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الرحيم الرحمن اللة بسمMuscle and neuron as excitable tissue
• In general sense all living cell are excitable because they respond to external stimuli.
• These cells have some properties which necessary for excitation:
1. They can maintain different concentration between positively and negatively charge
2. Their permeability change when they are stimulated
3. When stimulated only specific types of ions can pass in certain direction
Nerve and muscle cells are called excitable tissue because they respond to chemical, mechanical and electrical stimuli.
The membrane serves as both an insulator and a diffusion barrier to the movement of ions.
Anion and cation• An anion (−) is an ion with more electrons than
protons, giving it a net negative charge (since electrons are negatively charged and protons are positively charged).
• A cation (+) is an ion with fewer electrons than protons, giving it a positive charge.
ECF ICF
Cations (mmolL)
Na+ 140 10
K+ 4 145
Ca+ 5 1
Mg+ 2 40
Total 156 196
Anions (mmolL)
Cl- 110 3
HCO3- 31 10
Protein- 4 45
HPO4- 6 138
Total 156 196
The concentration of major cations and anions
Resting potential• When the membrane potential of a cell can go
for a long period of time without changing significantly, it is referred to as a resting potential or resting voltage. This term is used for the membrane potential of non-excitable cells, but also for the membrane potential of excitable cells in the absence of excitation.
• In case of resting membrane potential there is negatively voltage of about - 70 to -90 mV inside the cell with respect to outside because of the following:
1. The resting cell membrane is 10 - 100 times more permeable to K than to Na.
2. The non - diffusible anions (protein, sulphat and phosphate ions)can not leave the cell.
3. Avery small amount of Na diffuses into the cell down its concentration gradient.
The membrane potential has two basic functions:• First, it allows a cell to function as a battery,
providing power to operate a variety of voltage in the membrane.
• Second, in electrically excitable cells such as neurons and muscle cells, it is used for transmitting signals between different parts of a cell. Signals are generated by opening or closing of ion channels at one point in the membrane, producing a local change in the membrane potential.
Origin of resting membrane potential
A number of forces act on cell membranes. This force are responsible for:
1. Maintenance of resting membrane potential
2. Development of action potential.
3. Bringing the cell back to its resting state after the action potential.
These forces are:• Diffusion• Electrical gradient• Active transport
Ion transporter/pump
Is a trans-membrane protein that moves ions across a plasma membrane against their concentration gradient.
Ion channels allow ions to move across the membrane down those concentration gradients, a process known as active transport or facilitated diffusion.
Na K pumping
The most important of active transport ( facilitated diffusion) system is Na K pumping
, the ion transporter Na+/K+-ATPase pumps which transport sodium cations from the inside to the outside, and potassium cations from the outside to the inside of the cell. There is an enzyme called Na-K adenosine triphosphatase (ATPase) present on the cell membrane, which activated by Na and k to hydrolyse ATP and release the energy.
Voltage-dependent calcium channels (VDCC)• Are a group of voltage-gated ion channels
found in excitable cells ( muscle, neurons, etc.) with a permeability to the ion Ca2+.These channels are slightly permeable to sodium ions, so they are also called Ca2+-Na+ channels, but their permeability to calcium is about 1000-fold greater than to sodium under normal physiological conditions.
• At physiologic or resting membrane potential, VDCCs are normally closed.
• VDCC are activated ( opened) at depolarized membrane potentials and this is the source of the "voltage-dependent". Activation of particular VDCCs allows Ca2+ entry into the cell, which, depending on the cell type, results in muscular contraction, excitation of neurons, up-regulation of gene expression, or release of hormones or neurotransmitters.
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls. The action potential is a sudden reversal of membrane polarity produced by a stimulus. Action potential occur in living organism to produce physiological effects such as:• Transmission of impulses• Release of neurosecretions or chemical transmtters in
synapses.• Contraction of muscle.• Activation or inhibition of glandular secretion
Development of action potential
When a cell membrane is stimulated by a physical or chemical stimulus, the cell membrane permeability to Na is increased. Sodium channels open and the sodium ions rush through the channels to the inside of the cell. This is called depolarization. The membrane potential actually becomes reversed and reaches +35 mV.
At the end of depolarization, Na permeability stops and K permeability increased abruptly and K ions leaves the the cell down their concentration gradient causing the inside membrane return to its original potential. This called repolarization.
The duration of DP and RP in muscle and nerve about 1-5 ms (1/1000 s)
Threshold stimulus
Is a stimulus which is just strong enough to move the resting membrane potential from – 70 mV to – 55 mV that leads to production of action potential.
Synaptic transmission
A synapse is the junction between tow neurons where the electrical activity of one neuron is transmitted to the other. Most synapses occur between the axon terminals of one neuron and the cell body (dendrites). The presynaptic endings enlarge slightly to make the synaptic Knob.
The synaptic knob contains vesicles which contain a transmitter substance. When AP arrives from the axon it cause the calcium channels to open and increasing the membrane permeability to Ca.
Calcium attracts the vesicles to the membrane and once they are in contact they rupture and neurotransmitter is released into synaptic cleft and combine with specific receptors for that transmitter on the postsynabtic membrane. This changes the permeability of postsynaptic membrane to specific ions and results in postsynaptic potential.
The postsynaptic membrane usually contains no transmitter; this why nerve conduction occur only in one direction.
Neurotransmitter
Synaptic vesicles
Reuptake pump
Receptors
Voltage gated Ca++ channel
Axon terminal
Synaptic cleft
dendrite
Post-synabtic density
