A&P 1: Spinal Cord & Nerves

Spinal cord

Roughly cylindrical nervous system structure extending through vertebral column with cervical, thoracic, lumbar, sacral regions and cervical and lumbosacral enlargements (for limb nerves)

Continuous with medulla oblongata extending to inferior border of L1 vertebra (tapers to form conus medullaris)

3/4” diameter, 16-18” long

Posterior median sulcus and anterior median sulcus depressions extend full length

Spinal nerve

Mixed nerve anchored to spinal cord by posterior and anterior root each composed of multiple rootlets

Posterior root

Houses unipolar sensory neurons that extend to sensory receptors

Relays info to spinal cord

Posterior root ganglion

Structure outside of spinal cord containing cell bodies of sensory receptors

Anterior root

Houses multipolar motor neurons extending from spinal cord (internal cell body) to effectors

Relay signals from spinal cord to muscles or glands

Cauda equina

Spinal nerve roots within vertebral canal extending inferiorly from conus medullaris

Vertebral column

Bony framework of 26 stacked vertebrae which form vertebral canal

Physically protect spinal cord and cauda equina

Intervertebral foramen

Lateral opening between adjacent stacked vertebrae where spinal nerves exit exit spinal column

Exit horizontally for thoracic and cervical spine, inferiorly then horizontal for lower regions

Cervical vertebra exit at superior vertebrae (previous number), others at inferior (same number)

Spinal cord meninges

CT membranes that encapsulate spinal cord within vertebral canal

Pia mater

Delicate, innermost meninx composed of elastic and collage

Directly adheres to external surface of spinal cord

Forms denticulate ligaments: numerous, paired triangular extensions

Forms filumn terminale: thin strand of pia mater anchoring conus medullaris to coccyx extending within cauda equina

Arachnoid mater

Delicate web of collagen and elastic fibers (arachnoid trabeculae)

Superior to subarachnoid space which contains CSF

Dura mater

Single dense irregular CT layer with extensions ensheathing spinal nerve roots and merging with epineurium

Epidural space houses adipose and areolar CT and blood vessels external to dura mater

Subdural space separates from arachnoid space

Lumbar puncture

Prodecure to obtain CSF where needle is inserted through skin, subcutaneous layer, back muscle, ligamentum flavum (connect vertebrae laminae), epidural space, dura mater to subdural space

Must be performed inferior to L1 to avoid spinal cord injury, typically L3-L5

Posterior horn

Left/right posterior gray matter mass formed by dendrites and cell bodies of interneurons (located completely in CNS)

Sensory neurons extend through posterior root and synapse with interneurons within this region

Contains both somatic (posterior) and visceral (lateral/posterior)

Anterior horn

Left/right anterior gray matter mass formed by dendrites and cell bodies of somatic motor nuclei with axons extending to innervate somatic effector

Lateral horn

Left/right lateral gray matter mass located in T1-T2 containing dendrites and cell bodies of autonomic motor neurons (autonomic motor nuclei)

Innervate visceral effectors (cardiac/smooth muscle, glands, other involuntary effectors)

Poliomyelitis

Infection caused by poliovirus (FTO transmission)

Often mild but can spread to nervous system and attack anterior horn causing paralysis

Gray commissure

Bar of gray matter connecting left and right sides of horns

Houses unmyelinated axons

Central canal

Small, internal channel extending through gray commissure containing CSF from fourth ventricle

Funiculus

Region of white matter in spinal cord; columns

May be posterior, lateral or anterior

White commissure

Connects anterior funiculi

Fasiculi

Smaller bundles of axons within funiculi with common origin/destination, similar function; tracts

Conduct signals from brain to spinal cord

Symmetric with sensory and motor tracts on both sides of spinal cord

Sensory/afferant: all

Motor/efferant: lateral, anterior

Naming: origin + destination

Conduction pathway

Series of neurons and associated structures that relay nerve signals between brain and specific body part

Can be sensory (ascending) or motor (descending)

Characterized by

• paired tracts

• 2+ neurons

• common neuron cell body location (PRG, gray horn, brain nuclei)

• common axon location

• decussation (crossing > contralateral relationship, >90%)

