Overview of Spinal Cord Anatomy and Functions

Spinal cord

Information highway between brain and torso/limbs; extends through vertebral column to L1.

Functions of spinal cord

Conduction, locomotion, reflexes.

Conduction

Move information up/down cord, connects different levels of trunk to each other and to brain.

Locomotion

Repetitive movements coordinated in spinal cord; brain initiates motor neurons, determines speed, distance, and direction.

Reflexes

Involuntary stereotyped responses to stimuli; involve brain, spinal cord, and peripheral nerves.

Regions of the Spinal Cord

Spinal cord has 4 regions: cervical, thoracic, lumbar, and sacral regions; does not extend past L1.

Cervical enlargement

Nerves of upper limbs.

Lumbar enlargement

Nerves of pelvic region and lower limbs.

Medullary cone

Cord tapers to form point; lumbar and medullary cone give rise to nerves of the cauda equina (travel through L2-S5).

Meninges

3 fibrous membranes; dura, arachnoid, and pia mater.

Dura mater

Forms loose dural sheath around spinal cord; tough, rubbery collagen.

Epidural space

Space between dura mater and bone; contains blood vessels, adipose, and loose connective tissue.

Arachnoid mater

Arachnoid membrane adheres to dura and has a layer of loose mesh of collagen and elastic fibers spanning space until pia mater.

Subarachnoid space

Gap filled with cerebrospinal fluid (CSF).

Pia mater

Deepest layer, delicate membrane closely following contours of spinal cord; continues beyond medullary cone as the terminal filum.

Denticulate ligaments

Extensions that anchor cord to dura to prevent side-to-side movements.

Spina Bifida

Condition where 1 in 1000 born with 1+ vertebrae fail to form complete arch, thus enclosing spinal cord; severity depends on number of vertebrae affected.

Gray matter

Dull in color due to little myelin; contains somas, dendrites, proximal parts of axons; involved in synaptic transmission and integration of information within CNS.

White matter

Abundance of myelinated axons; carries signals within CNS.

Gray commissure

Connects left-right halves of gray matter.

Dorsal root

Carries sensory fibers into dorsal horn; fibers either continue up spinal cord or may synapse with an interneuron.

Ventral root

Axons of motor fibers leaving spinal cord; somas in ventral horns.

Spinal tracts

Ascending and descending tracts; all fibers in a tract have similar origin, destination, and function.

Ascending tracts

Sensory signals travel via 3 neurons to sensory areas of brain.

Descending tracts

Typically have 2 neurons; upper motor neuron has a soma in cerebral cortex or brain stem and axon that ends at a lower motor neuron.

Nerve

cordlike organ of few-1 mil axons bound together by connective tissue

PNS nerves

ensheathed in Schwann cells

Neurilemma

the outermost layer of PNS nerves

Perineurium

wraps fibers bundled into fascicles

Epineurium

wraps fascicles bundled together to form a nerve

Dorsal root ganglion

somas of afferent neurons that form prior to rootlets branching to spinal cord

Spinal nerves

31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal

Dorsal rami

innervates muscles/joints in region and skin of back

Ventral rami

innervates ventral and lateral muscles/skin of trunk and forms nerves of limbs

Meningeal branch

reenters vertebral canal to innervate meninges, vertebrae, and spinal ligaments

Nerve plexuses

ventral rami branch and merge to form 5 weblike nerve plexuses from neck to coccyx

Shingles

a painful condition caused by the chickenpox virus that remains in dorsal root ganglia for life

Radial nerve injury

vulnerable to injury due to location, such as being pinched against humerus

Sciatic nerve injury

vulnerable to injury due to location, such as herniated disc or pressure due to pregnancy

Dermatosome

specific skin region from which each spinal nerve (except C1) receives sensory information

Dermatosome map

a diagram of skin regions innervated by each spinal nerve

Properties of a reflex

require stimulation, quick with few-no interneurons, involuntary, and stereotyped

Somatic reflexes

involuntary contraction of muscle or glandular secretion occurring via a reflex arc

Muscle spindle

stretch receptors within skeletal muscle that serve as proprioceptors