SPINAL CORD

Cells of the Spinal Cord

The spinal cord is composed of many neurons, axons, and supporting cells . In addition, it must have many trophic factors in order to grow and function properly

1. Neurons- nerve cells that provide the communication pathway through the body . There are as many as 10,000 subtypes of neurons which are specialized to receive and send information , detect changes in the environment, and organize immediate and long-term responses to change. Each neuron is made up of a cell body, axons, and dendrites The nucleus is located in the cell body which, along with the dendrites and neuroglial cells makes up the butterfly shaped "gray matter" in the center of the spinal cord .

a. Dendrites- the input centers of the neuron. They are tree-like extensions from the cell body, that receive signals from other cells at the synaptic junction

b. Axons- the output centers of the neuron that extend from the cell body. They connect with either the dendrite of another neuron, a muscle, or a gland, where they release neurotransmitters of the activities of other cells to propagate the nerve impulse . Most axons are covered by a myelin sheath, which insulates the axon and gives them their white color. Thus, the outer region of the spinal cord where the axons are located is called "white matter"

2. Glial cells- supporting cells of the nervous system that serve many functions: a. physical support b. response to injury c. the maintenance of chemical balance of surrounding cells d. aid in the blood-brain and blood-spinal-cord barriers e. The formation of the myelin sheath of neural pathways f. The regulation of neurons during development. They may also produce substances that inhibit the regeneration of neurons after injury to the spinal cord. The most common types of glial cells in the central nervous system are astrocytes, oligodendrocytes, and microglia.

3. Astrocytes- the largest, most abundant , and principle glial cells of the central nervous system . Astrocytes aid neurons by providing extra energy and by producing neurotrophic factors necessary for neuron sustenance . They contribute to the blood-brain barrier and maintain the chemical environment by destroying harmful proteins and chemicals that could damage the neurons . After a spinal cord injury, astrocytes multiply and contribute to the formation of the glial scar

4. Oligodendrocytes- a type of glial cell located in the brain and spinal cord that produces myelin that surrounds the axons making impulse conduction faster and more reliable . Oligodendrocytes also manufacture substances that prevent axon regeneration in the adult central nervous system . Picture provided by The National Institutes of Health A. Oligodendrocyte; B. Axons; C. Node of Ranvier; D. Myelin; E. Myelin Sheaths

5. Microglia- immune cells located in the brain that become functional only after injury. When activated by injury microglial cells aid in ridding the injury of dying cells, dead cells, and other wastes that accumulate at the injury site. Microglia also manufacture cytokins, molecules that activate other immune cells to aid at the injury site .

Impluse Transmission
Nerve impulses are transmitted through the body by neurons. When a neuron is at rest (not transmitting an action potential) it maintains a voltage difference across its plasma membrane; the cytoplasmic fluid next to the membrane is negatively charged in comparison to the interstitial fluid outside the membrane. This voltage difference is called the resting membrane potential .

Picture adapted from Starr and Taggart

Sodium-Potassium Pump: maintains the resting membrane potential across the neuron plasma membrane. A. Sodium leaks into the neuron by diffusion; B. Sodium is actively pumped out of the cell through channel proteins by active transport; C. Potassim is actively pumped into the neuron through channel proteins; D. Potassium leaks into the neuron by diffusion; E. Potassium leaks out of the neuron by diffusion; F. Carrier protein that serves as the sodium-potassium pump to actively transport sodium and potassium into and out of the cell to maintain the membrane potential; G. Outside the neuron; H. The plasma membrane; I. Inside the neuron

Signals arise in the neuron's trigger zone and when they reach the neuron's input zone, usually the dendrite, the neuron is stimulated by an action potential: a very brief voltage reversal across the plasma membrane. An action potential can arise, however only if the voltage reversal reaches the threshold level. Once the threshold level is reached in one area of the neuron, the action potential triggers the voltage reversal at an adjacent area of membrane, making the action potential self-propagate along the neuron .

Picture adapted from Starr and Taggart

A disturbance causes the action potential to propagate down the axon The disturbance at a. creates another action potential at b. and c. This distrubance causes sodium gates at the nodes to open, and sodium flows in to create another action potential.

Action potentials travel rapidly along the axons of both sensory and motor neurons due to the myelin sheaths that surrounds them . The myelin sheath is made by oligodendrocytes and it is important because it insulates the axons and greatly increases the speed of neural transmission.

Picture provided by The National Institutes of Health

A. Synapses coming from different nerve cells B. Axon C. Synaptic vessicles that contain neurotransmitter molecules D. Synapse E. Synaptic Cleft F. Neurotransmitter receptors on post-synaptic cell membrane G. synaptic vessicle releasing neurotransmitter H. Node of Ranvier I. Myelin Sheath J. Oligodendrocyte K. nucleus L. nerve cell body M. dendrites

Small gaps called nodes of Ranvier separate the cells, and the action potential jumps from node to node making neural transmission very fast. As the action potential reaches the output zone, gated channels for calcium ions, which extend across the membrane, open. The ions travel down the concentration gradient, which induces synaptic vesicles containing neurotransmitters to fuse with the membrane .

