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gray matter

(1) being primarily composed of the dendrites and cell bodies of neurons and (2) functioning as a processing center. IN SPINAL CHORD - is centrally located - shape resembles a letter H or a butterfly - SUBDIVISON on each side of the spinal cord: --- a posterior horn, a lateral horn, an anterior horn, and a bar of gray matter that connects the left and right sides called the gray commissur

Distribution of White Matter

(1) primarily composed of myelinated axons and (2) functioning to relay nerve signals IN SPINAL CHORD - is external to the gray matter and on each side of the cord is partitioned into three distinct anatomic structural regions based upon their location within the spinal cord -

visceral sensory nuclei!

- (dark blue-shaded region) is the location for synapses between visceral sensory neurons (dark blue line) that extend from visceral sensory receptors (e.g., baroreceptors Page 546of the urinary wall) to the interneurons within the posterior horns of the spinal cord. (Recall that visceral refers to an internal organ.)

somatic sensory nuclei!

- (light blue-shaded region) is the site for synapses between somatic sensory neurons (light blue line) that extend from somatic sensory receptors (e.g., tactile receptors within the skin; see section 16.2a) and the interneurons within the posterior horns of the spinal cord. (Recall that somatic refers to "body" in general).

lateral corticospinal tract

- (which composes 85% of the direct pathway) innervates skeletal muscles that control skilled movements in the limbs, such as playing a guitar, dribbling a soccer ball, or typing on your computer keyboard.

polysynaptic reflex

- A reflex may also be classified by the number of neurons participating in the reflex. - A polysynaptic reflex has one or more interneurons positioned between the sensory and the motor neuron. These reflex arcs are more complicated and not as rapid.

monosynaptic reflex

- A reflex may also be classified by the number of neurons participating in the reflex. - this only a sensory neuron and a motor neuron. - The axon of the sensory neuron synapses directly on the motor neuron, whose axon projects to the effector. --- Thus, there is only one synapse between neurons. - Monosynaptic reflexes are the simplest, and they are the most rapid. With only one synaptic delay, the response is very prompt.

vertebral canal

- All of the stacked vertebral foramina collectively form the vertebral canal, which houses both the spinal cord and the cauda equina

decussation

- Most pathways include neurons that cross over, or decussate to make in the form of an X), from one side of the body to the other side at some point along the pathway—within either the spinal cord or the brain. - This means that the left side of the brain receives sensory input from or initiates motor output to the right side of the body, whereas the right side of the brain receives sensory input from or initiates motor output to the left side of the body

descending nerve tracts

- Motor pathways include the series of motor neurons that relay motor output from the brain. - Motor pathways are also called descending pathways because the nerve signals are relayed from the brain inferiorly to the body's muscles and glands.

somatic reflex

- This classification criterion is determined by the type of effector that is stimulated by the motor neurons involved in the reflex. - Somatic reflexes involve skeletal muscle as the effector. - Visceral (or autonomic) reflexes involve cardiac muscle, smooth muscle, or a gland as the effector.

posterior root

- houses sensory neurons that extend from sensory receptors - these sensory neurons relay nerve signals from the sensory receptors to the spinal cord. - the sensory neurons that compose the spinal nerves are unipolar neurons - dendrites of these sensory neurons form the sensory receptors and their axons extend from the dendrites to the spinal cord - cell bodies of these sensory neurons (which are positioned along the length of the axon) are located external to the spinal cord and form the posterior root ganglion

radial n.

- extends along the posterior side of the arm and then along the radial side of the forearm. - The radial nerve innervates the posterior arm muscles (forearm extensors) and the posterior forearm muscles (extensors of the wrist and digits, and the supinator of the forearm). - It receives sensory nerve signals from the posterior arm and forearm surface and the dorsolateral side of the hand.

arachnoid mater

- external to the pia mater. - partially composed of a delicate web of both collagen and elastic fibers termed the ARACHNOID TRABECULAE.

phrenic n.

- formed primarily from the C4 nerve and some contributing axons from C3 and C5. The phrenic nerve extends through the thoracic cavity to innervate the thoracic diaphragm, which is the primary skeletal muscle of breathing

subdivisions of spinal chord

CERVICAL PART - continuous with the medulla oblongata THORACIC PART LUMBAR PART SACRAL PART -- ENLARGEMENTS-- - cervical enlargement: wider In the cervical part - lumbosacral enlargement: wider in the lumbar and sacral parts -- enlarged due to presence of a greater number of neurons within the spinal nerves extending from these spinal cord parts to innervate the upper and lower limbs, respectively.

