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)

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

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”)

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

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

Spinal Nerve Anatomy

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

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

Reflex Arcs

• 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

• 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

Reflex Arcs

• Reflex Arcs provide an illustration of homeostasis

Stretch Reflex

• 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

• 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

Stretch Reflex• The muscle spindle organ

“notifies” the spinal cord if it is being stretched

Crossed Extensor Reflex

• 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)

• 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

Movements of the Lower Leg

Involuntary movement

Voluntary movement