Physiology of the Nerve Lecture 1

Study Notes

The nerve cell (neuron) is the structural unit of the nervous system.
A neuron is composed of two main parts: the cell body (soma) and the cell processes.

Contains a large central nucleus with a nucleolus.
Cytoplasm includes neurofibrils (delicate strands), Nissl bodies (large triangular masses high in RNA), and other organelles like the Golgi apparatus and endoplasmic reticulum.
The neuron lacks a centrosome and cannot divide.

A single, long process originating from the axon hillock (thickened area of the cell body).
Conducts nerve impulses away from the cell body and dendritic zone.
Ends in terminal buttons (axon telodendria) containing neurotransmitters (e.g., acetylcholine).
Axons are also called nerve fibers.

The axon is covered by a myelin sheath, a protein-lipid complex acting as an electrical insulator.
The myelin sheath is interrupted at nodes of Ranvier.
An outer layer called the neurilemmal sheath (or Schwann sheath) surrounds the myelin sheath. It regenerates the nerve fiber if it's cut. (The CNS lacks this layer, using oligodendrocytes instead.)
Axons are classified according to the presence of myelin: A (thick myelinated), B (thin myelinated), and C (non-myelinated).
Conduction speeds vary: A fibers have high speed (around 100m/s), followed by B (around 10 m/s), and then C (around 1 m/s).

The ability of living tissue to respond to a stimulus.
Nerve tissue is highly excitable.
A stimulus is any change in the environment causing a tissue to react (e.g., electrical, chemical, mechanical).

Illustrates the relationship between stimulus strength and duration required to elicit a response.
Key factors: rheobase (minimum intensity for stimulation), chronaxie (time needed for a stimulus twice the intensity of the rheobase to evoke a response), and utilization time (time a stimulus takes, starting at the rheobase).
The greater the excitability, the shorter the chronaxie. Excitability varies between different tissues (e.g. nerve > skeletal muscle > cardiac muscle).

Used to record electrical activity of neurons.
Measures rapid events in milliseconds and millivolts.
Utilizes microelectrodes (fine glass pipettes) filled with KCl as electrical conductors.

Potential difference between the inner and outer sides of the membrane at rest.
Typically, -70 mV (negative inside).
Caused by selective membrane permeability (more permeable to K+ than Na+) and the Na⁺-K⁺ pump, which actively transports Na⁺ outwards and K⁺ inwards.

The resting membrane is more permeable to K+ than Na+ (about 50-100 times).
K+ channels are open at rest and there are factors like hydration energy which further explain this permeability.

An active transport mechanism.
Pumps 3 Na+ ions out and 2 K+ ions in, maintaining the concentration gradient and membrane potential.
Inhibited by low temperature or a drug like ouabain.