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Chapter 12 – The spinal cord

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    This refers to a blundle of axons in the PNS. 3 Types:
    Mixed: sensory and motor (Spinal nerves)
    Afferent: sensory, carry impulses TOWARD the CNS
    Efferent: motor, carry impulses AWAY from the CNS
    Spinal nerves
    Spinal nerves
    These type of nerves carry mixed nerves: both sensory and motor fibers. They are formed outside the CNS as they exit the intervertebral foramen. There are 31 pairs:
    8 pairs of cervical nerves (7 bones)
    12 pairs of thoracic nerves (12 ribs)
    5 pairs of lumbar nerves (5 bones)
    5 pairs of sacral nerves (5 openings)
    1 pair of coccygeal nerves.
    Named after where they come out (not where they originate)
    They refer to dorsal and ventral roots combined.
    Cranial nerves
    These type of nerves can be:
    Sensory: CNI, II, VIII
    Motor: III, IV, VI, XI, XII
    Mixed: V, VII, IX, X
    Spinal cord
    This part of the body is about 42cm (17in) long and 1.8cm (3/4in) thick. It provides a 2 way conduction pathway to and from the brain. It begins at the Foramen magnum and ends at L1-L2 (L3 in infants) in Cauda equina. It is a major reflex center, they are initiated and completed at that same level. It is protected by bone, meninges, and CSF.
    Cauda Equina
    Cauda Equina
    “Horse’s tail” This refers to the collection of nerve roots at the inferior end of the vertebral canal. Its strange arrangement reflects the fact that during fetal developmental, the vertebral column grows faster than the spinal cord, forcing the lower spinal nerve roots to “chase” their exit points inferiorly through he vertebral canal.
    Spinal segment
    This refers to an area of the spinal cord form which a pair of spinal nerves arises
    This refers to the fibrous connective tissue membrane that protects the spinal cord. Same name as the brain ones but different structures. They are continuous though. 3 layers:
    Dura mater + Subdural space
    Arachnoid mater + subarachnoid space
    Pia mater
    Dura and arachnoid layers extend to S2, well beyond end of spinal cord at the Conus medullaris.
    Dura mater
    This single layer is not attahced to the bony walls of the vertebral column. It has a small subdural space.
    Arachnoid mater
    This helps protect the spinal cord. It has a subarachnoid space with CFS (Here’s where you do a spinal tap).
    Pia mater
    This layer is stronger than the one in the brain and anchors the spinal cord. The FILUM TERMINALE anchors cord distally (from conus medullaris to the coccyx) and the DENTICULATE LIGAMENTS anchors laterally to vertebrae, one on ea side.
    Conus Medullaris
    Conus Medullaris
    This is where the spinal cord terminates in a tapering cone shaped structure.
    Epidural space
    This part of the Spinal cord is b/w the bony vertebrae and the spinal dura mater and it is filled with adipose tissue and loose connective tissue
    Lumbar puncture
    AKA spinal tap. Anything inferior to L1 and L2 and the CSF is taken from the subarachnoid space.
    Denticulate ligament
    Denticulate ligament
    These are saw-toothed shelves of pia mater that secure the spinal cord lateraly to the tough dura mater meninx thorught its lenght.
    Filum terminale
    Filum terminale
    This is a fibrous extension of the conus covered by pia mater, extends inferiorly from the conus medullaris to the cocccyx, where it anchors to the spinal cord so it is not jostled by body movement.
    Vertebra cross section
    Vertebra cross section
    Composed of gray matter in the center and white mater on the outside. 2 Groves divide it: the ventral (anterior) median fissure, and the dorsal (posterior) median sulcus. Both sides are connected by the gray commisure. Axons coming out from the ventral area come together to become the ventral roots of the spinal cord (Somatosensory motor). Axons coming out from the dorsal area come together to become the dorsal roots of the spinal cord (Sensory neurons). The central canal is filled with CSF.
    Ventral median fissure
    Ventral median fissure
    Dorsal median sulcus
    Dorsal median sulcus
    Gray matter
    Gray matter
    This portion of a vertebra cross section looks like a butterfly. It is located in its core. It is connected by the gray commisure (crossbar). Parts:
    Dorsal (posterior) horns – SS (Interneuronos receiving input from somatic sensory neurons), VS (Interneurons receiving input from visceral sensory neurons)
    Ventral (Anterior) horns – VM (Visceral motor (autonomic) neurons, SM (Somatic motor neurons)
    lateral horns, and central canal (CSF). It can be divided into
    Dorsal horns
    Dorsal horns
    This part of gray matter contains interneurons, afferent somatosensory and visceral sensory neurons.