• limited ipsilateral pathways (same side, ~10%)

General sense receptors

Receptors lacking specialization that transmit sensory info to CNS

May be somatic (somatosensory, tactile or proprioceptor) or visceral (stretch)

Somatic pathways contain primary neuron (receptor to CNS), secondary neuron (interneuron to tertiary or cerebellum), tertiary (interneuron from secondary to cerebrum)

Posterior funiculus-medial lemniscal pathway

Three-neuron chain communicating tactile or proprioception input

Identifies object by touch or position of body in space

1) Primary neuron ascends posterior funiculus (fasiculus cuneatus or fasciculus gracilis), synapses with medulla gray matter (nucleus cuneatus or nucleus gracilis)

2) Secondary neuron extends from medulla, projects within medial lemnicus to thalamus

3) Tertiary neuron extends from thalamus to cerebrum in postcentral gyrus

Anterolateral pathway

Spinothalamic pathway; three-neuron chain communicating skin and mucous membrane input (crude touch, pressure, pain, temperature; itch, tickle, etc…)

1) Primary neuron synapses with secondary neuron in posterior horn

2) Secondary neuron extends from spinal cord to thalamus via anterior funiculus

3) Tertiary neuron extends from thalamus to cerebrum in postcentral gyrus)

Spinocerebellar pathway

Two-neuron chain to communicate subconscious postural input from proprioceptor to cerebellum

1) Primary neuron extends from proprioceptor to spinal cord via posterior root

2) Secondary neuron extends from spinal cord to cerebellum within spinocerebellar tract

Motor pathway

Descending pathway for brain to control effector

Upper motor neuron housed in cerebral cortex or nuclei of cerebral/brainstem nuclei; may synapse with LMN (direct) or interneuron (indirect)

Lower motor neuron housed in anterior horn of spinal cord innervates skeletal muscle

Direct pathway

Pyramidal pathway; two-neuron chain communicating from primary motor cortex to lower motor neuron

1) Upper motor neuron extends from primary motor cortex through internal capsule and cerebral peduncles and corticospinal tract

2) Lower motor neuron extends from anterior horn to target muscle

Housed in lateral (skilled limb movement, 85%) or anterior (axial, 15%) corticospinal tract

Indirect pathway

Motor pathway with more than one upper neuron in brainstem nuclei taking a complex, circuitous route rough brain to spinal cord

Modifies or helps control pattern of somatic motor activity (excitingation/inhibition of lower neurons).

Tracts grouped based on primary function

Lateral pathway

Regulates and controls precise, discrete movements and tone in flexor muscles of limbs

Consists of rubrospinal tracts originating in red nucleus of midbrain

Medial pathway

Regulates reflexive muscle tone and gross movements of head, neck, proximal parts of limbs, trunk

Three groups of tracts

Reticulospinal tracts

Medial pathway with tracts originating from reticular formation in midbrain

Help control reflexive movements related to posture and balance

Tectospinal tracts

Medial pathway extending from superior and inferior colliculi in tectum of midbrain

Regulates reflexive positional changes of upper limbs, eyes, head, neck in response to visual/auditory stimuli

Vestibulospinal tracts

Medial pathway originating within vestibular nuclei of brainstem to regulate reflexive muscle activity to maintain balance while sitting, standing and walking

Spinal nerves

31 pairs of nerves (C1-C8, T1-T12, L1-L5, S1-S5, Co1)

Sensory receptor through posterior root and motor neuron extending from anterior horn

Each pair extends through intervertebral foramen to exit vertebral column (except C1 and Co1) and split into rami

Posterior ramus

Spinal nerve branch innervating deep and skin muscles of back

Smaller of two main branches

Anterior ramus

Larger main branch which splits in multiple other branches

Innervates skeletal muscles anterior & lateral trunk muscles and skin, upper & lower limbs

Many branches go on to form nerve plexuses

Rami communicantes

Rami associated with spinal nerves and ANS

Extend between spinal nerve and sympathetic trunk ganglion

Dermatome

Specific skin segment innervated by single spinal nerve

Only C2 does not innervate skin

Clinically important due to anesthesia and referred visceral pain

Shingles

Herpes zoster viral infection migration and latency in posterior root ganglia that becomes reactivated and proliferates to dermatone causing rash, blisters and intense pain

