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PHYSIOLOGY OF NERVOUS TISSUE
Like muscle fibers, neurons are electrically excitable. They communicate with one another using two types of electrical signals:
(1) Graded potentials are used for short-distance communication only.
(2) Action potentials allow communication over long distances within the body.
When an action potential occurs in a neuron (nerve cell), it is called a nerve action potential (nerve impulse).
The production of graded potentials and action potentials depends on two basic features of the plasma membrane of excitable cells: the existence of a resting membrane potential and the presence of specific types of ion channels.
Like most other cells in the body, the plasma membrane of excitable cells exhibits a membrane potential, an electrical potential difference (voltage) across the membrane. In excitable cells, this voltage is termed the resting membrane potential.
Graded potentials and action potentials occur because the membranes of neurons contain many different kinds of ion channels that open or close in response to specific stimuli.
ION CHANNELS:
When ion channels are open, they allow specific ions to move across the plasma membrane, down their electrochemical gradient.
Ion channels can open and close due to the presence of “gates.” The gate is a part of the channel protein that can seal the channel pore shut or move aside to open the pore. The electrical signals produced by neurons and muscle fibers rely on four types of ion channels: leakage channels, ligand-gated channels, mechanically gated channels, and voltage-gated channels.
The gates of leakage channels randomly alternate between open and closed positions.
A ligand-gated channel opens and closes in response to a specific chemical stimulus. A wide variety of chemical ligands including neurotransmitters, hormones, and particular ions can open or close ligand-gated channels.
A mechanically gated channel opens or closes in response to mechanical stimulation in the form of vibration (such as sound waves), touch, pressure, or tissue stretching. The force distorts the channel from its resting position, opening the gate.
A voltage-gated channel opens in response to a change in membrane potential (voltage). Voltage- gated channels participate in the generation and conduction of action potentials.
Resting membrane potential: The resting membrane potential exists because of a small buildup of negative ions in the cytosol along the inside of the membrane, and an equal buildup of positive ions in the extracellular fluid along the outside surface of the membrane.
In neurons, the resting membrane potential ranges from -40 to -90 mV. The minus sign indicates that the inside of the cell is negative relative to the outside. A cell that exhibits a membrane potential is said to be polarized.
A graded potential is a small deviation from the membrane potential that makes the membrane either more polarized (inside more negative) or less polarized (inside less negative). When the response makes the membrane more polarized (inside more negative), it is termed a hyperpolarizing graded potential. When the response makes the membrane less polarized (inside less negative), it is termed a depolarizing graded potential.
Graded potentials have different names depending on which type of stimulus causes them and where they occur. For example, when a graded potential occurs in the dendrites or cell body of a neuron in response to a neurotransmitter, it is called a postsynaptic potential. On the other hand, the graded potentials that occur in sensory receptors and sensory neurons are termed receptor potentials and generator potentials.
GENERATION OF ACTION POTENTIAL:
An action potential (AP) or impulse is a sequence of rapidly occurring events that decrease and reverse the membrane potential and then eventually restore it to the resting state.
An action potential has two main phases: Depolarizing phase: the negative membrane potential becomes less negative, reaches zero, and then becomes positive.
Repolarizing phase: the membrane potential is restored to the resting state of - 70 mV.
Each voltage-gated Na+ channel has two separate gates, an activation gate and an inactivation gate. In the
resting state of a voltage-gated Na+ channel, the inactivation gate is open, but the activation gate is
closed. As a result, Na+ cannot move into the cell through these channels.
At threshold, voltage-gated Na+ channels are activated. In the activated state of a voltage-gated Na+ channel, both the activation and inactivation gates in the channel are open and Na+^ inflow begins. As more channels open, Na+^ inflow increases, the membrane depolarizes further, and more Na+ channels open.
Repolarizing Phase: Shortly after the activation gates of the voltage-gated Na+ channels open, the inactivation gates close. Now the voltage-gated Na+ channel is in an inactivated state. In addition to opening voltage-gated Na+ channels, a threshold level depolarization also opens voltage-gated K+ channels. Because the voltage- gated K+ channels open more slowly, their opening occurs at about the same time the voltage-gated Na+ channels are closing.
Slowing of Na+^ inflow and acceleration of K+^ outflow causes the membrane potential to change from +30 mV to -70 mV. Repolarization also allows inactivated Na+ channels to revert to the resting state.
After-hyperpolarizing Phase: While the voltage-gated K+ channels are open, outflow of K+^ may be large enough to cause an after- hyperpolarizing phase of the action potential. During this phase, the voltage-gated K+ channels remain open and the membrane potential becomes even more negative (about -90 mV). As the voltage-gated K+ channels close, the membrane potential returns to the resting level of -70 mV.
Refractory Period: The period of time after an action potential begins during which an excitable cell cannot generate another action potential in response to a normal threshold stimulus is called the refractory period
SIGNAL TRANSMISSION AT SYNAPSE:
At a synapse between neurons, the neuron sending the signal is called the presynaptic neuron , and the neuron receiving the message is called the postsynaptic neuron.
