Anatomy and Physiology I

Chapter 13The Spinal Cord, Spinal Nerves,

and Somatic Reflexes

Spinal Cord Functions

• Conduction– Bundles of nerve fibers

• conduct info up and down the cord

– Sensory info reaches brain– Motor commands reach the effectors

• Locomotion– Walking– Central pattern generators- produce sequence of walking

• Reflexes– Involuntary, stereotyped responses to stimuli

Spinal Cord

• Cylinder of nervous tissue• Arises from brainstem• L1• 31 pairs of spinal nerves• Divided into cervical,

thoracic, lumbar, and sacral regions

The Meninges• 3 fibrous connective tissue

membranes– Dura mater

• Forms loose sleeve- dura sheath• Epidural space- between vertebrae and sheath

– Arachnoid mater• Loose mesh of fibers• Subarachnoid space- between arachnoid membrane and pia mater

– Filled with CSF

– Pia mater• Delicate membrane• Follows contours of spinal cord

Spinal Cord

• Consists of 2 kinds of nervous tissue– Gray matter• Dull color• Little myelin

– White matter• Bright, pearly white• Abundance of myelin• Surrounds gray matter• Composed of tracts

– Carry signals from one part of the CNS to another

Gray Matter

• Central core- butterfly, H shaped

• 2 dorsal horns• 2 ventral horns• Gray commissure-

connects R and L side– Central canal

Gray Matter

• Branches into ventral and dorsal roots

• Dorsal root- carries sensory nerve fibers to the spinal cord– Enter posterior horn

• Ventral root- motor nerve fibers exit and go to skeletal muscles

Spinal Tracts

• Ascending tracts – carry sensory information up the

spinal cord to the brain

• Descending tracts– conduct motor impulses down

the brain stem and the spinal cord• Muscles or organs

• Tracts cross- decussation

Spinal Nerves

• Spinal cord communicates with body– Spinal nerves

• Nerve– Composed of numerous nerve fibers (axons)

bound together by connective tissue• PNS nerve fibers enclosed by Schwann cells

Spinal Nerves

• Endoneurium- thin layer of connective tissue – surrounds nerve fiber

• Fascicles- bundles of nerve fibers

• Perineurium- sheath– surrounds fascicle

• Epineurium- surrounds several bundles of fascicles– Whole nerve– Protects nerve

Peripheral Nerve Fibers

• Sensory fibers- afferent fibers– Signals from sensory receptors to CNS

• Motor fibers- efferent fibers– Signals from CNS to muscles and glands

Spinal Nerves

• Sensory nerves- afferent fibers• Motor nerves- efferent fibers• Mixed nerve- both afferent and efferent fibers– Conducts signals in 2 directions

• Ganglion (knot)- cluster of neurosomas outside the CNS

Spinal Nerves

• 31 pairs• 8 cervical• 12 thoracic• 5 lumbar• 5 sacral• 1 coccygeal

Nerve Plexus• Nerves merge to form web-like nerve plexus• Cervical plexus

• Brachial plexus

• Lumbar plexus

• Sacral plexus and Coccygeal plexus

Dermatomes

• Specific area of the skin the sends sensory information to the spinal cord

• Spinal nerves overlap– Need to severe 3 successive

spinal nerves to lose sensation in one dermatome area

• Spinal nerve damage assessed with pin pricks

Reflexes

• Quick, involuntary, stereotyped reactions of glands or muscles to stimulation

• 1. Require stimulation- not spontaneous reactions

• 2. Quick- minimum synaptic delay

• 3. Involuntary- difficult to suppress– Occur w/out intent and awareness

• 4. Stereotyped- same way every time

Somatic Reflex

• Involuntary contraction of skeletal muscle• Employs a reflex arc- signal travels along a pathway

– 1. Somatic receptors• Skin, muscle, tendon

– 2. Afferent nerve fibers• Carry info into dorsal horn of spinal cord

– 3.Integrating center• interneurons

– 4. Efferent nerves• Carry info from ventral horn out to the skeletal muscle

– 5. Skeletal muscles• Effectors that carry out response

Muscle Spindle

• Stretch receptors imbedded in skeletal muscle– Proprioceptors- sense organs specialized to monitor

body position and movement of body parts• Function: inform brain of muscle length and

movement– Brain sends motor commands back to muscles

The Stretch Reflex

• When muscle is stretched– “fights back”– Contracts– Feels stiffer

• This helps maintain equilibrium and posture• Extension creates stretch reflex in flexors• Flexion creates stretch reflex in extensors• Stretch reflexes stabilize joints and coordinate

vigorous and precise movements

The Tendon Reflex• Reflexive contraction of muscle when tendon is tapped

• Depends on Reciprocal inhibition– Prevents muscles from working against each other- inhibits

antagonist

The Withdrawal (Flexor) Reflex

• Quick contraction of flexor muscles – resulting in the withdrawal of a limb from an injurious

stimulus• Contraction of flexors and

relaxation of extensors

The Crossed Extension Reflex

• Contraction of extensor muscles in the limp opposite from the one withdrawn

• Maintains balance, prevents falling

• Shift in center of gravity