NCERT Textbook - Neural Control and Coordination NEET Notes | EduRev

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NEET : NCERT Textbook - Neural Control and Coordination NEET Notes | EduRev

 Page 1


NEURAL C ONTROL AND COORDINATION 315
As you know, the functions of the organs/organ systems in our body
must be coordinated to maintain homeostasis. Coordination is the
process through which two or more organs interact and complement the
functions of one another. For example, when we do physical exercises,
the energy demand is increased for maintaining an increased muscular
activity. The supply of oxygen is also increased. The increased supply of
oxygen necessitates an increase in the rate of respiration, heart beat and
increased blood flow via blood vessels. When physical exercise is stopped,
the activities of nerves, lungs, heart and kidney gradually return to their
normal conditions. Thus, the functions of muscles, lungs, heart, blood
vessels, kidney and other organs are coordinated while performing physical
exercises. In our body the neural system and the endocrine system jointly
coordinate and integrate all the activities of the organs so that they function
in a synchronised fashion.
The neural system provides an organised network of point-to-point
connections for a quick coordination. The endocrine system provides
chemical integration through hormones. In this chapter, you will learn
about the neural system of human, mechanisms of neural coordination
like transmission of nerve impulse, impulse conduction across a synapse
and the physiology of reflex action.
NEURAL CONTROL AND
COORDINATION
CHAPTER  21
21.1 Neural System
21.2 Human Neural
System
21.3 Neuron as
Structural and
Functional Unit
of Neural
System
21.4  Central Neural
System
21.5 Reflex Action
and Reflex Arc
21.6 Sensory
Reception and
Processing
2015-16(19/01/2015)
Page 2


NEURAL C ONTROL AND COORDINATION 315
As you know, the functions of the organs/organ systems in our body
must be coordinated to maintain homeostasis. Coordination is the
process through which two or more organs interact and complement the
functions of one another. For example, when we do physical exercises,
the energy demand is increased for maintaining an increased muscular
activity. The supply of oxygen is also increased. The increased supply of
oxygen necessitates an increase in the rate of respiration, heart beat and
increased blood flow via blood vessels. When physical exercise is stopped,
the activities of nerves, lungs, heart and kidney gradually return to their
normal conditions. Thus, the functions of muscles, lungs, heart, blood
vessels, kidney and other organs are coordinated while performing physical
exercises. In our body the neural system and the endocrine system jointly
coordinate and integrate all the activities of the organs so that they function
in a synchronised fashion.
The neural system provides an organised network of point-to-point
connections for a quick coordination. The endocrine system provides
chemical integration through hormones. In this chapter, you will learn
about the neural system of human, mechanisms of neural coordination
like transmission of nerve impulse, impulse conduction across a synapse
and the physiology of reflex action.
NEURAL CONTROL AND
COORDINATION
CHAPTER  21
21.1 Neural System
21.2 Human Neural
System
21.3 Neuron as
Structural and
Functional Unit
of Neural
System
21.4  Central Neural
System
21.5 Reflex Action
and Reflex Arc
21.6 Sensory
Reception and
Processing
2015-16(19/01/2015)
316 BIOLOGY
21.1 NEURAL SYSTEM
The neural system of all animals is composed of highly specialised cells
called neurons which can detect, receive and transmit different kinds of
stimuli.
The neural organisation is very simple in lower invertebrates. For
example, in Hydra it is composed of a network of neurons. The neural
system is better organised in insects, where a brain is present along with
a number of ganglia and neural tissues. The vertebrates have a more
developed neural system.
21.2 HUMAN NEURAL SYSTEM
The human neural system is divided into two parts :
(i) the central neural system (CNS)
(ii) the peripheral neural system (PNS)
The CNS includes the brain and the spinal cord and is the site of
information processing and control. The PNS comprises of all the nerves
of the body associated with the CNS (brain and spinal cord). The nerve
fibres of the PNS are of two types :
(a) afferent fibres
(b) efferent fibres
The afferent nerve fibres transmit impulses from tissues/organs to
the CNS and the efferent fibres transmit regulatory impulses from the
CNS to the concerned peripheral tissues/organs.