Posterior/anterior root composition

POSTEROR - posterior root contains sensory neurons that extend from sensory receptor -- these sensory neurons include somatic sensory neurons, which relay nerve signals from somatic sensory receptors, and visceral sensory neurons, which relay nerve signals from visceral sensory receptors - BOTH types of sensory neurons synapse with interneurons within the gray matter of the posterior horns ANTERIOR - contains motor neurons that extend to effectors - motor neurons include somatic motor neurons, which extend to skeletal muscle, and autonomic (or visceral) motor neurons, which extend to autonomic effectors - a posterior root has an associated ganglion (posterior root ganglion) and an anterior root does not.

tibial nerve?

is formed from the anterior divisions of the sciatic nerve

primary, secondary and tertiary neurons

primary neuron (or first-order neuron) - is the first neuron in the chain of neurons. - The primary neuron extends from the sensory receptor to the CNS (brain or spinal cord), where it synapses with a secondary neuron. secondary neuron (or second-order neuron) - is an interneuron that extends from the primary neuron to either the tertiary neuron or the cerebellum. tertiary neuron (or third-order neuron) is also an interneuron. - It extends from the secondary neuron to the cerebrum (specifically, the primary somatosensory cortex of the parietal lobe; see section 13.3c). -Pathways that lead to the cerebellum do not have a tertiary neuron.

Conus medullaris

superior end of the spinal cord is continuous with the medulla oblongata of the brain, and its inferior end tapers (narrows) to form the conus medullaris

cervical nerves

wonder why there are eight cervical nerves and only seven cervical vertebrae? - first seven cervical spinal nerves (C1-C7) exit the vertebral canal and extend through an intervertebral foramen that is superior to the vertebra of the same number - eg, the C2 spinal nerve exits the vertebral canal through the intervertebral foramen between the C1 and C2 vertebrae. - The eighth cervical spinal nerve, in comparison, exits the intervertebral foramen inferior to the C7 vertebra. - All remaining spinal nerves inferior to the C8 nerve exit the vertebral canal and extend through an intervertebral foramen inferior to the vertebra of the same number. -- Thus, for example, the T2 spinal nerve exits the vertebral canal through the intervertebral foramen inferior to the T2 vertebra.

General Distribution of Spinal Nerves

- Each spinal nerve (except C1 and Co1) extends through an intervertebral foramen to exit the vertebral column - Each spinal nerve splits almost immediately into two primary branches - TERMED RAMI/RAMUS (read more)

intervertebral foramina

- Each spinal nerve exits the vertebral column through an intervertebral foramen (which is a lateral opening between two adjacent stacked vertebrae - each of the more superior spinal nerves (i.e., cervical and thoracic) extends horizontally through its associated intervertebral foramen at the same level - inferior spinal nerves (i.e., lumbar, sacral and coccygeal) have roots that extend inferiorly as part of the cauda equina, and each of these spinal nerves then extends through its intervertebral foramen, which is inferior to where the roots are anchored to the spinal cord

subarachnoid space

- Immediately deep to the arachnoid mater is the subarachnoid space. --- Cerebrospinal fluid (CSF) circulates within this space (both around the spinal cord and around the brain). -------Cerebrospinal fluid can be analyzed (e.g., for infectious agents) following its removal from the subarachnoid space by the clinical procedure called a lumbar puncture

nerve tract

- Tracts are myelinated axons that have a common origin, a common destination, and a similar function - name of each tract reflects its origin and destination - sensory tracts usually begin with the prefix spino-, indicating that they originate in the spinal cord - second part of the name provides its destination - Motor pathways begin either with cortico-, indicating an origin in the cerebral cortex - or with the name of a brainstem nucleus (such as rubro-, indicating an origin within the red nucleus of the midbrain; see section 13.5a). ---Thus, the corticospinal tract extends from the cerebral cortex to the spinal cord. - eg, is the sensory spinothalamic tract, which extends from the spinal cord to the thalamus -- spinocerebellar tract, which extends from the spinal cord to the cerebellum.

sciatic n. (tibial n. + common fibular n.)

