Spinal Cord, Nerves, and Reflexes

Chapter 13: The Spinal Cord, Spinal Nerves, and Somatic Reflexes

Introduction

• Thousands of Americans get paralyzed by spinal cord injuries each year.
• The spinal cord serves as the "information highway" connecting the brain to the lower body.
• This chapter focuses on the spinal cord and its nerves.

The Spinal Cord Functions

• Conduction takes place when nerve fibers carry sensory and motor information up and down the spinal cord.
• Neural integration happens when spinal neurons receive input from multiple sources, integrate it and decide on the appropriate output like bladder control.
• Locomotion involves central pattern generators helping the spinal cord coordinate repetitive muscle contraction for walking.
• Reflexes are involuntary, rapid responses to stimuli that are crucial for maintaining posture, coordination, and protection.

Spinal Cord Anatomy

• The spinal cord is a cylinder of nervous tissue originating from the brainstem at the foramen magnum of the skull.
• It occupies the upper two-thirds of the vertebral canal.
• The spinal cord's inferior margin ends at L1 or slightly beyond.
• On average, the spinal cord measures 1.8 cm thick and 45 cm long.
• The spinal cord gives rise to 31 pairs of spinal nerves.
• A segment is defined as the part of the spinal cord supplied by each pair of spinal nerves.
• Anterior median fissure and posterior median sulcus are longitudinal grooves on the anterior and posterior sides.
• Spinal cord divided into the cervical, thoracic, lumbar, and sacral regions.
• Cervical enlargement indicates nerves supplying upper limb.
• Lumbosacral enlargement refers to the nerves supplying the pelvic region and lower limbs.
• The medullary cone (conus medullaris) is where the cord tapers to a point inferior to the lumbosacral enlargement.
• The cauda equina is a bundle of nerve roots that occupies the vertebral canal from L2 to S5.

Meninges

• Meninges are three fibrous membranes that enclose the brain and spinal cord.
• They separate soft tissue of the central nervous system from the bones of the cranium and vertebral canal.
• The three layers of the meninges, from superficial to deep, are the dura mater, arachnoid mater, and pia mater.
• Dural sheath surrounds the spinal cord and is separated from vertebrae by the epidural space.
• The arachnoid membrane adheres to the dura and is separated from the pia by CSF filled subarachnoid space.
• A lumbar puncture (spinal tap) takes a sample of CSF.
• The Pia is a delicate membrane following the spinal cord contours and continues inferiorly as a fibrous terminal filum that fuses with the dura to form the coccygeal ligament.

Cross Sectional Anatomy

• The central area of gray matter is shaped like a butterfly and surrounded by white matter in three funiculi.
• Gray matter consists of neuron cell bodies with little myelin and serves as the site of information processing and synaptic integration.
• White matter is made up of abundantly myelinated axons that carry signals from one part of the CNS to another.
• The spinal cord has a central core of gray matter that looks butterfly- or H-shaped in cross section.
• Gray matter includes a pair of posterior (dorsal) horns.
• The posterior (dorsal) root of spinal nerve carries only sensory fibers.
• Gray matter also has a pair of thicker anterior (ventral) horns, from which the anterior (ventral) root of spinal nerve carries only motor fibers.
• The gray commissure connects the right and left sides and has a central canal lined with ependymal cells and containing CSF.
• Lateral horn is only visible from T2 through L1 and contain neurons of the sympathetic nervous system.

White Matter

• White matter of the spinal cord surrounds the gray matter.
• It consists of bundles of axons that course up and down the cord providing communication between different levels of the CNS.
• Funiculi (columns) are three pairs of white matter bundles with posterior (dorsal), lateral, and anterior (ventral) funiculi on each side.
• Tracts (fasciculi) are subdivisions of each funiculus.