The PNS is divided into two divisions called somatic neural system
and autonomic neural system. The somatic neural system relays
impulses from the CNS to skeletal muscles while the autonomic neural
system transmits impulses from the CNS to the involuntary organs and
smooth muscles of the body. The autonomic neural system is further
classified into sympathetic neural system and parasympathetic neural
system.
21.3 NEURON AS STRUCTURAL AND FUNCTIONAL UNIT OF
NEURAL SYSTEM
A neuron is a microscopic structure composed of three major parts,
namely, cell body, dendrites and axon (Figure 21.1). The cell body
contains cytoplasm with typical cell organelles and certain granular bodies
called Nissl’s granules. Short fibres which branch repeatedly and project
out of the cell body also contain Nissl’s granules and are called dendrites.
These fibres transmit impulses towards the cell body.   The axon is a long
2015-16(19/01/2015)
Page 3


NEURAL C ONTROL AND COORDINATION 315
As you know, the functions of the organs/organ systems in our body
must be coordinated to maintain homeostasis. Coordination is the
process through which two or more organs interact and complement the
functions of one another. For example, when we do physical exercises,
the energy demand is increased for maintaining an increased muscular
activity. The supply of oxygen is also increased. The increased supply of
oxygen necessitates an increase in the rate of respiration, heart beat and
increased blood flow via blood vessels. When physical exercise is stopped,
the activities of nerves, lungs, heart and kidney gradually return to their
normal conditions. Thus, the functions of muscles, lungs, heart, blood
vessels, kidney and other organs are coordinated while performing physical
exercises. In our body the neural system and the endocrine system jointly
coordinate and integrate all the activities of the organs so that they function
in a synchronised fashion.
The neural system provides an organised network of point-to-point
connections for a quick coordination. The endocrine system provides
chemical integration through hormones. In this chapter, you will learn
about the neural system of human, mechanisms of neural coordination
like transmission of nerve impulse, impulse conduction across a synapse
and the physiology of reflex action.
NEURAL CONTROL AND
COORDINATION
CHAPTER  21
21.1 Neural System
21.2 Human Neural
System
21.3 Neuron as
Structural and
Functional Unit
of Neural
System
21.4  Central Neural
System
21.5 Reflex Action
and Reflex Arc
21.6 Sensory
Reception and
Processing
2015-16(19/01/2015)
316 BIOLOGY
21.1 NEURAL SYSTEM
The neural system of all animals is composed of highly specialised cells
called neurons which can detect, receive and transmit different kinds of
stimuli.
The neural organisation is very simple in lower invertebrates. For
example, in Hydra it is composed of a network of neurons. The neural
system is better organised in insects, where a brain is present along with
a number of ganglia and neural tissues. The vertebrates have a more
developed neural system.
21.2 HUMAN NEURAL SYSTEM
The human neural system is divided into two parts :
(i) the central neural system (CNS)
(ii) the peripheral neural system (PNS)
The CNS includes the brain and the spinal cord and is the site of
information processing and control. The PNS comprises of all the nerves
of the body associated with the CNS (brain and spinal cord). The nerve
fibres of the PNS are of two types :
(a) afferent fibres
(b) efferent fibres
The afferent nerve fibres transmit impulses from tissues/organs to
the CNS and the efferent fibres transmit regulatory impulses from the
CNS to the concerned peripheral tissues/organs.
The PNS is divided into two divisions called somatic neural system
and autonomic neural system. The somatic neural system relays
impulses from the CNS to skeletal muscles while the autonomic neural
system transmits impulses from the CNS to the involuntary organs and
smooth muscles of the body. The autonomic neural system is further
classified into sympathetic neural system and parasympathetic neural
system.