- also known as the ischiadic (is-kē-at′ik; hip joint) nerve - is the largest and longest nerve in the body. - is formed from portions of both the anterior and posterior divisions of the sacral plexus. - This nerve projects from the pelvis through the greater sciatic notch of the os coxae and extends into the posterior region of the thigh. - is actually composed of two divisions—the tibial division and the common fibular division—wrapped in a common sheath.

intercostal nerves

- anterior rami of spinal nerves T1-T11 are called intercostal nerves ecause they are located within the intercostal space sandwiched between two adjacent ribs - exception of T1, the intercostal nerves do not form plexuses - intercostal nerves innervate much of the torso wall and portions of the upper limb\ The specific innervation pattern of the T1-T12 nerves is as follows: - A portion of the anterior ramus of T1 helps form the brachial plexus, but a branch of it is housed within the first intercostal space. - anterior ramus of nerve T2 emerges from its intervertebral foramen and innervates the intercostal muscles of the second intercostal space. Additionally, a branch of T2 transmits sensory information from the skin covering the axilla and the medial surface of the arm. - Anterior rami of nerves T3-T6 follow the costal grooves of the ribs to innervate the intercostal muscles and receive sensations from the anterior and lateral chest wall. - Anterior rami of nerves T7-T12 innervate not only the inferior intercostal spaces but also the abdominal muscles and their overlying skin.

meninges (sl. meninx)

- are connective tissue membranes that protect and encapsulate the spinal cord within the vertebral canal. - are continuous with the cranial meninges layered from innermost to outermost, are: PIA MATET ARACHNOID MATER DURA MATER

anterior horn

- both the left and right anterior masses of gray matter. - gray matter of the anterior horns is due to the presence of the dendrites and cell bodies of somatic motor neurons. - Collectively, they form the somatic motor nuclei (red shaded area), which forms the entire anterior horn on each side of the spinal cord. - The axons of somatic motor neurons (red line) extend to and innervate a somatic effector. - The somatic effector includes only the muscle that can be controlled consciously or voluntarily (i.e., skeletal muscle).

lateral horn

- both the left and right lateral masses of gray matter - located only within the T1-L2 parts of the spinal cord, not the entire length of the spinal cord. - gray matter of the lateral horns is due to the presence of the dendrites and cell bodies of autonomic motor neurons - Collectively, they form the autonomic motor nuclei (orange shaded region), which makes up the entire lateral horn on each side of the spinal cord. - The axons of autonomic motor neurons (orange line) extend to and innervate autonomic (or visceral) effectors. -- Autonomic effectors include those body structures that are not controlled consciously or voluntarily (i.e., cardiac muscle, smooth muscle, and glands).

posterior root ganglion

- cell bodies of these sensory neurons (which are positioned along the length of the axon) are located external to the spinal cord and form the posterior root ganglion

anterior root

- contains motor neurons that extend to effectors (muscle or glands) - motor neurons relay nerve signals from the spinal cord and control muscles and glands. - motor neurons that compose the spinal nerves are multipolar neurons - Both the dendrites and the cell bodies of motor neurons, unlike those of sensory neurons, are housed within the spinal cord --- thus, the anterior root does not contain a ganglion along its length - axons exit from the spinal cord within the anterior root and extend through the spinal nerve to their terminal ends, which innervate an effector - Thus, a significant difference between anterior and posterior roots is that each anterior root lacks a ganglion along its length. --- because, as mentioned, the dendrites and cell bodies of motor neurons are within the spinal cord and the anterior root contains only the axons of these neurons.

ulnar n.

- descends along the medial side of the arm. - It extends posterior to the medial epicondyle of the humerus and then extends along the ulnar side of the forearm. - It innervates some of the anterior forearm muscles (the medial region of the flexor digitorum profundus and all of the flexor carpi ulnaris). -- also innervates most of the intrinsic hand muscles, including the hypothenar muscles, the palmar and dorsal interossei, and the medial two lumbricals (see section 11.8e). - receives sensory input from the skin of the dorsal and palmar aspects of the medial 1½ fingers (the pinky finger and the medial half of the ring finger).

comparing cross sections of cerv, thor, lum, and sacral

- difference in the relative amounts of gray matter and white matter reflects the function of that part of the spinal cord. -- For example, the lumbar part of the spinal cord has a greater amount of gray matter because more neuron cell bodies are located there that have axons extending from there to innervate the lower limbs.

