Fundamentals of the Nervous System and Nervous Tissue

Basic Functions of the Nervous System

The nervous system is responsible for controlling and integrating all body activities. It is essential for communication within the body and for responding to internal and external stimuli.

• Sensory Input: Gathering information from sensory receptors about internal and external changes.

• Integration: Processing and interpreting sensory input to determine an appropriate response.

• Motor Output: Activating effector organs (muscles and glands) to cause a response.

Structural and Functional Divisions of the Nervous System

The nervous system is divided both structurally and functionally to efficiently manage its complex roles.

• Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for integration and command.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS; connects the CNS to limbs and organs.

• Functional Divisions:

  • Somatic Nervous System (SNS): Controls voluntary movements of skeletal muscles.

  • Autonomic Nervous System (ANS): Regulates involuntary functions (e.g., heart rate, digestion); subdivided into sympathetic and parasympathetic divisions.

Structural Components of a Neuron and Their Functional Roles

Neurons are the basic functional units of the nervous system, specialized for rapid communication.

• Cell Body (Soma): Contains the nucleus and organelles; integrates incoming signals.

• Dendrites: Short, branched extensions that receive signals from other neurons.

• Axon: Long projection that transmits electrical impulses away from the cell body.

• Axon Terminals: Release neurotransmitters to communicate with other cells.

Classification of Neurons by Structure and Function

Neurons can be classified based on their structure and function.

• Structural Classification:

  • Multipolar: Many dendrites, one axon (most common in CNS).

  • Bipolar: One dendrite, one axon (found in sensory organs).

  • Unipolar: Single process that splits into two branches (sensory neurons in PNS).

• Functional Classification:

  • Sensory (Afferent) Neurons: Transmit impulses toward the CNS.

  • Motor (Efferent) Neurons: Carry impulses away from the CNS to effectors.

  • Interneurons: Connect sensory and motor neurons within the CNS.

Functions of Neuroglia

Neuroglia (glial cells) support and protect neurons in both the CNS and PNS.

• Astrocytes: Maintain the blood-brain barrier, provide structural support, regulate ion balance.

• Microglia: Act as immune cells, removing debris and pathogens.

• Ependymal Cells: Line ventricles of the brain and spinal cord, produce cerebrospinal fluid.

• Oligodendrocytes (CNS): Form myelin sheaths around CNS axons.

• Schwann Cells (PNS): Form myelin sheaths around PNS axons.

• Satellite Cells (PNS): Surround neuron cell bodies in ganglia, regulate environment.

Structure and Function of the Myelin Sheath

The myelin sheath is a fatty layer that insulates axons, increasing the speed of nerve impulse conduction.

• In the CNS: Formed by oligodendrocytes; one cell can myelinate multiple axons.

• In the PNS: Formed by Schwann cells; each cell myelinates a single axon segment.

• Function: Increases conduction velocity and maintains signal integrity.

Nucleus vs. Ganglion; Nerve vs. Tract

These terms distinguish between structures in the CNS and PNS.

• Nucleus: Cluster of neuron cell bodies in the CNS.

• Ganglion: Cluster of neuron cell bodies in the PNS.

• Nerve: Bundle of axons in the PNS.

• Tract: Bundle of axons in the CNS.

Types of Membrane Ion Channels

Ion channels are proteins that allow specific ions to cross the neuronal membrane, crucial for generating electrical signals.

• Leak Channels: Always open; maintain resting membrane potential.

• Gated Channels: Open or close in response to stimuli.

  • Voltage-Gated: Respond to changes in membrane potential.

  • Chemically-Gated (Ligand-Gated): Open in response to binding of a chemical messenger.

  • Mechanically-Gated: Open in response to physical deformation of the membrane.

Resting Membrane Potential and Its Electrochemical Basis

The resting membrane potential is the voltage difference across the membrane of a resting neuron, typically around -70 mV.

• Established by: Unequal distribution of Na+ and K+ ions, maintained by the sodium-potassium pump.

• Electrochemical Gradient: Combination of concentration and electrical gradients drives ion movement.

Equation:

$ V_m = \frac{RT}{F} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right) $

Additional info: This is the Nernst equation for potassium, a major determinant of resting potential.

Graded Potentials

Graded potentials are short-lived, localized changes in membrane potential that can be depolarizing or hyperpolarizing.

• Occur in: Dendrites and cell bodies.

• Examples: Receptor potentials, postsynaptic potentials.

• Characteristics: Magnitude varies with stimulus strength; decay with distance.

Comparison: Graded Potentials vs. Action Potentials

Graded and action potentials are two types of electrical signals in neurons.

Generation and Propagation of Action Potentials

Action potentials are rapid, large changes in membrane potential that travel along axons.

• Generation: Triggered when depolarization reaches threshold, opening voltage-gated Na+ channels.

• Phases: Depolarization, repolarization, hyperpolarization.

• Propagation: Sequential opening of voltage-gated channels along the axon.

Absolute and Relative Refractory Periods

Refractory periods ensure unidirectional propagation of action potentials and limit their frequency.

• Absolute Refractory Period: No new action potential can be initiated, regardless of stimulus strength.

• Relative Refractory Period: A stronger-than-normal stimulus can initiate another action potential.

Saltatory vs. Continuous Conduction

Saltatory conduction occurs in myelinated axons, while continuous conduction occurs in unmyelinated axons.

• Saltatory Conduction: Action potentials jump from one node of Ranvier to the next, increasing speed.

• Continuous Conduction: Action potentials propagate along every part of the axon membrane; slower.

Example: Myelinated motor neurons use saltatory conduction for rapid signal transmission.