21.3 NEURON AS STRUCTURAL AND FUNCTIONAL UNIT OF
NEURAL SYSTEM
A neuron is a microscopic structure composed of three major parts,
namely, cell body, dendrites and axon (Figure 21.1). The cell body
contains cytoplasm with typical cell organelles and certain granular bodies
called Nissl’s granules. Short fibres which branch repeatedly and project
out of the cell body also contain Nissl’s granules and are called dendrites.
These fibres transmit impulses towards the cell body.   The axon is a long
2015-16(19/01/2015)
NEURAL C ONTROL AND COORDINATION 317
fibre, the distal end of which is branched. Each
branch terminates as a bulb-like structure called
synaptic knob which possess synaptic vesicles
containing chemicals called neurotransmitters.
The axons transmit nerve impulses away from the
cell body to a synapse or to a neuro-muscular
junction. Based on the number of axon and
dendrites, the neurons are divided into three types,
i.e., multipolar (with one axon and two or more
dendrites; found in the cerebral cortex), bipolar
(with one axon and one dendrite, found in the retina
of eye) and unipolar (cell body with one axon only;
found usually in the embryonic stage). There are
two types of axons, namely, myelinated and non-
myelinated. The myelinated nerve fibres are
enveloped with Schwann cells, which form a myelin
sheath around the axon. The gaps between two
adjacent myelin sheaths are called nodes of
Ranvier. Myelinated nerve fibres are found in spinal
and cranial nerves. Unmyelinated nerve fibre is
enclosed by a Schwann cell that does not form a
myelin sheath around the axon, and is commonly
found in autonomous and the somatic neural
systems.
21.3.1 Generation and Conduction of
Nerve Impulse
Neurons are excitable cells because their membranes are in a polarised
state. Do you know why the membrane of a neuron is polarised? Different
types of ion channels are present on the neural membrane. These ion
channels are selectively permeable to different ions. When a neuron is not
conducting any impulse, i.e., resting, the axonal membrane is
comparatively more permeable to potassium ions (K
+
) and nearly
impermeable to sodium ions (Na
+
). Similarly, the membrane is
impermeable to negatively charged proteins present in the axoplasm.
Consequently, the axoplasm inside the axon contains high concentration
of K
+
 and negatively charged proteins and low concentration of Na
+
. In
contrast, the fluid outside the axon contains a low concentration of K
+
, a
high concentration of Na
+
 and thus form a concentration gradient. These
ionic gradients across the resting membrane are maintained by the active
transport of ions by the sodium-potassium pump which transports 3
Na
+
 outwards for 2 K
+
 into the cell. As a result, the outer surface of the
axonal membrane possesses a positive charge while its inner surface
Figure 21.1 Structure of a neuron
2015-16(19/01/2015)
Page 4


NEURAL C ONTROL AND COORDINATION 315
As you know, the functions of the organs/organ systems in our body
must be coordinated to maintain homeostasis. Coordination is the
process through which two or more organs interact and complement the
functions of one another. For example, when we do physical exercises,
the energy demand is increased for maintaining an increased muscular
activity. The supply of oxygen is also increased. The increased supply of
oxygen necessitates an increase in the rate of respiration, heart beat and
increased blood flow via blood vessels. When physical exercise is stopped,
the activities of nerves, lungs, heart and kidney gradually return to their
normal conditions. Thus, the functions of muscles, lungs, heart, blood
vessels, kidney and other organs are coordinated while performing physical
exercises. In our body the neural system and the endocrine system jointly
coordinate and integrate all the activities of the organs so that they function
in a synchronised fashion.
The neural system provides an organised network of point-to-point
connections for a quick coordination. The endocrine system provides
chemical integration through hormones. In this chapter, you will learn
about the neural system of human, mechanisms of neural coordination
like transmission of nerve impulse, impulse conduction across a synapse
and the physiology of reflex action.
