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Nervous Tissue

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Nervous Tissue

Nervous tissue is the master controller and communication system of the body. It forms the brain, spinal cord, and peripheral nerves, and its primary function is to regulate and integrate all body functions by rapidly transmitting electrical signals.

neuron structure 1
Figure 01. neuron structure 1. Original image

The Two Main Cell Types

The nervous system is comprised of two principal types of cells that work in concert.

Neurons (Nerve Cells)

These are the primary functional cells that are specialized to transmit electrical signals (nerve impulses). They send and receive messages using chemical signals called neurotransmitters across junctions known as synapses.

Neuroglia (Glial Cells)

These are the non-excitable, supporting cells of the nervous system. They provide physical and metabolic support, insulation (myelin), and immune defense for the neurons. Examples include Astrocytes, Oligodendrocytes, and Schwann Cells.

General Characteristics

  • Primary Function: To receive stimuli, transmit electrical impulses, and process information to control the body's responses.
  • Location: Makes up the Central Nervous System (CNS)—the brain and spinal cord—and the Peripheral Nervous System (PNS)—the peripheral nerves.

Key Properties of Neurons

Excitability

The ability to respond to a stimulus by generating an electrical change across its membrane (membrane potential).

Conductivity

The ability to propagate these electrical signals (nerve impulses or action potentials) rapidly along the cell membrane.

Neuron Structure and Classification

Neurons are the excitable cells responsible for transmitting electrical signals. They are typically long-lived, amitotic (do not divide in their mature form), and have a very high metabolic rate, requiring a continuous supply of oxygen and glucose to function.

Neuroglial cells of the CNS and myelin sheath formation 1
Figure 02. Neuroglial cells of the CNS and myelin sheath formation 1. Original image

1. Structural Components of a Typical Neuron

Cell Body (Soma)

The neuron's main nutritional and metabolic center. It contains the nucleus, most organelles, and prominent Nissl bodies (rough ER), reflecting its high rate of protein synthesis.

Dendrites

Numerous, short, highly branched processes that act as the main receptive regions. They receive incoming signals from other neurons and convey them towards the cell body.

Axon

A single, long process that acts as the conducting region, generating and transmitting nerve impulses (action potentials) away from the cell body. It terminates in branches called axon terminals.

The Myelin Sheath

Many axons are covered by a fatty, insulating layer called the myelin sheath, which is formed by glial cells (Schwann cells in the PNS and oligodendrocytes in the CNS). This sheath dramatically speeds up nerve impulse transmission. The gaps between the myelin segments are called Nodes of Ranvier, where the action potential "jumps" from node to node (saltatory conduction).

2. Functional Classification of Neurons

Sensory (Afferent)

Transmit impulses from sensory receptors towards the CNS.

Motor (Efferent)

Transmit impulses from the CNS to effector organs (muscles/glands).

Interneurons

Lie between sensory and motor neurons within the CNS to integrate information. Most neurons are interneurons.

Nervous tissue teaching diagram
Figure 03. Nervous tissue teaching diagram. Original image

3. Structural Classification of Neurons

Multipolar

Three or more processes (one axon, many dendrites). The most common type in the CNS.

Bipolar

Two processes (one axon, one dendrite). Rare; found in special sense organs like the retina.

Unipolar

A single, short process that divides T-like. Found in most sensory neurons in the PNS.

Neuroglia: Supporting Cells of Nervous Tissue

Neuroglia are non-excitable cells that surround, support, insulate, and protect neurons. They are far more numerous than neurons and can divide throughout life. There are six types of neuroglia: four in the CNS and two in the PNS.

Neuroglia Glial Cells 1
Figure 04. Neuroglia Glial Cells 1. Original image

A. Neuroglia of the Central Nervous System (CNS)

  • Astrocytes: Help control the chemical surroundings of neurons and support the blood–brain barrier.
  • Oligodendrocytes: Form myelin around axons in the CNS. One cell can myelinate several axon segments.
  • Microglia: Provide immune defence by removing damaged material and other debris.
  • Ependymal cells: Line the brain ventricles and spinal cord central canal; specialised cells contribute to cerebrospinal fluid production.

B. Neuroglia of the Peripheral Nervous System (PNS)

  • Schwann cells: Support peripheral axons. Each myelinating Schwann cell wraps one segment of one axon.
  • Satellite cells: Surround neuron cell bodies in peripheral ganglia and help maintain their local chemical environment.

For the cell types and their functions, see OpenStax: Nervous Tissue.

Nerve Impulse (Action Potential) Generation and Transmission

The ability of neurons to communicate relies on their ability to generate and transmit electrical signals, a process that involves several key stages.

