Spinal Cord and Spinal Nerves

Spinal cord functions (4)

conduction, neural integration, locomotion, reflexes

Conduction

sensory information ascends, motor commands descend

Neural integration

processing of information from diverse sources

Locomotion

central pattern generators coordinate simple repetitive movements

Reflexes

involuntary, stereotyped responses to stimuli

Spinal cord regions (4)

cervical, thoracic, lumbar, sacral

Spinal cord enlargements (2)

cervical, lumbosacral

Medullary cone

tapered end of spinal cord

Terminal filum

fibrous strand extending from medullary cone

Cauda equina

bundle of nerve roots resembling a horse tail

Meninges (3)

dura mater, arachnoid mater, pia mater

Dura mater

tough outer layer of dense collagen fibers

Epidural space

space containing fat and blood vessels around dura mater

Arachnoid mater

middle layer with subarachnoid space containing CSF

Subarachnoid space

shock absorber and diffusion medium for CSF

Pia mater

delicate inner layer with elastic and collagen fibers

Gray matter

contains little myelin; site of synapses and integration

Gray matter horns (3)

posterior (dorsal), anterior (ventral), lateral (thoracic and lumbar regions)

White matter

myelinated axons organized into tracts (fasciculi)

White matter columns (3)

posterior (dorsal), lateral, anterior (ventral)

Nerve

bundle of axons wrapped in connective tissue

Endoneurium

wraps one axon

Perineurium

wraps a fascicle of axons

Epineurium

wraps entire nerve

Nerve fiber types (6)

afferent, efferent, somatic, visceral, general, special

Afferent fibers

carry sensory signals from receptors to CNS

Efferent fibers

carry motor signals from CNS to effectors

Somatic fibers

innervate skin, skeletal muscles, bones, and joints

Visceral fibers

innervate blood vessels, glands, and viscera

General fibers

innervate widespread organs such as muscles, glands, skin, and vessels

Special fibers

innervate localized organs in the head (eyes, ears, taste, smell, facial muscles)

Spinal nerves (31 pairs)

C1–C8, T1–T12, L1–L5, S1–S5, Co1

Proximal branches

dorsal (posterior) root, ventral (anterior) root

Dorsal root ganglion

cluster of sensory neuron cell bodies outside CNS

Distal branches

posterior ramus, anterior ramus, meningeal branch

Posterior ramus

innervates muscles, joints, and skin of the back

Anterior ramus

innervates anterior and lateral skin and muscles; gives rise to limb nerves

Meningeal branch

reenters vertebral canal to innervate meninges, vertebrae, and ligaments

Ganglion

cluster of cell bodies outside CNS containing sensory neurons

Posterior root ganglia

example: varicella-zoster infection

Nerve plexuses (5)

cervical, brachial, lumbar, sacral, coccygeal

Cervical plexus (C1–C5)

innervates neck muscles, skin, diaphragm, upper chest; phrenic nerve

Brachial plexus (C5–T1)

innervates pectoral girdle, upper limbs, chest; ulnar nerve

Lumbar plexus (T12–L4)

innervates pelvic girdle and lower limbs

Sacral plexus (L4–S4)

innervates lower limbs and pelvis; sciatic nerve

Coccygeal plexus

innervates lower sacrum and coccyx

Cutaneous innervation

each spinal nerve receives sensory input from specific skin areas

Dermatome map

diagram showing skin regions innervated by spinal nerves

Dermatome clinical use

used to locate nerve damage based on sensory loss

Anesthesia

loss of sensation due to nerve block; injected into epidural space

Epidural anesthesia

regional pain reliever blocking pain sensation

Subarachnoid anesthesia

local anesthesia delivered into subarachnoid space

Reflex properties (5)

require stimulation, rapid, involuntary, stereotyped, maintain homeostasis

Reflex types

innate (pain withdrawal) or acquired (learned, e.g., braking a car)

Reflex arc

pathway followed by nerve impulse in a reflex

Reflex arc steps (5)

1. Arrival of stimulus and activation of receptor 2. Sensory neuron (afferent) activated 3. Integration in CNS (interneurons process and release neurotransmitters) 4. Motor neuron (efferent) activated 5. Peripheral effector (muscle or gland) responds

Spinal shock

paralysis and absence of reflexes after spinal cord transection

Neurogenic shock

dangerous drop in blood pressure from loss of sympathetic tone

Paraplegia

paralysis of both lower limbs (lesions from T1–L1)

Quadriplegia

paralysis of all four limbs (lesions above C5)

Hemiplegia

paralysis on one side of body due to stroke or brain lesion