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PHYSIOLOGY
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ALL ABOUT WHAT HAPPENING IN THE CELL,FOR EXAMPLE WHAT IS HAPPENING IN NUCLEUS
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Neurophysiology (Nervous system physiology)NB: THESE NOTES DO NOT REPLACE THE USAGE OF THE PRESCRIBED TEXTBOOK, use as a guideline and
summary of the chapter.
Action potential/nerve impulse – neurons are highly excitable (responsive to
stimuli). When a neuron is adequately stimulated, an electrical impulse is
generated and conducted along the length of its axon. This response called the
action potential or nerve impulse is always the same, regardless of the source of
type of stimulus and it virtually underlies all functional activities of the nervous
system.
Now we are going to discuss how neurons are excited or inhibited and how they
communicate with other cells. First of all, we need to understand the concept of
‘resting membrane potential’. The resting potential exists only across the cell
membrane (in this case, it is neuron plasma membrane). The resting membrane
potential is generated by differences in the ionic make up of the intracellular and
extracellular fluids and by the differential permeability of the plasma membrane
to those ions.
When one microelectrode of the volt meter is inserted into the neuron and the
other is in the extracellular fluid, a voltage across the membrane of approximately
– 70 mv is recorded. The cytoplasmic side (inside) of the membrane is negatively
charged relative to the outside. This potential difference in a resting neuron is
called the resting membrane potential and the membrane is said to be polarized.
The value of resting membrane potential varies from (-40mv to -90mv) in
different types of neurons under normal circumstances – This value of resting
membrane potential (approximately -70mv) is maintained by following factors:
• The concentration of Na⁺ and K⁺ on each side of the plasma membrane of
cell (inside and outside cells).
• The permeabilities of Na⁺ and K⁺ across the plasma membrane of cell.
Na⁺ concentration is higher outside the cell (extracellular fluid)
K⁺ concentration is higher inside the cell
1
, Although there are many solutes (C1⁻, glucose, urea) in both fluids, potassium (K⁺)
plays the most important role in generating the membrane potential.
Some K⁺ is always leaking out of the cell and some Na⁺ is always leaking in due to
differential permeability of plasma membrane to various ions. This permeability is
regulated by ATP-driven sodium-potassium pump which stabilizes the resting
membrane potential by maintaining the concentration gradients for sodium and
potassium.
Membrane potentials that act as signals
Neurons use changes in their membrane potential as communication signals for
receiving, integrating and sending information. The change in membrane
potential can be produced by anything that causes
• Changes in the ion concentration on two sides of plasma membrane
• Change in plasma membrane permeability to any ion.
• Changes in membrane potential can produce two types of signals:
Graded potentials
Action potentials
Let us discuss these potentials in brief:
Graded potentials (GP):
It arises in the cell body and dendrites
It travels short distance – typically within cell body to axon hillock (0.1 – 1.0 mm)
It declines with distance
The stimulus for opening of ion channels is chemical (neurotransmitter) or
sensory stimulus (e.g. light, pressure, temperature)
Repolarization is voltage in dependent and occurs when stimulus is no longer
present
When GP has excitatory effect – it opens Na⁺ and K channels, when GP inhibitory
– opens K⁺ or C1⁻ channels
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