotion and behavior may be physically manifested via the hypothalamus, which receives input from the limbic system and prefrontal cortex. ▪ Circadian rhythms 3  THE BRAINSTEM In general, the brainstem: o Connects tracts that travel between the spinal cord, diencephalon, and cerebrum. o Contains reflex centers for respiration and cardiovascular control and consciousness o Contains cranial nerve nuclei for CNs III though XII The brainstem is divided into the hindbrain, which is both the medulla oblongata (myelencephalon) and the pons (metencephalon), plus the midbrain (mesencephalon). o Medulla oblongata ▪ The medulla oblongata is bounded caudally by the spinal cord and rostrally by the pons. ▪ Surface anatomy ❖ Anterior surface o The pyramids, which are divided from one another by the midsagittal anterior median fissure, contain the major white matter motor/efferent tracts (corticospinal & corticobulbar). o At the inferior/caudal end of the medulla, the pyramidal fibers that make up the pyramids decussate in an obvious-appearing structure, the decussation of the pyramids. 4 ❖ The superior lateral surface is remarkable for the presence of the olives, which are two bumps on the lateral medulla and lateral to the pyramids; they contain the olivary nuclei. ❖ Posterior surface o Complex appearance o Dorsal-most part of the medullary tegmentum (which contain patches of gray matter from cranial nerve nuclei, nuclei of the reticular formation, and other nuclei as well as the white matter of the sensory/ascending spinal tracts) and is continuous with the rostral pontine tegmentum o The superior portion comprises a portion of the floor of the 4th ventricle. The vagal and hypoglossal triangles are small projections that are formed by their respective cranial nerve nuclei. o The posterior median sulcus is midsagittal, separating the two gracile tubercles from each other. ▪ Lateral to the gracile tubercles are the cuneate tubercles. ▪ The gracile and cuneate nuclei are found within their respective tubercles and receive touch sensation from the spinal cord (via the gracile and cuneate funiculi – which are components of the afferent posterior column-medial lemniscal pathway). o Olivary nuclei are the inferior olivary nuclei, with dorsal and medial accessory olivary nuclei. They are involved with voluntary movement (motor coordination), sending decussating fibers to the cerebellum. o Nuclei: ▪ Nucleus ambiguous (CNs IX, X, and XI) control muscles of the throat via many cranial nerves. ▪ Solitary tract nucleus (CNs VII, IX, X) receive visceral sensation and taste from several CNs that help make the solitary tract. ▪ Dorsal nucleus of the vagus (CN X) control parasympathetic functions of the vagus nerve throughout the body. 5  ▪ Hypoglossal nucleus (CN XII) ▪ Vestibulonuclear complex (CN VIII; at the pontine-medullary/pontomedullary junction) o Pons ▪ “Pons” means “bridge,” refers to the many transverse fibers that cross its ventral surface. ▪ Pons is located ventral to the cerebellum, inferior/caudal to the midbrain, and superior/rostral to the medulla oblongata. ▪ Surface anatomy ❖ Anterior surface is remarkable for many visible transverse fibers, within which is the basilar groove, which runs vertically down the midline. ❖ Lateral surface is remarkable for the middle cerebellar peduncles, which connect the brainstem to the cerebellum. ❖ Posterior surface is covered by the cerebellum. Upon removal of the cerebellum, the triangular posterior pons is visible. o The dorsal surface has several landmarks: ▪ Superior cerebellar peduncles form the lateral limits of the dorsal pons. ▪ Median sulcus, with the medial eminences on either side. ▪ The facial colliculi are the expanded inferior ends of the medial eminences. ▪ General internal structure ❖ Pontine tissue anterior to the trapezoid body is the basal part. o Basal part contains pontine nuclei, where corticopontine fibers from the cerebral cortex terminate, synapsing on the neurons of the nuclei. o The pontine nuclei axons are the transverse fibers, which decussate on the anterior surface of the pons and enter the middle cerebellar peduncles. o This comprises most of the pathway between the cerebrum and cerebellum. o Pontine tissue posterior to the trapezoid body is the tegmentum. Within the tegmentum is: ▪ Facial nucleus (CN VII; at the inferior pons) is immediately posterior to the trapezoid bodies. ▪ Abducens nucleus (CN VI; at the inferior pons) is in the posterior tegmentum. The axons of the facial tract loop posterior to it, forming the facial colliculi. ▪ Trigeminal nerve nuclei (CN V; all throughout the pons). 