Fundamentals of the Nervous System and Nervous Tissue (Chapter 11) - 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: 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 produce a response.

• 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 control.

  • Peripheral Nervous System (PNS): Composed of nerves and ganglia outside the CNS; connects the CNS to the rest of the body.

• Functional Divisions:

  • Sensory (Afferent) Division: Transmits sensory information to the CNS.

  • Motor (Efferent) Division: Carries commands from the CNS to effectors.

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

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

      • Sympathetic Division: Mobilizes body systems during activity.

      • Parasympathetic Division: Conserves energy and promotes housekeeping functions.

Structural Components of a Neuron and Their Functional Roles

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

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

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

• Axon: Long, slender projection; transmits electrical impulses away from the cell body.

• Axon Hillock: Region where the axon originates; site of action potential initiation.

• Axon Terminals: Endings that release neurotransmitters to communicate with other cells.

• Myelin Sheath: Insulating layer around axons; increases speed of impulse transmission.

• Example: Motor neurons have long axons to reach muscles.

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 special senses).

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

• Functional Classification:

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

  • Motor (Efferent) Neurons: Carry commands 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.

• Central Nervous System (CNS):

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

  • Microglia: Immune defense; phagocytize debris and pathogens.

  • Ependymal Cells: Line ventricles; produce and circulate cerebrospinal fluid.

  • Oligodendrocytes: Form myelin sheath in CNS.

• Peripheral Nervous System (PNS):

  • Satellite Cells: Surround neuron cell bodies; regulate environment.

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

Structure and Function of the Myelin Sheath

The myelin sheath is a fatty layer that insulates axons and speeds up electrical transmission.

• CNS: Myelin formed by oligodendrocytes; one cell can myelinate multiple axons.

• PNS: Myelin formed by Schwann cells; each cell myelinates a single axon segment.

• Function: Increases conduction velocity, prevents signal loss, and aids in repair.

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

Nucleus vs. Ganglion; Nerve vs. Tract

Terminology distinguishes structures in the 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 the neuronal 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 (Ligand-Gated): Open/close in response to binding of a chemical (e.g., neurotransmitter).

  • Mechanically-Gated: Open/close 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.

• Definition: The inside of the neuron is negatively charged relative to the outside, typically about -70 mV.

• Electrochemical Basis: Maintained by differences in ion concentrations and selective permeability of the membrane.

• Sodium-Potassium Pump: Actively transports 3 Na+ out and 2 K+ in, maintaining gradients.

• Equation: $V_m = \frac{RT}{F} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$ 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 (EPSPs and IPSPs), receptor potentials in sensory neurons.

• 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 and cell body.

• Action Potentials: All-or-none, non-decremental, occur in axons, propagate long distances.

• Table:

Property	Graded Potential	Action Potential
Amplitude	Variable	Fixed (all-or-none)
Propagation	Decreases with distance	Self-propagating
Location	Dendrites, cell body	Axon
Threshold	No threshold	Requires threshold
Summation	Possible	Not possible

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; voltage-gated Na+ channels open.

• Propagation: Depolarization spreads, opening adjacent channels; signal moves down the axon.

• Equation: $I = g(V_m - E_{ion})$ Additional info: This is the ionic current equation, where $g$ is conductance.

• 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 Na+ channels reset, K+ channels still open.

• Function: Prevents overlap of signals and ensures discrete impulses.

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: Saltatory conduction is faster and more energy-efficient.

• Example: Myelinated motor neurons use saltatory conduction; pain fibers (unmyelinated) use continuous conduction.