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1
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nerve plexuses
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Meninges
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brain and spinal cord covered by membranes called meninges
three layers - dura mater, arachnoid, pia matter
subarachnoid space between arachnoid and pia matter contains cerebrospinal fluid
arachnoid granulations, sites for loss of cerebrospinal fluid into venous blood
dural septa - two layers of meningeal dura come together after separating from periosteal dura during formation of dural sinus to form a dural septa, including: falx cerebri (between left and right cerebral hemispheres), tentorium cerebelli (supporting cerebrum to keep it from crushing the cerebellum), falx cerebelli separates right and left cerebellar lobes; diaphragma sellae (roof over hypophyseal fossa and invaginated by hypophysis)
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arteries of dura mater
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innervation of dura mater
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5
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dural sinuses
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dura mater is composed of two layers that separate in the region of a venous sinus into an outer periosteal layer, which lines the calvarium and an inner meningeal layer, which forms the unattached boundaries of the sinus.
in the region of a sinus two meningeal dural layers come together after forming the sinus to creat a septa or dural fold
upper group of dural sinuses - supperior sagittal sinus, inferior sagittal sinus, straight sinus, confluence of the sinuses, transverese sinus, sigmoid sinus
lower group of dural sinuses - cavernous sinus, anterior intercavrnoud sinus, posterior intercavernous sinus, sphernoparietal sinus, superior petrosal sinus, inferior petrosal sinus
the right an left cavrnous sinuses connect via the intercavernous sinuses that pass around the hypophysis, which sits in the hypophyseal fossa after invaginating the diaphragma sellae. Coronal section cuts through internal carotid artery twice due to 18- degree syphon
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central and peripheral nervous systems
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CNS consists of the brain and spinal cord, which constitute a functional unit
PNS consists of the nerves emerging from the brain and spinal cord
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Neurons
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the nervous system is composed of neurons and supporting neuroglial cells, which vastly outnumber them
each neuron contains a body with one axon and one or more dentrites (receptor segments). The release of neurotransmitters at synapses creates an excitatory or inhibitory postsynaptic potential at the target neuron. If the depolarization threshold of the neuron is exceeded it fires.
Certain gligal cells with lipid-rich membranes may myelinate axons. Myelination electrically insulates axons, thereby increasing impulse conduction speed. In the CNS one oligodendrocyte myelinates one intermode on multiple axons, in the PNS one Schwann cell
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Gray and white matter in the CNS
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nerve cell bodies appear gray in cross inspection, whereas nerve cell processes and their insulating myelin sheats appear white
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Development of brain
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neural tube to 3 primary vesicles - prosencephalon, mesencephalon and rhombencephalon
prosencephalon (forebrain) - telecephalon (cerebrum) into cerebral cortex, white matter and basal ganglia; diencephalon into epithalamus (pineal), dorsal thalamus, subthalamus and hypothalamus
mesencephalon (midbrain) - tectum, tegmentum and cerebral pednuncles
rhombencephalon (hindbrain) - metencephalon - cerebellum into cerebral cortex, nuclei and peduncles and pons into nuclei and fiber tracts; myelencephalon - medulla oblongata into nuclei and fiber tracts
mesencephalon, pons and medulla oblongata known as brainstem
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adult brain
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telencephalon
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divided into cerebral cortex, white matter and basal ganglia. The cerebral cortex is further divided into allocortex and isocortex
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Basal ganglia
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essential component of motor system
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allocortex
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three-layered allocortex consists of olfactory cortex (blue), and hippocampus (pink)
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isocortex
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morphological considerations didive the isocortex into six horisontal layers, functional into cortical columns
it can also be divided into association areas - lobes
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telencephalon - hippocampal formation
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hippocampal formation - hippocampus, fornix and amygdala are the major components of limbic system
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diencephalon
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the major components of the diencephalon are the thalamus, hypothalamus, and hypophysis
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structures of diencephalon
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preoptic recess, optic chiasm, 3rd ventricle, optic tract, infundibulum, thalamus: reticular nucleus of thalamus, external medullary lamina, bentrolateral thalamic nuclei, internal medullary lamina, medial thalamic nuclei, anterior thalamic nuclei, paraventricular nuclei; subthalamic nucleus, substantia nigra, mammillothalamic fasciculus, mammillary body
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structures of telencephalon
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corpus callosum, septum pellucidum, lateral ventricle, fornix, caudate nucleus, internal capsule, putamen, globus pallidus, cavum septi pellucidi, anterior commisure, lateral olfactory stria, chroroid plexus, basal ganglia, amygdala, hippocampus
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cerebellum
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axons enter or leave the cerebellum through cerebellar penduncles, afferent axons originate in the spinal cord, vstibular organs, inferior olive and pons, efferent in cerebellar nuclei
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brainstem
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the site of emergence and enrty of 10 pairs of true CN III-XII
21
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circulation of cerebrospinal fluid
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the brain and spinal cord are suspended in the choroid plexus, CSF occupies the subarachnoid space and ventricles of the brain and drains through the arachnoid granulations into the dural venous system of the cranial cavity. Smaller amounts drain along proximal portions of the spial nerves into venous plexuses or lymphatic pathways
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ventricular system
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the ventricular system is a continuation of the central spinal canal into the brain. Cast specimens are used to demonstrate the connections beween the four ventricular cavities
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veins in the brain - superficial lateral view
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veins in the brain - superficial medial view
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veins in the brain - basal system
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veins in the brainstem
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arteries in the brainstem and cerebellum
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arteries of the brain - basal
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arteries of the brain
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Divisions of the nervous system
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organization of the grey matter
the grey matter of the spinal cord is divided into three columns (horns).
