www.pharmanotes.or
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.
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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.
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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.
1|Page
, www.pharmanotes.or
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.
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