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

the spinal cord’s four principal functions

conduction, neural integration, locomotion, and reflexes

meninges

the spinal cord and brain are enclosed in three of these fibrous membranes (dura mater, arachnoid mater, and pia mater)

dura mater

forms a loose-fitting sleeve called the dural sheath around the spinal cord; it is a tough membrane about as thick as a rubber kitchen glove, composed of multiple layers of dense irregular connective tissue

arachnoid mater

consists of the arachnoid membrane—five or six layers of squamous to cuboidal cells adhering to the inside of the dura—and a looser array of cells and collagenous and elastic fibers spanning the gap between the arachnoid membrane and the pia mater

pia mater

a delicate, transparent membrane composed of one or two layers of squamous to cuboidal cells and delicate collagenous and elastic fibers; it closely follows the contours of the spinal cord

subarachnoid space

filled with cerebrospinal fluid

gray matter

contains the cell bodies, dendrites, and proximal parts of the axons of neurons; it is the site of synaptic contact between neurons, and therefore the site of all neural integration in the spinal cord

white matter

composed of axon bundles that course up and down the spinal cord, providing avenues of communication between different levels of the CNS

ascending tracts

carry sensory information up the cord

descending tracts

conduct motor signals down the cord

decussation

when spinal tracts, as they pass up or down the brainstem and spinal cord, cross over from the left side of the body to the right, or vice versa

nerve

a cordlike organ composed of numerous nerve fibers (axons) bound together by connective tissue

ganglion

a cluster of nerve cell bodies outside the CNS

spinal nerves

there are 31 pairs: 8 cervical (C1-C8), 12 thoracic (T1-T12), 5 lumbar (L1-L5), 5 sacral (S1-S5), and 1 coccygeal (Co1)

reflexes

quick, involuntary, stereotyped reactions of glands or muscles to stimulation

stretch (myotatic) reflex

helps to maintain equilibrium and posture

flexor reflex

the quick contraction of flexor muscles resulting in the withdrawal of a limb from an injurious stimulus

crossed extension reflex

the contraction of extensor muscles in the limb opposite from the one that is withdrawn; it extends and stiffens that limb and enables you to keep your balance

tendon reflex

a response to excessive tension on the tendon; it moderates muscle contraction before it tears a tendon or pulls it loose from the muscle or bone