Neural Physiology: Organization, Cell Function, and Neural Signaling

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Neural Physiology: Organization & Cell Function

Overview of the Nervous System

The nervous system is a complex network responsible for communication, integration, and regulation of bodily functions. It is divided into the central nervous system (CNS) and peripheral nervous system (PNS), each with specialized roles in processing and transmitting information.

Central Nervous System (CNS): Consists of the brain and spinal cord. It integrates sensory information and coordinates motor output.
Peripheral Nervous System (PNS): Composed of neurons outside the CNS. It transmits sensory input to the CNS and carries motor commands to effectors.

Sensory (Afferent) vs. Motor (Efferent) Neurons

Sensory (Afferent) Neurons: Carry information from sensory receptors toward the CNS.
Motor (Efferent) Neurons: Transmit commands from the CNS to muscles and glands.

Cell-to-Cell Communication

Cells communicate via local and long-distance mechanisms, using both chemical and electrical signals.

Local Communication:
- Gap junctions (direct electrical/chemical connection)
- Contact-dependent signals (membrane-bound molecules)
- Autocrine and paracrine signaling (diffusion through extracellular fluid)
Long-Distance Communication:
- Endocrine system (hormones via blood, chemical signals)
- Nervous system (electrical signals—action potentials—and chemical signals—neurotransmitters)

Neurons: Structure and Function

Neurons are the functional units of the nervous system, specialized for rapid communication.

Dendrites: Receive input from other neurons.
Cell Body (Soma): Contains the nucleus and organelles; site of most protein synthesis.
Axon: Conducts electrical impulses away from the cell body toward axon terminals.
Axon Hillock: The trigger zone for action potential initiation.
Axon Terminals: Release neurotransmitters to communicate with target cells.
Synapse: The junction where an axon terminal communicates with another cell (neuron, muscle, or gland).

Synaptic Transmission

Pre-synaptic Cell: Neuron sending the signal.
Post-synaptic Cell: Cell receiving the signal.
Synaptic Cleft: Small gap between pre- and post-synaptic cells where neurotransmitters diffuse.

Homeostasis and Neural Integration

The nervous system maintains homeostasis by detecting changes (stimuli), integrating information, and initiating responses.

Example: In response to cold, thermoreceptors signal the brain, which activates skeletal muscles to induce shivering and restore body temperature.

Axonal Transport

Axonal transport is essential for moving proteins, vesicles, and organelles between the cell body and axon terminals.

Problem: Proteins are synthesized in the cell body but needed at axon terminals.
Solution: Axonal transport moves materials along microtubules.
Fast Axonal Transport: Up to 400 mm/day; includes anterograde (cell body to terminal) and retrograde (terminal to cell body) directions.

Glial Cells: Supportive Cells of the Nervous System

Glial cells provide structural and functional support for neurons. They are classified based on their location:

Glial Cell Type	Location	Main Function(s)
Ependymal cells	CNS	Produce cerebrospinal fluid
Astrocytes	CNS	Blood-brain barrier, nutrient support, ion balance
Microglia	CNS	Immune defense (scavengers)
Oligodendrocytes	CNS	Form myelin sheaths
Schwann cells	PNS	Form myelin sheaths
Satellite cells	PNS	Support cell bodies

Myelin Sheath: Insulating layer formed by oligodendrocytes (CNS) or Schwann cells (PNS) that increases the speed of nerve impulse conduction.

Neural Physiology: Graded Potentials

Membrane Potentials and Excitability

Neurons are excitable cells capable of generating graded and action potentials. The resting membrane potential (RMP) is typically around -70 mV, determined by ion concentrations and membrane permeability to Na+ and K+.

Depolarization: Membrane potential becomes less negative (more positive) than RMP.
Hyperpolarization: Membrane potential becomes more negative than RMP.
Repolarization: Return to RMP after depolarization or hyperpolarization.