Afferent, blue, and efferent, red, are clustered according to function
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muscle innervation
indicator muscles are innervated by motor neurons in the anterior horn of one spinal cord segment, most muscles receive innervation from a motor column, a vertical arrangement of motor nuclei spanning several segments
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reflexes
muscular function at the uncoscious level controlled by the grey matter
polysynaptic reflexes may be mediated by receptors inside of or remote from the muscle, they act via interneurons to stimulate muscle contraction
intrisic facilities are the intrisic circuits, allowing axons to ascend and descend to coordinate spinal reflexes for multisegmental muscles
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sensory and motor systems
the sensory and motor systems are interrelated and they may be described as one
white matter of the spinal cord contains ascending tracts (afferent) and descending (efferent), CNS equivalent of peripheral nerves
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ascending paths of spinal cord
aterior spinothalamic tract - in the anterior funiculus, pathway for crude touch and pressure sensation; 1st afferent neurons in spinal ganglia, contain 2nd neurons and cross in the anterior commisure
lateral spinothalamic tract - in the anterior and lateral funiculi, pathway for pain, temperature, tickle, itch and sexual sensation; contains 1st afferent located in spinal ganglia and 2nd neurons and cross in the anterior comissure
anterior and posterior spinocerebellar tracts - lateral funiculus, pathway for unconscious coordination of motor activities (proprioception, automatic processes) to the cerebellum, projection (2nd) neurons reveive proprioreceptive signals from 1st afferent fibers originating at the 1st neurons of spinal ganglia
fasciculus cuneatus and gracilis - posterior funiculus, pathway for position sense and fine cutaneous sensation; cuneatus conveys information from upper limb, gracilis from lower; cell bodies of 1st neuron located in spinal ganglion, pass uncrossed to the dorsal column nuclei
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sensory pathways of the ascending tracts
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motor pathways descending tracts
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descending tracts of the spinal cord
pyramidal tract - anterior and lateral corticospinal tract, most important pathway for volunary motor function, originates in the motor cortex, corticonuclear fibers to motor nuclei of cranial nerves, corticospinal fibers to motor cells in anterior horn of spinal cord, corticoreticular fibers to nuclei of the reticular formation
extrapyradimal motor system - rubrospinal, reticulospinal, vestibulospinal, tectospinal tracts - pathways for autonomic and learned motor processes
olivospinal tract - pathway for autonomic and learned motor processes
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sensory systems
vision - optic nerve
balance and hearing - vestibulocochlear nerve - vestibular for balance and cochlear for hearing
taste - facial, glossopharyngeal and vagus nerves
smell - olfactory nerve
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reflexes of the visual system
reflexes are mediated by the optic and oculomotor nerves
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balance
regulated by visual, proprioreceptive and vestibular systems. All three send afferent fibers to vestibular nuclei, then get distributed to spinal cord(motor support), cerebellum(fine motor function) and brainstem(oculomotor function). Proprioception - perception of limb position in space
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vestibular system and nuclei
receptors of vestibular system are in the membraneous labirynth, the maculae of the utricle and saccule respond to linear acceleration, semicircular duct organs in the ampullary crests respond to angular (rotational) acceleration
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oculomotor nuclei
oculomotor nuclei receive efferent fibers from both the vestibular and visual systems, conjugate eye movement requires the activity of multiple extraocular muscles and therefore coordinated at a supranuclear level by premotor nuclei
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auditory system
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gustatory system
when specialized epithelial cells in the tongue are chemically stimulated, the cell bases release glutamate, stimulating the peripheral processes of afferent cranial nerves VII, IX, and X
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olfactory system
the only sensory system not relayed in the thalamus before reaching the cortex (prepriform area - the primary olfactory cortex)
