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Nervous Tissue: Structure, Function, and Synaptic Transmission

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Chapter 12: Nervous Tissue

Introduction to Nervous Tissue

The nervous system is a complex network responsible for coordinating the body's activities by transmitting signals to and from different parts. It is divided into the central and peripheral nervous systems, each with specialized structures and functions.

  • Neural tissue consists of two main cell types: neurons (excitable cells that transmit signals) and neuroglia (supporting cells).

  • The nervous system is organized into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

Organization of the Nervous System

Central Nervous System (CNS)

  • Composed of the brain and spinal cord.

  • Responsible for processing and integrating information, and issuing commands.

Peripheral Nervous System (PNS)

  • Consists of all neural tissue outside the CNS.

  • Divided into afferent (sensory) and efferent (motor) divisions.

  • The efferent division includes the Somatic Nervous System (SNS) (controls skeletal muscles) and the Autonomic Nervous System (ANS) (regulates glands, smooth and cardiac muscle).

  • The ANS is further divided into Sympathetic and Parasympathetic divisions.

Neurons and Neuroglia

Neurons

Neurons are the functional units of the nervous system, specialized for the conduction of electrical impulses.

  • Cell body (soma): Contains the nucleus and organelles.

  • Dendrites: Receive incoming signals.

  • Axon: Conducts impulses away from the cell body.

  • Synapse: Junction between neurons or between a neuron and an effector cell.

Neuroglia

Neuroglia are supporting cells that protect, nourish, and insulate neurons.

  • Astrocytes, oligodendrocytes, microglia, and ependymal cells are found in the CNS.

  • Schwann cells and satellite cells are found in the PNS.

Generation of an Action Potential

Steps in Action Potential Generation

An action potential is a rapid change in membrane potential that travels along the axon.

  1. Depolarization to threshold: Stimulus causes the membrane potential to become less negative.

  2. Activation of sodium channels: Na+ influx causes rapid depolarization.

  3. Inactivation of sodium channels and activation of potassium channels: K+ efflux repolarizes the membrane.

  4. Return to resting potential: Ion gradients are restored by the sodium-potassium pump.

Absolute refractory period: No new action potential can be generated. Relative refractory period: A stronger stimulus is required to initiate another action potential.

Graphical Representation

The action potential is depicted as a sharp rise (depolarization), followed by a fall (repolarization), and a brief undershoot (hyperpolarization).

Synaptic Transmission

Cholinergic Synapse (Acetylcholine as Neurotransmitter)

A cholinergic synapse uses acetylcholine (ACh) to transmit signals between neurons or from neurons to muscles.

  1. Arrival of action potential: Depolarizes the synaptic terminal.

  2. Calcium influx: Voltage-gated Ca2+ channels open, Ca2+ enters the terminal.

  3. Release of ACh: Synaptic vesicles fuse with the membrane, releasing ACh into the synaptic cleft.

  4. Binding to receptors: ACh binds to postsynaptic receptors, opening ion channels and generating a postsynaptic potential.

  5. Termination: ACh is broken down by acetylcholinesterase, ending the signal.

Neurotransmitter Mechanisms of Action

  • Direct effect: Neurotransmitter binds to and opens/closes ion channels directly (e.g., ACh at cholinergic synapses).

  • Indirect effect: Neurotransmitter binds to a receptor that activates a G protein, which then activates a second messenger (e.g., cAMP, cGMP, IP3, Ca2+), leading to slower but longer-lasting effects (e.g., norepinephrine at adrenergic synapses).

Table: Comparison of CNS and PNS Neuroglia

Neuroglia

Location

Function

Astrocytes

CNS

Blood-brain barrier, support, repair

Oligodendrocytes

CNS

Myelination of CNS axons

Microglia

CNS

Phagocytosis, immune defense

Ependymal cells

CNS

Produce and circulate cerebrospinal fluid

Schwann cells

PNS

Myelination of PNS axons

Satellite cells

PNS

Support neuron cell bodies in ganglia

Key Equations

  • Nernst Equation (for equilibrium potential):

  • Ohm's Law (for membrane current):

Summary Table: Steps of Synaptic Transmission at a Cholinergic Synapse

Step

Description

1

Action potential arrives at synaptic terminal

2

Voltage-gated Ca2+ channels open

3

Synaptic vesicles release ACh into synaptic cleft

4

ACh binds to postsynaptic receptors

5

Postsynaptic cell depolarizes

6

ACh is broken down by acetylcholinesterase

Additional info:

  • Neurotransmitters can be excitatory or inhibitory, depending on the type of receptor and ion channel they affect.

  • Myelination increases the speed of action potential propagation via saltatory conduction.

  • Disorders of synaptic transmission or myelination can lead to neurological diseases (e.g., multiple sclerosis).

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