Physiology of the Nerve Lecture 1

Physiology of the Nerve (Lecture 1)

• 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.

The Cell Body (Soma)

• 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.

The Cell Processes

• Two types: dendrites and axons.

Dendrites

• Short, branching processes extending from the cell body.
• Increase the surface area of the cell body (receptive field).
• Conduct nerve impulses towards the cell body.

Axon

• 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.

Nerve Fiber Covering

• 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).

Excitability

• 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).

Factors Affecting Stimulus Response

• The rate of stimulus application.

• The strength of the stimulus.

• The duration of the stimulus.

• A strong, sudden stimulus is more effective than a weak, gradual one.

Strength-Duration Curve

• 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).

Cathode Ray Oscilloscope (CRO)

• 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.

Resting Membrane Potential (RMP)

• 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.

Selective Permeability of the Membrane

• 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.

The Na⁺-K⁺ Pump

• 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.