6 o Midbrain ▪ Connects pons and cerebellum with the diencephalon and cerebrum. ▪ Surface anatomy ❖ Anterior surface: Remarkable for the interpeduncular fossa bounded laterally by the two crus cerebri (cerebral peduncles) and perforated by small blood vessels (making the region known as the posterior perforated substance). 7  ❖ On the dorsal surface is the corpora quadrigemina, a collection of four colliculi, which are gray matter centers. Vertical and transverse grooves divide them into superior and inferior, right and left. o The two superior colliculi deal with visual reflexes (i.e., tracking eye movements) o The two inferior colliculi deal with auditory localization (i.e., where is the sound?) ❖ Lateral surfaces reveal the superior and inferior brachia. o The superior brachium connects the superior colliculus with the lateral geniculate nucleus (LGN) of the thalamus (the LGN is the thalamic relay between the superior colliculus and the visual cortex within the occipital lobe) o The inferior brachium connects the inferior colliculus with the medial geniculate nucleus (MGN) of the thalamus (the MGN is the thalamic replay between the inferior colliculus and the auditory cortex within the temporal lobe) ▪ General internal structure ❖ Tectum is the posterior-most portion of midbrain. It contains the corpora quadrigemina, and other tissue as well, including the pretectal nuclei that are positioned anteriorly to the superior colliculi and involved in the pupillary light reflex. The tectum is located posterior to the cerebral aqueduct. ❖ The cerebral aqueduct runs through the midbrain, connecting the 3rd and 4th ventricles. Surrounding the cerebral aqueduct is a ring of gray matter called the periaqueductal gray. o Certain portions of this area make up the nuclei for the oculomotor nerve (CN III; at the level of the superior colliculi), the trochlear nerve (CN IV; at the level of the inferior colliculi), and the mesencephalic portion of the trigeminal nerve (CN V). o More on CN III (refer to image on right): Parasympathetic efferents come from cell bodies in the Edinger-Westphal Nucleus of the midbrain (near the oculomotor nucleus). 8  The axons travel along in the oculomotor tracts and nerves, before branching in the inferior division upon entering the orbit. Parasympathetic preganglionic axons travel to the ciliary ganglion via the parasympathetic roots, where they synapse. Postganglionic axons exit the ciliary ganglion in the short ciliary nerves. They travel into the eye, supplying the iris pupillary sphincter muscle and the ciliary muscle. ❖ Tegmentum is the portion of the midbrain anterior to the cerebral aqueduct but posterior to the substantia nigra. o Substantia nigra is a pigmented strip of gray matter that functions as a motor nucleus. ▪ The pigment is from melanin granules in the cytoplasm of neurons. ▪ Connects to the cerebral cortex, spinal cord, hypothalamus and basal nuclei to help control muscle activity. o Red nucleus is found in the tegmentum, at the level of the superior colliculi. Its reddish hue is from both high vascularity and iron inclusions in the cytoplasm of the gray matter neurons. ❖ Anterior to the substantia nigra is the crus cerebri (cerebral peduncle), which connects the cerebral cortex with the spinal cord, pons, cerebellum, and cranial nerve nuclei. 9 10  RETICULAR FORMATION Network of nerve cells that runs vertically from the spinal cord to the thalamus and receives input from most sensory systems as well as transmitting efferent information that influences most CNS processes. Gross arrangement of reticular formation o One median column o Two (right and left) medial columns o Two (right and left) lateral columns Functions of reticular formation are numerous and variable. o Affects control of skeletal muscle – even eye movements! o Affects control of somatic and visceral sensations o Affects autonomic nervous system (i.e., cardiovascular, respiratory control) o Sensory modulator for afferent information from spinal cord to thalamus o May influence “biological clocks” ▪ Arousal: the reticular formation acts as a sensory filter, allowing sensation to go to cerebral cortex, causing the sleeper to awaken. ▪ States of consciousness: differing degrees of wakefulness depend upon the activity of the reticular formation in filtering sensations. 