Nervous Tissue

a. Structural Organization

The Nervous System consists of two subdivisions: the Central nervous system (CNS) and the Peripheral nervous system( PNS).

• The central nervous system (CNS) includes the Brain and Spinal Cord.

  • The brain is protected and enclosed within the skull, whereas the spinal cord is housed and protected within the vertebral canal.

• The peripheral nervous system (PNS) includes the Cranial nerves (nerves that extend from the brain), Spinal nerves (nerves that extend from the spinal cord) and Ganglia.

b. Functional Organisation

The nervous system has two functional divisions: the sensory nervous system and the motor nervous system. Both divisions have CNS and PNS components, which together perform three general functions:

• Collect Information

• Process and Evaluate Information

• Initiate Response to Information

a. Neurons

• The basic structural unit of the nervous system is the Neuron.

• Neurons conduct nerve impulses from one part of the body to another.

• They have several Special characteristics:

  • Neurons have a high metabolic rate.

  • Neurons have extreme longevity.

  • Neurons typically are non-mitotic.

  • Neurons are excitable because they respond to a stimulus.

  • Neurons exhibit conductivity when an electrical charge is quickly propagated along their plasma membrane.

• Neuron Structures:

  • Neurons come in all shapes and sizes, but all neurons share certain basic structural features.

  • The cell body, also called a soma, serves as the neuron's control center and is responsible for receiving, integrating and sending nerve impulses.

  • The cell body is enclosed by a plasma membrane and contains cytoplasm surrounding a nucleus. The nucleus contains a prominent nucleolus, reflecting the high metabolic activity of neurons, which require the production of many proteins.

  • Dendrites tend to be shorter, smaller processes that branch off the cell body.

    • Some neurons have only one dendrite, whereas other have many.

    • Dendrites conduct nerve impulses toward the cell body; in essence, they receive input and then transfer it to the cell body for processing.

  • The axon or nerve fiber, is typically a longer nerve cell process emanating from the cell body to make contact with other neurons, muscle cels or gland cells.

    • Neurons have either one axon or no axon at all.

    • The axon transmits a nerve impulse away from the cell body toward another cell; in essence, the axon transmits output information to other cells.

    • The axons extends from a triangular region of the cell body called the axon hillock.

• Neuron Classification

  • Structural Classification

    • Unipolar Neurons: have a single, short process that emerges from the cell body.

    • Bipolar Neurons: have two processes that extend from the cell body, one axon and one dendrite.

    • Multipolar Neurons: are the most common type of neurons

  • Functional Classifications

    • Sensory Neurons (Afferent Neurons): transmit nerve impulses from sensory receptors to the CNS.

    • Motor Neurons (Efferent Neurons): transmit nerve impulses from the CNS to muscles or glands.

    • Interneurons (Association Neurons): lie entirely within the CNS and are multipolar structures. 

      • They receive nerve impulses from other neurons and carry out the integrative functions of the nervous system, that is, they retrieve, process and store information and decide how the body responds to stimuli.

      • Thus, interneurons facilitate communication between sensory and motor neurons.

b. Glial Cells

• Glial cells, sometimes referred to as neuroglia, occur within both the CNS and the PNS.

  • Glial cells differ from neurons in that they are smaller and capable of mitosis.

  • Glial cells do not transmit nerve impulses, but they do assist neurons with their functions.

  • Collectively, the glial cells physically protect and help nourish neurons, and provide an organized, supporting framework for all the nervous tissue.

• Glial Cells of the CNS

  • Four types of glial cells occur in the central nervous system: astrocytes, ependymal cells, microglial cells and oligodendrocytes.

  • They can be distinguished on the basis of size, intracellular organization, and the presence of specific cytoplasmic processes.

    • Astrocytes

      • Appearance

        • Large cells with numerous cell processes, in contact with neurons and capillaires, most common type of glial cells.

      • Functions:

        • Helps form the blood-brain barrier.

        • Regulates tissue fluid composition.

        • Provides structural support and organization to CNS.

        • Replaces damaged neurons.

        • Assists with neuronal development.

        • Helps regulate synaptic transmission.

    • Ependymal Cells

      • Appearance

        • Simple cuboidal epithelial cell lining cavities in brain and spinal cord, cilia on apical surface.

      • Functions

        • Lines ventricules of brain and central canal of spinal cord.

        • Assists in production and circulation of CSF.

    • Microglial Cells

      • Appearance

        • Small cells with slender branches from cell body, least common type of glial cell.

      • Functions

        • Defends against pathogens.

        • Removes debris.

        • Phagocytizes wastes.

    • Oligodendrocytes

      • Appearance

        • Rounded, bulbous cell with slender cytoplasmic extensions, extensions wrap around CNS axons.

      • Functions

        • Myelinates and insulates CNS axons.

        • Allows faster nerve impulses conduction through the axon.

• Glial Cells of the PNS

  • The two glial cell types in the PNS are satellite cells and neurolemmocytes.

    • Satellite Cells

      • Appearance

        • Flattened cell clustered around neuronal cell bodies in a ganglion.

      • Functions

        • Protects and regulates nutrients for cell bodies in ganglia.

    • Neurolemmocyte

      • Appearance

        • Flattened cell wrapped around a portion of an axon in the PNS.

      • Functions

        • Myelinates and insulates PNS axons.

        • Allows for faster nerve impulses conduction through the axon.

