(FE) 8 IV: Spinal Cord and Spinal Nerves

Spinal Cord Function

Conduction: Afferent and Efferent. Structural and functional link between brain and body. Sensory input from body to brain. Motor commands from brain to body. Neural integration. Minimal - most “thinking, processing, and decision-making” occurs at level of brain. Reflexes.

Reflexes

Responses that do not involve the brain. Fast reaction to a stimulus

Spinal Cord Gross Anatomy

Extends inferiorly from brain’s medulla through vertebral canal. Four parts: cervical, thoracic, lumbar, and sacral. Ends at L1 vertebra with conus medullaris. Cauda equina. Two widen regions with greater number of neurons: Cervical and Lumbar Enlargement.

Cauda Equina

“Horse tail”. Spinal nerve roots extend inferiorly

Cervical Enlargement

Neurons innervating upper limbs

Lumbar Enlargement

Neurons innervating lower limbs

Nerve

Cablelike bundle of axons. 31 pairs of spinal nerves. Names begin with first letter of spinal cord region to which it attaches, followed by a number (e.g., T7). 8 Cervical, 12 Thoracic, 5 Lumbar, 5 Sacral, and 1 Coccyx

Connective Tissue Wrappings for Nerves

Epineurium (around nerve), Perineurium (around fascicle), and Endoneurium (around axon)

Dorsal Root/Posterior Root

Afferent sensory (somatic and visceral). Cell body in dorsal/posterior root ganglion. Axon projects into dorsal/posterior gray matter of spinal cord.

Ventral Root/Anterior Root

Efferent motor (somatic and visceral). Cell body in ventral/anterior horn.

What Protects and Support the Spinal Cord?

Protects by bone, meninges, and cerebrospinal fluid. Vertebral column houses the spinal cord. Cord passes through the vertebral canal. Each epinal nerve exits through an intervertebral foramen.

Spinal Cord Meninges

Dura mater, Arachnoid mater, and Pia mater

Dura Mater

Tough, outermost layer. One layer of dense irregular connective tissue that stabilizes spinal cord. Epidural space and subdural space.

Epidural Space

Between dura and vertebra. Houses adipose, areolar connective tissue, blood vessels.

Subdural Space

Between dura and arachnoid

Arachnoid Mater

Web-like layer, external to pia. Arachnoid trabeculae and subarachnoid space.

Arachnoid Trabeculae

Fibrous extensions of the membrane

Subarachnoid Space

Area deep to arachnoid through which CSF flows

Pia Mater

Delicate layer adhering to spinal cord. Made of elastic and collagen fibers. Denticulate ligaments and filum terminale.

Denticulate Ligatments

Lateral extensions of pia; help suspend spinal cord

Filum Terminal

Pia anchoring inferior end of spinal cord to coccyx

Lumbar Puncture

Procedure for obtaining CSF for medical diagnosis. Needle passes through skin, back muscles, ligamentum flavum, epidural space, dura mater, arachnoid mater into subarchnoid space. Adult spinal cord ends at L1. Lumbar puncture below this, just above or below L4. Spinous process of L4 at highest points of iliac crests.

Gray Matter

Made of neuron’s cell bodies, dendrites, and unmyelinated axons; also glial cells.

Posterior Horns

House axons of sensory neurons and cell bodies of interneurons

Anterior Horns

House cell bodies of somatic motor neurons

Lateral Horns

House cell bodies of autonomic motor neurons. Only present in parts T1-L2.

Gray Commissure

Horizontal band of gray matter surrounding central canal. Contains unmyelinated axons connecting left and right gray matter. Nuclei.

Nuclei

Groups of cell bodies

Sensory Nuclei

In posterior horn contain interneurons. Somatic sensory nuclei receive signals from skin, muscle, joints. Visceral sensory nuclei receive signals from blood vessels, viscera.

Motor Nuclei

In anterior and lateral horns contain motor neurons. Somatic motor nuclei (anterior) innervate skeletal muscle. Autonomic motor nuclei (lateral) innervate smooth muscle, heart, glands.

White Matter

Myelinated axons to and from the brain.