Spinal Tracts

• Fibers in a given tract have similar origins, destinations, and functions.
• Ascending tracts carry sensory information up.
• Descending tracts carry motor information down.
• Decussation is the crossing of the midline which allows the brain to sense and control the contralateral side of the body.
• Contralateral means that the origin and destination of a tract are on opposite sides of the body.
• Ipsilateral means that the origin and destination of a tract are on the same side of the body; it does not decussate.

Ascending Tracts

• Ascending tracts carry sensory signals up the spinal cord.
• These tracts include the gracile fasciculus, cuneate fasciculus, spinothalamic tract, spinoreticular tract, posterior (dorsal) and anterior (ventral) spinocerebellar tracts.
• Sensory signals travel across three neurons from origin (receptors) to destinations in the sensory areas of the brain.
• First-order neurons detect the stimulus and transmit signal to spinal cord or brainstem.
• Second-order neurons continue to the thalamus at the upper end of the brainstem.
• Third-order neurons carry the signal the rest of the way to the sensory region of the cerebral cortex.

Gracile Fasciculus

• The gracile fasciculus carries signals from the midthoracic and lower parts of the body.
• Below T6 it composes the entire Posterior Funiculus.
• Above T6, it is accompanied by the cuneate fasciculus.
• The gracile fasciculus consists of first-order nerve fibers traveling up the ipsilateral side of the spinal cord.
• It terminates at the gracile nucleus of the medulla oblongata.
• It carries signals for vibration, visceral pain, deep and discriminative touch, and proprioception from lower limbs and lower trunk.
• Proprioception is for nonvisual sense of the position and movements of the body.

Cuneate Fasciculus

• At T6 and above, the cuneate fasciculus occupies the lateral portion of the posterior funiculus.
• Second order neurons of gracile and cuneate nuclei decussate and form the medial lemniscus—a tract leading to the thalamus.
• Third-order neurons go from the thalamus to the cerebral cortex, carrying signals to the cerebral hemisphere.
• Signals are from the upper limb and chest.
• Due to crossing, the left hemisphere processes stimuli from the right side of the body, and vice versa.

Spinothalamic Tract

• The spinothalamic tract is part of the anterolateral system that passes up the anterior and lateral funiculi of the spinal cord.
• It carries signals for pain, pressure, temperature, light touch, tickle, and itch.
• The tract is made up of axons of second-order neurons.
• First-order neurons end in the posterior horn of spinal cord
• Second-order neurons start in the posterior horn, then decussate and form the spinothalamic tract.
• Third-order neurons continue from there to the cerebral cortex.
• Signals are sent to the cerebral hemisphere contralateral to the site of the stimulus.

Spinoreticular Tract

• The spinoreticular tract travels up the anterolateral system.
• It carries pain signals resulting from tissue injury.
• First-order neurons enter the posterior horn and synapse with second-order neurons.
• Second-order neurons decussate to the opposite anterolateral system; ascend the cord and end in the reticular formation within the medulla and pons.
• Third-order neurons continue from the pons to the thalamus.
• Fourth-order neurons complete the path to the cerebral cortex.

Spinocerebellar Tracts

• Anterior and posterior spinocerebellar tracts travel through the lateral funiculus.
• This spinocerebellar tract carries proprioceptive signals from the limbs and trunk up to the cerebellum.
• First-order neurons originate in the muscles and tendons and end in the posterior horn of the spinal cord.
• Second-order nerves ascend spinocerebellar tracts and end in the cerebellum providing it with feedback needed to coordinate movements.
• The posterior spinocerebellar tract stays ipsilateral.
• The anterior spinocerebellar tracts cross over and travel up the contralateral side, but cross back to end in the ipsilateral cerebellum.

Descending Tracts

• Descending tracts carry motor signals down the brainstem and spinal cord.
• They involve two motor neurons.
• The upper motor neuron originates in the cerebral cortex or brainstem and terminates on a lower motor neuron.
• The lower motor neuron's neurosoma is in the brainstem or spinal cord, and its axon leads to a muscle or other target organ.

