Chapter 11: Fundamentals of the Nervous System and Nervous Tissue – Study Guide

Fundamentals of the Nervous System and Nervous Tissue

Basic Functions of the Nervous System

The nervous system is responsible for controlling and coordinating the activities of the body. It enables rapid communication between different parts of the body and responds to internal and external stimuli.

• Sensory Input: Detects changes (stimuli) inside and outside the body via sensory receptors.

• Integration: Processes and interprets sensory input, deciding what action is needed.

• Motor Output: Responds by activating effector organs (muscles or glands).

• Example: Touching a hot object triggers sensory input, integration in the brain, and motor output to withdraw the hand.

Structural and Functional Divisions of the Nervous System

The nervous system is divided into structural and functional components for organization and specialization.

• Structural Divisions:

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

  • Peripheral Nervous System (PNS): Includes all nerves outside the CNS; connects CNS to the rest of the body.

• Functional Divisions:

  • Sensory (Afferent) Division: Carries information from sensory receptors to the CNS.

  • Motor (Efferent) Division: Transmits commands from CNS to effector organs.

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

  • Autonomic Nervous System: Regulates involuntary functions (smooth muscle, cardiac muscle, glands).

Structural Components of a Neuron and Their Functional Roles

Neurons are specialized cells for transmitting electrical signals. Each part has a distinct function.

• Cell Body (Soma): Contains nucleus and organelles; metabolic center.

• Dendrites: Receive incoming signals and convey them toward the cell body.

• Axon: Conducts electrical impulses away from the cell body.

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

• Example: Motor neuron axon transmits impulses to muscle fibers.

Classification of Neurons by Structure and Function

Neurons are classified based on their shape and their role in the nervous system.

• Structural Classification:

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

  • Bipolar: One dendrite, one axon (found in retina, olfactory mucosa).

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

• Functional Classification:

  • Sensory (Afferent) Neurons: Carry impulses toward CNS.

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

  • Interneurons: Connect sensory and motor neurons; found in CNS.

Types and Functions of Neuroglia

Neuroglia (glial cells) support and protect neurons. They are essential for nervous system function.

• Astrocytes: Support neurons, regulate environment, form blood-brain barrier.

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

• Ependymal Cells: Line CNS cavities, produce and circulate cerebrospinal fluid.

• Oligodendrocytes: Form myelin sheaths in CNS.

• Schwann Cells: Form myelin sheaths in PNS.

• Satellite Cells: Support neurons in PNS ganglia.

Structure and Function of the Myelin Sheath

The myelin sheath is a fatty layer that insulates axons, increasing the speed of electrical transmission.

• CNS: Myelin formed by oligodendrocytes.

• PNS: Myelin formed by Schwann cells.

• Function: Increases conduction velocity, prevents signal loss.

• Example: Multiple sclerosis is caused by myelin loss in CNS.

Nucleus vs. Ganglion; Nerve vs. Tract

These terms describe collections of nerve cell bodies and fibers in the nervous system.

• Nucleus: Cluster of neuron cell bodies in CNS.

• Ganglion: Cluster of neuron cell bodies in PNS.

• Nerve: Bundle of axons in PNS.

• Tract: Bundle of axons in CNS.

Types of Membrane Ion Channels

Ion channels are proteins that allow ions to cross the cell membrane, crucial for electrical signaling.

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

• Gated Channels: Open in response to specific stimuli.

  • Voltage-gated: Open/close in response to changes in membrane potential.

  • Chemically-gated: Open/close in response to binding of a chemical (e.g., neurotransmitter).

  • Mechanically-gated: Open/close in response to physical deformation.

Resting Membrane Potential and Its Electrochemical Basis

The resting membrane potential is the voltage difference across the cell membrane when the neuron is not transmitting signals.

• Typical Value: About -70 mV in neurons.

• Basis: Due to differences in ion concentrations (Na+, K+) and selective permeability of the membrane.

• Maintained by: Sodium-potassium pump ($\text{3 Na}^+ \text{out}, 2 K^+ \text{in}$) and leak channels.

• Equation: $V_m = \frac{RT}{F} \ln \left( \frac{[K^+]_{\text{out}}}{[K^+]_{\text{in}}} \right)$ Additional info: This is the simplified Nernst equation for potassium.

Graded Potentials and Examples

Graded potentials are local changes in membrane potential that vary in size and decrease with distance.

• Characteristics: Short-lived, localized, can be depolarizing or hyperpolarizing.

• Examples: Postsynaptic potentials, receptor potentials.

• Function: Initiate action potentials if threshold is reached.

Comparison of Graded Potentials and Action Potentials

Both are electrical signals, but differ in properties and functions.

Property	Graded Potential	Action Potential
Amplitude	Variable	All-or-none
Propagation	Decreases with distance	Self-propagating, does not decrease
Location	Dendrites, cell body	Axon
Function	Initiate action potentials	Transmit signals long distances

Generation and Propagation of Action Potentials

Action potentials are rapid, all-or-none electrical signals that travel along axons.

• Generation: Triggered when membrane potential reaches threshold.

• Phases: Depolarization (Na+ influx), repolarization (K+ efflux), hyperpolarization.

• Propagation: Action potential moves along axon by opening voltage-gated channels.

• Equation: $I = g(V_m - E)$ Additional info: This is the general formula for ionic current.

Absolute and Relative Refractory Periods

These periods ensure unidirectional propagation and limit firing rate of neurons.

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

• Relative Refractory Period: Action potential can be generated, but requires stronger stimulus.

Saltatory vs. Continuous Conduction

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

• Saltatory Conduction: Action potential jumps from node to node (Nodes of Ranvier), increasing speed.

• Continuous Conduction: Action potential travels along entire axon membrane, slower.

• Example: Myelinated axons in PNS use saltatory conduction; unmyelinated axons use continuous conduction.