Olfactory system is linked to other brain areas and can therefore evoke complex emotional and behavioural responses - mediated by hypothalamus, thalamus and limbic system
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limbic system
exhanges and integrates information between the telencephalon, diencephalon and mesencephalon, regulates drive and affective behaviour, crucial role in memory and learning
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limbic system nuclei
neuronal circuit establishes a connection between information stored at the conscious and unconscious levels
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autonomic nervous system
innervates the smooth and cardiac muscle and glands, it is subdivided into the sympathetic and parasympathetic nervous systems, which often act in antagonistic fashion to regulate blood flow, secretions and organ function. the sympathetic synapse occurs within the paired paravertebral ganglia of the sympathetic trunk or one of the unpaired prevertebral ganglia located at the base of the artery for which the ganglion was named. Except in the head the parasympathetic synapse occurs in the terminal ganglion within the wall of the target organ. The four parasympathetic ganglia associated with one of the parasympathetic cranial nerves are as follows
ciliary - CN III
pterygopalatine - CN VII
submandibular - CN VII
otic ganglion - CN IX
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effects of parasympathetic and sympathetic nervous systems
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autonomic nervous system circuitry
body wall only receives sympathetic postganglionic innervation. Somatic efferent sympathetic preganglionic fibers exit the anterior root of the spinal cord and synapse in the closest paravertebral ganglion, accessed via the white ramus communicans. Postganglionic sympathetic fibers join the anterior and posterior rami via the gray ramus communicans. Visceral efferent fibers pass through paravertebral ganglion and continue out to a prevertebral ganglion via a splanchnic nerve. They synapse in a distant ganglion, the postganglionic sympathetic fibers passing into the target organ by following its arterial supply. Visceral efferent preganglionic parasympathetic fibers transverese the vagus nerve to the prevertebral ganglion, pass through it and synapse in an intramural ganglion within the wall of the target organ.
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modulation of response at target cells by neurotransmitters of the ANS
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blood pressure regulation
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organization of the nervous system
Divided into the CNS and PNS.
CNS - brain and spinal cord, enclosed within the cranial cavity and vertebral column of the axial skeleton
PNS - CN, spinal nerves, autonomic nerves and enteric nervous system
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Development of nervous system
during the third week of the development of the outermost layer of the embryo - ectoderm - thickens to form the neural plate. This develops longitudinally running neural groove, which deepens so that it is flanked on either sides by neural folds. These folds further develop and then fuse during neurulation to form a long tube-like structure called naural tube. Fusion of the tube starts at the midpoint and extends cranially and caudally so that the tube is fully formed by the fourth week.
Continued cell proliferation of the cells at the cephalic end cause the tube to dilate and form the three primary brain vesicles: prosencephalon(forebrian), mesencephalon(midbrain), rhombencephalon (hindbrain). The neural tube lengthens and narrows to form spinal cord
neural canal - cavities of ventricular system and in the brain and central canal of spinal cord
PNS - CN, spinal nerves, spinal ganglia, enteric system and autonomic ganglia. Formed by nerve fibers that extend out the CNS and by neurons and their fibers that develop from migratory neural crest cells.
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Cellular components
primary components - nerve cells and glial cells. Neurochemical signaling carried out through a complex series of physiological connections between adjoining neurons. Glial cells regulate the contents of extracellular space and neurotransmitters at synaptic junction.
Neurons consist of a cell body (or soma), which contains
the cell nucleus, short processes called dendrites for receiving
input from other neurons, and long processes called axons,
which conduct signals away from the cell body.
Depending on their location, neuronal morphology can be
quite variable. The majority of mammalian neurons are
multipolar, indicating that there are several dendrites from
one end and a single axon that branches extensively at its
terminus. Some additional neuronal types are
bipolar, unipolar, and pseudounipolar.