NEURAL CONTROL AND
COORDINATION
CHAPTER  21
21.1 Neural System
21.2 Human Neural
System
21.3 Neuron as
Structural and
Functional Unit
of Neural
System
21.4  Central Neural
System
21.5 Reflex Action
and Reflex Arc
21.6 Sensory
Reception and
Processing
2015-16(19/01/2015)
316 BIOLOGY
21.1 NEURAL SYSTEM
The neural system of all animals is composed of highly specialised cells
called neurons which can detect, receive and transmit different kinds of
stimuli.
The neural organisation is very simple in lower invertebrates. For
example, in Hydra it is composed of a network of neurons. The neural
system is better organised in insects, where a brain is present along with
a number of ganglia and neural tissues. The vertebrates have a more
developed neural system.
21.2 HUMAN NEURAL SYSTEM
The human neural system is divided into two parts :
(i) the central neural system (CNS)
(ii) the peripheral neural system (PNS)
The CNS includes the brain and the spinal cord and is the site of
information processing and control. The PNS comprises of all the nerves
of the body associated with the CNS (brain and spinal cord). The nerve
fibres of the PNS are of two types :
(a) afferent fibres
(b) efferent fibres
The afferent nerve fibres transmit impulses from tissues/organs to
the CNS and the efferent fibres transmit regulatory impulses from the
CNS to the concerned peripheral tissues/organs.
The PNS is divided into two divisions called somatic neural system
and autonomic neural system. The somatic neural system relays
impulses from the CNS to skeletal muscles while the autonomic neural
system transmits impulses from the CNS to the involuntary organs and
smooth muscles of the body. The autonomic neural system is further
classified into sympathetic neural system and parasympathetic neural
system.
21.3 NEURON AS STRUCTURAL AND FUNCTIONAL UNIT OF
NEURAL SYSTEM
A neuron is a microscopic structure composed of three major parts,
namely, cell body, dendrites and axon (Figure 21.1). The cell body
contains cytoplasm with typical cell organelles and certain granular bodies
called Nissl’s granules. Short fibres which branch repeatedly and project
out of the cell body also contain Nissl’s granules and are called dendrites.
These fibres transmit impulses towards the cell body.   The axon is a long
2015-16(19/01/2015)
NEURAL C ONTROL AND COORDINATION 317
fibre, the distal end of which is branched. Each
branch terminates as a bulb-like structure called
synaptic knob which possess synaptic vesicles
containing chemicals called neurotransmitters.
The axons transmit nerve impulses away from the
cell body to a synapse or to a neuro-muscular
junction. Based on the number of axon and
dendrites, the neurons are divided into three types,
i.e., multipolar (with one axon and two or more
dendrites; found in the cerebral cortex), bipolar
(with one axon and one dendrite, found in the retina
of eye) and unipolar (cell body with one axon only;
found usually in the embryonic stage). There are
two types of axons, namely, myelinated and non-
myelinated. The myelinated nerve fibres are
enveloped with Schwann cells, which form a myelin
sheath around the axon. The gaps between two
adjacent myelin sheaths are called nodes of
Ranvier. Myelinated nerve fibres are found in spinal
and cranial nerves. Unmyelinated nerve fibre is
enclosed by a Schwann cell that does not form a
myelin sheath around the axon, and is commonly
found in autonomous and the somatic neural
systems.
21.3.1 Generation and Conduction of
Nerve Impulse
Neurons are excitable cells because their membranes are in a polarised
state. Do you know why the membrane of a neuron is polarised? Different
types of ion channels are present on the neural membrane. These ion
channels are selectively permeable to different ions. When a neuron is not
conducting any impulse, i.e., resting, the axonal membrane is
comparatively more permeable to potassium ions (K
+
) and nearly
impermeable to sodium ions (Na
+
). Similarly, the membrane is
impermeable to negatively charged proteins present in the axoplasm.