Regions of white matter

Posterior Funiculus, Lateral Funiculus, Anterior Funiculus

Posterior Funiculus

Sits between posterior gray horns and posterior median sulcus. Contains sensory tracts (axon bundles called fasciculi)

Lateral Funiculus

Sits on lateral sides of spinal cord. Contains sensory (ascending) and motor (descending) tracts.

Anterior Funiculus

Sits between anterior gray horns and anterior median fissure. Left and right anterior funiculi are interconnected by white commissure. Contains sensory (ascending) and motor (descending) tracts.

Sensory Pathways Summary

Ascend toward the brain. Posterior funiculus- medial lemniscal pathway. Anterolateral pathway. Spinocerebellar pathway.

Motor Pathways

Descend from brain. Upper motor neuron and lower motor neuron. Direct (pyramidal) pathway. Lateral Corticospinal Tracts and Anterior Corticospinal Tracts. Indirect Pathway: Lateral pathway and Medial pathway.

Common Pathway Characteristic.

Pathways are paired: there is a left and right tract. Cell locations: axons are in spinal cord tracts; cell bodies are in ganglia, spinal cord gray horns, and brain gray matter. Each pathway is made of a chain of two or more neurons. Most pathways decussate; axons cross midline so brain processes information for contralateral side. Uncrossed pathways work on the ipsilateral side of the body.

Sensory Pathway in Depth

Sensory input transmitted through spinal cord originate from general sense receptors, of which there are two categories: Somatic Sensory (somatosensory) Receptors and Visceral Sensory Receptors

Somatic Sensory Receptors

(Somatosensory). 2 types. Tactile receptors: detect characteristics of an object. Proprioceptors: detect stretch in joints, muscles, tendons

Visceral Sensory Receptors

Detect changes (e.g., stretch) in an organ.

Posterior Funiculus- Medial Lemniscal Pathway

Signals about proprioception, touch, pressure, and vibration with a three neuron chain. Primary neuron relays from skin to brainstem. Secondary neuron relays signal from medulla to thalamus. Tertiary neuron relays signal to primary somatosensory cortex (postcentral gyrus).

Anterolateral Pathways

Signals related to crude touch, pressure, pain, and temperature with a three-neuron chain. Primary neuron relays signal from skin to spinal cord. Secondary neuron relays signal from spinal cord to thalamus: Axon contact tertiary neuron in thalamus. Tertiary neuron relays signal from thalamus to cerebral cortex.

Spinocerebellar Pathway

Signals about proprioception with a two-neuron chain. Primary neuron relays signal from skin to spinal cord. Secondary neuron relays signal from spinal cord to cerebellum.

Motor Pathways in Depth

Control effectors such as skeletal muscles. Start in brain and includes at least two neurons: Upper motor neuron and Lower motor neuron.

Upper Motor Neuron

In motor cortex, cerebral nucleus or brainstem nucleus; contacts lower motor neuron

Lower Motor Neuron

In cranial nerve nucleus or spinal cord anterior horn; excites muscle.

Direct Pathway

(Pyramidal). Pathways between brain and skeletal muscles. Includes Lateral corticospinal tracts and Anterior corticospinal tracts.

Lateral Corticospinal Tracts

Upper motor neuron axons decussate within medulla’s pyramids. Lower motor neurons innervate limb muscles for skilled movements.

Anterior Corticospinal Tracts

Upper motor neuron axons form white tracts in anterior funiculi. Lower motor neurons innervate axial skeletal muscle.

Indirect Pathways

Upper motor neurons originate in brainstem nuclei and take complicated route to spinal cord. Includes Lateral pathway and Medial pathway.

Lateral Pathway

Regulates precise movement and tone in flexor limb muscles. Consists of rubrospinal tracts originating in midbrain.

Medial Pathway

Regulates muscle tone and movements of head, neck, proximal limb, trunk. Consists of reticulospinal, tectospinal, vestibulospinal tracts.

Reticulospinal Tracts

From reticular formation. Help control reflexes related to posture and balance.

Tectospinal Tracts

From superior and inferior colliculi. Regulate reflexive orienting responses to visual and auditory stimuli.