11  CEREBELLUM The cerebellum integrates proprioceptive information (movement of voluntary muscle, tendons, joints) with balance and sight information, and sends inhibitory signals to the nuclei of origin of certain efferent tracts to the spinal cord and motor cortex, helping “fine tune” muscle movements. Gross structure o There are two cerebellar hemispheres connected in the middle by the vermis. o The cerebellar hemispheres have fine, tightly-packed gyri and sulci arranged into three lobes each. ▪ Anterior lobe is visible on the superior view. It is separated from the middle lobe by the primary fissure. ▪ Middle (posterior) lobe borders the primary fissure and the posterolateral fissure. ▪ Flocculonodular lobe is posterior to posterolateral fissure o There is also a horizontal fissure that has no functional significance, but merely separates the superior portion of the cerebellum from the inferior. o The cerebellum is connected to the posterior brainstem by the superior, middle, and inferior cerebellar peduncles. o Cerebellar cortex organization ▪ Cerebellar cortex is comprised of gray matter. It overlies internal white matter. Cerebellar cortex is divided into three layers: ❖ Molecular layer (external) contains basket cells (inhibitory interneurons), tangential processes of granular cells and the highly-arborized dendritic tree of the Purkinje cells. ❖ Purkinje cell layer is comprised of the soma of many Purkinje cells. Their dendrites are in the molecular layer and feed into the soma. Their axons pass through the granular layer into the deep white matter, before finally synapsing on an intercerebellar nucleus. ❖ Granular layer (internal) contains the soma of granular cells, which release axons that travel to the molecular layer along with Golgi cells (inhibitory interneurons that inhibits cells within this layer via a negative feedback loop) are located here, too. 12 o Cerebellar circuit ▪ Afferent input to the cerebellar cortex is via the climbing and mossy fibers, usually via the inferior and middle cerebellar peduncles. ❖ Climbing fibers arise from the contralateral olivary nuclei of the medulla oblongata. They ascend to the molecular layer of the cerebellar cortex before forming excitatory synapses with the dendrites of the Purkinje cell. ❖ Mossy fibers arise from all other inputs to the cerebellum. These axons reach the granular layer and synapse with the excitatory granular cells or inhibitory Golgi cells. ❖ Granular cells send their axons to the molecular layer, where they split into horizontally- running parallel processes. These form an excitatory synapse with the dendritic trees of Purkinje cells. ▪ Efferent output from cerebellar cortex is entirely via the axons of the Purkinje cells. ❖ Purkinje dendrites have spines, which interact with the parallel processes of granular cells approximately 200,000 times per Purkinje cell. ❖ Purkinje axons exit through the white matter of the cerebellum to the cerebellar nuclei, where they synapse. o Axon tracts arise from the cerebellar nuclei to exit the cerebellum, usually via the superior cerebellar peduncle. o These signals work to “sharpen” the muscular movement of the body. 13  SPINAL CORD AND ASCENDING/DESCENDING TRACT PATHWAYS The most caudal portion of the CNS is the spinal cord. Ascending and descending tracts travel through this structure to take information to and from the brain and PNS. External structure o There are 31 spinal nerve pairs that emerge from each spinal segment (remember, 8 cervical; 12 thoracic; 5 lumbar; 5 sacral and 1 coccygeal). o The spinal cord is roughly cylindrical in shape and stretches from the foramen magnum to the L1 vertebrae. It travels in the vertebral canal of the vertebral column. o There are anterior and posterior median fissures that divide the cord into right and left halves. o Meninges surround the spinal cord as they do the brain. ▪ Dura mater in the spinal cord is loosely connected to the vertebral canal. It ends at S2. ▪ Arachnoid mater tends to follow the dura mater. ▪ Pia mater closely follows the spinal cord. ▪ Ligamentum denticulatum are connections of the pia mater to the arachnoid matter. These thicken in between the nerve roots, giving the structure a toothy appearance. ▪ When the spinal cord ends at the conus medullaris (at L1), the pia collapses upon itself and forms the thin filium terminale, which travels among the cauda equina within the subarachnoid space until it meets the dura and arachnoid mater at S2. Internal structure of spinal cord (Gray Matter) o Gray matter core of the spinal cord is “H-shaped.” ▪ Anterior and posterior horns on each side of the cord. ▪ Gray matter commissure connecting the horns with central canal located in the center of the commissure. o Anterior (ventral) horn