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

Introduction to Neurophysiology

Relevance of the Nervous System in Exercise Science

The nervous system plays a crucial role in controlling muscle contractions and movement, which are fundamental to athletic performance, coordination, and rehabilitation. Understanding its function is essential for optimizing various aspects of physical performance.

• Muscle Control: The nervous system regulates voluntary and involuntary muscle actions.

• Athletic Performance: Efficient neural control enhances speed, strength, and coordination.

• Rehabilitation: Neural pathways are targeted in recovery from injury.

• Performance Aspects: Includes reaction time, motor learning, and adaptation.

• Example: Sprinters rely on rapid neural signaling for explosive movement.

Organisation of the Nervous System

Overview of Nervous System Structure

The nervous system is organized into distinct divisions, each with specialized functions in processing and transmitting information throughout the body.

• Central Nervous System (CNS): Composed of the brain and spinal cord; responsible for integrating sensory information and coordinating responses.

• Peripheral Nervous System (PNS): Connects the CNS to limbs and organs; subdivided into:

  • Afferent Division: Carries sensory information 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 Organisation

Sensory information about the external and internal environment is transmitted to the CNS, where it is processed and integrated. The CNS then generates appropriate responses via the efferent division.

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

• Autonomic Nervous System: Regulates involuntary functions such as heart rate, digestion, and respiratory rate.

  • 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.

• Structure: Consists of dendrites (receive inputs), cell body (integrates signals), axon (transmits output), and axon terminals (communicate with other cells).

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

• Diversity: Neurons vary in shape and function, adapted to specific roles.

Glial Cells

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

• Types: Oligodendrocytes (CNS) and Schwann cells (PNS) create myelin sheaths.

• Myelination: Fatty layering around axons increases the speed of electrical signal transmission.

• Clinical Relevance: Loss of myelin (e.g., multiple sclerosis) impairs neural signaling.

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 signals.

• Typical Value: Approximately -70 mV (inside negative relative to outside).

• Generation: Caused by differences in ionic concentrations and membrane permeability.

Key Mechanisms

• Ionic Concentration Differences: High [K+] and negatively charged proteins inside the cell; high [Na+] and [Cl-] outside.

• Membrane Permeability: More permeable to K+ than Na+; maintained by the Na+/K+ ATPase pump.

Equation:

The Nernst equation describes the equilibrium potential for a particular ion:

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

Action Potentials

Generation and Propagation

Action potentials are rapid, transient changes in membrane potential that propagate along axons, enabling long-distance communication.

• Threshold: The minimum depolarization required to trigger an action potential.

• All-or-None Principle: Action potentials are uniform in magnitude and duration, regardless of stimulus size (once threshold is reached).

• Propagation: Action potentials travel without diminishing in strength.

Phases of Action Potential

• Depolarization: Rapid influx of Na+ ions.

• Repolarization: Efflux of K+ ions restores resting potential.

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

Equation:

Action potential generation involves voltage-gated ion channels:

$I = g_{ion} (V - E_{ion})$

Graded Potentials

Local and Postsynaptic Potentials

Graded potentials are small, variable changes in membrane potential that occur at synapses or dendrites. They can be excitatory or inhibitory and influence the likelihood of action potential generation.

• Summation: Multiple graded potentials can combine to reach threshold.

• Types: Excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs).

Synaptic Transmission

Mechanisms of Communication Between Neurons

Synaptic transmission is the process by which neurons communicate via chemical messengers called neurotransmitters.

• Neurotransmitter Release: Triggered by action potential arrival at axon terminal.

• Receptor Types: Fast responses via ligand-gated ion channels; slow responses via G protein-coupled receptors.

• Examples: Glutamate (excitatory), GABA (inhibitory), acetylcholine, noradrenaline, serotonin (5-HT).

Steps in Synaptic Transmission

1. Action potential arrives at presynaptic terminal.

2. Neurotransmitter is released into synaptic cleft.

3. Neurotransmitter binds to receptors on postsynaptic cell.

4. Graded potential is generated in postsynaptic neuron.

Essential Terminology

• Nervous system

• Central nervous system

• Peripheral 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, Coordination
Peripheral Nervous System (PNS)	Nerves, Ganglia	Connects CNS to body
Afferent Division	Sensory Neurons	Transmits sensory info to CNS
Efferent Division	Motor Neurons	Transmits motor commands from CNS
Enteric Nervous System	Neural networks in GI tract	Regulates digestion

Additional info: This guide expands on brief lecture points to provide a comprehensive overview suitable for exam preparation in Anatomy & Physiology.