Vestibulospinal Tracts

From vestibular nuclei of brainstem. Help maintain balance during sitting, standing, walking.

Treating Spinal Cord Injuries

May leave individuals paralyzed and unable to perceive sensations. A) Prompt use of steroids after injury: May preserve muscle function. B) Early antibiotics: Have reduced number of deaths due to pulmonary and urinary infections. C) Neural stem cells: May be used in future to regenerate CNS axons.

General Distribution of Spinal Nerve

After intervertebral foramen, spinal nerve spilts. Posterior ramus = small branch. Anterior ramus = large branch. Rami communicantes = small branches of autonomic fibers.

Posterior Ramus

Small branch. Innervates muscles and skin of back.

Anterior Ramus

Large branch. Spilts into multiple other branches. At different levels, this ramus innervates anterior and lateral trunk, upper limb, lower limb. Participate in plexuses.

Rami Communicantes

Small branches of autonomic fibers. Extend between spinal nerve and sympathetic trunk ganglion. Ganglia interconnected in sympathetic trunk parallel to vertebral column.

Dermatomes

Segment of skin supplied by single spinal nerve. Some overlap in innervated regions. Can help localize damage to one or more spinal nerves. Involved in referred visceral pain.

Shingles

Reactivation of chickenpox infection. Virus remaining latent in posterior root ganglia. Reactivated, travels through sensory axons to dermatone. Rash and blisters along the dermatone. Burning and tingling pain. Antiviral medication to reduce severity. Vaccine to prevent or reduce disease severity.

Nerve Plexuses

Network of interweaving anterior rami of spinal nerves. Four main plexuses occur bilaterally: cervical, brachial, lumbar, and sacral plexuses. (Most thoracic spinal nerves and nerves S5-Co1 do not form plexuses.) Individual rami branch repeatedly. Damage to one nerve or spinal segment does not deprive a muscle or skin region of all innervation.

Intercostal Nerves

Located between the ribs.

Cervical Plexuses

Anterior rami of C1-C4. Branches innervate: anterior neck muscles, skin of neck, portions of head and shoulders. From rami of C3-C5 it gives rise to phrenic nerve innervating diaphragm.

Brachial Plexuses

From anterior rami of C5-T1. Trunks divide into anterior and posterior divisions. Axons innervate anterior and posterior parts of upper limbs.

Brachial Plexus Injuries

Minor injuries treated with rest. Severe injuries may require nerve grafts or transfers.

Axillary Nerve Injury

Can be compressed in axilla or damaged if neck of humerus broken. Difficulty abducting the arm and anesthesia along superolateral skin

Radial Nerve Injury

By humeral shaft fractures or injuries to lateral elbow. Causes paralysis of extensor muscles of forearm, wrist, fingers. Causes anesthesia along posterior arm, forearm, part of hand.

Posterior Cord Injury

(Axillary and Radial Nerves). May be injured by improper use of crutches (crutch palsy).

Median Nerve Injury

May be compressed in carpal tunnel syndrome. Causes paralysis of the ulnar muscles, lateral lumbricals; anesthesis in part of hand

Ulnar Nerve Injury

May be injured by fractures or dislocations of elbow. Causes paralysis of most intrinsic hand muscles; sensory loss on medial hand

Superior Trunk Injury

(C5-C6). Injury by excessive separation of neck and shoulder. Any brachial plexus branch that has these nerves affected.

Inferior Trunk Injury

(C8-T1). Can be injured if arm excessively abducted. Any brachial plexus branch from these nerves affected to some degree.

Lumbar Plexuses

From anterior rami of L1-L4. Includes femoral nerve, obturator nerve, and smaller nerves of lumbar plexus.

Femoral Nerve

Main nerve in posterior division of plexus. Innervates anterior thigh muscles and satorius. Sensory input from anterior and inferomedial thigh and medial leg.

Obturator Nerve

Main nerve in anterior division of plexus. Innervates medial thigh muscles. Sensory input from superomedial skin of skin. Smaller nerves of lumbar plexus. Innervate abdominal wall, portions of external genitalia.

