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 body parts and responds to internal and external stimuli.

• Sensory Input: Detects changes in the environment (internal and external) via sensory receptors.

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

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

• Example: Touching a hot object triggers sensory input, integration in the brain/spinal cord, 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 command.

  • Peripheral Nervous System (PNS): Composed of nerves and ganglia 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 effectors.

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

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

      • Sympathetic Division: "Fight or flight" responses.

      • Parasympathetic Division: "Rest and digest" responses.

Structural Components of a Neuron and Their Functional Roles

Neurons are specialized cells for transmitting electrical signals.

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

• Dendrites: Receive incoming signals from other neurons.

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

• Axon Hillock: Initiates action potentials.

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

• Example: Motor neuron cell body in spinal cord, axon extending to muscle.

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, e.g., motor neurons).

  • Bipolar: One dendrite, one axon (e.g., retina, olfactory epithelium).

  • Unipolar: Single process that splits into two branches (e.g., 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 within CNS.

Types and Functions of Neuroglia

Neuroglia (glial cells) support and protect neurons.

• Central Nervous System (CNS):

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

  • Microglia: Immune defense, phagocytize debris.

  • Ependymal Cells: Line ventricles, produce cerebrospinal fluid.

  • Oligodendrocytes: Form myelin sheath in CNS.

• Peripheral Nervous System (PNS):

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

  • Schwann Cells: Form myelin sheath in PNS, aid in regeneration.

Structure and Function of the Myelin Sheath

The myelin sheath is a protective, insulating layer around axons, increasing the speed of electrical transmission.

• CNS: Myelin formed by oligodendrocytes.

• PNS: Myelin formed by Schwann cells.

• Function: Increases conduction velocity, insulates axons, facilitates saltatory conduction.

• Example: Multiple sclerosis is a disease where myelin is damaged.

Nucleus vs. Ganglion; Nerve vs. Tract

Terminology distinguishes structures in CNS and PNS.

• 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.

• Example: Dorsal root ganglion (PNS), corticospinal tract (CNS).

Types of Membrane Ion Channels

Ion channels regulate the movement of ions across neuronal membranes, crucial for electrical signaling.

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

• Gated Channels:

  • Voltage-Gated: Open in response to changes in membrane potential.

  • Chemically-Gated (Ligand-Gated): Open in response to binding of a chemical (e.g., neurotransmitter).

  • Mechanically-Gated: Open in response to physical deformation (e.g., touch).

Resting Membrane Potential and Its Electrochemical Basis

The resting membrane potential is the voltage difference across the membrane of a resting neuron.

• Typical Value: About -70 mV (inside negative relative to outside).

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

• Maintained by: Sodium-potassium pump ($\text{Na}^+/\text{K}^+$ ATPase$) 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: Can be depolarizing or hyperpolarizing; not all-or-none.

• Examples: Postsynaptic potentials (EPSP, IPSP), receptor potentials.

• Function: Initiate action potentials if threshold is reached.

Comparison of Graded Potentials and Action Potentials

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

• Graded Potentials: Variable amplitude, decremental, occur in dendrites/cell body, can summate.

• Action Potentials: All-or-none, constant amplitude, propagate along axon, do not summate.

• Table:

Property	Graded Potential	Action Potential
Amplitude	Variable	Constant
Propagation	Decremental	Non-decremental
Location	Dendrites/cell body	Axon
Summation	Possible	Not possible
Threshold	No threshold	Requires threshold

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; involves opening of voltage-gated Na+ and K+ channels.

• Propagation: Depolarization spreads, opening channels along the axon.

• Equation: $\text{AP}: \text{Depolarization} \rightarrow \text{Repolarization} \rightarrow \text{Hyperpolarization}$

• Example: Nerve impulse transmission in motor neurons.

Absolute and Relative Refractory Periods

Refractory periods ensure unidirectional propagation and limit firing rate.

• Absolute Refractory Period: No new action potential can be generated; Na+ channels are inactivated.

• Relative Refractory Period: Action potential possible with stronger stimulus; some K+ channels still open.

• Function: Prevents overlap of action potentials, ensures discrete signaling.

Saltatory vs. Continuous Conduction

Action potentials travel differently in myelinated and unmyelinated axons.

• Saltatory Conduction: In myelinated axons, action potentials "jump" between nodes of Ranvier, increasing speed.

• Continuous Conduction: In unmyelinated axons, action potentials propagate smoothly along the entire membrane.

• Comparison Table:

Type	Speed	Location
Saltatory	Fast	Myelinated axons
Continuous	Slow	Unmyelinated axons

Example: Motor neurons use saltatory conduction for rapid muscle control.