Consequently, the axoplasm inside the axon contains high concentration
of K
+
 and negatively charged proteins and low concentration of Na
+
. In
contrast, the fluid outside the axon contains a low concentration of K
+
, a
high concentration of Na
+
 and thus form a concentration gradient. These
ionic gradients across the resting membrane are maintained by the active
transport of ions by the sodium-potassium pump which transports 3
Na
+
 outwards for 2 K
+
 into the cell. As a result, the outer surface of the
axonal membrane possesses a positive charge while its inner surface
Figure 21.1 Structure of a neuron
2015-16(19/01/2015)
318 BIOLOGY
becomes negatively charged and therefore is polarised. The electrical
potential difference across the resting plasma membrane is called as the
resting potential.
You might be curious to know about the mechanisms of generation
of nerve impulse and its conduction along an axon. When a stimulus is
applied at a site (Figure 21.2 e.g., point A) on the polarised membrane,
the membrane at the site A becomes freely permeable to Na
+
. This leads
to a rapid influx of Na
+
 followed by the reversal of the polarity at that site,
i.e., the outer surface of the membrane becomes negatively charged and
the inner side becomes positively charged. The polarity of the membrane
at the site A is thus reversed and hence depolarised. The electrical potential
difference across the plasma membrane at the site A is called the
action potential, which is in fact termed as a nerve impulse. At sites
immediately ahead, the axon (e.g., site B) membrane has a positive charge
on the outer surface and a negative charge on its inner surface. As a
result, a current flows on the inner surface from site A to site B. On the
outer surface current flows from site B to site A (Figure 21.2) to complete
the circuit of current flow. Hence, the polarity at the site is reversed, and
an action potential is generated at site B.  Thus, the impulse (action
potential) generated at site A arrives at site B. The sequence is repeated
along the length of the axon and consequently the impulse is conducted.
The rise in the stimulus-induced permeability to Na
+
 is extremely short-
lived. It is quickly followed by a rise in permeability to K
+
. Within a fraction
of a second, K
+
 diffuses outside the membrane and restores the resting
potential of the membrane at the site of excitation and the fibre becomes
once more responsive to further stimulation.
- -
-
- - - - - - - -
-
-
-
- -
- - - -
+
+ +
+ +
+ + + + + +
+
+ + + + +
+ +
+
+
+ + +
- -
- -
-
- - - - - - - -
-
-
-
- -
- - - -
+
+ +
+ +
+ + + + + +
+
+ + + + +
+ + +
+
+ + +
- -
A
Na
B
Na
Figure 21.2 Diagrammatic representation of impulse conduction through an axon
(at points A and B)
2015-16(19/01/2015)
Page 5


NEURAL C ONTROL AND COORDINATION 315
As you know, the functions of the organs/organ systems in our body
must be coordinated to maintain homeostasis. Coordination is the
process through which two or more organs interact and complement the
functions of one another. For example, when we do physical exercises,
the energy demand is increased for maintaining an increased muscular
activity. The supply of oxygen is also increased. The increased supply of
oxygen necessitates an increase in the rate of respiration, heart beat and
increased blood flow via blood vessels. When physical exercise is stopped,
the activities of nerves, lungs, heart and kidney gradually return to their
normal conditions. Thus, the functions of muscles, lungs, heart, blood
vessels, kidney and other organs are coordinated while performing physical
exercises. In our body the neural system and the endocrine system jointly
coordinate and integrate all the activities of the organs so that they function
in a synchronised fashion.
The neural system provides an organised network of point-to-point
connections for a quick coordination. The endocrine system provides
chemical integration through hormones. In this chapter, you will learn
about the neural system of human, mechanisms of neural coordination
like transmission of nerve impulse, impulse conduction across a synapse
and the physiology of reflex action.
