1. Anatomy and Physiology I
Chapter 13
The Spinal Cord, Spinal Nerves, and Somatic Reflexes

2. Spinal Cord Functions Conduction Locomotion Reflexes
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

3. Spinal Cord Cylinder of nervous tissue Arises from brainstem L1
31 pairs of spinal nerves
Divided into cervical, thoracic, lumbar, and sacral regions

4. 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

5. 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

6. Gray Matter Central core- butterfly, H shaped 2 dorsal horns
2 ventral horns
Gray commissure- connects R and L side
Central canal

7. 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

8. Spinal Tracts Ascending tracts Descending 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

9. Spinal Nerves Spinal cord communicates with body Nerve
Composed of numerous nerve fibers (axons) bound together by connective tissue
PNS nerve fibers enclosed by Schwann cells

10. 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

11. Peripheral Nerve Fibers
Sensory fibers- afferent fibers
Signals from sensory receptors to CNS
Motor fibers- efferent fibers
Signals from CNS to muscles and glands

12. 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

13. Spinal Nerves 31 pairs 8 cervical 12 thoracic 5 lumbar 5 sacral
1 coccygeal

14. Nerve Plexus Nerves merge to form web-like nerve plexus
Cervical plexus
Brachial plexus
Lumbar plexus
Sacral plexus and Coccygeal plexus

15. 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

16. 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

17. 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

18. 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

19. 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

20. The Tendon Reflex
Reflexive contraction of muscle when tendon is tapped
Depends on Reciprocal inhibition
Prevents muscles from working against each other- inhibits antagonist

21. 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

22. 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