Most synapses are either: axodendritic (from axon to dendrite), axosomatic (from axon to cell body), axoaxonic (from axon to axon).
The two types of synapses: electrical and chemical.
At an electrical synapse, action potentials (impulses) conduct directly between adjacent cells through structures called gap junctions.
NEUROHUMORAL TRANSMISSION:
Neurohumoral transmission implies to the transmission of messages by nerves across synapses and neuroeffector junctions by the release of humoral (chemical) messengers.
Chemical synapse: Although the plasma membranes of presynaptic and postsynaptic neurons in a chemical synapse are close, they do not touch. They are separated by the synaptic cleft, a space of 20 – 50 nm* that is filled with interstitial fluid.
In response to a nerve impulse, the presynaptic neuron releases a neurotransmitter that diffuses through the fluid in the synaptic cleft and binds to receptors in the plasma membrane of the postsynaptic neuron. The postsynaptic neuron receives the chemical signal and in turn produces a postsynaptic potential , a type of graded potential. Thus, the presynaptic neuron converts an electrical signal (nerve impulse) into a chemical signal (released neurotransmitter)
A typical chemical synapse transmits a signal as follows:
- The depolarizing phase of the nerve impulse opens voltage gated Ca2+ channels, which are present in the membrane of synaptic end bulbs. Because calcium ions are more concentrated in the extracellular fluid, Ca2+^ flows inward through the opened channels.
A neurotransmitter causes either an excitatory or an inhibitory graded potential. A neurotransmitter that depolarizes the postsynaptic membrane is excitatory because it brings the membrane closer to threshold. A depolarizing postsynaptic potential is called an excitatory postsynaptic potential (EPSP).
A neurotransmitter that causes hyperpolarization of the postsynaptic membrane is inhibitory. A hyperpolarizing postsynaptic potential is termed an inhibitory postsynaptic potential (IPSP).
Neurotransmitter receptors are classified as either ionotropic receptors or metabotropic receptors based on whether the neurotransmitter binding site and the ion channel are components of the same protein or are components of different proteins.
Ionotropic Receptors: An ionotropic receptor is a type of neurotransmitter receptor that contains a neurotransmitter binding site and an ion channel. An ionotropic receptor is a type of ligand- gated channel. In the absence of neurotransmitter (the ligand), the ion channel component of the ionotropic receptor is closed. When the correct neurotransmitter binds to the ionotropic receptor, the ion channel opens, and an EPSP or IPSP occurs in the postsynaptic cell.
Metabotropic Receptors: A metabotropic receptor is a type of neurotransmitter receptor that contains a neurotransmitter binding site, but lacks an ion channel as part of its structure. However, a metabotropic receptor is coupled to a separate ion channel by a type of membrane protein called a G protein. When a neurotransmitter binds to a metabotropic receptor, the G protein either directly opens (or closes) the ion channel or it may act indirectly by activating another molecule, a “second messenger,” in the cytosol, which in turn opens (or closes) the ion channel.
Removal of the neurotransmitter: Removal of the neurotransmitter from the synaptic cleft is essential for normal synaptic function. Neurotransmitter is removed in three ways:
- Diffusion: Some of the released neurotransmitter molecules diffuse away from the synaptic cleft. Once a neurotransmitter molecule is out of reach of its receptors, it can no longer exert an effect.
- Enzymatic degradation: Certain neurotransmitters are inactivated through enzymatic degradation. For example, the enzyme acetylcholinesterase breaks down acetylcholine in the synaptic cleft.
- Uptake by cells: Many neurotransmitters are actively transported back into the neuron that released them (reuptake). Others are transported into neighboring neuroglia (uptake). The membrane proteins that accomplish such uptake are called neurotransmitter transporters.
NEUROTRANSMITTERS:
Neurotransmitter is a chemical substance that acts as a mediator for the transmission of nerve impulse from one neuron to another neuron through a synapse.
Criteria for Neurotransmitter: To consider a substance as a neurotransmitter, it should fulfill certain criteria as given below:
1. It must be found in a neuron
2. It must be produced by a neuron
3. It must be released by a neuron
4. After release, it must act on a target area and produce some biological effect
5. After the action, it must be inactivated.
Classificationof Neurotransmitters:
1. Depending Upon Function : Some of the neurotransmitters cause excitation of
postsynaptic neuron while others cause inhibition. Thus, neurotransmitters are classified into two types:
Excitatory neurotransmitters: Glutamate, Aspartate, Histamine, Nitric Oxide, Noradrenaline
Inhibitory neurotransmitters: GABA, Glycine, Dopamine, Serotonin.
2. Dependingonsize:
Small molecule neurotransmitters: Acetylcholine, Glutamate, Aspartate, Noradrenaline, ATP and other purines, and nitric oxide
Neuropeptides: SubstanceP,encephalin,endorphinanddynorphin