    Blue – Interneurons receiving input from somatic sensory neurons
    Green – Interneurons receiving input from visceral sensory neurons.
    Yellow – Visceral motor (autonomic) neurons
    Red – Somatic motor neurons
    This refers to a group of roots.
    Dorsal root
    This refers to incoming sensory fibers that arise from sensory neurons in the dorsal root ganglia and conduct impulses from peripheral receptors to the spinal cord.
    Dorsal root ganglia
    Dorsal root ganglia
    The cell bodies of the associated sensory neurons are found in this enlarged region of the dorsal root. (Unipolar neurons)
    Ventral horns
    This part of the gray matter contains efferent somatic motor neurons.
    Ventral root
    this refers to outgoing motor fibers that arise from ventral horn motor neurons and extend to and innervate the skeletal muscles.
    Lateral horns
    These areas of the gray matter are only found in Thoracic and Lumbar regions and contain sympathetic region neurons. Visceral sensory and visceral motor (autonomic) neurons.
    White matter
    White matter
    This part of a vertebra cross section is on the outside and is composed of myelinated and nonmyelinated nerve fibers (axons = tracts) that allow communication b/w diff. parts of the spinal cord and b/w the cord and brain. Three directions:
    Ascending (up to higher centers, sensory inputs) originate in the dorsal root horn
    Descending (down from the brain or within, motor outputs) end in the ventral root
    Transverse (across from one side of the cord to the other, commisural fibers).
    It has 6 bundles (3 pairs, 3 ea side) called columns or finiculi: Ventral, lateral, and dorsal.
    Bundles of fiber tracts or columns of white matter and each is made up of axons with similar destination and functions.Ventral, dorsal and lateral (picture). 2 for each side.
    Branching of spinal nerves
    Branching of spinal nerves
    Dorsal Rami: short, to skin and muscles of back. A
    Ventral rami: longer, from plexus to anterior trunk and limbs. B
    Meningeal branches: supply meninges, vertebrae and blood vessels. C
    Both ventral and dorsal are similar in that they contain both sensory and motor fibers.
    This forms when fibers from diff. ventral rami criss-cross and become resdistributed. Each contains fibers form several nerves and they travel to body periphery via several diff. routes. This allows for function retention in case of damage of one particular nerve. It wont have debilitating effects.
    Ventral Rami
    It is composed of nerve pelxes that form webs in cervical, lumbar, sacral and coccyx area. It includes:
    -Cervical plexus (C1-C5)
    -Brachial plexus (C5-T1)
    -Lumbar plexus (L1-L4)
    -Sacral plexus (L4-S4)
    -Coccygeal plexus (S4-CO1)
    Cervical plexus (C1-C5)
    Cervical plexus (C1-C5)
    It supplies parts of head, neck and shoulders. It is burried deep in the neck under the sternocleidomastoid muscle. Most of its branches are cutaneous nerves that supply only the skin. The most important nerve is the Phrenic nerve (C3-C5) it supplies both sensory and motor fibers to the diaphragm.
    Phrenic nerve
    Phrenic nerve
    It is located in the cervical plexus b/w C3-C5, it supplies both sensory and motor fibers to the diaphragm. If damaged, it can cause respiratory arrest. Irritation of it causes hiccups.
    Brachial plexus
    Brachial plexus
    This plexus goes from C5-T1 to shoulder and upper limbs, passes superior to 1st rib, under clavicle. Nerves:
    1. Axillary (C5-C6): deltoid, teres minor, shoulder joint
    2. Musculocutaneous (C5-C7): elbow flexors (biceps brachii, brachialis)
    3. Radial (C5-C8,T1): Shoulder, elbow extensors (triceps brachii, brachioradialis)
    4. Median (C8-T1, C5-C7): forearm flexors, pronator teres, palmaris longis, muscles of hand.
    5. Ulnar (C8-T1): flexor carpi ulnaris, many small muscles of hand.
    Muscles: Deltoid, teres minor, biceps brachii, brachialis, triceps brachii, brachioradialis, pronator teres, palmaris longis, flexor carpi ulnaris.