Sacral Plexuses

Anterior rami of L4-S4. Anterior division nerves tend to innervate flexor muscles. Posterior division nerves tend to innervate extensor muscles. Sciatic nerve, tibial nerve, common fibular nerve, deep fibular nerve, superficial fibular nerve.

Sciatic Nerve

Largest and longest nerve in body. Formed from portions of anterior and posterior sacral plexus. Composed of tibial division and common fibular division. The two divisions spilt into two separate nerves just above popliteal fossa.

Tibial Nerve

(from anterior division of sciatic). Innervates hamstring and hamstring part of adductor magnus muscle. Receives sensory signals from skin on sole of foot.

Common Fibular Nerve

(from posterior division of sciatic). Innervates short head of biceps femoris muscle. Spilts into deep fibular nerve and superficial fibular nerve.

Deep Fibular Nerve

Innervates anterior leg muscles and muscles on dorsum of foot. Sensory input from skin between dorsum of first and second toes.

Superficial Fibular Nerve

Innervates lateral compartment muscles of leg. Sensory input from dorsal foot and anteroinferior leg.

Sacral Plexus Injuries

Superior or inferior gluteal nerves. Can be injured by poorly placed gluteal injection. Sciatica.

Sciatica

Injury to sciatic nerve. Characterized by extreme pain down posterior thigh and leg. May be caused by herniated intervertebral disc.

Common Fibular Nerve

Prone to injury due to fracture of neck or compression from cast. May cause paralysis of anterior and lateral leg muscles. Person unable to dorsiflex and evert the foot.

Reflexes

Rapid, preprogrammed, involuntary responses of muscles or glands to a stimulus A stimulus is required to initiate a reflex. Response is rapid; involves a chain of only a few neurons. The response is preprogrammed; always the same. The response is involuntary; no intent or awareness of the reflex before it happens. A reflex is a survival mechanism.

Reflex Arc

Neural pathway responsible for generating the response.

Pathway of Reflex Arc

1. Somatic receptors. In skin, muscles, or tendons.

2. Afferent nerve fibers. Carry information from receptors to posterior horn of spinal cord or the brainstem

3. Integrating center. A point of synaptic contact between neurons in gray matter of spinal cord or brainstem. Determines whether efferent neurons issue signal to muscles

4. Efferent nerve fiber. Carry motor impulses to skeletal muscle.

5. Effectors. The somatic effectors carry out the response.

Spinal/Cranial Reflexes

Is the spinal cord or brain the reflex integration center?

Somatic/Visceral Reflexes

Is the effector a skeletal muscle or is it cardiac muscle, smooth muscle, a gland?

Monosynaptic/Polysynaptic Reflexes

Do sensory neurons synapse directly with motor neurons or are there interneurons in the reflex arc?

Ipsilateral/Contralateral Reflexes

Are receptor and effector on the same side of the body or on opposite sides?

Innate/Acquired Reflexes

Are you born with the reflex or do you develop it after birth?

Monosynaptic Reflex

Direct communication between sensory and motor neuron (e.g., stretch reflex)

Polysynaptic Reflex

Interneuron facilitates sensory-motor communication (e.g., withdrawal reflex)

Spinal Reflexes

Four common spinal reflexes: stretch reflex, Golgi tendon reflex, withdrawal reflex, and crossed-extensor reflex. Stretch and golgi tendon reflexes rely on proprioceptors.

Muscle Spindle

A proprioceptor that detects stretch in a muscle. Spindle contains intrafusal muscle fibers innervated by gamma motor neurons and wrapped by sensory. Fibers not within spindle are extrafusal muscle fibers innervated by large alpha motor neurons. When stretched, spindle’s sensory axon fires impulses that are conducted to the spinal cord.

Withdrawal Reflex

Pulls a body part away from a painful stimulus. Stimulus excites nociceptor sensory neuron that transmits signal to spinal cord and excites interneurons. Interneurons excite motor neurons of flexors so flexor muscles (e.g., hamstrings) contract and limb is withdrawn. Simultaneously, other interneurons reciprocally inhibit motor neurons of extensors so that extensor muscles (e.g., quadriceps) relax and withdrawal happens quickly.