Nerve plexus

Network of interweaving anterior rami of spinal nerves associated

Formed on both sides then split multiple named nerves innervating various body structures

E.g. cervical, brachial, lumbar, sacral

Terminal branches contain different spinal nerves so damage to single nerve does not loss total loss of innervation

Intercostal nerves

Spinal nerves T1-T11 located within intercostal space (T12 is subcostal)

Do not form plexuses except T1 (brachial plexus)

T2: intercostal muscles and sensory info from skin

T3-6: intercostal muscle and lateral chest wall sensations

T7-12: inferior intercostal spaces and abdominal muscles and skin

Cervical plexuses

Network of nerves that innervates anterior neck muscles, skin of all neck, part of head, shoulders

Formed by anterior rami of C1-C4; C5 contributes some axons

Contains phrenic nerve

Phrenic nerve

Formed by C4 along with parts of C3, C5

Innervates thoracic diaphragm

Brachial plexus

Network of nerves supplying upper limb and pectoral girdle

Formed by anterior rami of C5-T1

Extend laterally: neck > superior to first rib > axilla

Anterior rami emerge and unite forming superior (C5-6), middle (C7) and inferior trunks (C8-T1)

Trunks divide deep to clavicle into anterior and posterior division

Converge at axilla forming posterior (C5-T1), medial (C8-T1) and lateral cords (C5-C7

Axillary nerve

Nerve innervating deltoid and teres minor

Receives sensory input from superolateral portion of arm

Formed from posterior cord of brachial plexus

Median nerve

Nerve innervating most of anterior forearm, thenar and lateral lumbricals

Receives sensory input from palmar side of lateral 3 1/2 fingers

Formed by medial and lateral cords of brachial plexus along midline of forearm deep to carpal tunnel

Musculocutaneous nerve

Nerve innervating anterior arm to flex humerus, forearm or both

Formed from lateral cord of brachial plexus along midline of arm

Radial nerve

Nerve innervating posterior arm muscles (forearm extensors) and posterior forearm muscles (wrist/digit extensors, forearm supinator)

Receives sensory input from posterior arm and forearm surface and dorsolateral side of hand

Formed from posterior cord of brachial plexus from posterior side of arm through radial side of forearm

Ulnar nerve

Innervates anterior forearm muscles (medial region) and intrinsic hand muscles

Receives sensory input form dorsal and palmar skin of pinky and medial half of ring finger

Formed from medial cord of brachial plexus descending along medial side of arm then ulnar side of forearm

Brachial plexus injury

Common ailment for teenagers and young adults

Damage occurs in fractures to humeral neck (axillary), humeral shaft (radial), improper use of crutches (posterior), carpal tunnel syndrome or wrist laceration (median), elbow fracture or dislocation (ulnar)

Also includes superior (excessive separation of neck/shoulder) or inferior (outstretched fall, pull arm) trunk injury

Lumbar plexuses

Network of nerves innervating inferior abdominal wall, anterior & medial thigh, skin of medial leg

Formed from anterior rami of L1-L4

Subdivided into anterior and posterior divisions

Femoral nerve

Innervates anterior thigh and receives sensory input form skin of anterior and anteromedial thigh, medial leg

Main nerve of lumbar plexus posterior division

Obtruator nerve

Innervates medial thigh (adduction), conducts sensory input from superomedial skin of thigh

Smaller branches innervate abdominal wall, portions of external geniltalia, inferior portions of abdominal muscles

Main nerve of lumbar plexus anterior division

Sacral plexuses

Network of nerves innervating gluteal region, pelvis, perineum, posterior thigh, most of leg and foot

Organized into anterior and posterior division

Formed by anterior rami of L4-S4

Sciatic nerve

Projects from pelvis through posterior thigh with tibial and common fibular division wrapped in common sheath above popliteal fossa

Composed of both anterior and posterior sacral plexus divisions

Largest and longest nerve

Tibial nerve

Innervates hamstrings except (biceps femoris short head), plantar and toe flexors, and receives sensory input from sole of foot