or column contains α-efferent and γ-efferent neuron cell bodies, which are large, multipolar cells that exit via the anterior spinal root to innervate extrafusal skeletal muscle and muscle spindles, respectively. o Posterior (dorsal) horn or column has several cell body groups: ▪ Substantia gelatinosa cell group is at the apex of the posterior horn. Touch, pain, and temperature signals. ▪ Nucleus proprius cell group is a large group of large neurons that receive fibers from the posterior white column of spinal cord (mainly ascending sensory information). Deals with proprioception, resolution (two-point discrimination) and vibration. ▪ Nucleus dorsalis (Clark’s column) cell group is only found from T1 to L3. It contains large neurons, some of which are the soma for dendrites from the neuromuscular spindles and Golgi tendon organs. Thus, this area deals with proprioception. 14  o Lateral horn: There is a lateral horn or column present in the thoracic and upper lumbar segments. It contains a group of small neurons, which are the autonomic neurons. Since these are only located in the thoracic and upper lumbar segments, the soma associated with these horns are soma of sympathetic preganglionic neurons. (Note: The autonomic tracts lie just lateral to the lateral horn.) ▪ There are also small groups of neurons in the sacral portion of the spinal cord that give rise to preganglionic parasympathetic fibers. o Interneurons: Throughout the gray matter core of the spinal cord are interneurons (internuncial neurons), small neurons that links two longer neurons together and are most often inhibitory and associated with a reflex arc to inhibit the action of the antagonistic muscle. Internal structure of spinal cord (White Matter) o The grey matter core is surrounded by white matter, which is divided into anterior, posterior, and lateral columns. o There are a variety of myelinated nerve fiber tracts, which ascend to the brain, descend from the brain, or are intersegmental (travel from one place in the spinal cord to another). o Ascending and Descending Pathways ▪ Ascending neuronal pathways: Carry afferent signals from the spinal cord to the brain. ❖ Afferent signals follow a general course which travels along a chain of three neurons: 1. First-order neuron stretches from the sensory receptor in the periphery to the spinal cord. Its cell body is in the dorsal root ganglion. Its axon ends on the dendrites or soma of the second-order neuron in the CNS. 2. Second-order neuron has its cell body in the gray matter of the spinal cord or brainstem. Second-order neurons usually decussate (and after the soma of the second-order neuron and its location can vary based on the specific afferent tract). Ascends to the brain to synapse on the third-order neuron. 3. Third-order neuron normally has its cell body in the thalamus. It sends its axon to a specific area of the cerebral cortex (i.e., primary sensory cortex of the parietal lobe) NOTE: There are some exceptions to this rule as some pathways have less than three neurons; however, none of these pathways were discussed in this class. 15 ❖ Specific ascending spinal tracts: o Spinothalamic (anterolateral) tract transmits course touch and pressure as well as pain and temperature sensation. ▪ First-order neurons travel from free nerve endings and other receptors to enter the spinal cord via the dorsal root and synapse on second-order neurons within the posterior gray horn (specifically the substantia gelatinosa). ▪ Second-order neurons arise from their cell bodies in the substantia gelatinosa and surrounding areas. The second order axons immediately decussate (cross over to the contralateral side) at or slightly above this same level via the anterior white commissures. The axons then ascend in the contralateral anterior and lateral columns (white matter) through the spinal cord, medulla, pons and midbrain. As they travel through the medulla, the fibers begin to be collectively known as the spinal lemniscus. The axons of the spinal lemniscus finally end in the ventral posterior nucleus (VPN) of the thalamus where they synapse on third-order neurons. ▪ Soma of third-order neurons within the VPN. Axons arise from this nucleus and travel through the internal capsule and corona radiata (also contain efferent first- order axons that will eventually form the corticospinal tract in the brainstem), before finally ending in the primary somatosensory cerebral cortex. 