Nervous System and Nervous Tissue: Structure, Function, and Electrophysiology

Chapter 11: Introduction to the Nervous System and Nervous Tissue

Overview of the Nervous System

The nervous system is responsible for controlling perception, experience, and many aspects of homeostasis. It is divided into central and peripheral components, each with distinct roles.

• Nervous system: Directs voluntary movement, consciousness, personality, learning, and memory.

• Regulates homeostasis: Controls respiratory rate, blood pressure, body temperature, sleep-wake cycle, and blood pH.

Structural Divisions of the Nervous System

The nervous system is anatomically divided into:

• Central Nervous System (CNS): Includes the brain and spinal cord, protected by bones of the skull and vertebral column.

• Peripheral Nervous System (PNS): Bundles of axons, blood vessels, and connective tissue outside the protection of the skull and vertebral column.

• Cranial nerves: Originate from the brain.

• Spinal nerves: Originate from the spinal cord.

Functional Divisions of the Nervous System

The nervous system performs three main functional categories:

• Sensory functions: Gather information from sensory receptors.

• Integrative functions: Analyze and interpret sensory information, determine appropriate responses.

• Motor functions: Actions performed in response to integration; motor output is the response.

Sensory Divisions of the PNS

• Somatic sensory division: Neurons carry signals from sensory organs (vision, hearing, taste, smell, balance).

• Visceral sensory division: Neurons carry signals from viscera (organs).

Neurons and Neuroglia

Structure and Function of Neurons

Neurons are excitable cells responsible for sending and receiving signals in the form of action potentials. They consist of several key parts:

• Cell body (soma): Metabolically active region, manufactures proteins, houses organelles.

• Dendrites: Short, branched processes that receive input and transmit signals to the cell body.

• Axon: Single process that can generate and conduct action potentials; may have distinct regions (axon hillock, telodendria, axon terminals, synaptic bulbs, axolemma).

Functional Regions of Neurons

• Receptive region: Dendrites and cell body.

• Conducting region: Axon.

• Secretory region: Axon terminals.

Classification of Neurons by Structure

Type	Description	Example
Multipolar	Single axon, multiple dendrites	Most neurons in CNS
Bipolar	One axon, one dendrite	Retina, olfactory epithelium
Pseudounipolar	Single process splits into two branches	Sensory neurons in PNS

Classification of Neurons by Function

Type	Function
Sensory (afferent)	Carry information from sensory receptors to CNS
Interneurons	Association neurons; process and relay information within CNS
Motor (efferent)	Carry information from CNS to muscles/glands

Neuroglia (Glial Cells)

Neuroglia provide structural support and protection for neurons and help maintain their environment. Types include:

• Astrocytes

• Oligodendrocytes

• Microglia

• Ependymal cells

• Schwann cells (PNS)

• Satellite cells (PNS)

Myelin Sheath

The myelin sheath is composed of repeating layers of plasma membrane of glial cells. It insulates axons and increases the speed of action potential conduction.

• Myelination: Process by which glial cells wrap axons in myelin.

• Occurs in both CNS (oligodendrocytes) and PNS (Schwann cells).

Regeneration of Nervous Tissue

Regeneration is limited in the CNS and more possible in the PNS, provided the cell body remains intact.

Electrophysiology of Neurons

Introduction to Electrophysiology

Neurons respond to various stimuli (chemical, electrical, mechanical) by generating electrical changes across their membranes.

• Electrical changes: Rapid conduction along the membrane.

• Two forms: local potentials and action potentials.

Principles of Electrophysiology

• Resting membrane potential: The electrical charge difference across the plasma membrane when the cell is at rest.

Definition: The resting membrane potential is typically about -70 mV in neurons.

How is voltage established? By the difference in ion concentrations across the membrane, mainly sodium (Na+) and potassium (K+).

Membrane potential equation:

$V_m = V_{inside} - V_{outside}$

Generation of Resting Membrane Potential

• Diffusion of potassium ions out of the cell through leak channels.

• Sodium ions diffuse into the cell.

Changes in Membrane Potential: Ion Movements

• Depolarization: Membrane potential becomes more positive due to influx of positive ions.

• Repolarization: Membrane potential returns to resting value.

• Hyperpolarization: Membrane potential becomes more negative than resting value.

Local and Action Potentials

• Local potentials: Small changes in membrane potential, can trigger action potentials if threshold is reached.

• Action potentials: Large, rapid changes in membrane potential that propagate along the axon.

Action potential phases:

1. Depolarization

2. Repolarization

3. Hyperpolarization

Action potential equation:

$I = g(V - E)$

Where $I$ is the current, $g$ is the conductance, $V$ is the membrane potential, and $E$ is the equilibrium potential.

All-or-None Principle

• Action potentials occur only if threshold is reached; otherwise, no action potential is generated.

Propagation of Action Potentials

• Action potentials are self-propagating and travel in one direction along the axon.

• Continuous conduction: Occurs in unmyelinated axons.

• Saltatory conduction: Occurs in myelinated axons, action potential jumps between nodes of Ranvier.

Neuronal Synapses

Overview of Neuronal Synapses

Synapses are junctions where neurons communicate with other neurons or effector cells.

• Presynaptic neuron: Sends the signal.

• Postsynaptic neuron: Receives the signal.

• Synaptic cleft: Space between presynaptic and postsynaptic neurons.

Types of Synapses

• Chemical synapses: Use neurotransmitters to transmit signals.

• Electrical synapses: Cells are electrically coupled via gap junctions.

Synaptic Transmission

• Transfer of chemical or electrical signals between neurons at a synapse.

• Neurotransmitters bind to receptors on the postsynaptic membrane, causing a response.

Neural Integration: Summation of Stimuli

• Summation: The process by which multiple synaptic potentials combine to influence the postsynaptic neuron.

• Action potential is generated only if the sum of inputs reaches threshold.

Neurotransmitters

Neurotransmitter Receptors

• Ionotropic receptors: Ligand-gated ion channels, direct effect on ion flow.

• Metabotropic receptors: Linked to metabolic processes, often via G-proteins.

Major Neurotransmitters

Type	Example	Function
Acetylcholine (ACh)	Widely used in CNS and PNS	Excitatory and inhibitory effects
Biogenic amines	Dopamine, serotonin, norepinephrine	Regulate mood, attention, and arousal
Amino acid neurotransmitters	Glutamate, GABA, glycine	Excitatory and inhibitory effects
Neuropeptides	Substance P, opioids	Modulate pain and other functions

Functional Groups of Neurons

Neuronal Pools

Neuronal pools are groups of interneurons within the CNS that process information.

Neural Circuits

Neural circuits are patterns of synaptic connection between neuronal pools, allowing for complex processing and integration of information.

Additional info: Some explanations and tables have been expanded for clarity and completeness based on standard Anatomy & Physiology textbook content.