To prevent the loss of linear signal propagation, glial
cells form a phospholipid-based layer of insulation called
the myelin sheath along the length of the axon (eFig. 9.7).
The myelin sheath is formed by oligodendrocytes in the CNS
and Schwann cells in the PNS. Interspersed between the
segments of myelin are exposed segments of the axon called
nodes of Ranvier, which have a large population of voltagegated ion channels
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parts of typical neuron
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nervous system functional organization
organized into somatic and visceral
somatic - nerves that carry conscious sensation from peripheral regions back to CNS and nerves that exit CNS and innervate voluntarily (skeletal) muscles
visceral - nerves that carry sensory info into and motor(autonomic) innervation out of the CNS to regulate homeostasis functions
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spinal cord
continuous with medulla oblongata, occupies vertebral canal of vertebral column until L1-L2, numerous ascending and descending axonal tracts course through it and connect to the brain
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spinal meninges
like the brain surrounded by 3 concentric meninges - dura mater, arachnoid and pia mater. Spinal dura mater is continous with inner meningeal layer of the cranial dura mater and extends inferiorly to the posterior surface of the vertebral body of S2. Separated from bony vertebral canal by epidural space.
unlike in the cranial cavity, arachnoid is not tightly adherent to dura, has subdural space, overall structure of arachnoid is the same.
subarachnoid extends to the level of S2, due to superior termination of spinal cord at L2 this space is safe for accessing CSF
Pia mater is highly vascular and adherent to surface of spinal cord, midway between anterior and posterior roots the pia mater forms a flat continuous sheet - denticulate ligament. At the posterior and anterior rootlets,
sleevelike projections from the denticulate ligament
extend out through the arachnoid mater to attach onto
the dura mater. These delicate attachments anchor and
position the spinal cord within the central area of the
subarachnoid space.
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external features of spinal cord
anterior and posterior surfaces have longitudinally running fissures and sulci
along midline on anterior surface is a deep separation - anterior medial fissure
posteriorly - shallower separation, posterior median sulcus, flanked by postero-lateral sulcus
emerging are rootlets, which coalesce to form anterior and posterior roots - converge from 31 pairs of spinal nerves.
along length of spinal cord two regions are enlarged to accommodate numerous neurons innervating the upper and lower extremities - C5 to T1 and innervates upper extremities, lower extremities portion is L2 to S3
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internal features of spinal cord
H-shaped gray mater consisting of neuronal cell bodies and outer white mater composed of myelinated neuronal axons. Ventral or anterior horns of grey mater contain cell bodies of motor neurons
Posterior horn contains cell bodies receiving sensory information
An enlargement of the lateral portion of the grey mater, intermediolateral cell column, from T1 to L2 - enlarges to accommodate preganglionic cell bodies of sympathetic nervous system.
diverse cytoarchitecture of the gray mater - divided into 10 zones known as rexed's laminae, these will be referred to later as they relate to ascending and descending tracts within spinal cord
anterior funculus of white mater consists of motor axons, posterior of axons conveying sensory information, lateral has a mixture of both
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ascending tracts in spinal cord
sensory info from peripheral receptors is conducted through a series of neurons that synapse with targets in the spinal cord, cerebral cortex, and other structures - pain, temp, tactile and proprioceptive input. Conscious perception of sensory input is transmitted through neuronal pathways, which reach the primary somatosensory region of cerebral cortex.