pia mater

- directly adheres to the external surface of the spinal cord. - It is the delicate, innermost meningeal layer, - is a meshlike membrane composed of both elastic and collagen fibers. - extensions form two structures (Both types of pia mater extensions help stabilize the spinal cord within the vertebral canal.): --- DENTICULATE LIGAMENTS ---- FILUMTERMIMALE

reflex arc

- includes a sensory receptor, an effector, and the neural wiring between the two. - always begins at a receptor in the PNS, communicates with the CNS, and ends at a peripheral effector, either a muscle or a gland. - number of intermediate steps varies, depending upon the complexity of the reflex. Generally, five steps are involved in a reflex: A stimulus activates a sensory receptor - A sensory receptor (dendritic endings of a sensory neuron or specialized receptor cells) responds to external and internal stimuli, such as temperature, pressure, or tactile changes. -- Proprioceptors are sensory receptors found in muscles and tendons, and a stimulus to a proprioceptor (such as the tapping of tendon) may initiate a reflex as well The sensory neuron transmits a nerve signal to the CNS. - A sensory neuron transmits a nerve signal from the receptor to the spinal cord (or brain). Information from the nerve signal is processed in the integration center by interneurons - More complex reflexes may use a number of interneurons within the CNS to integrate and process incoming sensory nerve signals and transmit information to the motor neuron. The simplest reflexes do not involve interneurons; rather, the sensory neuron synapses directly on a motor neuron in the CNS. The motor neuron transmits a nerve signal from the CNS to an effector - A motor neuron transmits a nerve signal from the CNS to a peripheral effector organ—a gland or a muscle. The effector responds to the nerve signal from the motor neuron. An effector is a muscle or a gland that responds to the nerve signal from the motor neuron. This response is intended to counteract or remove the original stimulus.

musculocutaneous n.

- innervates the anterior arm muscles (coracobrachialis, biceps brachii, and brachialis), which flex the humerus, flex the forearm, or both. - It also receives sensory information from the lateral surface of the forearm.

nerve plexus

- is a network of interweaving anterior rami of spinal nerves - anterior rami of most spinal nerves form nerve plexuses on both the right and left sides of the body. - These nerve plexuses then split into multiple "named" nerves that innervate various body structures. The main plexuses are the cervical plexuses, brachial plexuses, lumbar plexuses, and sacral plexuses - Nerve plexuses are organized such that axons from each anterior ramus extend to body structures through several different branches. - each terminal branch of the plexus houses axons from several different spinal nerves --- thus, damage to a single segment of the spinal cord or damage to a single spinal nerve generally does not result in complete loss of innervation to a particular muscle or region of skin. - Most of the thoracic spinal nerves, as well as nerves S5-Co1, do not form plexuses

subdural space

- is a potential space internal to the dura mater (between the arachnoid mater and dura mater).

central canal

- is a small, internal channel that extends through the center of the gray commissure along the entire length of the spinal cord - As with the ventricles of the brain, the central canal is formed during embryonic development from the neural canal within the neural tube - contains cerebrospinal fluid (CSF), which enters this space from the fourth ventricle of the brain

epidural space

- is a space external to the dura mater - is a clinically significant area that houses adipose and areolar connective tissue, as well as blood vessels. -- Epidural anesthetics, such as may be used to lessen pain during childbirth, are introduced into this space.

filum terminale!

- is a thin strand of pia mater that anchors the conus medullaris to the coccyx bone

anterior rami (ramus sl.)

- is the larger of the two main branches. - anterior ramus splits into multiple other branches --- which innervate skin and skeletal muscles of the anterior and lateral portions of the trunk, the upper limbs, and the lower limbs. - Many of the anterior rami go on to form NERVE PLEXUS

posterior rami

- is the smaller of the two main branches. It innervates the deep muscles of the back (e.g., erector spinae and transversospinalis and the skin of the back.