• The main activity of axons is nerve impulse conduction.

• A nerve impulse or action potential is the rapid movement of an electrical change along an axon's plasma membrane.

• Myelination

  • It is the process by which part of an axon is wrapped with a myelin sheath, the insulating covering around the axon consisting of concentric layers of myelin.

    • In the CNS, a myelin sheath forms from Oligodendrocytes.

    • In the PNS, it forms from Neurolemmocytes.

  • Myelin mainly consists of the plasma membranes of these glial cells and contains a large proportion of fats and a lesser amount of proteins.

  •  The high lipid content of the myelin sheath gives the axon a distinct, glossy-white appearance and serves to effectively insulate it.

• Nerve Impulse Conduction

  • The myelin sheath supports, protects and insulates an axon.

  • The small spaces interrupt the myelin sheath between adjacent oligodendrocytes or neurolemmocytes, these gaps are called neurofibril nodes or nodes of ranvier.

    • At these nodes, and only at these nodes, can change in voltage occur across the plasma membrane and result in the movement of a nerve impulse.

  • Thus, in a myelinated axon, the nerve impulse seems to jump from neurofibril node to neurofibril node, a process called saltatory conduction. In an myelinated axon, the nerve impulse must travel the entire length of the axon membrane, a process called continuous conduction.

• The success of PNS axon regeneration depends upon two primary factors:

  • The amount of damage

  • The distance between the site of the damaged axon and the structure it innervates.

• Neurolemmocytes play an active role in regeneration and it follows these stages;

  • Trauma severs axons

  • The proximal portion of each severed axon seals off the swells. The distal portion of axon and myelin sheath disintegrate, the neurilemma survives.

  • Neurilemma and endoneurium form a regeneration tube.

  • Axon regenerates and remyelination occurs.

  • Innervation to effector is restored.

• A nerve is a cablelike bundle of parallel axons.

• A nerve has three successive connective tissue:

  • An individual axon in a myelinated neuron is surrounded by neurolemmocytes and then wrapped in the endoneurium, a delicate layer of areolar connective tissue that separates and electrically isolates each axon.

  • Groups of axons are wrapped into separate bundles called fascicles by a cellular dense irregular connective tissue layer called perineurium.

  • All of the fasicicles are bundled together by a superficial connective tissue covering the epineurium.

• Axons terminate as they contact other neurons, muscle cells or gland cells, at specialized junctions called synapses, where the nerve impulse is transmitted to the other cell.

• A typical synapse in the CNS consists of the close association of presynaptic and a postsynaptic neuron at a space called the synaptic cleft. 

  • Presynaptic neurons transmit nerve impulses through their axons toward a synapse; postsynaptic neurons conduct nerve impulses through their dendrites and cell bodies away from the synapse.

• Axons may establish synaptic contacts with any portions of another neuron, except those regions covered by a myelin sheath. Three common types of synapses are axodendritic, axosomatic and axoaxonic.

  • The axodendritic synapse is the most common type. It occurs between the synaptic knobs of a presynaptic neuron and the dendrites of the postsynaptic neuron.

  • The axosomatic synapse occurs between synaptic knobs and the cell body of the postsynaptic neuron.

  • The axoaxonic synapse is the least common synapse and far less understood. It occurs between the synaptic knob of a presynaptic neuron and the synaptic knob of a postsynaptic neuron.

• Synaptic Communication

  • Synapses are termed electrical when a flow of ions passes from the presynaptic cell through gap junctions; synapses are termed chemical when a nerve impulse causes the release of a chemical neurotransmitter from the presynaptic cell that induces a response, in the postsynaptic cell.

  • A myelinated axon conducts impulses faster than an unmyelinated axon, and the larger the diameter of the axon, the faster is the rate of conduction.

• A very precise sequence of events is required for the conduction of a nerve impulse from the presynaptic neuron to the postsynaptic neuron:

  • A nerve impulse travels through the axon and reaches it synaptic knob.

  • The arrival of the nerve impulse at the synaptic knob causes an increase in calcium ion movement into the synaptic knob through voltage-regulated calcium ion channels in the membrane.

  • Entering calcium ions cause synaptic vesicles tonmove to and bind to the inside surface of the membrane, neurotransmitter molecules within the synaptic vesicles are released into the synaptic cleft by exocytosis.

  • Neurotransmitter molecules diffuse across the synaptic cleft to the plasma membrane of the postsynaptic cell.

  • Neurotransmitter molecules attach to specific protein receptors in the plasma membrane of the post synpatic cell, causing ion gates to open.

  • An influx of sodium ions moves into the postsynaptic cell through the open gate, affecting the charge across the membrane.

  • Change in the postsynaptic cell voltage causes a nerve impulse to begin in the postsynaptic cells

  • The enzyme acteylcholinesterase (AChE) resides in the synaptic cleft and rapidly breaks down molecules of ACh that are released into the synaptic cleft. Thus, AChE is needed so that ACh will not continuously stimulate the postsynaptic cell.

• Interneurons are organized into neuronal pools, which are groups of interconnected neurons with specific functions.

• In a converging circuit, neurons synapse on the same postsynaptic neuron.

• A diverging circuit spreads information to several neurons.

• In the reverberating circuit, neurons continue to restimulate presynaptic neurons in the circuit.

• A parallel-after-discharge circuit involves parallel pathways that process the same information over different amounts of time and deliver that information to the same output cell.