Anatomy Ch.12 Spinal Cord

5. Ganglion

Ganglion are clusters of cell bodies that lie along nerves in the PNS. 1. The cell bodies of the associated sensory neurons are found in an enlarged region of the dorsal root called the dorsal root ganglion or spinal ganglion. 2. First-order neurons, whose cell bodies reside in a ganglion (dorsal root or cranial), conduct impulses from the cutaneous receptors of the skin and from proprioceptors to the spinal cord or brain stem, where they synapse with second-order neurons. 3. Neuron crest cells that come to lie alongside the cord form the dorsal root ganglia containing sensory neuron cell bodies.

9. Sensory (afferent) tract

Sensory (afferent) tracts are ascending pathways that that conduct sensory impulses upward, typically through chains of three consecutive neurons to various areas of the brain. 1. Ascending a descending tracts make up most of the white matter. 2. In general, somatosensory information travels along three main pathways on each side of the spinal cord. Two of these pathways (the dorsal column-medial lemniscal and spinothalamic pathways) transmit impulses via the thalamus to the sensory cortex for conscious interpretation. 3. Collectively, the inputs of these sister tracts provide discriminative touch and conscious proprioception. Both pathways decussate-- the first in the medullaand the second in the spinal cord.

3. Dorsal root

The dorsal roots are afferent fibers that carry impulses from peripheral sensory receptors to the spinal cord. 1. The cell bodies of the associated sensory neurons are found in an enlarged region of the dorsal root called the dorsal root ganglion or spinal ganglion. 2. The dorsal and ventral roots are are very short and fuse laterally to form the spinal nerves.

10. Spinal cord

The spinal cord is enclosed in the vertebral column and provides a two-way conduction pathway to and from the brain. 1. The spinal cord is a major reflex center: Spinal reflexes are initiated and completed at the spinal cord level. 2. Like the brain, the spinal cord is protected by bone, meninges, and cerebrospinal fluid. The single-layered spinal dura mater is not attached to the bony walls of the vertebral column. 3. The spinal cord extends from the foramen magnum of the skull to the first or second lumbar vertebrae, just inferior to the ribs.

7. Motor (efferent) tract

Motor (efferent) tracts are descending pathways that deliver efferent impulses from the brain to the spinal cord. 1. Ascending a descending tracts make up most of the white matter. 2. The descending pathways are divided into two groups: (1) the direct pathways, equivalent to the pyramidal tracts, and (2) the indirect pathways, essentially all others. 3. Motor pathways involve two neurons, referred to as the upper and lower motor neurons: -Upper motor neurons are the pyramidal cells of the motor cortex and the neurons of the subcortical motor nuclei. -Lower motor neurons are the ventral horn motor neurons. These directly innervate the skeletal muscles (their effectors).

12. Ventral root

The ventral roots are fused ventral rootlets that send the axons of motor neurons to the skeletal muscles. 1. The dorsal and ventral roots are very short and fuse laterally to form the spinal nerves. 2. Severe damage to ventral root or ventral horn cells results in flaccid paralysis of the skeletal muscles served. Nerve impulses do not reach these muscles, which consequently cannot move either voluntary or involuntarily. Without stimulation, the muscles atrophy.

6. Gray matter

Gray matter is composed of neuron cell bodies, dendrites, associated glia and blood vessels, but no myelinated fiber tracts. 1. The gray matter of the spinal cord is located in its core. In cross section the gray matter of the cord looks like the letter H or like a butterfly. It consists of mirror image lateral gray masses connected by a crossbar of gray matter, the gray commissure, that encloses the central canal. 2. The amount of ventral gray matter present at a given level of the spinal cord reflects the amount of skeletal muscle innervated at that level.

11. Spinal nerves

Spinal nerves are the 31 pairs of nerves that arise from the spinal cord and supply all parts of the body except the head and some areas of the neck. 1. All spinal nerves are mixed nerves. 2. Each spinal nerve connects to the spinal cord by a dorsal and ventral root. 1. Each nerve exits from the vertebral column by passing superior to its corresponding vertebrae via the intervertebral foramen, and travels to the body region it serves.

1. Arachnoid mater

The arachnoid mater is the middle meninx that forms a loose brain covering that never dips into the sulci at the cerebral surface. 1. Beneath the arachnoid membrane is the wide subarachnoid space. Spiderweb-like extensions span this space and secure the arachnoid mater to the underlying pia mater. The sub arachnoid space is filled with cerebrospinal fluid and also contains the largest blood vessels serving the brain. 2. Inferiorly, the dural and arachnoid membranes extend to the level of S2, well beyond the end of the spinal cord. The spinal cord typically ends between L1 and L2.

2. Central canal

The central canal runs through the core of each osteon and contains the small blood vessels and nerve fibers that serve the osteon's cells. 1. The fourth ventricle lies in the hindbrain dorsal to the pons and superior medulla. It is continuous with the central canal of the spinal cord inferiorly. 2. In cross section the gray matter of the cord looks like the letter H or like a butterfly. It consists of mirror image lateral gray masses connected by a crossbar of gray matter, the gray commissure, that encloses the central canal.

4. Dura mater

The dura mater is the strongest meninx. 1. Where it surrounds the brain, it is a two-layered sheet of fibrous connective tissue. The more superficial periosteal layer attaches to the inner surface of the skull (periosteum). (There is no dural periosteal layer surrounding the spinal cord.) The deeper meningeal layer forms the true external covering of the brain and continues caudally in the vertebral canal as the spinal dura mater. 2. Inferiorly, the dural and arachnoid membranes extend to the level of S2, well beyond the end of the spinal cord. The spinal cord typically ends between L1 and L2.

8. Pia mater

The pia mater is the only meninx that clings tightly to the brain like cellophane wrap, following its every convolution. 1. The pia mater, meaning "gentle mother," is composed of delicate connective tissue and richly invested with tiny blood vessels. Small arteries entering the brain tissue carry ragged sheaths of pia mater inward with them for short distances. 2. Cerebrospinal fluid fills the subarachnoid space between the arachnoid and pia mater.

13. White matter

White matter of the spinal cord is composed of the myelinated and nonmyelinated nerve fibers that allow communication between different parts of the spinal cord and between the cord and the brain. 1. Ascending and descending tracts make up most of the white matter. 2. The white matter onn each side of the cord is divided into three white columns, or funiculi, named according to their position as dorsal (posterior), lateral, and ventral (anterior) funiculi.