NEURAL CONTROL AND
COORDINATION
CHAPTER  21
21.1 Neural System
21.2 Human Neural
System
21.3 Neuron as
Structural and
Functional Unit
of Neural
System
21.4  Central Neural
System
21.5 Reflex Action
and Reflex Arc
21.6 Sensory
Reception and
Processing
2015-16(19/01/2015)
316 BIOLOGY
21.1 NEURAL SYSTEM
The neural system of all animals is composed of highly specialised cells
called neurons which can detect, receive and transmit different kinds of
stimuli.
The neural organisation is very simple in lower invertebrates. For
example, in Hydra it is composed of a network of neurons. The neural
system is better organised in insects, where a brain is present along with
a number of ganglia and neural tissues. The vertebrates have a more
developed neural system.
21.2 HUMAN NEURAL SYSTEM
The human neural system is divided into two parts :
(i) the central neural system (CNS)
(ii) the peripheral neural system (PNS)
The CNS includes the brain and the spinal cord and is the site of
information processing and control. The PNS comprises of all the nerves
of the body associated with the CNS (brain and spinal cord). The nerve
fibres of the PNS are of two types :
(a) afferent fibres
(b) efferent fibres
The afferent nerve fibres transmit impulses from tissues/organs to
the CNS and the efferent fibres transmit regulatory impulses from the
CNS to the concerned peripheral tissues/organs.
The PNS is divided into two divisions called somatic neural system
and autonomic neural system. The somatic neural system relays
impulses from the CNS to skeletal muscles while the autonomic neural
system transmits impulses from the CNS to the involuntary organs and
smooth muscles of the body. The autonomic neural system is further
classified into sympathetic neural system and parasympathetic neural
system.
21.3 NEURON AS STRUCTURAL AND FUNCTIONAL UNIT OF
NEURAL SYSTEM
A neuron is a microscopic structure composed of three major parts,
namely, cell body, dendrites and axon (Figure 21.1). The cell body
contains cytoplasm with typical cell organelles and certain granular bodies
called Nissl’s granules. Short fibres which branch repeatedly and project
out of the cell body also contain Nissl’s granules and are called dendrites.
These fibres transmit impulses towards the cell body.   The axon is a long
2015-16(19/01/2015)
NEURAL C ONTROL AND COORDINATION 317
fibre, the distal end of which is branched. Each
branch terminates as a bulb-like structure called
synaptic knob which possess synaptic vesicles
containing chemicals called neurotransmitters.
The axons transmit nerve impulses away from the
cell body to a synapse or to a neuro-muscular
junction. Based on the number of axon and
dendrites, the neurons are divided into three types,
i.e., multipolar (with one axon and two or more
dendrites; found in the cerebral cortex), bipolar
(with one axon and one dendrite, found in the retina
of eye) and unipolar (cell body with one axon only;
found usually in the embryonic stage). There are
two types of axons, namely, myelinated and non-
myelinated. The myelinated nerve fibres are
enveloped with Schwann cells, which form a myelin
sheath around the axon. The gaps between two
adjacent myelin sheaths are called nodes of
Ranvier. Myelinated nerve fibres are found in spinal
and cranial nerves. Unmyelinated nerve fibre is
enclosed by a Schwann cell that does not form a
myelin sheath around the axon, and is commonly
found in autonomous and the somatic neural
systems.
21.3.1 Generation and Conduction of
Nerve Impulse
Neurons are excitable cells because their membranes are in a polarised
state. Do you know why the membrane of a neuron is polarised? Different
types of ion channels are present on the neural membrane. These ion
channels are selectively permeable to different ions. When a neuron is not
conducting any impulse, i.e., resting, the axonal membrane is
comparatively more permeable to potassium ions (K
+
) and nearly
impermeable to sodium ions (Na
+
). Similarly, the membrane is
impermeable to negatively charged proteins present in the axoplasm.