Subconscious sensory input - to cerebellum
Two somatosensory pathways to cortex -
anterolateral - pain, temp and crude touch
posterior column-medial lamniscal pathway - sensation of discriminative or fine touch, vibration, and conscious perception
both pathways transmit info through three neurons
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anterolateral pathways
3 tracts - spinothalamic, spinoreticular and spinomesencephalic
spinothalamic - separate aspects of pain; first-order neuronal cell body of axons forming the sphinothalamic tract is in a spinal ganglion, axons enter the spinal cord through posterior root to reach posterior horn; axons either synapse immediately on second-order neurons in the posterior horn grey mater (lamina 1 and 5), or others have axonal collaterals that ascend or descend one to two spinal cord segments in the posterolateral tract of Liassauer before synapsing. Second-order cross obliquely two or three spinal cord segments within the anterior commissure of the spinal cord to join the anterolateral tract on the contralateral side. These second-order ascend the spinal cord through CNS to reach third-order neuronal cell bodies in the ventral posterior lateral nucleus of thalamus. Axons from third-order project through posterior limb of internal capsule to reach primary somatosensory cortex
Spinoreticular and spinomesencephalic tracts have a similar beginning, only, the target structure of the second-order axons is instead of the thalamus, spinoreticular to reticular formation in the brainstem to convey emotional and arousal aspects of pain; spinomesencephalic - periaqueductal gray mater and superior colliculi in midbrain for central modulation of pain
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Posterior column-medial lemniscal pathway
first-order cell bodies located in a spinal ganglion - axons enter spinal cord through posterior root to reach gracile fasciculus (carries info from lower limb and trunk) or the cuneate fasciculus (carries info from upper limb and neck), axons ascend ipsilaterally to caudal medulla and synapse with second-order within nucleus gracilis and nucleus cuneatus - axons of second-order cross over as the arcuate fibers to form medial lemniscus in contralateral medulla, second-orders ascend through brainstem to third-order neuronal cell bodies in ventral posterior lateral nucleus of thalamus. Axons from 3-order neurons project through posterior limb of internal capsule to reach primary somatosensory cortex
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descending tracts in spinal cord
voluntary movements - postural, coordination of head, neck and eye.
Originate from cerebral cortex and brainstem and are influenced by sensory input and feedback circuitry from cerebellum and basal ganglia
tracts of medial and lateral motor systems - composed of upper motor neuron with cell bodies located in cerebral cortex or brainstem, and lower motor neuron with cell bodies located in the spinal cord grey matter
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Descending tracts - lateral motor systems - lateral corticospinal tract
lateral corticospinal tract and rubrospinal tract - both in lateral column of spinal cord white matter and synapse on lower motor neuronal cell bodies in lateral aspect of the anterior horn gray matter
lateral corticospinal tract - clinically most important (controlling movement of upper and lower extremities). Cell bodies forming this tract are located in the primary motor cortex, axons of these upper motor neurons converge in the corona radiate and descend through posterior limb of internal capsule to reach crus cerebri of midbrain, these axons continue through anterior aspect of pons as small bundles to accommodate the transverse pontocerebellar fibers, which are also present in this location. Once they reach the medulla, again grouped together and form a large swelling called pyramid. At caudal medulla before transitioning into spinal cord, most of the axons decussate over to contralateral side to form lateral corticospinal tract - remaining stay ipsilateral and form anterior corticospinal tract - included in the medial motor systems. After forming lateral corticospinal tract - descend through spinal cord to synapse on cell bodies of lower motor neurons in lateral portion of anterior horn gray matter, axons of these lower motor neurons exit spinal cord through anterior root.
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Descending tracts - medial motor systems - anterior corticospinal tract
tracts of medial motor systems regulate axial or truncal muscles involved in maintainig posture, balance, automatic gait-related movement and orientating movements of head and neck. Unlike the lateral motor, the tracts project bilaterally on interneurons within spinal cord, making it difficult to test each tract individually in clinical system
anterior corticospinal tract - formed by remaining descending upper motor neurons that did not decussate in the caudal medulla, these ipsilateral neurons form the anterior corticospinal tract, descend through the medial aspect of the anterior spinal cord to the level of upper thoracic region. They then project bilaterally to synapse on cell bodies of lower motor neurons in the medial portion of the anterior horn gray matter, axons of these lower motor neurons then exit the spinal cord through the anterior root
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Descending tracts - medial motor systems - tectospinal tract
Tectospinal tract axons arise from cell bodies located in the superior colluculus of the dorsal midbrain. These axons decussate in the dorsal tegmental decussation shortly after leaving the nucleus to form the tectospinal tract along the middle of the brainstem. It continues through brainstem near the medial lonhitudinal fasciculus and into cervical regions of spinal cord near anterior medial fissure. Withing cervical SC, axons project bilaterally to synapse on cell bodies of interneurons in the anterior horn gray matter. As the superior colluculus receives visual input, it is believed the tectospinal tract modulates reflex posural movements in response to visual stimuli.