Formed from anterior division of sciatic nerve

Splits into lateral and medial plantar nerves in foot

Common fibular nerve

Innervates short head of biceps femoris then branches

Deep branch innervates anterior leg and dorsum to dorsiflex foot, extend toes; receives sensory input from first and second toe webs

Superior branch innervates lateral compartment muscles for eversion, weak plantar flexion; receives sensory input from dorsal surface and anteroinferior part of leg

Formed from posterior division of sciatic nerve

Sacral plexus injury

Damage can occur due to gluteal injection error or herniated disc (sciatica)

Fibular neck fx or compression from leg cast can cause common fibular nerve injury preventing dorsiflexion (foot drop) and eversion

Reflexes

Rapid, preprogrammed, involuntary responses of muscles or glands to stimulus; multiple classifications

Processing site: spinal or cranial

Type of effector: somatic (skeletal muscle) or visceral (autonomic)

Synapse count: monosynapic (1) or polysynaptic (2+)

Body side: ipsilateral (same) or contralateral (opposite)

Development: innate or acquired

Reflex arc

1) Stimulus activates sensory receptor

2) Sensory neuron transmits to CNS

3) Interneurons process info in integration center (complex reflex)

4) Motor neuron transmits to effector

5) Effector responds to nerve signal to counteract/remove stimulus

Muscle spindle

Propriocentor detecting stretch within muscle; stretch receptor

Composed of intrafusal muscle fibers surrounded by CT capsule contractile distally only (no myofilaments in center)

Innervated by sensory and gamma motor neurons (control sensitivity)

Surrounded by extrafusal muscle fibers innervated by alpha motor neurons (large diameter)

Stretch reflex

Reflex initiated by muscle spindle proprioceptor

Muscle reflexively contracts in response to stretch

1) Muscle spindle detects stretch

2) Nerve signal to CNS

3) AMN neuron stimulation

4) Extrafusal muscle fiber contract

5) Interneurons synapse for reciprocal inhibition of antagonist AMN

Reciprocal inhibition

Polysynaptic process where stretch receptor stimulates antagonist effectors

During synapse in spinal cord some axons synapse with interneurons which synapse with alpha motor neurons of antagonist muscle to inhibit contraction so that agonist contraction will not be opposed

Tendon reflex

Reflex initiated by Golgi tendon organ proprioceptor (detect tendon stretch) to prevent excessive tension

1) Muscle contraction > tension detected by Golgi tendon organ

2) Sensory signal to CNS

3) Synapse with interneurons

4) Interneurons inhibit agonist AMN, stimulate antagonist AMN

5) Agonist relaxes, antagonist contracts (reciprocal activation)

Reciprocal activation

Tendon reflex process where antagonist AMN are stimulated.

Sensory neuron synapses with some interneurons for antagonist to stimulate contraction during agonist relaxation

Withdrawal reflex

Reflex initiating muscle contraction away from painful stimulus

1) Nociceptors detect pain

2) Sensory info to interneurons

3) Interneurons stimulate motor neurons for contraction and reciprocal inhibition, others cross for cross-extensor reflex

Cross-extensor reflex

Reflex often occurring in conjunction with withdrawal reflex where the antagonist of the other limb contract

Aids in balance of lower limbs when shifting weight as part of withdrawal

Hypoactive reflex

Diminished or absent reflex

May indicate damage to spinal cord segment or neuromuscular junction or muscle disease

Hyperactive reflex

Abnormally strong response

May be accompanied by clonus (rhythmic extension/flexion oscillation)

May indicate damage to brain or spinal cord

Neural canal

Neural tube structure which develops into central canal of spinal cord Growith is hypertrophic and not hyperplasic with rapid differentiation to white and gray matter in weeks 4-5

Sulcus limitans

Horizontal groove dividing neural tube into basal and alar regions

Forms by week 6 in lateral walls of central canal

Basal plates

Region anterior to sulcus limitans which develops into anterior and lateral horns and anterior gray commissure

Alar plates

Region posterior to sulcus limitans which develops into posterior horns and posterior gray commissure by week 9