Neurophysiology Study Guide: Structure and Function of the Nervous System

Unit III – Neurophysiology

Lecture 1: Overview of the Nervous System

The nervous system is essential for communication, integration, and control within the body. This section introduces its main functions, organization, and cellular components.

• Main Functions of the Nervous System:

  • 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.

• Anatomical Organization:

  • Central Nervous System (CNS): Brain and spinal cord; integration and command center.

  • Peripheral Nervous System (PNS): Cranial and spinal nerves; communication lines between the CNS and the rest of the body.

• Physiological Organization:

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

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

  • Somatic Nervous System: Voluntary control of skeletal muscles.

  • Autonomic Nervous System: Involuntary control of smooth muscle, cardiac muscle, and glands.

• Cellular Components:

  • Neurons: Functional units of the nervous system; transmit electrical signals.

  • Neuroglia (Glial Cells): Support, protect, and insulate neurons.

• Neuron Structure: Cell body (soma), dendrites (receive signals), axon (transmits signals), axon hillock (action potential initiation), and synaptic terminals.

• Classification of Neurons: Based on structure (multipolar, bipolar, unipolar) and function (sensory, motor, interneurons).

Example: Sensory neurons detect changes in temperature, interneurons process this information, and motor neurons activate muscles to respond.

Lecture 2: Membrane Potentials and Action Potentials

Neurons communicate via electrical signals, which depend on the movement of ions across their membranes. This section covers the basis of these signals and their propagation.

• Depolarization and Hyperpolarization:

  • Depolarization: Membrane potential becomes less negative (e.g., from -70 mV to -55 mV).

  • Hyperpolarization: Membrane potential becomes more negative than the resting potential.

• Action Potential: A rapid, temporary change in membrane potential that travels along the axon.

• Phases of Action Potential:

  • Resting state

  • Depolarization (Na+ influx)

  • Repolarization (K+ efflux)

  • Hyperpolarization

• Refractory Periods:

  • Absolute Refractory Period: No new action potential can be initiated.

  • Relative Refractory Period: A stronger stimulus is required to initiate another action potential.

Equation:

$I = g(V_m - E_{ion})$

Where $I$ is the ionic current, $g$ is the conductance, $V_m$ is the membrane potential, and $E_{ion}$ is the equilibrium potential for the ion.

Example: During an action potential, voltage-gated Na+ channels open, causing depolarization.

Lecture 3: Conduction and Synaptic Transmission

This section explores how electrical signals are conducted along neurons and transmitted between cells.

• Insulators and Conductors:

  • Insulator: Material that resists the flow of electric current (e.g., myelin sheath).

  • Conductor: Material that allows electric current to flow (e.g., cytoplasm).

• Action Potential Propagation: Action potentials are regenerated at each segment of the axon, allowing the signal to travel long distances without decrement.

• Saltatory Conduction: In myelinated axons, action potentials jump from node to node, increasing conduction speed.

• Synaptic Transmission: Neurotransmitters are released from the presynaptic neuron and bind to receptors on the postsynaptic cell, causing a response.

• Types of Synapses: Electrical (direct ion flow) and chemical (neurotransmitter-mediated).

Table: Comparison of A, B, and C Nerve Fibers

Fiber Type	Diameter	Myelination	Conduction Speed
A fibers	Large	Myelinated	Fastest
B fibers	Medium	Lightly myelinated	Intermediate
C fibers	Small	Unmyelinated	Slowest

Additional info: A fibers are typically involved in motor and sensory functions requiring rapid response, while C fibers are associated with slow pain transmission.

Lecture 4: Action Potentials and Energy

Action potentials are the fundamental electrical signals of the nervous system. This section details their generation, propagation, and energy requirements.

• Action Potential: A self-propagating, all-or-none electrical event triggered by threshold depolarization.

• Threshold: The critical level of depolarization required to initiate an action potential.

• Energy Use: The Na+/K+ ATPase pump restores ion gradients, consuming ATP.

• Factors Affecting Conduction Velocity: Axon diameter, myelination, and temperature.

Equation:

$V = IR$

Where $V$ is voltage, $I$ is current, and $R$ is resistance (Ohm's Law).

Example: Myelinated axons conduct impulses more rapidly due to saltatory conduction.

Lecture 5: Synaptic Integration and Neurotransmitters

Neurons communicate at synapses, where neurotransmitters mediate signal transmission. This section covers synaptic types, integration, and neurotransmitter effects.

• Types of Synapses: Axodendritic, axosomatic, and axoaxonic.

• Neurotransmitters: Chemical messengers such as acetylcholine, norepinephrine, dopamine, and serotonin.

• Excitatory vs. Inhibitory Synapses: Excitatory synapses depolarize the postsynaptic membrane; inhibitory synapses hyperpolarize it.

• Summation: Temporal (rapid, repeated stimuli) and spatial (multiple simultaneous stimuli) summation integrate signals at the postsynaptic neuron.

Example: Acetylcholine at the neuromuscular junction causes muscle contraction.

Lecture 6: Neural Circuits and Processing

Neural circuits are organized patterns of connectivity that determine how information is processed in the nervous system.

• Types of Neural Circuits: Diverging, converging, reverberating, and parallel after-discharge circuits.

• Serial vs. Parallel Processing: Serial processing involves a single pathway; parallel processing involves multiple pathways for simultaneous information handling.

Example: Reflex arcs are examples of serial processing, while higher-order brain functions use parallel processing.