brachial plexus

- left and right brachial plexuses are networks of nerves that supply the upper limb. - Each brachial plexus is formed by the anterior rami of spinal nerves C5-T1 - components of the brachial plexus extend laterally from the neck, pass superior to the first rib, and then continue into the axilla. - Each brachial plexus innervates the pectoral girdle and the entire upper limb of one side. - each brachial plexus is more complex than a cervical plexus --- is composed of anterior rami, trunks, divisions, and cords when examined from a medial to lateral perspective - ANTERIOR RAMI (roots) of brachial plexus are simply the continuations of the anterior rami of spinal nerves C5-T1 -- These rami emerge through the intervertebral foramina and extend to the neck . - five rami unite in the posterior triangle of the neck to form the superior, middle, and inferior trunks - Nerves C5 and C6 unite to form the superior trunk; nerve C7 remains as the middle trunk; and nerves C8 and T1 unite to form the inferior trunk.

cervical plexus

- left and right cervical plexuses are located deep on each side of the neck, immediately lateral to cervical vertebrae C1-C4 - formed primarily by the anterior rami of spinal nerves C1-C4 - fifth cervical spinal nerve is not considered part of the cervical plexus, although it contributes some axons to one of the plexus branches - Branches of the cervical plexuses innervate anterior neck muscles as well as the skin of the neck and portions of the head and shoulders

lumbar plexus

- left and right lumbar plexuses are formed from the anterior rami of spinal nerves L1-L4 located lateral to the L1-L4 vertebrae and along the psoas major muscle in the posterior abdominal wall - innervates the inferior abdominal wall, anterior thigh, medial thigh, and skin of the medial leg - is structurally less complex than the brachial plexus - like the brachial plexus, the lumbar plexus is subdivided into an anterior division

posterior horn

- left and right posterior masses of gray matter. - gray matter forming the posterior horns is due to the presence of the dendrites and cell bodies of interneurons (the neurons that are located completely within the CNS - Sensory neurons within the spinal nerves extend through the posterior root and synapse with the dendrites and cell bodies of the interneurons within the posterior horn. - The posterior horn gray matter on each side of the spinal cord is subdivided regionally into both SOMATIC SENSORY nuclei and VISCERAL SENSORY nuclei based upon the specific type of sensory neurons that synapse there.

sacral plexus

- left and right sacral plexuses are formed from the anterior rami of spinal nerves L4-S4 and are located immediately inferior to the lumbar plexuses - The lumbar and sacral plexuses are sometimes considered together as the lumbosacral plexus. - nerves emerging from a sacral plexus innervate the gluteal region, pelvis, perineum, posterior thigh, and almost all of the leg and foot. - anterior rami of the sacral plexus are organized into an anterior division and a posterior division. - nerves that are formed from the anterior division tend to innervate muscles that flex (or plantar flex) parts of the lower limb, whereas the posterior division nerves tend to innervate muscles that extend (or dorsiflex) part of the lower limb

obturator n

- main nerve of the anterior division is the obturator nerve, - extends through the obturator foramen of the os coxae to the medial thigh. -- There, the nerve innervates the medial thigh muscles (which adduct the thigh, see section 11.9a) and conducts sensory input from the superomedial skin of the thigh. - smaller branches of each lumbar plexus innervate the abdominal wall, portions of the external genitalia, and the inferior portions of the abdominal muscles

femoral n.

- main nerve of the posterior division of the lumbar plexus is the femoral nerve. - innervates the anterior thigh muscles, such as the quadriceps femoris (knee extensor) and the sartorius, psoas, and iliacus (hip flexors, see sections 11.9a and b). - also receives sensory input from the skin of the anterior and inferomedial thigh as well as the medial aspect of the leg.

denticulate ligaments

- numerous paired, triangular extensions present along the spinal cord. - These pia mater extensions suspend and anchor the spinal cord laterally to the arachnoid and dura mater

dura mater

- outermost layer of meninges - composed of dense irregular connective tissue - dura mater associated with the spinal cord has only one layer (unlike the dura mater covering the brain, which is composed of both a periosteal layer and meningeal layer) - Extensions of the dura mater ensheathe the spinal nerve roots and merge with the connective tissue layer that surrounds the spinal nerves Two spaces are associated with the dura mater: - SUBDURAL SPACE - EPIDURAL SPACE

reflex

- rapid, preprogrammed, involuntary responses of muscles or glands to a stimulus. - survival mechanism PROPERTIES - A stimulus is required to initiate a reflex. - A rapid response requires that few neurons are involved and synaptic delay is minimal. - A preprogrammed response occurs the same way every time. - An involuntary response requires no conscious intent or preawareness of the reflex activity. Thus, reflexes are usually not suppressed.

