1. Spinal Cord, Spinal nerves & Reflexes
Exercise 21 & 22
Spinal Cord, Spinal nerves
& Reflexes

2. Structure of a Nerve
Figure 13.3b

3. Spinal Nerves
Thirty-one pairs of mixed nerves arise from the spinal cord and supply all parts of the body except the head
They are named according to their point of issue
8 cervical (C1-C8)
12 thoracic (T1-T12)
5 Lumbar (L1-L5)
5 Sacral (S1-S5)
1 Coccygeal (C0)

4. Spinal Nerves
Figure 13.6

5. Spinal Nerves: Roots
Each spinal nerve connects to the spinal cord via two medial roots
Each root forms a series of rootlets that attach to the spinal cord
Ventral roots arise from the anterior horn and contain motor (efferent) fibers
Dorsal roots arise from sensory neurons in the dorsal root ganglion and contain sensory (afferent) fibers

6. Spinal Nerves: Roots
Figure 13.7a

7. Spinal Nerve Innervation
Figure 13.7b

8. Cervical Plexus
Figure 13.8

9. Sacral Plexus
Figure 13.11

10. Lumbar Plexus
Figure 13.10

11. Brachial Plexus: Distribution of Nerves
Figure 13.9c

12. Brachial Plexus
Figure 13.9a

13. Reflexes A reflex is a rapid, predictable motor response to a stimulus
Reflexes may:
Be inborn (intrinsic) or learned (acquired)
Involve only peripheral nerves and the spinal cord
Involve higher brain centers as well

14. Reflex Arc There are five components of a reflex arc
Receptor – site of stimulus
Sensory neuron – transmits the afferent impulse to the CNS
Integration center – either monosynaptic or polysynaptic region within the CNS
Motor neuron – conducts efferent impulses from the integration center to an effector
Effector – muscle fiber or gland that responds to the efferent impulse

15. Reflex Arc Spinal cord (in cross-section) Stimulus Integration center2
Sensory neuron 3
Integration center 1
Receptor 4
Motor neuron
Skin 5
Effector
Interneuron
Figure 13.14

16. Crossed Extensor Reflex
Figure 13.19

17. Golgi Tendon Reflex
Figure 13.18

18. Stretch Reflex
Figure 13.17

19. Dermatomes
A dermatome is the area of skin innervated by the cutaneous branches of a single spinal nerve
All spinal nerves except C1 participate in dermatomes

20. Dermatomes
Figure 13.12