Nervous Tissue: The Body's Communication Network

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.

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.

The Neuron (Nerve Cell): The Signaling Unit

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

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.

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 (Glial Cells): The Supporting Cast

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.

A. Neuroglia of the Central Nervous System (CNS)

Astrocytes (Star Cells)

Most abundant and versatile. Anchor neurons to blood vessels (form the blood-brain barrier) and regulate the chemical environment.

Microglial Cells

The resident macrophages of the CNS. They monitor neuron health and phagocytize microorganisms and debris.

Ependymal Cells

Line the central cavities of the brain and spinal cord. Their cilia help circulate cerebrospinal fluid (CSF).

Oligodendrocytes

Form the myelin sheaths around axons in the CNS. One oligodendrocyte can myelinate several axons.

B. Neuroglia of the Peripheral Nervous System (PNS)

Satellite Cells

Surround neuron cell bodies in PNS ganglia, providing support and regulating the chemical environment.

Schwann Cells

Form the myelin sheaths around thicker axons in the PNS. One Schwann cell myelinates one segment of one axon. Crucial for regeneration.

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.

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.