1. Spinal Cord: Meninges
The spinal meninges (dura mater, arachnoid mater, and pia mater) are layers of connective tissue that protect the spinal cord and supply it with nutrients
They are continuous with the cranial meninges, which perform the same functions for the brain
Between the arachnoid mater and the pia mater is the subarachnoid space, which contains cerebrospinal fluid (CSF)

2. Spinal Cord: Spinal Tap
A spinal tap (lumbar puncture) is a procedure in which a needle is inserted into the arachnoid space in the lumbar region to withdraw CSF or administer medication

3. Spinal Cord: External Anatomy
There are 31 pairs of spinal nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal)
Bundles of axons called roots connect each nerve to the spinal cord
The dorsal root is sensory, the ventral root is motor
Each dorsal root has a “swelling” composed of cell bodies of sensory neurons, called the dorsal root ganglion
Nerves to and from the upper limbs form the cervical enlargement, nerves to and from the lower limbs form the lumbar enlargement
The spinal cord tapers at its end to form the conus medullaris
Lumbar, sacral, and coccygeal nerves hang below the conus medullaris to form the cauda equina (“horse’s tail”)

4. Spinal Cord: Internal Anatomy
The white matter is permeated by the anterior median fissure and the posterior median sulcus
The gray matter forms an “H” or butterfly, with the central canal in the middle of the gray commissure
The ventral horns contain somatic motor nuclei, whereas the dorsal horns contain somatic and autonomic sensory nuclei
The anterior white commissure connects the white matter on right and left sides
The ventral and dorsal gray horns divide the white matter into the ventral white columns, dorsal white columns, and lateral white columns

5. Spinal Nerve Anatomy Nerves are similar in organization to muscles
Groups of axons are bundled in fascicles
The entire nerve is wrapped in the epineurium
The perineurium surrounds each fascicle
The endoneurium surrounds each axon

6. Spinal Nerve Anatomy

7. Spinal Cord Tracts
Spinal tracts are the “highways” for information traveling between the brain and the body
Sensory tracts take information from sensory organs to the brain
Motor tracts carry impulses from the brain to muscles and target organs
Spinal cord gray matter is a site where excitatory and inhibitory impulses are summed

8. Dermatomes
Dermatomes provide sensory input to the CNS via the dorsal roots of spinal nerves, or via the trigeminal nerve (cranial nerve V)
Can be used diagnostically to assess spinal nerve damage
Can explain referred pain
Can be used to determine how to administer anesthesia to portions of the body (e.g. phantom pain)
May be a remnant of our earlier segmentation

9. Reflex Arcs
The pathway followed by a nerve impulse that produces a reflex is a reflex arc
A sensory receptor responds to a stimulus
A sensory neuron conveys the impulse to the spinal cord
Integration takes place in the spinal cord, conveying the impulse to a motor neuron through a monosynaptic or polysynaptic reflex arc
The motor neuron conveys the impulse to the part of the body that will respond
The effector organ (an organ, muscle, or gland) responds to the motor impulse
Somatic reflexes involve skeletal muscles, autonomic reflexes involves smooth muscle and glands

10. Reflex Arcs
Reflex Arcs provide an illustration of homeostasis

12. Stretch Reflex
The stretch reflex occurs in response to stretching of muscle
The stretch reflex is monosynaptic and ipsilateral
Stretching simulates the muscle spindle organ
Impulse is sent along a somatic sensory neuron to the dorsal horn
In the spinal cord the sensory neuron synapses with a motor neuron in the ventral horn
An excitatory motor impulse is conveyed along the motor neuron to the muscle, which contracts
The sensory neuron also synapses with another motor neuron
An inhibitory motor impulse is sent to the antagonistic muscle

13. Stretch Reflex
The muscle spindle organ “notifies” the spinal cord if it is being stretched

14. Crossed Extensor Reflex
The crossed extensor reflex is polysynaptic and contralateral
The crossed extensor reflex is an intersegmental reflex
Painful stimulus to right foot sends impulse along sensory neuron
In the spinal cord the sensory neuron synapses with several neurons on the same side and on the opposite side
Motor neurons on the same side cause flexion of the limb
Motor neurons on the opposite side cause extension of the opposite limb
Inhibitory motor neurons cause inhibition of necessary antagonistic muscles on both sides (not shown)

15. Movements of the Lower Leg
Involuntary movement
Voluntary movement