1. Ch 12 & 13 Spinal Cord and Spinal Nerves
ANATOMY & PHYSIOLOGY
Ch 12 & 13
Spinal Cord and Spinal Nerves

2. Spinal Cord
This part of the central nervous system (CNS) is enclosed in the vertebral cavity
It s protected by:
Vertebrae
Meninges –
dura, arachnoid and pia
covered in previous lecture
Cerebrospinal Fluid (CSF)
Epidural space – space between the vertebrae and the dural sheath (dura mater)

3. Spinal Cord
The spinal cord extends inferiorly from the medulla oblongata to the lumbar region
Along its length there are two notably thicker regions: The Cervical and Lumbosacral enlargements
These are sites where the nerves serving the limbs emerge
Conus medullaris – the end of the spinal cord in the lumbar region
Cauda equina – the tail-like collection of spinal nerve roots inferior to the conus medullaris
Spinal nerves (PNS) will be covered below.

4. Spinal Cord Gray Matter
Gray matter consists of cell bodies, unmyelinated processes, and neuroglia
Gray commissure – connects masses of gray matter on the left and right
The central canal runs through the gray commissure
Posterior (dorsal) horns – interneurons (sensory)
Anterior (ventral) horns – some interneurons but mostly somatic motor neurons
Lateral horns – contain autonomic nerve fibers

5. Gray Matter and Spinal Roots

6. Cross Section of Spinal Cord

7. Reflex Arc
The reflex arc is the simplest network of cells capable of receiving a stimulus and producing a response
A reflex is an automatic response to a stimulus that occurs without conscious thought.
Components of a reflex arc
Sensory receptor is stimulated and transmits an action potential to
Sensory neuron. (in the case of pain reception 2-in-1 cell) To-
Interneurons. To-
Motor neuron. To-
Effector organ which responds with a reflex

8. Spinal Nerves
31 pairs of spinal nerves supply all of the body except head
Each nerve is named according to its nearby vertebra
8 cervical (C1-C8)
12 thoracic (T1-T12)
5 Lumbar (L1-L5)
5 Sacral (S1-S5)
1 Coccygeal (C0)

9. Spinal Nerves Each spinal nerve is formed from two roots
Dorsal root – sensory roots and ganglia
Ventral root – motor roots
A dorsal root and a ventral root fuse laterally (on the side) to form spinal nerves

10. Spinal Nerves: Roots
Each spinal nerve arises from the spinal cord via two roots
Each root is formed from a set of rootlets that attach to the spinal cord
Ventral roots arise from the ventral horn of the spinal cord and contain motor (efferent) fibers
Dorsal roots arise from sensory neurons in the dorsal root ganglia and contain sensory (afferent) fibers

11. Spinal Nerves: Roots

12. Spinal Nerve Rami Spinal nerves are actually quite short.
Two spinal nerve roots give rise to each spinal nerve.
The dorsal root carries afferent (sensory) neurons.
The ventral root carries efferent (motor) neurons.
So each spinal nerve is a mixed nerve carrying both sensory and motor neurons.
Each spinal nerve, in turn, gives rise to two spinal nerve rami (singular: ramus).
Both the dorsal and ventral spinal nerve rami carry both sensory and motor neurons.

13. Spinal Nerve: Rami

14. Ventral Rami Form Nerve Plexuses
All ventral rami except T2-T12 form interlacing networks called plexuses (i.e. spaghetti bowls)
Plexuses are found in the:
Cervical region
Brachial region
Lumbar region
Sacral region
The last two will be dealt with as the Lumbosacral plexus
Each resulting branch of a plexus contains fibers from several spinal nerves
Each muscle receives a nerve supply from more than one spinal nerve
Damage to one spinal segment cannot completely paralyze a muscle

15. Cervical Plexus The cervical plexus is formed by ventral rami of C1-C4
Most branches are cutaneous nerves of the neck, ear, back of head, and shoulders
The most important nerve of this plexus is the phrenic nerve .
The phrenic nerve is the major motor and sensory nerve of the diaphragm.
As long as an injury to the spinal cord occurs inferior to the emergence of the phrenic nerve the ability to breathe independently may be spared.

16. Brachial Plexus Formed by C5-C8 and T1
In some individuals C4 and T2 may also contribute to this plexus.
The brachial plexus gives rise to nerves supplying the arm.
The five major branches
of this plexus are:
Axillary
Radial
Musculocutaneous
Ulnar
Median

17. Brachial Plexus for PT and OT
Additional detail is provided for those headed to PT and OT programs:
Five ventral rami (C5-T1) form
three trunks that separate into six divisions then form
three cords that give rise to:
Branches/nerves
Axillary
Radial
Musculocutaneous
Ulnar
Median
Smaller nerves, too.

18. Lumbosacral Plexus (Lumbar and Sacral Plexuses)
Arises from L1-S4 and innervates the thigh, abdominal wall, psoas muscle, the buttocks, lower limb, pelvic structures, and the perineum
The major nerves are the
Femoral (Lumbar)
Obturator (Lumbar)
Sciatic (Sacral)
Tibial
Common fibular (peroneal)

20. Practice Questions

21. Which of the following is NOT a nerve plexus? 
The thoracic rami 
The lumbosacral plexus 
The cervical plexus  
The brachial plexus 

22. Which of the following nerves does NOT arise from the brachial plexus? 
The musculocutaneous nerve 
The radial nerve 
The axillary nerve 
The ulnar nerve 
The phrenic nerve

23. The muscle fiber or gland cell that responds to the efferent impulses is known as the  
Effector  
Motor neuron 
Integration center 
Sensory neuron 
Receptor

24. The basic pattern of the CNS in the spinal cord is a central cavity surrounded by a gray matter core and a white matter layer lying outside of that.   True  False

25. Although the pairs of spinal nerves correspond to the number of vertebrae in the each area, there are eight pairs of cervical cranial nerves, but only seven cervical vertebrae.   True  False

26. Each muscle in a limb receives its nerve supply from one spinal nerve
Each muscle in a limb receives its nerve supply from one spinal nerve.   True  False

27. A sensory neuron transmits afferent impulses to the CNS.   True  False