فسيولوجيا الأعصاب: الإثارة والتوصيل

Physiology of Nerves: Excitation and Conduction

• This lecture is part of Theoretical Physiology I, Stage 2, Lecture 2.
• The nervous system is a complex organ system, composed of three main parts: sensory input (sensing), central nervous system (integrating), and motor output (responding).
• Sensory receptors detect changes inside and outside the body using sense organs and nerve endings.
• The brain processes information from sensations and sends appropriate signals through the motor output.
• The nervous system consists of two main types of cells: neurons and neuroglia (glial cells).
• Neurons are excitable, impulse-conducting cells performing the work of the nervous system.
• Neuroglia protect neurons and support their function.
• Neurons and glial cells, along with brain capillaries, form a functional unit essential for normal brain function, including synaptic activity, extracellular fluid homeostasis, energy metabolism, and neural protection. Imbalances in these elements can lead to neurological disorders.

Divisions of the Nervous System

• The central nervous system (CNS) includes the brain and spinal cord.
• The peripheral nervous system (PNS) is a vast network of nerves throughout the body, connecting the CNS to the rest of the body.
• The PNS can be divided into sensory (afferent) and motor (efferent) divisions.
• The sensory division carries signals from nerve endings to the CNS, which includes somatic and visceral sensory branches.
• The motor division transmits information from the CNS to the rest of the body and includes somatic and autonomic motor branches.

Nerve Cells

• Neurons are the basic units of the nervous system.
• A typical neuron contains four main parts: cell body, dendrites, axon, and axon terminals.
• The cell body contains the nucleus and is the neuron's metabolic center.
• Dendrites extend outward from the cell body and receive signals.
• The axon is a long, fibrous structure that transmits signals away from the cell body.
• Axon terminals release signals to other neurons or target tissues.
• A typical neuron also has an axon hillock, a thickened area where the axon originates.
• The axon has initial segments and branches into presynaptic terminals.
• Terminal buttons contain synaptic vesicles.
• Many neurons have myelin sheaths around their axons, insulating the axon and increasing the speed of signal conduction. Gaps in the myelin sheath are called nodes of Ranvier.

Classification of Neurons

• Neurons are classified based on the number of processes (extensions) from the cell body: unipolar, bipolar, and multipolar.

Nerve Excitation and Conduction

• Nerve cells have excitable membranes, responding to electrical, chemical, or mechanical stimuli, resulting in local and/or propagated potentials

• A membrane potential exists due to a slight excess of negative ions inside and positive ions outside the membrane.

Resting Membrane Potential (RMP)

• The resting membrane potential (RMP) represents the steady state when excitable cells are not transmitting signals.
• In neurons, the RMP is typically around −70 mV.
• The RMP arises primarily from the higher concentration of potassium ions inside the cell and sodium ions outside the cell.

Action Potential Propagation

• Action potentials are rapid changes in membrane potential that are propagated along the nerve fiber.
• The action potential is initiated at the axon hillock and propagates along the axon by opening and closing of voltage-gated ion channels.

Depolarization

• During depolarization, the membrane's permeability to Na+ ions increases and positively charged Na+ ions rush into the axon.

Repolarization

• Repolarization is the change back to the resting negative membrane potential, due to the opening of voltage-gated K+ channels and the outflow of K+ ions.

Propagation (continued)

• In unmyelinated axons, the depolarization wave travels continuously along the axon membrane.
• In myelinated axons, depolarization jumps between nodes of Ranvier. This "saltatory" conduction is much faster than continuous conduction.