Physiology of the Nerve PDF

PHYSIOLOGY OF THE NERVE LECTURE (1) The Nerve Cell (Neuron): The nerve cell (neuron) is the structural unit of the nervous system. STRUCTURE OF THE NEURON: Each neuron is formed of 2 main parts: 1. The cell body (Soma). 2. The cell processes. 1. THE CELL BODY: It...

PHYSIOLOGY OF THE NERVE LECTURE (1) The Nerve Cell (Neuron): The nerve cell (neuron) is the structural unit of the nervous system. STRUCTURE OF THE NEURON: Each neuron is formed of 2 main parts: 1. The cell body (Soma). 2. The cell processes. 1. THE CELL BODY: It contains a large central nucleus with a well-marked nucleolus. The cytoplasm contains: a. Neurofibrils: delicate strands that pass the cell body and processes. b. Nissel Bodies: large triangular masses with high RNA content. They are absent from the axon hillock. Severe activity and anoxia cause disappearance of Nissel granules (Chromatolysis). c. Other organelles as in other cells (as Golgi apparatus, ER…etc). The neuron doesn't contain a centrosome and can never divide. 2. THE CELL PROCESSES: Are of 2 types; dendrites & axon. A. Dendrites: Which are short processes that extend out from the cell body and branch extensively. Functions: 1. Increase the surface area of the cell body (i.e., receptive field of neuron). 2. Conduct the nerve impulse towards the cell body. B. Axon: Which is a single long process that originates from a thickened area of the cell body called the axon hillock. Functions: 1. Conduct the nerve impulse away from the cell body and the dendritic zone. 2. The axon ends in a number of synaptic terminal buttons or axon telodendria. They contain vesicles filled with the chemical transmitter (e.g., Acetylcholine). The axons are called the nerve fibres.  Figure 1: Structure of the nerve cell The axon is covered by: 1. The myelin sheath: it is a protein-lipid complex and act as an electric insulator. The myelin sheath is interrupted at nodes of Ranvier. 2. An outer nucleated layer called neurilemmal sheath or Schwann sheath. It regenerates the nerve fibre when it is cut and forms myelin sheath. It is absent in neurons of the CNS (the myelin sheath in CNS is formed by another type of cells called oligodendrocytes). According to myelin sheath, nerve fibres are classified into: A fibres (thick myelinated, ν of conduction ≈ 100 m/s) B fibres (thin myelinated, ν of conduction ≈ 10 m/s) C fibres (non-myelinated, ν of conduction ≈ 1 m/s) Figure 2: Myelin & Schwann sheathes Excitability: It is the ability of any living tissue to respond to a stimulus. It is a property of life. The nerve is one of the most excitable tissues. A Stimulus: It is any change in the environment surrounding a living tissue that causes it to react. Types of stimuli: Electrical – Chemical – Mechanical - Thermal. Electrical stimulus is preferred in practical experiments because: 1. They are similar to natural signals inside the body. 2. Their intensity and duration of application can be easily controlled. 3. They produce response without causing damage (i.e. They are reproducible or can be repeated) Factors affecting the degree of response to a particular stimulus: 1. The rate of application. 2. The strength of the stimulus 3. The duration of application. 1. Effect of the rate of application: A suddenly applied stimulus of certain intensity is more effective than when a weaker stimulus is applied and then gradually increased to the same intensity as the previous one. 2. Effect of the strength of the stimulus and its duration of application: This is best illustrated by the Strength-Duration (SD) Curve. Figure 3: The Strength Duration (SD) Curve From the curve above, it is concluded that: The stronger is the stimulus, the shorter is the time needed to excite and vice versa. There is a minimal or threshold intensity called (Rheobase, R) which can stimulate. It is the minimal intensity of a current of a very long duration which can stimulate or, it is the minimal galvanic current which can stimulate. The Utilization Time (UT): it is the time needed by the Rheobase (R, Threshold) to stimulate. The Chronaxie (C): It is the time needed by a current of double the rheobase (2 R or 2 threshold) intensity to stimulate.  Significance: It is used to compare excitability of different tissues. The shorter the chronaxie, the greater is the excitability. (Excitability of the Nerve ˃ Skeletal m. ˃ Cardiac m. ˃ Smooth m.) The Minimal Duration (t): it is minimal time needed for a stimulus to produce a response. If the duration of application is less than (t), no response whatever the strength of the stimulus is. Significance: In diathermy, the use of high voltage alternating currents for a very short period of time less than (t) during each phase causes rapid oscillation of ions leading to heating of tissues without stimulation. This is used in electrocautery to stop bleeding. _____________________________________________________________ N.B.: Cathode Ray Oscilloscope (CRO); is used to record the electrical activity of the neuron. It registers rapid events measured in milliseconds (ms) and rapid changes in potential in millivolts (mV). Microelectrodes: Very fine glass capillary pipettes with pointed tips and filled with KCl (as electric conductor). Figure 4: Cathode Ray Oscilloscope THE RESTING MEMBRANE POTENTIAL (RMP): Definition: It is the potential difference between the inner and outer sides of the membrane under resting conditions. It is -70 mV (with the negativity inside). Causes of RMP: 1. Selective permeability of the membrane. 2. The Na+ - K+ pump. I. Selective Permeability of the membrane: The resting membrane is 50-100 times more permeable to K+ than Na+. This is due to: 1. The membrane Na+ channels are closed under resting conditions, while the K+ channels are opened. 2. The Na+ channels are guarded by Ca2+ from outside. 3. The hydration energy for Na+ is greater than that for K+. Thus, the size of hydrated Na+ ion is greater than that of K+ (i.e., the hydration energy is inversely proportionate with the atomic number). II. The Na+- K+ Pump: It is an electrogenic pump. It transmits 3 Na+ ions to the exterior for each 2 K+ ions transmitted to the interior. Thus, creating negativity (-) inside and positivity (+) outside the nerve membrane. The pump is an active process (i.e., it requires ATP and Na+- K+ ATPase). It is stimulated by either excess Na+ or excess ATP inside the cell, and excess K+ outside the cell. It is inhibited by decreased temperature (i.e., cooling) or by a drug called ouabain (a drug used in heart failure).