Corticospinal Tracts

• Corticospinal tracts carry signals from the cerebral cortex for precise, finely coordinated movements.
• Pyramids are ridges on the anterior surface of the medulla oblongata formed from fibers of this system.
• Most fibers decussate in the lower medulla forming the lateral corticospinal tract on the contralateral side of the spinal cord.
• Some fibers form the anterior (ventral) corticospinal tract that descends on the ipsilateral side of the spinal cord and decussates inferiorly.

Tectospinal Tract

• Tectospinal tract begins at the Tectum of the Midbrain.
• It crosses to the contralateral side of the midbrain and creates a reflex turning of the head in response to sights and sounds.

Lateral and Medial Reticulospinal Tracts

• These originate in the reticular formation of the brainstem.
• It controls muscles of the upper and lower limbs, especially those for posture and balance.
• It contains descending analgesic pathways, which reduce the transmission of pain signals to the brain.

Lateral and Medial Vestibulospinal Tracts

• Begin in the brainstem vestibular nuclei.
• Receives impulses for balance from the inner ear.
• Controls the extensor muscles of the limbs for balance control.

Spinal Nerves

• The spinal cord communicates with the rest of the body via spinal nerves.
• A nerve is a cord-like organ composed of numerous nerve fibers (axons) bound together by connective tissue.
• Mixed nerves contain both afferent (sensory) and efferent (motor) fibers.
• Nerve fibers of the peripheral nervous system are surrounded by Schwann cells forming the neurilemma and myelin sheath around the axon.
• Endoneurium is loose connective tissue external to the neurilemma.
• Perineurium refers to the layers of overlapping squamous cells that wrap fascicles, which are bundles of nerve fibers.
• Epineurium is dense irregular connective tissue that wraps the entire nerve..
• Sensory (afferent) nerves carry signals from sensory receptors to the CNS.
• Motor (efferent) nerves carry signals from the CNS to muscles and glands.
• Ganglion-cluster of neurosomas outside the CNS.
• Enveloped in an endoneurium continuous with that of the nerve.

Spinal Nerves Pairs

• 31 pairs of spinal nerves (mixed nerves).
• 8 cervical (C1-C8).
• 12 thoracic (T1-T12).
• 5 lumbar (L1-L5).
• 5 sacral (S1-S5).
• 1 coccygeal (Co1).

Proximal Branches of Spinal Nerves

• Each spinal nerve is formed from two roots (proximal branches).
• Posterior (dorsal) root is sensory input to the spinal cord and merges with the posterior (dorsal) root ganglion which contains the neurosomas of sensory neurons carrying signals to the spinal cord.
• Six to eight rootlets enter the posterior horn of the cord.
• Anterior (ventral) root is motor output out of spinal cord.
• and made up of six to eight rootlets leaving spinal cord and converging to form anterior root.
• Cauda equina: formed from roots arising from L2 to C01.

Distal Branches, Spinal Nerves

• Beyond the vertebra, the nerve divides into distal branches.
• Anterior ramus: In the thoracic region, it gives rise to the intercostal nerve; in other regions, anterior rami form plexuses, these control the muscles and joints in that region of the spine and the skin of the back.
• Posterior Ramus: Innervates the muscles and joints in that region of the spine and the skin of the back.
• Meningeal branch: Reenters the vertebral canal and innervates the meninges, vertebrae, and spinal ligaments.

Nerve Plexuses

• Anterior rami form five nerve plexuses.
• Cervical Plexus: Neck, C1 to C5, supplies neck and phrenic nerve to the diaphragm.
• Brachial Plexus: Shoulder, C5 to T1, supplies upper limb and some of shoulder and neck, this can also be the median nerve which is linked to carpal tunnel syndrome.
• Lumbar Plexus: Lower back, L1 to L4, supplies abdominal wall, anterior thigh, and genitalia.
• Sacral Plexus: Pelvis, L4, L5, and S1 to S4, supplies remainder of lower trunk and lower limb.
• Coccygeal Plexus: S4, S5, and Co1.
• Somatosensory Function: Carry sensory signals from bones, joints, muscles, and skin.
• Proprioception: Brain retrieves body information to the nerve that end in muscles and tendons.
• Motor Function: Primarily stimulates muscle contraction.