    From roots, to trunks, to posterior Divisions, to anterio divisions Cords) “Really Tired? Drink Coffee”
    Lumbar plexus
    Lumbar plexus
    This plexus arises from spinal nerves L1-L4 and lies within the psoas major muscles. It supplies abdominal wall and muscles, external genitals, anterior/medial thigh. Nerves:
    1. Femoral nerve (L2-L4) is the largest terminal nerve and runs deep to the inguinal ligament to enter the thigh and then divides into several large branches. If damaged, it causes inability to extend leg and loss of sensation in thigh.
    2. Obturator (L2-L4) enters the medial thigh via the obturator foramen and innervates the adductor muscles. If damaged, it causes paralysis of thigh abductors.
    3. Lateral femoral cutaneous (L2-L3) skin of lateral thigh
    4. Iliohypogastric (L1)Skin on side of buttock and above pubis
    5. Ilioinguinal (L1) Skin of external genitalia and proximal medial aspect of the thigh
    6. Genitofemoral (L1, L2) Skin of scrotum in males, labia majora in female.
    Sacral plexus (Coccygeal plexus)
    Sacral plexus (Coccygeal plexus)
    This plexus arises from nerves L3-S4 and it is part of the ventral rami. They are both anterior to the sacrum. They serve the buttock and lower limb and others innervate pelvic structures and the perineum. Nerves:
    1. Sciatic Nerve (L4, L5, S1-S3) this is the largest branch of the sacral plexus, thickes and longest nerve in the body! It supplies the entrire lower limb, except the anteromedial thigh. Composed of Tibial and common fibular nerves. Muscles: hamstring (all the extensors and knee flexors) and the adductor magnus. Pain: extends from the buttock down the leg to the foot. May be signs of: herniated disc, dislocated hip, osteoarthritis of lumbrosacral spine, pressure from uterus during pregnancy.
    a. Tibial nerve: supplies the posterior compartment muscles of the leg. it gives off the sural nerve (serves the skin of posterolateral leg). It divides into medial and lateral plantar nerves at the ankle. Injury: dorsiflexion of foot, shuffling, numbness.
    b. Common fibular nerve (peroneal) lateral side of the knee. Injure: foot drop or inversion or numbness.
    2. Superior gluteal (L4, L5, S1) buttocks and tensor facia lata muscles (inferior too)
    3. Inferior gluteal (L5-S2)
    4. Posterior femoral cutaneous (S1-S3)
    5. Pudendal (S2-S4) innervates muscles and skin in the perineum and helps stimulated erection and control urination.
    Reflex arc components
    1. Receptor – site of stimulus
    2 Sensory neuron – transmits afferent impulses to the CNS
    3. Integration center – gray matter in the spinal cord. 2 Types:
    a. monosynaptic reflex: 1 sensor neuron, 1 motor neurons
    b. Polysynaptic reflex: multiple synapses and chains of interneurons.
    4. Motor neuron – conducts efferent impulses from the integration center to the organ
    5. Effector – muscle fiber or gland that responds to efferent impulses.
    Integration center
    It is the gray mater in the spinal cord and it attends to somatic reflexes (skeletal muscle), Autonomic reflexes (smooth and cardiac muscle and glands), and cranial nerve reflexes.
    Spinal reflexes
    They respond quick to the environment but not spontaneously to sensory input. They are involuntary and difficult to suppress. They are unlearned and stereotypical (same way every time)
    stretch and tendon reflexes
    They help your nervous system smoothly coordinate the activity of your skeletal muscle. It requires information about the length of the muscle from the muscle spindles and the amount of tension in the muscle and its associated tendons from tendon organs.
    Muscle spindles
    It consist of 3-10 modified skeletal muscle fibers called:
    1. intrafusal muscle fibers, they are <1/4 the size of the effector fibers of the muscle called; These fibers have contractile regions at their ends called gamma efferent fibers.
    2. Extrafusal muscle. Anulospiral (Primary) endings and Flower spray endings (Secondary) send sensory inputs to the CNS. These fibers have alpha efferent fiber and alpha motor neurons.
    Stretch reflex
    Stretch reflex
    Example: Patellar or knee jerk reflex. This reflex makes sure that the muscle stays at that length (muscle contraction in response to increased muscle length, stretch). Prevents your knees from buckling when you are standing up right. All stretch reflexes are monosynaptic and ipsilateral reflex arc, except the part of the reflex arc that inhibits the motor neurons serving the antagonistic muscles, that is polysynaptic (reciprocal inhibition). Process:
    1. tapping the ligament stretches the muscle and excites the muscle spindles
    2. Impulse travels to the spinal cold. It synapses with motor and interneurons.
    3a. motor neurons send impulse to muscle = contraction, knee extension.
