Spinal Cord Protection and coverings Meninges (me NIN jeez) – layers of connective tissue around the spinal cord (spinal meninges) and brain (cranial meninges). Dura mater – tough outer layer

More on meninges Arachnoid mater – middle layer of collagen and elastic fibers that look like a spider web. Pia mater – inner clear layer of collagen and fibers which adheres to the brain and spinal cord. It contains many blood vessels.

Subarachnoid space – space between the pia mater and arachnoid mater which contains cerebrospinal fluid (a clear liquid which carries oxygen, glucose and other necessary chemicals to the neurons and neuroglia from the blood and removes wastes from those cells and puts it into the blood.)

Fat and connective tissue also form a cushion between the vertebrae and the dura mater of the spinal cord.

Diagram:

Spinal cord structure 16-18” long, from the lowest part of the brain to the upper border of the 2nd lumbar vertebra. Below the 2nd lumbar vertebra, nerves called cauda equina (“horse’s tail) angle down through the rest of the lumbar vertebrae and sacrum and look like wisps of hair.

31 pairs of spinal nerves extend from the spinal cord (8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal). (See p.248)

Diagram:

Spinal cord is divided into right and left halves by the anterior median fissure and the posterior median sulcus.

Gray matter fills the middle of the spinal cord. Gray matter contains cell bodies of neurons, neuroglia, unmyelinated axons and dendrites of interneurons and motor neurons.

Anterior (ventral) gray horns contain cell bodies of somatic motor neurons. Posterior (dorsal) gray horns contain somatic and autonomic sensory neurons.

Central canal – space containing cerebrospinal fluid in the center of the gray matter. White matter surrounds the gray matter.

Organized into anterior, posterior and lateral white columns. White matter Made of myelinated and unmyelinated axons of sensory, motor and interneurons. Organized into anterior, posterior and lateral white columns.

Tract – distinct bundle of axons within a white column. Sensory (ascending) tracts – axons that conduct information to the brain. Motor (descending) tracts – axons that conduct information from the brain.

A dorsal and ventral root extends from each side of the spinal cord where it attaches to a spinal nerve on each side. Posterior (dorsal) root – contains sensory axons. Their cell bodies are contained in a swelling called the posterior (dorsal) root ganglion. Anterior (ventral) root – contains axons of somatic and autonomic motor neurons.

Diagram: White matter Posterior (dorsal) root posterior median sulcus Anterior (ventral) root Posterior (dorsal) gray horns Gray matter Central canal Anterior (ventral) gray horns spinal nerve anterior median fissure posterior (dorsal) root ganglion.

Spinal Nerves Spinal nerves leave the vertebral column through the intervertebral foramina. Each spinal nerve divides into several branches after leaving its intervertebral foramen.

Plexus – network of nerves on the side of the body where the branching spinal nerves join with axons from the part of the body they lead to. Cervical plexus – connects to the skin and muscles of the posterior part of the head, neck and upper part of the shoulders and the diaphragm. Phrenic nerves – to diaphragm.

Brachial plexus – connects to arms, neck and shoulder muscles. Lumbar plexus – connects to abdominal wall, genitals and part of the legs. Sacral plexus – connects to the buttocks and legs.

Intercostal nerves – Do not have plexuses, but go directly to the muscles between the ribs, abdominal muscles and skin of the chest and back.

Reflexes A reflex is a fast, involuntary response to a stimulus. A reflex can be inborn, such as the withdrawal reflex of pulling your hand away if you touch something hot or sharp. A reflex can be learned, such as those you use when driving.

Spinal reflex - does not involve the brain. Cranial reflex – involves the brain stem. Reflex arc – the pathway of neurons involved in a reflex action.

Ex. Patellar reflex – knee jerk A sensory receptor detects the stimulus (muscle spindles in the patellar ligament detect stretching due the the hammer strike). A sensory neuron fires. (Signals are sent to the spinal cord and brain)

The integrating center in the spinal cord connects the sensory neuron to a motor neuron. Interneurons may also be involved. Motor neurons fire to cause the response (excitatory to the quadriceps so it contracts and inhibitory to the hamstrings – biceps femoris, semimembrinosis and semitendinosis).

Effector – the part of the body that responds – the quadriceps contracts and the leg kicks out.

Somatic reflex – the effector is a skeletal muscle Autonomic reflex – the effector is smooth muscle, cardiac muscle or a gland. Ex. Swallowing, urinating, defecating

Lack of a patellar reflex can indicate damage of the sensory or motor neurons, spinal cord injury. Lack of pupillary light reflex (pupil size decreases when you shine a light in the eye) can indicate brain damage since this reflex synapses in the lower brain regions.

Diagram of the patellar reflex: Excitatory Sensory neuron Interneuron Inhibitory Motor neurons