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Descending tracts - medial motor systems - vestibulospinal tract
Vestibulospinal tract - axons from vestibular nuclei located in pons and medulla. Medial vestibula nucles gives rise to medial vestibulospinal tract, which projects bilaterally to thoracic regions of the spinal cord and the lateral vestibulospinal tract, descending ipsilaterally through the entire length of the spinal cord to synapse on interneurons in the anterior horn gray matter. Vestibular nuclei receive sensory input from inner ear and cerebellum, this tract fascilitates activity of flexor muscles to maintain balance and posture - change in head position during tripping initiates extension of the upper limb to prevent falling
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Descending tracts - medial motor systems Reticulospinal tract -
Reticulospinal tract - axons arise from reticular formation in pons and medulla. Axons of pontine and medullary reticulospinal tracts descend ipsilaterally through the length of the spinal cord in the anterior white matter and synapse with interneurons in the anterior horn gray matter, they are believed to function in regulating voluntary movements in reflex activity and autonomic outflow
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Descending tracts - medial and lateral motor systems
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vascular supply to the spinal cord
supplied by 3 longitudinally running vessels and several segmental branches
anterior spinal artery and two posterior spinal arteries
posterior spinal arteries originate in cranial cavity as branches of either the vertebral artery or PICA, they descend along the length of posterior spinal cord on postolateral sulcus
anterior spinal artery - originates within cranial cavity from union of two contributing branches from vertebral arteries, it descends along length of anterior spinal cord on anterior median fissure
reinforcing blood supply provided by 8-10 segmental medullary arteries - largest segmental medullary artery is artery of Adamkiewicz in the lower thoracic or upper lumbar region
Venous drainage of the spinal cord occurs through a series of longitudinally running channels that connect with anterior and posterior spinal veins on the surface of the cord
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cerebral hemispheres
gray matter on the outside - white on the inside - opposite to spinal cord
composed of 6 layers of cell bodies
white matter - myelinated axonal processes of cells that extend into cerebral hemispheres
gyri - elevations on the surface
sulci - infoldings
4 lobes - frontal, parietal, occipital and temporal
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Brain structures - horizontal section
lobes separated along the middle by - longitudinal fissure, concealing a small cortex area called insula - fusion of telencephalon and diencephalon and can be seen by gently prying open the lateral sulcus
path to and from cerebral cortex is achieved through various white matter pathways coursing through the spinal cord, brainstem, and cerebral hemispheres
beneath gray matter of cortical surface - corona radiata - white matter - condenses to form internal capsule, contains axons traversing to and from cortical and deep nuclear structures
internal capsule divided into 3 parts
most anterior portion - anterior limb, bounded medially by the head of caudate and laterally by globus pallidus and putamen. The
anterior limb transitions into the genu (Latin for “knee”)
at the level of the interventricular foramen (of Monro) and
completes its course as the posterior limb, situated lateral
to the thalamus and medial to the globus pallidus and
putamen. In addition to this more vertical stream of axonal
connections is the horizontally running corpus callosum.
The corpus callosum is formed by myelinated
axons horizontally linking the two cerebral hemispheres to
one another, and it is divided into a rostrum, genu, body,
and splenium
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ventricular system of brain
derived from inner lumen of neural tube, as brain grows, caverns and canals adapt to shape of cerebral hemispheres, diencephalon, pons, medulla, cerebellum, which form the surrounding walls
inferior to corpus callosum - two large fluid-filled cavities - beginning of ventricular system - two C shaped lateral ventricles
as lateral ventricles extend through all lobes - 5 names parts: in frontal lobe - anterior horn, projecting into occipital lobe is the posterior horn, inferior horn in temporal lobe, body within frontal and parietal lobes. All horns and body come together near splenium of corpus callosum - atrium of lateral ventricles.
lining ventricles is choroid plexus - ependymal cells responsible for producing 0.5L of CSF a day
From the lateral ventricles, CSF flows through the interventricular foramen (of Monro) to the slitlike third ventricle,
which is surrounded by the thalamus and hypothalamus. The third ventricle communicates with the
fourth ventricle via the cerebral aqueduct (aqueduct of
Sylvius), which courses through the midbrain.
Surrounded by the pons and medulla anteriorly and the
cerebellum posteriorly, the fourth ventricle sends CSF out
of the ventricular system and into the subarachnoid space
via the lateral foramina of Luschka and midline foramen
of Magendie
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Descending tracts - lateral motor systems -rubrospinal tract
Rubrospinal tract - cell bodies of upper motor neurons in pathway begin in red nucleus of midbrain, after than cross the midline as ventral tegmental decussation and descend as the rubrospinal tract through the brainstem and lateral column of spinal cord white matter. Axons only descend to cervical regions and synapse with interneurons in anterior horn gray matter to facilitate flexor muscle activity and inhibit extensor muscle activity of upper limb