    3b. interneurons make inhibitory synapses with ventral horn neurons = prevent antagonist muscle from resisting contraction of other muscle.
    reciprocal inhibition
    This type of inhibition happens when branches of afferent fibers also synapse with interneurons that inhibit motor neurons controlling antagonistic muscles (parallel processing). This causes the antagonists to relax so that they cannot resist the shortening of the “stretched” muscle. This part of the reflex arc is polysynaptic.
    Tendon reflex
    Opposite effect of the stretch reflex: muscles relax and lengthen in response to tension. This is a polysynaptic reflex. Tendons are activated when muscle tension increases during contraction or passive stretching. Afferent impulses go to the spinal cord, then to the cerebellum where info is sent to adjust muscle tension. At the same time, motor neurons supplying the contracting muscle are inhibited and antagonist muscles are activated (reciprocal activation) = contracting muscle relaxes as its antagonist is activated. Process:
    1. muscle contracts, Golgi tendon organs are activated
    2. Afferent fibers synapses w interneurons in the spinal cord.
    3a. Efferent impulses to muscle w stretched are damped = muscle relaxes, reducing tension.
    3b. Efferent impulses to antagonist muscle = contraction.
    reciprocal activation
    This happens when motor neurons in the spinal cord circuits supplying the contracting muscle are inhibiter and antagonist muscles are activated simultaneously. this results in the contracting muscles relaxing as the antagonist is activated.
    Somatic spinal reflexes
    2 types of receptors:
    1. muscle spindle apparatus – monitor stretch in skeletal muscles
    2. Golgi tendon organs – monitor tension in tendons
    Effectors = skeletal muscles.
    Protective. Also known as the withdrawal reflex, causes automatic withdrawal of the threatened body part from the stimulus. They are ipsilateral and polysynaptic (particularly when several muscles are needed to withdraw the body part). They can override spinal pathways and prevent other reflexes from using them at the same time. They can also be overridden if a painful stimulus expected.
    Cross-extensor reflex
    It is particularly important to maintain balance and can work together with the flexor reflex. Consists of ipsilateral withdrawal reflex and contralateral extensor reflex. Ex. foot on broken glass, lift one foot, extend/stand on other leg for support. Or, one arm grabbed, the other one extends.
    Superficial reflexes
    These reflexes are elicited by gentle cutaneous stimulation. Two best known: Plantar and abdominal reflexes.
    They help detect disorders/injury (MS, Neurosyphilis, lesion of nervous system, electrolyte imbalance).
    Plantar reflexes
    This type of reflex tests the integrity of the spinal cord from L4-S2 and indirectly determines if the corticospinal tracts are functioning properly. Stroke the lateral margin of the sole of foot. Normal response: toes flex downward (curl). Negative response: great toe dorsiflexes and the others fan laterally. This indicates that the primary motor cortex or corticospinal tract is damaged. This response is normal in infants 12 months or younger.
    Abdominal reflexes
    This type of reflex check the integrity of the spinal cord and ventral rami from T8-T12. Stroking the skin of the lateral abdomen above, to the side, or below the umbilicus induces a reflex contraction of the abdominal muscles in which the umbilicus moves towards the stimulated site. Absence of response: indicates lesions in the corticospinal tract.
    Spinal cord trauma
    The spinal cord is elastic but any localized injury to it or its roots leads to some functional loss. May lead to Paresthesias and paralysis (Flaccid, spastic, paraplegia, quadriplegia, and hemiplegia)
    This refers to the damage of the dorsal roots or sensory tracts and results in abnormal sensations or sensory loss.
    This is refers to damage to the ventral roots or ventral horn cells. It is loss of motor function. Two Types: Flaccid and Spastic.
    4 functional types: parasthesias, paraplegia paralysis, quadriplegia paralysis, hemiplegia paralysis.
    Flaccid paralysis
    Occurs when the spinal cord or ventral roots are injured = nerve impulses do not reach the affect muscles = muscles cannot be moved voluntarily or involuntarily. W/o stimulation muscles atrophy.
    Spastic paralysis
    Only occurs when the upper motor neurons of the primary motor cortex (higher axons in the spinal cord) are damaged. Spinal neurons remain intact; muscles are stimulated by reflex activity but no voluntary control of muscle. Muscles may shorten permanently.
    A cross-section injury at any level of the spinal cord will result in total motor and sensory loss in regions inferior to the cut.
