Chapter 13
Nervous Tissue


     The nervous and endocrine systems control and adjust the activities of other organ systems.
Overview
     The nervous system has two anatomical subdivisions:
Central Nervous System (CNS) – The CNS consists of the brain and the spinal cord. The CNS integrates, processes and coordinates.
Peripheral Nervous System (PNS) – The PNS is all of the nervous tissue outside of the CNS. The PNS can be further subdivided into:
Afferent Division – Receptors of various kinds provide sensory input to the CNS.
Efferent Division – carries motor commands effectors that include muscle cells and gland cells. The efferent division can be divided into:
Somatic Nervous System (SNS) – The SNS controls skeletal muscle contractions.
Autonomic Nervous System (ANS) – The ANS regulates smooth muscle, cardiac muscle and glandular activity.

Cellular Organization in Neural Tissue
     Neural tissue contains two cell types:
1. Neurons – These cells transfer and process information.
2. Neuroglia – These cells, glial cells, serve various supportive roles for neurons. These cells comprise about half the volume of nervous tissue but there are about 5 times as many neuroglia as neurons.

  Neuroglia
     CNS – Four types of glial cells are found in the CNS:            
1. Astrocytes (astro –star)
     The largest and most numerous glial cells, these cells have numerous processes that contact the surfaces of neurons and capillaries. Astrocytes physically isolate and support the neurons. The functions of astrocytes include:
a. Maintain the blood-brain barrier – Chemicals in the blood would disrupt neuron function. Astrocytes form a cytoplasmic boundary around capillaries that maintain the highly restrictive permeability characteristics of the endothelial cells of the capillaries in the CNS.
b. Create a three-dimensional framework for the CNS – The processes of astrocytes are reinforced by cytoskeletal elements and these processes form a framework that supports the neurons.
c. Performs repairs in damaged neural tissue – Astrocytes stabilize damaged tissue by producing scar tissue.
d. Guide neuron development – Astrocytes guide the growth and interconnection of neurons in the embryo.
e. Controls the interstitial environment – Astrocytes optimize the interstitial environment by promoting rapid exchange of materials between the blood and interstitial fluid. Astrocytes also absorb and recycle neurotransmitters.
2. Oligodendrocytes (oligo – few + dendro – branch)
     Oligodendrocytes have fewer cell processes than astrocytes. Oligodendrocyte processes:
a. tie clusters of axons together.
b. regulate extracellular ion concentration.
c. provide myelination for CNS axons.
     Myelination consists of multiple layers of the cell membrane of either oligodendrocytes or Schwann cells (neurolemmocytes) wrapped around the axons of neurons. The cell membrane wrapping is called myelin. A number of glial cells are necessary to form the myelination along the length of an axon. The myelination contributed by each glial cell is called an internode. The small gaps between internodes are nodes or nodes of Ranvier.
     White and gray matters refer to two regions of the CNS. Regions dominated by myelinated axons appear glossy white. Regions dominated by neuron cell bodies, dendrites and unmyelinated axons appear dusky gray.
3. Microglia
     These are the smallest glial cells and have slender cytoplasmic processes with many fine branches. Microglia are phagocytic cells that engulf cellular debris, waste products and pathogens. They make up only 5 percent of glial cells but increase dramatically during injury or infection.
4. Ependymal Cells
     The fluid-filled spaces within the CNS are lined by ependymal cells. Ependymal cells are epithelial cells that monitor the composition of cerebrospinal fluid (CSF). Specialized ependymal cells also participate in the secretion of CSF.
     PNS – Two types of neuroglial cells are found in the PNS:
1. Satellite Cells
     These cells surround and support neuron cell bodies in ganglia. These cells regulate the exchange of materials between the cell and the extracellular environment.
2. Schwann Cells (neurolemmocytes)
     These cells surround and support the axons of the PNS. When the axon is unmyelinated, a single Schwann cell may surround and support several axons. When myelinated the Schwann cell surrounds a small segment (about 1 mm) of a single axon and many Schwann cells are needed to myelinate an axon depending on the length.
  Terminology:
Axolemma (lemma – husk) – This is the cell membrane of the axon.
Neurolemma – This refers to the cytoplasmic covering provided by the Schwann cell.

  Neurons
     Cellular anatomy:
Cell body (soma, perikaryon) - This is the part of the neuron that contains the nucleus.
Dendrite – These are processes that receive information primarily at the dendritic spines.
Axon – This is the process that carries information away from the cell body. The axon has the following parts:
Axon hillock (little hill) – Elevated surface of the cell body that gives rise to the axon.
Initial segment – The proximal part of the axon that is unmyelinated and is designed to trigger an impulse.
Axoplasm – cytoplasm of the axon.
Collaterals – side branches of the main axon.
Telodendria – fine terminal branches of the axon.
Synaptic terminal – end of telodendria where the axon makes contact with another neuron or effector. Often forms a simple round synaptic knob, also known as a synaptic end bulb.
     Communication between neurons:
     The communication between neurons occurs at the synapse. Communication involves the release of chemical messengers called neurotransmitters which are released when an electrical impulse arrives at the synapse.
     Neuron Classification:
Structural classification
Anaxonic neurons – Found only in CNS, these neurons are small with processes whose axons cannot be distinguished from dendrites.
Bipolar neurons – The branches of a dendrite fuse to form a single dendrite and the cell body lies between this dendrite and the axon. Unmyelinated and rare, these cells are associated with the special senses of sight, hearing and smell.
Pseudounipolar neurons – The dendritic and axonal processes are continuous. Inpulses are initiated at the base of dendritic branches and the remainder of the fused processes acts essentially as an axon. Most sensory neurons of the PNS are of this type.
Multipolar neurons – These neurons are the most common type in the CNS. Multipolar neurons have several dendrites and one axon. All motor neurons that control skeletal muscles are multipolar.
Functional classification
Sensory neurons
    Sensory neurons deliver information from the peripheral receptors to the CNS along afferent fibers. Sensory neurons are subclassified into:
 Somatic sensory neurons which include:
Exteroceptors  provide information about the external environment including touch, temperature, pressure, special senses and pain that affects the limbs and body surfaces.
Proprioceptors monitor the position of muscles and joints.
Visceral sensory neurons or interoceptors:
These neurons monitor deep pressure and pain in various internal organ systems. Chemical concentrations and pressure in the cardiovascular system. Chemicals in the oral cavity and pharynx are detected and perceived as taste.
Motor neurons
     Motor neurons stimulate or modify the activity of peripheral tissue, organs or organ systems along efferent fibers. Motor neurons are subclassified into:
Somatic motor neurons
     Somatic motor neurons innervate skeletal muscles by neurons whose cell bodies are in the CNS and whose axons extend to the muscle fibers they control. Most are consciously controlled.
Visceral motor neurons
     Visceral motor neurons innervate effectors other than skeletal muscle. There are two types of motor neurons that carry commands to effector organs in sequence (relay):
Preganglionic fibers - These neurons have cell bodies in the CNS and synapse on postganglionic neurons in peripheral ganglia. (Ganglion - An aggregation of neuron cell bodies in the PNS.)
Postganglionic fibers - These neurons have there cell bodies in peripheral ganglia and synapse onto the peripheral effectors.
Interneurons
     Interneurons are also called association neurons. These neurons are located entirely in the CNS and analyze the sensory input and coordinate motor output. They greatly outnumber sensory and motor neurons. They may either be excitatory or inhibitory depending on their effects on other neurons.