Cutaneous Innervation and Dermatomes

• Dermatome is a specific area of skin that conveys sensory input to a spinal nerve.
• The dermatome map is a diagram of the cutaneous regions innervated by each spinal nerve.
• Dermatomes overlap their edges as much as 50%.
• Necessary to anesthetize three successive spinal nerves to produce a total loss of sensation in one dermatome.

Somatic Reflexes

• Reflexes are quick, involuntary, stereotyped reactions of glands or muscle to stimulation.
• Reflexes include glandular secretion and contraction of all three types of muscle.
• Somatic reflexes involve the somatic nervous system innervating skeletal muscle.
• The 1st step in the Pathway of a somatic reflex arc is when somatic receptors in skin or muscle receives stimulation.
• The 2nd step requires afferent nerve fibers, carrying information from receptors to posterior horn of spinal cord or to the brainstem.
• The 3rd step involves the integrating center – a point of synaptic contact between neurons in gray matter of cord or brainstem.
• Where the brain determines whether to signal the muscles
• The 4th step consist of efferent nerve fibers that carry motor impulses to muscles
• The final 5th step involves the effectors that is, the muscles carrying out the response.

The Muscle Spindle

• The muscle spindle involves stretch receptors embedded in skeletal muscles.
• Proprioceptors are specialized sense organs that monitor position and movement of body parts.
• Muscle spindles inform the brain of muscle length and change of body movement.
• This enables the brain to send motor commands back to the muscles. that control coordinated movement, corrective reflexes, muscle tone, and posture.
• A spindle contains seven or eight intrafusal muscle fibers within it.
• Rest of the muscle's fibers (those generating force for movement) are extrafusal fibers.
• A gamma motor neuron innervates the ends of an intrafusal fiber and keeps it taut.
• The midportion of the intrafusal fiber contains sensory nerve fibers.
• Primary afferent fibers monitor fiber length and speed of length changes.
• Secondary afferent fibers monitor length only.

The Stretch Reflex

• Stretch (myotatic) reflex is when a muscle is stretched, it fights back and contracts.
• Helps maintain equilibrium and posture.
• Stabilizes joints by balancing tension in extensors and flexors, smoothing muscle actions.
• The stretch reflex is mediated primarily by the brain, but its spinal component can be more pronounced if the muscle is suddenly stretched by a tendon tap (knee jerk).
• Knee-jerk (patellar) reflex is a monosynaptic reflex that has one synapse between the afferent and efferent neurons.
• Reciprocal inhibition occurs as a reflex phenomenon that prevents muscles from working against each other by inhibiting the antagonist when the agonist is excited.

The Flexor (Withdrawal) Reflex

• Flexor reflex is the quick contraction of flexor muscles resulting in the withdrawal of a limb from an injurious stimulus.
• Triggers contraction of the flexors and relaxation of the extensors in that limb.
• The Polysynaptic reflex arc is when signals travel over many synapses on their way to the muscle.

The Crossed Extension Reflex

• The Crossed extension reflex is the contraction of extensor muscles in limb opposite of the one that is withdrawn.
• Maintains balance by extending other leg.
• Flexor reflex uses an ipsilateral reflex arc whereas a crossed extension reflex uses a contralateral reflex arc.
• Intersegmental reflex is one in which the input and output occur at different levels (segments) of the spinal cord.

The Tendon Reflex

• Tendon organs: Proprioceptors in a tendon near its junction with a muscle.
• Consists of a Golgi tendon organ: 1 mm long, nerve fibers entwined in collagen fibers of the tendon.
• Tendon reflex: response to excessive tension on the tendon.
• Inhibits muscle from contracting strongly.
• Moderates muscle contraction before it tears a tendon or pulls it loose from the muscle or bone.