    Paraplegia paralysis
    This refers to paralysis of both lower extremities from lesions: T1 – L1
    Quadriplegia paralysis
    This refers to paralysis of all 4 limbs from lesions above C5
    This refers to paralysis of one side of the body from trauma to one side of spinal cord or stroke/brain lesions on contralateral side of the body.
    Spinal shock
    This refers to a transient period of functional loss that follows the injury. It depresses all reflex activity caudal to the lesion site. Neural function typ. returns w/in few hours. If it does not resume w/in 48 hours, paralysis is permanent.
    “gray matter;inflamation of the spinal cord” results form the poliovirus (through feces contaminated water) destroys ventral horn motor neurons. It initially causes muscle pain and weakness, later, paralysis develops and the muscles served atrophy. Victim may die from paralyzed respiratory muscles. survivors may experience postpolio syndrome (burning pains in muscles, and muscle weakness and atrophy).
    Amyotrophic Lateral Sclerosis (ALS)
    AKA Lou Gehrig’s disease, neuromuscular condition that progressively destroys ventral horn motor neurons and fibers of the pyramidal tracts (major motor pathway). The victim will eventually lose the ability to speak, swallow, and breath. Death typ. happens w/in 5 years. 10% inherited mutations, spontaneous mutations the rest.
    Neuronal Pathway
    It refers to the sequence of structures a tract visits along the way to its destination. Many decussate, most consist of 3 neurons and area paired symmetrically, and most exhibit somatotopy
    This refers to pathways that cross form one side of the CNS to the other at some point along their journey. Tracts that decussate have a contralateral side response above the decussate. If they do not, the destination is ipsilateral. Their purpose is to strengthen the spinal cord structure. The crossing happens at various points of the spinal cord and CNS depending on the type of tract to avoid lumps.
    This refers to the chain of 2-3 neurons that contribute to successive tracts of the pathway
    Somatotopy of Pathaways
    This refers to the precise relationship among the tract fibers that reflects the orderly mapping of the body (ex. homunculus). The correspondence b/w the position of a receptor in part of the body and the corresponding area of the cerebral cortex that is activated by it. Adjacent areas on the skin are represented by adjacent neurons in the same pathway.
    Spinal tracts
    All nerve fibers (axons) in a particular tract have the same origin, destination, and function. The name indicates orgin and destination.
    Ascending tracts/pathways
    Ascending tracts/pathways
    They carry sensory signals up the spinal cord. Typically require 3 neurons. (1st. 2nd, and 3rd order neurons). These are located on the white matter of the spinal cord. 3 Different pathways:
    Dorsal white column: Gracile and cuneate fasciculus.
    Spinocerebellar tracts (lateral): Dorsal and ventral
    Spinothalamic tracts: Lateral and ventral
    1st order neurons
    This neuron’s cell body can be found in ganglion; dorsal root ganglia. Impulses are conducted from the cutaneous receptor of the skin and from proprioceptors to the spinal cord or brain stem. Here they synapse with 2nd order neuron.
    Impulses from the facial area are transmitted by cranial nerves, rest of the body by spinal nerves (31 pairs). Mostly unipolar neurons
    2nd order neuron
    This neuron’s cell body is often found in the dorsal horn of spinal cord (interneurons), can be in other areas (e.g. medulla). The axons transmit impulses to the thalamus (the gateway to the cortex where signals are interpreted, they synapse) or to the cerebellum. They typ. decussate. The thalamus receives “left-sided” information on the right side of the thalamus and vise versa.
    3rd order neuron
    This neuron’s cell body is in the thalamus. They relay impulses to somatosensory cortex of the cerebrum (postcentral gyrus) (not in the cerebellum). The area of sensory cortex devoted to a body region is relative to the number of sensory receptors.
    Dorsal column-medial lemniscal pathway (DCML)
    Dorsal column-medial lemniscal pathway (DCML)
    AKA posterior pathway. It mediates precise, straight-through transmission of inputs from a single type (or a few related types) of sensory receptor that can be localized precisely on the body surface, such as discriminative touch and vibrations. It decussates in the nucleus gracilis and nucleus cuneatus in the medulla. These pathways are formed by paired tracts of the dorsal white column of the spinal cord – Passes through the dorsal funiculus (gracilis and cuneatus) and the medial lemniscus which terminates in the thalamus. From there, impulses are forwarded to specific areas of the somatosensory cortex. 1st order neuron is in the dorsal ganglia. 2nd order neuron body is in the dorsal nucleus gracilis and cuneatus. It decussates in the nucleus gracilis and cuneatus in the medulla. 3rd order neuron is in the Thalamus.