16  o Dorsal column (posterior funiculus) – medial lemniscal tract transmits light touch, vibration, proprioception from joints and musculature (body posture, balance) and 2- point discrimination sensation. ▪ First-order neurons begin at specialized receptors (i.e., Meissner’s corpuscle, neuromuscular spindles, Golgi tendon organs) with cell bodies in the dorsal root ganglia; its axons enter the spinal cord via the dorsal spinal root. These axons go directly to the ipsilateral posterior white column. The posterior white column (posterior fasciculus) is divided into two ascending tracts along most of its length: o Fasciculus gracilis (medial) runs along the entire length of the spinal cord, transmitting sensory information from the lower trunk and limbs. o Fasciculus cuneatus (lateral) is only present in the upper thoracic and cervical spinal cord, transmitting sensory information from the upper trunk, arms and neck. ▪ The ascending axons of the fasciculi gracilis and cuneatus end in the nuclei gracilis and cuneatus of the dorsal medulla, where they synapse onto the second-order neurons. ▪ Second-order neurons are the soma of the nucleus gracilis or nucleus cuneatus, which give off axons that decussate in the medulla oblongata via the sensory decussation, then ascend in the medial lemniscus through the medulla, pons, and midbrain, before ending in the ventral posterolateral nucleus of the thalamus (as with the second-order neurons from the spinothalamic tract), synapsing on third-order neurons. ▪ Third-order neurons are found in the ventral posterolateral nucleus of the thalamus. Axons arise that travel through the internal capsule and corona radiata, before terminating on the postcentral gyrus (the primary somatosensory cortex). ▪ Descending neuronal pathways: Carry efferent signals from the brain to the spinal cord, allowing muscle and glandular activity. ❖ Efferent signals follow a general course that travels over two main neurons: 1. Upper motor neurons start with the neuronal soma in the cerebral cortex, and deep cerebral and brainstem nuclei. The axons exit these cell bodies to descend to different spinal segments, where they synapse on neuronal cell bodies in the anterior gray horn of the spinal cord. The descending spinal tracts we will discuss are composed of upper motor neurons. Many descending neurons decussate as they pass through the medulla oblongata. 17  2. Lower motor neurons have their soma in the anterior gray horn of the spinal cord. The axons exit the spinal cord in the anterior spinal root and run to muscles and glands. There are often interneurons between the upper and lower motor neurons. There are many different efferent signals that provide converging input upon each lower motor neuron. The total of the diverse signals determines the action of the muscle or gland. ❖ Corticospinal tracts send efferent information that controls voluntary, skilled movements. o Upper motor neurons are pyramid cells (one of the largest neurons in the body – these are multipolar neurons and are the primary neuron type in the CNS), the axons of which descend from the cerebral cortex. o Most fibers are from the primary motor cortex, but fibers can also originate from the primary sensory cortex, secondary motor centers and the cingulate gyrus. o The pyramid axons descend through the corona radiata, the posterior limb of the internal capsule, cerebral peduncle of the midbrain, the anterior pons and medulla. o Some axons branch off here, travelling to the different nuclei of the brainstem. This provides these nuclei with information regarding voluntary movement that is about to take place, so they can send appropriate signals in response via different tracts. o The sheer volume of pyramid cells that come together in the medulla oblongata causes two triangular-shaped protuberance in the anterior surface of the medulla called the pyramids. o Most (~90%) of these fibers decussate at the inferior/caudal border of the medulla oblongata at the area called the decussation of the pyramids and enter the contralateral lateral corticospinal tract. ▪ These decussated fibers descend through the spinal cord in the contralateral corticospinal tracts in the white column and terminate in the ventral gray horn of the spinal cord; these fibers innervate generally innervate the skeletal muscles of the appendages (arms/hands/fingers and legs/feet/toes) ▪ The 10% of pyramidal fibers that do not decussate descend in the ipsilateral anterior corticospinal tract in the white column, decussate at the spinal cord level of the lower motor neuron soma, then terminate in the cervical ventral gray horn; these fibers innervate skeletal muscles of the neck and trunk. 18