1. Resting Membrane Potential

A neuron at rest has a voltage difference across its membrane of about -70mV. This is maintained by the sodium-potassium pump and ion leak channels.

2. Graded Potentials

Short-lived, localized changes in membrane potential. If a graded potential is strong enough to reach the threshold potential (~ -55mV) at the axon hillock, it triggers an action potential.

3. Action Potential (Nerve Impulse)

A brief, rapid, all-or-none electrical impulse that travels down the axon. It involves a depolarization phase (Na⁺ rushes in) followed by a repolarization phase (K⁺ rushes out).

4. Synapses

The junction where information is transferred. An arriving action potential causes the release of chemical messengers called neurotransmitters across a tiny gap (the synaptic cleft), which then bind to the next cell.

Lesson map

Further reading

For deeper study, compare this lesson with current guidance and standard references from recognised medical and public-health organisations.

Test Your Knowledge

Check your understanding of the concepts covered in this post.

1. Which of the following is the primary function of nervous tissue?

  • Support and protect organs
  • Contract and generate force
  • Transmit electrical signals and process information
  • Form linings and glandular secretions
Rationale: This is the overarching function of nervous tissue, allowing for rapid communication and complex thought processes.

2. The two main types of cells found in nervous tissue are:

  • Epithelial cells and glial cells
  • Neurons and muscle cells
  • Neurons and neuroglia
  • Fibroblasts and oligodendrocytes
Rationale: Neurons are the excitable, signaling cells, while neuroglia (glial cells) are the supporting cells.

3. Which part of a neuron typically receives incoming signals from other neurons?

  • Axon
  • Axon terminal
  • Dendrite
  • Myelin sheath
Rationale: Dendrites are the highly branched receptive regions that receive neurotransmitter signals from other neurons.

4. The ability of a neuron to respond to a stimulus by changing its membrane potential is called:

  • Conductivity
  • Contractility
  • Excitability
  • Secretion
Rationale: Excitability is the fundamental property of neurons (and muscle cells) to respond to stimuli by generating an electrical impulse.

5. Which glial cell type forms the myelin sheath in the Central Nervous System (CNS)?

  • Schwann cells
  • Astrocytes
  • Microglial cells
  • Oligodendrocytes
Rationale: Oligodendrocytes are the glial cells responsible for myelinating axons in the CNS.

6. Which type of neuron transmits impulses from sensory receptors towards the CNS?

  • Motor (efferent) neuron
  • Sensory (afferent) neuron
  • Interneuron
  • Neuroglia
Rationale: Afferent neurons carry sensory information towards the CNS.

7. Gaps in the myelin sheath along an axon are called:

  • Synaptic clefts
  • Nodes of Ranvier
  • Axon hillocks
  • Dendritic spines
Rationale: These unmyelinated gaps allow for saltatory conduction, where the action potential "jumps," speeding up transmission.

8. Which glial cells are phagocytic and act as the "immune cells" of the CNS?

  • Ependymal cells
  • Satellite cells
  • Microglial cells
  • Astrocytes
Rationale: Microglial cells are the resident immune cells of the CNS, functioning as phagocytes to remove debris and pathogens.

9. The region where an action potential is typically generated in a neuron is the:

  • Dendrite
  • Cell body
  • Axon hillock
  • Axon terminal
Rationale: The axon hillock is the "trigger zone" where graded potentials summate, and if threshold is reached, an action potential is initiated.

10. Which statement best describes the primary role of the axon?

  • To integrate incoming signals.
  • To produce neurotransmitters.
  • To transmit nerve impulses away from the cell body.
  • To form the blood-brain barrier.
Rationale: The axon's primary role is to conduct the action potential from the cell body to the axon terminals, propagating the signal.

11. The long, single process of a neuron that transmits signals away from the cell body is called the _____________.

Rationale: This describes the core function and structure of the axon.

12. The fatty, insulating layer that speeds up nerve impulse transmission is the _____________.

Rationale: The myelin sheath is critical for efficient and rapid signal propagation in many neurons.

13. In the Peripheral Nervous System (PNS), the glial cells that form myelin sheaths are called _____________.

Rationale: This distinguishes the myelin-forming cells in the PNS from those in the CNS (oligodendrocytes).

14. The specialized junctions where neurons communicate with other cells by releasing neurotransmitters are called _____________.

Rationale: Synapses are the crucial points of information transfer between neurons or between neurons and effector cells.

15. Star-shaped glial cells that help form the blood-brain barrier and regulate the chemical environment in the CNS are called _____________.

Rationale: Astrocytes are the most abundant and versatile glial cells in the CNS, with numerous vital supportive roles.

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