Generation of Graded Potentials

Graded potentials are small, localized changes in membrane potential caused by the opening or closing of ion channels in response to stimuli.

Excitatory Postsynaptic Potential (EPSP): Caused by Na+ or Ca2+ influx, leading to depolarization.
Inhibitory Postsynaptic Potential (IPSP): Caused by K+ efflux or Cl- influx, leading to hyperpolarization.

Key Mechanisms

Opening of Na+ or Ca2+ channels → cations enter cell → depolarization (EPSP)
Opening of K+ channels → K+ exits cell → hyperpolarization (IPSP)
Opening of Cl- channels → Cl- enters cell → hyperpolarization (IPSP)

Properties of Graded Potentials

Amplitude reflects stimulus strength (not all-or-none).
Travel short distances; decrease in strength due to cytoplasmic resistance and current leak.

Factors Affecting Graded Potentials

Cytoplasmic Resistance: Internal resistance to ion flow within the cell.
Membrane Resistance: Resistance to ion leakage across the membrane.

Neural Physiology: Action Potentials

Action Potential Generation

Action potentials are large, rapid depolarizations that propagate along the axon without losing strength. They are triggered if graded potentials reach threshold at the axon hillock (trigger zone).

Voltage-Gated Na+ Channels: Open rapidly in response to depolarization, allowing Na+ influx (depolarization phase).
Voltage-Gated K+ Channels: Open more slowly, allowing K+ efflux (repolarization and hyperpolarization phases).

Phases of the Action Potential

Depolarization: Na+ channels open, Na+ enters cell.
Repolarization: Na+ channels inactivate, K+ channels open, K+ exits cell.
Hyperpolarization: K+ channels remain open briefly, membrane potential becomes more negative than RMP.

Refractory Periods

Absolute Refractory Period: No new action potential can be generated due to inactivated Na+ channels.
Relative Refractory Period: A stronger-than-normal stimulus is required to initiate another action potential (some Na+ channels have reset, but K+ channels are still open).

All-or-None Principle

Action potentials either occur fully or not at all; amplitude is always the same within a neuron.

Stimulus Strength Coding

Encoded by action potential frequency (number of APs per second, measured in Hertz).

Pharmacological Blockade

Local Anesthetics (e.g., Lidocaine): Block voltage-gated Na+ channels, preventing action potential generation and pain transmission.
Tetrodotoxin (TTX): Potent neurotoxin that blocks Na+ channels, leading to paralysis and potentially fatal respiratory failure.

Neural Physiology: Action Potential Propagation

Propagation Mechanisms

Action potentials propagate along axons by depolarizing adjacent membrane regions, ensuring one-way transmission due to refractory periods.

Continuous Conduction: Occurs in unmyelinated axons; each segment of the membrane must depolarize sequentially.
Saltatory Conduction: Occurs in myelinated axons; action potentials "jump" between nodes of Ranvier, increasing conduction speed.

Factors Affecting Conduction Velocity

Axon Diameter: Larger diameter = lower cytoplasmic resistance = faster conduction.
Myelination: Myelin increases membrane resistance and prevents ion leakage, greatly enhancing speed.

Demyelinating Diseases

Example: Multiple sclerosis results in loss of myelin, leading to impaired action potential conduction, muscle weakness, and neurological deficits.

Summary Table: Glial Cells of the CNS and PNS

Glial Cell	Location	Function
Ependymal cell	CNS	Produce and circulate cerebrospinal fluid
Astrocyte	CNS	Support neurons, maintain blood-brain barrier, regulate ions
Microglia	CNS	Immune defense, phagocytosis
Oligodendrocyte	CNS	Form myelin sheaths
Schwann cell	PNS	Form myelin sheaths
Satellite cell	PNS	Support neuron cell bodies

Additional info: This guide integrates foundational concepts from neural physiology, including cell structure, signaling, and support mechanisms, as relevant to introductory Anatomy & Physiology courses.