spinal cord

- roughly cylindrical nervous system structure that extends through the vertebral column to the inferior border of the L1 vertebra - superior end of the spinal cord is continuous with the medulla oblongata of the brain - inferior end tapers (narrows) to form the conus medullaris - A typical adult spinal cord is approximately ¾ inch in diameter and ranges between 16 and 18 inches (42 and 45 centimeters) in length - does not extend the entire length of the vertebral column, but ends at approximately the superior border of the small of your back. -- because growth of the individual vertebrae continues longer than the growth of the spinal cord; thus, an adult spinal cord is shorter than the vertebral column. TWO FUNCTIONS 1. provide an essential structural and functional link between the brain and the torso and limbs of the body - vital inputs and outputs are relayed along neuron pathways that are within the spinal cord and spinal nerves --- eg. when you pick up an object, sensory information about that object (shape, weight, temperature) is relayed along one or more spinal nerves and then through the spinal cord to reach the brain, where the information is interpreted --- movement of your limbs is controlled by nerve signals initiated in the brain. - both sensory input and motor output are relayed along the pathways within the spinal cord and spinal nerves. 2. role in spinal reflexes - involve nervous system responses that do not require the involvement of the brain, but instead have the spinal cord as the integration center. - initiate our quickest reactions to a stimulus - some functional independence from the brain - when you pull your hand back in response to a hot stimulus (e.g., touching a hot stove), this involves a spinal reflex - only when this sensory input reaches the brain that you consciously perceive that it is painful

ascending nerve tracts

- sensory pathways include the sensory neurons that relay sensory input to the brain. - Sensory pathways are also called ascending pathways because the nerve signals are relayed from the sensory receptors superiorly to the brain

spinal nerves

- spinal chord associated with 31 pairs of spinal nerves - Each spinal nerve is typically identified by the first letter of the spinal cord part to which it attaches, followed by a number -- each side of the spinal cord contains 8 cervical nerves (called C1-C8), 12 thoracic nerves (T1-T12), 5 lumbar nerves (L1-L5), 5 sacral nerves (S1-S5), and 1 coccygeal nerve (Co1) - different parts of the spinal cord do not match up exactly with the vertebrae of the same name -- eg.. the lumbar part of the spinal cord is actually closer to the inferior thoracic vertebrae than to the lumbar vertebrae. --- This apparent discrepancy is due to the continued growth of individual vertebrae after spinal cord growth is complete ANATOMY - Each spinal nerve anchors to the spinal cord by two roots, - POSTERIOR ROOT - ANTERIOR ROOT ---each of these roots is composed of multiple rootlets - Each spinal nerve forms where the posterior root (containing sensory neurons) and the anterior root (containing motor neurons) join - both sensory and motor neurons compose each spinal nerve, and it is classified as a MIXED NERVE - compare a spinal nerve to a cable composed of multiple wires, the "wires" within a spinal nerve are the sensory and motor axons, and each "wire" transmits signals in one direction only.

cauda equina

- spinal cord does not extend the entire length of the vertebral column, but typically ends at the inferior border of the L1 vertebra, as described. - Consequently, roots of the lumbar, sacral and coccygeal spinal nerves do not extend horizontally from the spinal cord --- Instead, these spinal nerve roots extend inferiorly from the conus medullaris until the specific location where they exit the vertebral column - Collectively, these spinal nerve roots form a structure called the cauda equina. conus medullaris (at the L1 vertebra) represents (1) the inferior end of the spinal cord and (2) the most superior portion of the spinal roots forming the cauda equina.

axillary n.

- traverses through the axilla and posterior to the surgical neck of the humerus. - The axillary nerve innervates both the deltoid and teres minor muscles. - It receives sensory nerve signals from the superolateral part of the arm.

Direct pathway

- uses a chain of only two motor neurons to communicate between the brain and the skeletal muscles - originates in the primary motor cortex of the cerebral frontal lobe. - only one upper motor neuron and one lower motor neuron. direct pathways are housed within one of two pathways within the spinal cord: LATERAL CORTIOSPINAL TRACTS ANTERIOR CORTICOSPINAL TRACTS

spinothalamic pathway (aka Anterolateral pathway)

