Neurophysiology 1: The Nervous System, Neurons, and Cell-Cell Communication

Neurophysiology 1: The Nervous System, Neurons & Cell-Cell Communication

Introduction

This study guide covers the fundamental organization and function of the nervous system, the diversity of neuron structure, the generation of membrane potentials, action potentials, and synaptic transmission. These topics are essential for understanding how the nervous system controls muscle contractions, movement, and overall physiological responses, especially in the context of exercise science.

Organization of the Nervous System

Divisions of the Nervous System

The nervous system is divided into several major components, each with distinct roles in processing and transmitting information throughout the body.

• Central Nervous System (CNS): Consists of the brain and spinal cord. Responsible for integrating and processing sensory information and coordinating responses.

• Peripheral Nervous System (PNS): All neural tissue outside the CNS. Subdivided into:

  • Afferent Division: Carries sensory information from receptors to the CNS.

  • Efferent Division: Transmits motor commands from the CNS to effectors (muscles and glands).

  • Enteric Nervous System: Regulates digestive processes independently of the CNS.

Functional Organization

• Somatic (Motor) Division: Controls voluntary movements by innervating skeletal muscles.

• Autonomic Nervous System (ANS): Regulates involuntary functions, including:

  • Sympathetic Division: Prepares the body for 'fight or flight' responses.

  • Parasympathetic Division: Promotes 'rest and digest' activities.

Cell Types in the Nervous System

Neurons

Neurons are the primary signaling cells of the nervous system, specialized for rapid communication.

• Dendrites: Receive input signals from other neurons.

• Cell Body (Soma): Integrates incoming signals.

• Axon: Transmits output signals to other neurons or effectors.

• Nerves: Bundles of axons surrounded by connective tissue, facilitating long-distance communication.

Glial Cells

Glial cells provide support, protection, and insulation for neurons.

• Oligodendrocytes (CNS) and Schwann Cells (PNS): Produce myelin, a fatty layer that insulates axons and increases the speed of signal transmission.

• Loss of Myelin: Conditions such as multiple sclerosis result in impaired signal conduction.

Membrane Potentials

Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the neuron's membrane when it is not actively transmitting a signal.

• Typical value: -70 mV (inside relative to outside).

• Generated by:

  • Differences in ionic concentrations (high [K+] inside, high [Na+] and [Cl-] outside).

  • Selective membrane permeability (more permeable to K+ than Na+).

  • Maintained by the Na+/K+ ATPase pump.

Equation:

$V_{m} = E_{K} = \frac{RT}{F} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$ (Nernst equation for K+)

Types of Membrane Potentials

• Action Potentials: Rapid, all-or-none electrical signals generated within neurons.

• Graded/Local/Postsynaptic Potentials: Variable amplitude signals generated at synapses; can be excitatory or inhibitory.

Action Potentials

Generation and Propagation

Action potentials are initiated when the membrane potential at the axon hillock reaches a threshold, typically around -55 mV.

• Threshold: Minimum depolarization required to trigger an action potential.

• All-or-None Principle: Action potentials have a fixed magnitude and propagate without decrement.

• Propagation: Action potentials travel along axons to communicate with other neurons or effectors.

Equation:

$I = g(V - E)$ (Ohm's Law for ionic currents)

Synaptic Transmission

Mechanism

Synaptic transmission is the process by which one neuron communicates with another at a synapse.

• Neurotransmitter Release: Action potential arrival triggers release of neurotransmitters into the synaptic cleft.

• Receptor Activation: Neurotransmitters bind to receptors on the postsynaptic neuron, generating graded potentials.

• Types of Responses:

  • Excitatory Postsynaptic Potentials (EPSPs): Depolarize the postsynaptic membrane, increasing likelihood of action potential generation.

  • Inhibitory Postsynaptic Potentials (IPSPs): Hyperpolarize the membrane, decreasing likelihood of action potential.

• Summation: Multiple graded potentials can combine to reach threshold and trigger an action potential.

Types of Synaptic Responses

• Fast Synaptic Responses: Mediated by ligand-gated ion channels (e.g., glutamate for excitation, GABA for inhibition).

• Slow Synaptic Responses: Mediated by G protein-coupled receptors (e.g., neuromodulators like noradrenaline, serotonin, acetylcholine).

Essential Terminology

• Nervous system

• Central nervous system

• Peripheral nervous system

• Somatic nervous system

• Autonomic nervous system

• Neuron

• Glial cell

• Myelin

• Membrane potential

• Depolarisation

• Hyperpolarisation

• Threshold

• Action potential

• Graded potential

• Voltage gated ion channel

• Ligand gated ion channel

• G protein coupled receptor

• Neurotransmitter

Summary Table: Divisions of the Nervous System

Division	Main Components	Function
Central Nervous System (CNS)	Brain, Spinal Cord	Integration, Processing
Peripheral Nervous System (PNS)	Nerves, Ganglia	Transmission of signals
Somatic Division	Motor Neurons	Voluntary movement
Autonomic Division	Sympathetic, Parasympathetic Neurons	Involuntary functions
Enteric Division	Neurons in GI tract	Digestive regulation

Application to Exercise Science

• The nervous system controls muscle contractions and movement, directly impacting athletic performance.

• Understanding neurophysiology is essential for training, rehabilitation, and optimizing performance.