Consequently, the axoplasm inside the axon contains high concentration
of K
+
 and negatively charged proteins and low concentration of Na
+
. In
contrast, the fluid outside the axon contains a low concentration of K
+
, a
high concentration of Na
+
 and thus form a concentration gradient. These
ionic gradients across the resting membrane are maintained by the active
transport of ions by the sodium-potassium pump which transports 3
Na
+
 outwards for 2 K
+
 into the cell. As a result, the outer surface of the
axonal membrane possesses a positive charge while its inner surface
Figure 21.1 Structure of a neuron
2015-16(19/01/2015)
318 BIOLOGY
becomes negatively charged and therefore is polarised. The electrical
potential difference across the resting plasma membrane is called as the
resting potential.
You might be curious to know about the mechanisms of generation
of nerve impulse and its conduction along an axon. When a stimulus is
applied at a site (Figure 21.2 e.g., point A) on the polarised membrane,
the membrane at the site A becomes freely permeable to Na
+
. This leads
to a rapid influx of Na
+
 followed by the reversal of the polarity at that site,
i.e., the outer surface of the membrane becomes negatively charged and
the inner side becomes positively charged. The polarity of the membrane
at the site A is thus reversed and hence depolarised. The electrical potential
difference across the plasma membrane at the site A is called the
action potential, which is in fact termed as a nerve impulse. At sites
immediately ahead, the axon (e.g., site B) membrane has a positive charge
on the outer surface and a negative charge on its inner surface. As a
result, a current flows on the inner surface from site A to site B. On the
outer surface current flows from site B to site A (Figure 21.2) to complete
the circuit of current flow. Hence, the polarity at the site is reversed, and
an action potential is generated at site B.  Thus, the impulse (action
potential) generated at site A arrives at site B. The sequence is repeated
along the length of the axon and consequently the impulse is conducted.
The rise in the stimulus-induced permeability to Na
+
 is extremely short-
lived. It is quickly followed by a rise in permeability to K
+
. Within a fraction
of a second, K
+
 diffuses outside the membrane and restores the resting
potential of the membrane at the site of excitation and the fibre becomes
once more responsive to further stimulation.
- -
-
- - - - - - - -
-
-
-
- -
- - - -
+
+ +
+ +
+ + + + + +
+
+ + + + +
+ +
+
+
+ + +
- -
- -
-
- - - - - - - -
-
-
-
- -
- - - -
+
+ +
+ +
+ + + + + +
+
+ + + + +
+ + +
+
+ + +
- -
A
Na
B
Na
Figure 21.2 Diagrammatic representation of impulse conduction through an axon
(at points A and B)
2015-16(19/01/2015)
NEURAL C ONTROL AND COORDINATION 319
21.3.2 Transmission of Impulses
A nerve impulse is transmitted from one neuron to another through
junctions called synapses. A synapse is formed by the membranes of a
pre-synaptic neuron and a post-synaptic neuron, which may or may not
be separated by a gap called synaptic cleft. There are two types of
synapses, namely, electrical synapses and chemical synapses. At electrical
synapses, the membranes of pre- and post-synaptic neurons are in very
close proximity. Electrical current can flow directly from one neuron into
the other across these synapses. Transmission of an impulse across
electrical synapses is very similar to impulse conduction along a single
axon. Impulse transmission across an electrical synapse is always faster
than that across a chemical synapse. Electrical synapses are rare in our
system.
At a chemical synapse, the membranes of the pre- and post-synaptic
neurons are separated by a fluid-filled space called synaptic cleft
(Figure 21.3). Do you know how the pre-synaptic neuron transmits an
impulse (action potential) across the synaptic cleft to the post-synaptic
neuron? Chemicals called neurotransmitters are involved in the
transmission of impulses at these synapses. The axon terminals contain
vesicles filled with these neurotransmitters. When an impulse (action
potential) arrives at the axon terminal, it stimulates the movement of the
synaptic vesicles towards the membrane where they fuse with the plasma
Figure 21.3 Diagram showing axon terminal and synapse
2015-16(19/01/2015)
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