BIOLOGY Volume 1 - A Guide to General Biology - 2004

6. HISTOLOGY

6.6. Nervous Tissue

Nervous Tissue consists of densely packed Nerve Cells (or Neurons), which are specialized for conducting nerve impulses. In addition, receptor cells and Schwann cells are also present (see below). Nervous tissue is often surrounded by Connective Tissue containing Blood Vessels.

6.6.1. Neurons

Neurons serve as the functional units of The Nervous system. They are capable of transmitting nerve impulses, which enables communication between receptors (cells or Organs that perceive stimuli) and effectors (tissues or organs that respond to stimuli, such as Muscles or glands; Fig. 6.27). Neurons that transmit impulses to the Central Nervous System (Brain AND SPINAL cord) are called sensory neurons, whereas motor neurons transmit impulses away from the central nervous system. Sensory neurons are frequently connected to motor neurons via interneurons (Relay neurons). The Structure of these neurons and their alternative names are illustrated in Fig. 6.28.

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Fig. 6.27. Simplified diagram of Nerve Impulse transmission. Alternative neuron terminology: sensory neuron — afferent neuron, receptor neuron; motor neuron — efferent neuron, effector; interneuron — intermediate neuron, connecting neuron, relay neuron. Use only one term for each type!

Fig. 6.28. Schematic representation of a sensory neuron (A), a motor neuron (B), and an interneuron (C).

Each neuron consists of a Cell body (Fig. 6.28) containing The Nucleus and most of the other cellular Organelles, along with a varying number of nerve processes extending from it. The Cell body also contains Nissl substance (or Nissl bodies), which consists of Ribosomes and rough Endoplasmic reticulum involved in Protein Synthesis, as well as the Golgi apparatus (Fig. 6.29).

Fig. 6.29. Neuronal cell bodies with synapses and Nissl bodies.

Nerve processes that conduct impulses toward the cell body are called dendrons (see Fig. 6.27). These are relatively short, slender processes that branch at their ends into fine terminal twigs known as dendrites (from the Greek déndron, meaning tree). Nerve processes that conduct impulses away from the neuron cell body to other cells or organs are called axons; they are thinner than dendrites and can reach several meters in length.

The distal portion of the axon branches into numerous fine twigs with swollen endings called synaptic knobs (or terminal buttons). These knobs do not make direct physical contact with the body of the adjacent nerve cell. A narrow gap remains between the synaptic knob and the neighboring cell body, across which a chemical substance — a neurotransmitter — must pass in order to stimulate that nerve cell (or effector). The neurotransmitter is released from the synaptic knob in response to a nerve impulse traveling down the axon. This specialized functional contact between two excitable cells is called a synapse (see Fig. 6.27).

Some nerve fibers are entirely covered by a fat-like myelin sheath that acts as an insulator. It is formed by specialized Schwann cells. The outer membrane of these cells stretches and wraps repeatedly around the nerve fiber like a rolled-up carpet (Fig. 6.30), forming the myelin sheath. This sheath consists primarily of Lipids, lacking the protein typically found in membranes. The Cytoplasm remains in a region called the neurilemma, which encloses the myelin sheath. Being lipid-rich, this sheath prevents the movement of Na⁺ and K⁺ ions into and out of the cell. The influx and efflux of these ions are essential for Nerve Impulse Conduction, meaning that if the sheath were continuous, nerve transmission would be impossible. However, the myelin sheath is interrupted at regular intervals (roughly every 1 mm) by the nodes of Ranvier (see Fig. 6.28). These nodes are located between adjacent Schwann cells, such that a single nucleus of a Schwann cell can be observed between any two nodes.

Nerve fibers surrounded by a myelin sheath (such as Spinal Nerves) are called myelinated, whereas those lacking such a sheath are termed unmyelinated. The latter lack nodes of Ranvier and are only partially enclosed by Schwann cells. Certain disorders, such as Tay-Sachs disease, involve The breakdown of myelin sheaths.

Fig. 6.30. Schematic representation of a myelinated nerve fiber.