1. Anatomy and Physiology
Marieb’s Human
Anatomy and Physiology
Ninth Edition
Marieb w Hoehn
Chapter 12
The Central Nervous System
Spinal Cord Reflexes Plexuses
94 min, 51 slides

2. Lecture Overview The spinal cord Spinal cord structure Spinal meninges
Cross-sectional anatomy of the spinal cord
Ascending and descending spinal tracts
Reflexes
Spinal Nerves
Nerve plexuses

3. Spinal Cord Structure
extends from the foramen magnum to 2nd lumbar vertebra
cervical and lumbar enlargements
cauda equina (horse’s tail) – thin nerve fibers that exit at different level than they arise (note that spinal cord does not extend into this area of the lumbar spine). Begins around L2 and extends to S5. Good area for lumbar puncture and collection of CSF.
Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007
Spinal cord extends entire length of vertebral column early in fetal development, to L3 at birth, and to L1 in an adult. Thus, it occupies only the upper two thirds of the vertebral column in an adult. The lower third of the vertebral column is occupied by the cauda equina.

4. Overview of the Spinal Cord
What type of vertebra is shown above?

5. Meninges of the Spinal Cord
Figure from: Marieb Human Anatomy & Physiology, Pearson 2013
Dura mater – thick, collagenous membrane (about thickness of rubber kitchen glove)
Arachnoid mater – simple squamous epithelium; loose meshwork of collagenous and elastic fibers spanning the gap (subarachnoid space)
Pia mater – delicate membrane, close follows contours of nervous tissue that it covers.
- dura mater – outer, tough (anchoring dural folds)
- arachnoid mater – web-like
- pia mater – inner, delicate
- Subdural space – like interstitial fluid
- Subarachnoid space – CSF

6. Organization of Spinal Cord
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

7. Organization of Spinal Gray Matter
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
Posterior gray horn = sensory
Lateral gray horn = visceral motor
Anterior gray horn = somatic motor
Anterior root
Posterior root
Gray matter = dendrites and unmyelinated axons

8. Spinal Cord and Nerve Roots
Ventral root - axons of motor neurons whose cell bodies are in spinal cord
Dorsal root - axons of sensory neurons in the dorsal root ganglion
Dorsal root ganglion - cell bodies of sensory neurons

9. Organization of Spinal White Matter
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
White matter = Myelinated axons

10. Functions of the Spinal Cord
a. is a conduit for nerve impulses to and from the brain (nerve tracts) b. is a center for spinal reflexes

11. Tracts of the Spinal Cord
Ascending tracts conduct sensory impulses to the brain
Descending tracts conduct motor impulses from the brain to motor neurons reaching muscles and glands
All the axons in a tract share a common origin and destination
Tracts are usually named for their place of origin (1st) and termination (2nd)
Most axons cross over during their travel. What will this mean clinically?

12. Ascending Tracts
fasciculus cuneatus/gracilis - fine touch, pressure, body movement - cross (decussate) in medulla
spinothalamic - crude pain, temperature, pressure, and touch - cross in spinal cord
spinocerebellar - subconscious coordination of muscle movements (1st and 2nd order neurons) - ipsilateral 3 2 1
Decussation
(crossing over)

13. 1st, 2nd, and 3rd Order Sensory Neurons
1st order neuron – from receptor to the spinal cord (cell bodies are located in the dorsal root ganglion)
2nd order neuron – from spinal cord to thalamus
3rd order neuron – from thalamus to sensory cerebral cortex - terminate in the cerebral cortex 3 2 1
Decussation

14. Descending Tracts
corticospinal (direct, pyramidal) - voluntary movement of skeletal muscles - lateral cross in medulla - contralateral
reticulospinal (indirect, extrapyramidal) subconscious muscle tone, sweat glands some lateral cross, anterior do not cross
rubrospinal (indirect, extrapyramidal) subconscious regulation of upper limb tone/movement cross in brain (less important in humans)
Upper motor – begin in precentral gyrus of cortex
Decussation
Lower
Upper MN – Cerebral cortex to spinal cord
Lower MN – Spinal cord to effector

15. Somatic Reflex Arcs
Reflexes – automatic, subconscious, quick, stereotyped responses to stimuli either within or outside the body
They occur in both the somatic and autonomic divisions

16. Knee-jerk (Stretch) Reflex
helps maintain posture
Monosynaptic, Ipsilateral

17. Withdrawal Reflex
protective
Polysynaptic, Ipsilateral, Intersegmental

18. Crossed-Extensor Reflex
flexor muscles contract
flexor muscles on opposite side inhibited
extensor muscles on opposite side contract for balance
Polysynaptic, Contralaterial, Intersegmental

19. Spinal Nerves mixed nerves 31 pairs 8 cervical (C1 to C8)
12 thoracic (T1 to T12)
5 lumbar (L1 to L5)
5 sacral (S1 to S5)
1 coccygeal (Co)
THIRTY ONEderful flavors of spinal nerves!
Below cervical spine, each spinal nerve leaves inferior to the same numbered vertebra
Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

20. Spinal Nerves
These are ‘mixed’ nerves (sensory and motor nerve fibers)
Ventral (anterior) ramus leads to formation of plexuses
Spinal nerves are named according to the level of the spinal cord from which they exit.

21. Cervical Plexus
Nerve plexus – complex network formed by anterior (ventral) branches of spinal nerves; fibers of various spinal nerves are sorted and recombined
Contains both sensory and motor fibers
Cervical Plexus
C1-C4
lies deep in the neck
supplies muscles and skin of the neck
contributes to phrenic nerve (diaphragm)
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

22. Brachial Plexus C5-T1 lies deep within shoulders
supplies shoulder and upper limbs
musculocutaneous nerves
supply muscles of anterior arms and skin of forearms
ulnar nerves
supply muscles of forearms and hands
supply skin of hands
radial nerves
supply posterior muscles of arms and skin of forearms and hands
axillary nerves
supply muscles and skin of superior, lateral, and posterior arms

23. Lumbosacral Plexus T12 – S5 supplies pelvis and lower limbs
extend from lumbar region into pelvic cavity
obturator nerves
supply adductors of thighs
femoral nerves
supply muscles and skin of thighs and legs
sciatic nerves
supply muscles and skin of thighs, legs, and feet
May be separated into lumbar, sacral, pudendal, and coccygeal plexuses

24. Classification of Nerve Fibers
SAME Sensory = Afferent Motor = Efferent
SOMAtic
- Skin - BOnes - Muscles - Articulations
Table from: Saladin, Anatomy & Physiology, McGraw Hill, 2007