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 (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/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): Brain and spinal cord; integration and command center.

  • Peripheral Nervous System (PNS): Nerves and ganglia outside the CNS; communication lines.

• Functional Divisions:

  • Sensory (Afferent) Division: Transmits impulses from receptors to CNS.

  • Motor (Efferent) Division: Transmits impulses from CNS to effectors.

  • Somatic Nervous System: Voluntary control of skeletal muscles.

  • Autonomic Nervous System (ANS): Involuntary control of smooth muscle, cardiac muscle, and glands; includes sympathetic and parasympathetic divisions.

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 signals from other neurons; conduct impulses toward cell body.

• Axon: Conducts impulses away from cell body; may be myelinated for faster transmission.

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

• Example: Motor neuron with multiple dendrites, a single long axon, and terminal branches.

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): Transmit impulses toward CNS.

  • Motor (Efferent): Transmit 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 in the CNS and PNS.

• 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, increasing the speed of electrical transmission.

• CNS: Myelin formed by oligodendrocytes.

• PNS: Myelin formed by Schwann cells.

• Function: Prevents ion leakage, speeds up impulse conduction.

• Nodes of Ranvier: Gaps in myelin where action potentials are regenerated.

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.

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:

  • 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: Unequal distribution of ions (Na+, K+) and selective permeability of membrane.

• Na+/K+ Pump: Maintains gradient by pumping 3 Na+ out and 2 K+ in.

• Equation:

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

Additional info: This is the Nernst equation for potassium, which contributes most to resting potential.

Graded Potentials and Examples

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

• Characteristics: Can be depolarizing or hyperpolarizing; not all-or-none.

• Examples: Postsynaptic potentials (EPSPs and IPSPs), 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, decay with distance, occur in dendrites/cell body.

• Action Potentials: All-or-none, do not decay, occur in axons, propagate long distances.

• Table:

Property	Graded Potential	Action Potential
Amplitude	Variable	Fixed (all-or-none)
Decay	Yes	No
Location	Dendrites/cell body	Axon
Threshold	No threshold	Requires threshold
Propagation	Local	Long-distance

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 (~ -55 mV).

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

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

• Equation:

$I = g(V_m - E)$

Additional info: This is the general formula for ionic current, where g is conductance, Vm is membrane potential, and E is equilibrium potential.

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.

Saltatory vs. Continuous Conduction

Action potentials travel differently in myelinated and unmyelinated axons.

• Saltatory Conduction: In myelinated axons, action potentials jump from node to node (Nodes of Ranvier); much faster.

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

• Example: Myelinated motor neurons use saltatory conduction for rapid muscle control.