    Damage: Los of sense of touch, proprioception, vibration in leg.
    Nucleus gracilis
    (Fasciculus gracilis) This is part of the dorsal column pathway and comes from the lower part of the body. It is made of sensory dorsal medial white tracts from lower extremities. It attends to fine touch, vibrations, deep pressure, visceral pain, and proprioception. The tracts ascend to the medulla oblongata the axons then synapse in the thalamus.
    Nucleus Cuneatus
    (Fasciculus cuneatus) This is also part of the dorsal column pathway. It carries sensory information on the dorsal lateral white tracts from the upper extremities. it ascents to the mdedula oblongata through the nucleus cuneatus. Axons then synapse in the thalamus.
    Medial lemniscus
    Medial lemniscus
    it arises in the medulla and terminates in the thalamus.
    Spinothalamic pathway
    Spinothalamic pathway
    This pathway decussates in the spinal cord and passes through the lateral and anterior funiculi. This is considered the major pain pathway, it transports temperature, pressure, tickle, itch, and crude touch sensory info.
    1st order neuron body is in the dorsal root ganglia. 2nd order neuron is in the dorsal horn. It decussates in the spinal cord. 3rd order neuron body is in the thalamus. Ends in the primary somatosensory cortex.
    3 primary sites at which modification of pain transmission can occur:
    1. The periphery: responsiveness of pain receptors is enhanced by prostaglandins (formed in response to tissue trauma, receptors). Endogenous compounds (histamine, serotonin) may be responsible for the actual pain sensation. Aspiring and ice affects prostaglandins.
    2. Reticular formation: pain evokes arousal, changes in heart rate, blood pressure, respiration and other activities
    3. Thalamus and cerebral cortex: appreciation or conscious awareness of pain.
    spinocerebellar pathway
    This pathway does not decussate and terminates in the cerebellum and only has 2 neurons. Passes through the lateral funiculus (ventral and dorsal spinocerebellar tracts). It sends propioceptive signals from limbs and trunk to cerebellum.
    1st order neuron body is in the dorsal root ganglia. 2nd order neuron body is in the dorsal horn. From there it goes to the cerebellum. They convey information about muscle or tendon stretch, it coordinates skeletal muscle activity. Do not contribute to conscious sensation. Anterior tracts decussate; posterior tracts do not.
    Descending tracts
    These are located on the white matter of the spinal cord. They carry motor signals down the spinal cord. It involves at least 2 neurons: upper and lower motor neuron. Deliver different impulses from brain to spinal cord through direct and indirect pathways. Different types:
    Corticospinal tracts (pyramidal tracts, pass through the pyramids of the medulla): lateral and ventral.
    Rubrospinal tract
    Reticulospinal tracts: medial and lateral
    Vestibulospinal tract
    Tectospinal tract
    Upper motor neurons
    “Pyramidal cells” These neurons are pyramidal cells of the motor cortex and the neurons of subcortical motor nuclei. They begin with the cerebral cortex.
    Lower motor neurons
    “Somatic motor neurons” These neurons are ventral horn motor neurons and directly innervate the skeletal muscle, their effectors. Their soma is in the brain stem or spinal cord, axon goes all the way to the effector.
    Direct pathways
    pyramidal tracts (voluntary) regulates fast and fine movements (writing or doing needle work) of skeletal muscles sent through the brain stem. They descend from the pyramidal cells w/o synapsing all they way to the spinal cord. There they synapse with with interneurons or ventral horn motor neurons.. They include tectospinal tracts, reticulospinal tracts, and vestibulospinal tracts (Do not memorize).
    Indirect pathways
    (all other (involuntary) muscles) These include brain stem motor nuclei and all motor pathways (except pyramidal pathways). They are complex and multisynaptic. They regulate: axial muscles that maintain balance and posture, coarse limb movements, head, neck, and eye movements that follow objects in the visual field. Many depend on reflex activity. Important tracts are: reticulospinal and vestibulospinal tracts, superior colliculi and tectospinal tracts, rubrospinal tracts.
    Rubrospinal tracts
    These efferent tracts control flexor muscles and maintain tone. It originates in the red nucleus, decussates in mid brain, into pons. Descend down the lateral funiculus, ventral horn. It is considered “extrapyramidal”. Its function is to maintain balance and muscle tone, controls gait.
    This refers to a change in sensitivity in the presence of a constant stimulus. The response is less responsive receptor membranes and decline in frequency or stop of receptor potentials. (new smell, good or bad)

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