Picture provided by The National Institutes of Health

The output zone of one neuron at the synaptic junction with the input zone of the second neuron. A. mitochondrion B. axon terminal C. synaptic vesicle releasing neurotransmitter molecules D. synaptic vessicles containing neurotransmitter molecules E. synapse F. synaptic cleft G.Neurotransmitter receptors on the post synaptic cell membrane

The neurotransmitter molecules diffuse across the synaptic cleft and bind with receptor proteins on the adjoining post synaptic cell. Binding causes channels to open through the protein allowing ions to cross the plasma membrane. Depending on the type of neurotransmitter and the post-synaptic cell, the neurotransmitter may have excitatory or inhibitory effects

picture adapted from Starr and Taggart

picture adapted from Starr and Taggart

neurotransmitter molecules diffusing across the membrane

receptor protein receiving the neurotransmitter molecule which allows sodium to cross the me

Anatomy of the Spinal Cord

The nervous system is made up of the central nervous system, the peripheral nervous system, and the cranial nerves. The spinal cord, part of the central nervous system, is generally around 17 inches long, and extends from the brain to the lower back. It is protected by the vertebral column (also known as the spinal column or backbone), which consists of 33 vertebrae . Three membranes, meninges, cover both the brain and spinal cord . The subarachnoid space is filled with cerebrospinal fluid which provides shock absorbance . The vertebrae are divided into sections depending on their location.

Picture provided by The National Institutes of Health Regions of the Spinal Cord: cervical, thoracic, lumbar and sacral

picture provided by Neurosurgery://OnCall Cross Section of the spinal cord

cervical vertebrae- the first 7 vertebrae, located in the neck

thoracic vertebrae- the next 12 vertebrae in the upper back

lumbar vertebrae- 5 vertebrae located in the lower back

5 fused sacral vertebrae located around the hip

4 fused vertebrae in the coccyx

The peripheral nerves branch off of the cervical, thoracic, lumbar and sacral regions, and are named according to their origin on the spinal column. Letters and numbers, which correspond to the level of the spine, are often used when referring to the location of the injury. For example, a C4 injury is located at the forth level in the cervical segment . The nerves leading out of each section of the spinal cord connect to specific regions of the body. The cervical nerves (C1-C8) control signals to the arms, neck and hands. The thoracic nerves (T1-T12) control the signals to the torso and parts of the arms. The lumbar nerves (L1-L5) relay signals to the legs and hips, and the sacral nerves (S1-S5) control signals to the toes, groin, and parts of the legs

Picture provided by The American Medical Association The human nervous system is comprised of the cranial nerves, the central nervous system, and the peripheral nerves. The central nervous system includes the brain and spinal cord. The peripheral nervous system includes the cervical, thoracic, lumbar, and sacral nerves that extend to the rest of the body from the spinal cord

All information traveling from the brain to the limbs must travel through the spinal cord, and conversely all sensory information coming from the body is relayed to the spinal cord and then to the brain . Due to this arrangement, the damage caused by a spinal cord injury is determined by the location of the injury

Physiology of the Spinal Cord

The spinal cord sends and receives information from the entire body and brain . The information controls sensations, movement, and autonomic function . Nerves traveling through the body relay the information from the spinal cord to target cells in all areas of the body. Axon bundles travel through the body in 2 pathways: ascending and descending. The descending pathway, which controls voluntary movement, carries information from the corticospinal tract in the brain to the motor neurons in the spinal cord and then to the final destination in the body. Information carried by the ascending pathway from the sensory neurons in the spinal cord to the brain controls sensory information about body position, temperature, pain, and touch . Each level of the spinal cord corresponds to the sensory perception at a particular part of the body. The lower the level of the spinal cord, the lower the area of the body that is controlled by that level. Thus, the lower the level of the spinal cord injury, the lower the parts of the body that are affected by the injury.

Picture provided by The National Institutes of Health The major pathways of the Spinal Cord: The ascending pathways (blue) carry sensory information from the body to the brain and spinal cord. The axons that comprise the ascending pathway end in the sensory cortex (blue) of the brain. The descending pathways (red) control motor function. The nerve cell bodies that control motor function are located in the motor cortex (red) of the brain.

Picture provided by The National Institutes of Health Cross Section of the Spinal Cord A. Descending (motor) pathways B. Ascending (sensory pathways) C. sensory neurons D. dorsal root ganglion (sensory) where the cell bodies of sensory neurons are located E. ventral root F. motor neurons (where axons descending from the motor cortex in the brain synapse with the cell bodies of spinal cord motor neurons.