- uses a chain of three neurons to communicate with the brain about a specific stimulus - This pathway originates at tactile somatosensory receptors within both the skin and mucous membranes. -This sensory input is providing information to the brain (specifically, the cerebral cortex) about crude touch and pressure as well as pain and temperature. - Typically, sensations that require us to act in response to the stimulus (such as either an itch that makes us want to scratch or tickling that makes us jerk away) are relayed through the anterolateral pathway. - name of this pathway (anterolateral pathway) is derived from the location of the two funiculi through which it ascends (anterior funiculus and lateral funiculus). -secondary name (spinothalamic pathway) is derived from the tracts that relay the nerve signals within the spinal cord to the thalamus. PATHWAY - axon of the primary neuron extends from the somatosensory receptor into the spinal cord (via the posterior root). The primary neuron synapses with the secondary neuron within the posterior horn of the spinal cord. - axon of the second neuron (blue line) extends from the spinal cord to the thalamus. The axons project within the spinothalamic tract—either within the anterior funiculus (via the anterior spinothalamic tract) or the lateral funiculus (via the lateral spinothalamic tract) to the thalamus. The thalamus "filters" the incoming sensory input. - (Decussation occurs to the opposite side within the spinal cord as the axons extend into the spinothalamic tract.) - axon of the tertiary neuron (green line) extends from the thalamus to the cerebrum (specifically, to a location within the primary somatosensory cortex housed within the postcentral gyrus of the parietal lobe). - Conscious perception of the tactile or proprioceptor sensory input occurs within the parietal lobe.

posterior column pathway

- uses a chain of three sensory neurons to communicate with the brain about a specific stimulus. - This pathway originates at either of the two types of somatosensory receptors: (1) tactile receptors housed within both the skin and mucous membranes or (2) proprioceptors within joints, muscles, and tendons. - this sensory input is providing information to the brain (specifically, the cerebral cortex) about discriminative touch, precise pressure, and vibration sensations from the tactile receptors of the skin and with conscious perception of the skeleton and skeletal muscles from proprioceptors. - eg, this pathway provides the information to your brain to identify an object in your hand (even if your eyes are closed) and where your arms are positioned (even if your eyes are closed). PATHWAY - The axon of the primary neuron (purple line) extends from the somatosensory receptor into the spinal cord (via the posterior root) and ascends within the posterior funiculus within the spinal cord. (The specific fasciculus is either the fasciculus cuneatus or the fasciculus gracilis.) -The primary neuron synapses with the secondary neuron within the gray matter of the medulla oblongata of the brain (specifically, the nucleus cuneatus and nucleus gracilis, respectively). - name of this pathway (posterior funiculus-medial lemniscal pathway) is derived from the two components of white matter that it extends through: the posterior funiculus within the spinal cord and medial lemniscus within the brain. -The axon of the secondary neuron (blue line) extends from the medulla oblongata and projects within the medial lemniscus to the thalamus. -The thalamus "filters" this incoming sensory input as described in section 13.4b. (Decussation occurs to the opposite side within the brain just prior to the medial lemniscus.) - The axon of the tertiary neuron (green line) extends from the thalamus to the cerebrum (specifically to a location within the primary somatosensory cortex housed within the postcentral gyrus of the parietal lobe; ). - Conscious perception of the tactile or proprioceptor sensory input occurs within the parietal lobe.

gray commissure

- within the spinal cord forms a bar of gray matter connecting the left and right sides of the posterior, lateral, and anterior horns - gray commissure is an unusual gray matter region because it primarily houses unmyelinated axons (which lack the whitish-colored myelin and, thus, appear gray in color) - bar of gray matter serves as a communication route between the right and left sides of the spinal cord.

anterior corticospinal tract

-(which composes the other 15% of the direct pathway) innervate axial skeletal muscle.

upper and lower neurons

UPPER - is the first neuron in a chain of neurons. - cell body of the upper motor neuron is housed within the cerebral cortex, cerebral nuclei, or a specific nucleus within the brainstem. - Axons of the upper motor neuron synapse either directly upon lower motor neurons (in direct pathways) or upon interneurons that ultimately synapse upon lower motor neurons (in indirect pathways). - The upper motor neurons either excite or inhibit the activity of lower motor neurons. LOWER - is the last neuron in the chain of neurons. - The cell body of a lower motor neuron is housed within the anterior horn of the spinal cord (as described in section 14.3a). - Axons of the lower motor neurons exit the spinal cord through the anterior root and project to and innervate a specific skeletal muscle. The lower motor neuron always excites the skeletal muscle fibers to contract.