STRUCTURE OF NEURON (= Nerve Cells):
A neuron is a structural and functional unit of the neural tissue and hence the neural system. Certain neurons may almost equal the length of body itself. Thus neurons with longer processes (projections) are the longest cells in the body. Human neural system has about 100 billion neurons.
Majority of the neurons occur in the brain. Fully formed neurons never divide and remain in interphase throughout life. Shortly after birth, new neurons do not develop. Certain neurons have flask shaped cytons and are called Purkinje cells, which occur in the cerebellum of the brain.
A neuron consists of main cell body and cytoplasmic processes arising from it.
(i) Cell body (= Cyton or Soma)
(ii) Dendrites (Dendrons)
(iii) Axon
(i) Cell body (= Cyton or Soma):
It varies in size and form. It may be up to 13.5 µm in diameter and may be irregular, spherical, oval, rounded, star-shaped or pyramidal. Like a typical cell it consists of cytoplasm, nucleus and cell membrane.
It has abundant cytoplasm, called neuroplasm and a relatively large spherical central nucleus with a distinct nucleolus. The cytoplasm has mitochondria, Golgi apparatus, rough endoplasmic reticulum, ribosomes, lysosomes, fat globules, pigment granules, neurofibrils, neurotubules and Nissl’s granules.
Presence of neurofibrils and Nissl’s granules is characteristic to all neurons. Neurofibrils play a role in the transmission of impulses. Neurotubules are in fact, microtubules which maintain the shape of the neuron.
The Nissl’s granules (also called Nissl’s bodies) are irregular masses of rough endoplasmic reticulum with numerous attached and free ribosomes and polysomes. The Nissl’s granules probably synthesize proteins for the cell.
Centrioles, formerly believed to be absent in mature neurons, have been described in neurons and may be associated with the production and maintenance of microtubules. The cytoplasm immediately surrounding the nucleus is loaded with protein-synthetic machinery, and is called perikaryon.
Previously the name perikaryon was given to the cyton (cell body or soma). Ageing neurons contain a pigment lipofuscin (made up of residual bodies derived from lysosomes). Cyton is concerned with metabolic maintenance and growth.
(ii) Dendrites (Dendrons):
The short cytoplasmic processes of cell body which receives stimulus from other neurone are called dendrites.The dendrites conduct nerve impulses induced by stimuli towards the cell body.The dendrites at their origin from cell body are 5-10 µm in thickness but gradually their thickness decreases by profuse branching.
Function:
Dendrites receives impulses from axon of another neurone through synapse and conducts the impulse towards the cell body ,therefore it is called the receptive organ.
(iii) Axon:
The long cytoplasmic process of cell body which transmits impulse from cell body to other neurone is termed axon.Axon is considerably longer than dendrites.The axon arises from the cell body in a conical elevation called axon hillock ,which is devoid of nissl's granules.
The length of axon is variable and depends on the functional relationship of the neurone.The cytoplasm of axon known as axoplasm contains mitochondria ,neurofibrils but no nissl's granules.The membrane covering axon is called axolemma.Axon can give of branches, called collaters along its course and near the end it undergoes considerable branching into axon terminals or end brush ,the last part of which is enlarged to form end bulb.Axon is present in white matter of central nervous system and peripheral nervous system.
The nerve fibres or axon are covered by a lipid rich membrane called myelin sheath.The myelin sheath is formed by schwann cells and each schwann cell covers a part of the nerve fibre.The region where axon is not covered by myelin sheath is the junction of adjacent myelinated segments called node of ranvier.
There are two types of axon namely myelinated and non-myelinated. In myelinated nerve fibres Schwann cells form myelin sheath around the axon. The gaps between two adjacent myelin sheaths are called nodes of Ranvier.
Myelinated nerve fibres are found in cranial and spinal nerves. In non-myelinated nerve fibres Schwann cells do not form myelin sheath and are without nodes of Ranvier. They are commonly found in autonomous and somatic neural systems.
Function :
Axon transmits impulse from cell body of one neurone to dendron of another neurone through synapse.
Types of Neurons:
Based on their structure and function, the neurons are classified into three:
1.Sensory Neurons
2.Motor Neurons
3.Interneurons or Connecting Neurons
1.Sensory Neurons:
The neurons that conduct impulses from the receptors or sense organs to the central nervous system are called the sensory neurons.
2.Motor Neurons:
The neurons that conduct impulses from the central nervous system to the effectors (muscles or glands) are called the motor neurons.
3.Interneurons or Connecting Neurons:
The neurons present in the brain that connect the sensory and the motor neurons are called interneurons or connecting neurons.
Types of Neurons on the Basis of Structure:
1. Non-Polar Neurons:
Each neuron has several branched processes (projections). These neurons are rare in vertebrates but occur in cnidarians (coelenterates) e.g., Hydra.
2. Unipolar Neurons:
The body has only one axon. It is found usually in the embryonic stage.
3. Pseudo unipolar Neurons:
A single process arises from the cyton and then divides into axon and dendrite. They are found in dorsal root ganglia of spinal nerves.
4. Bipolar Neurons:
Each bipolar neuron has one axon and one dendrite. They are present in the retina of eye.
5. Multipolar Neurons:These neurons have several dendrites and an axon. They are found in cerebral cortex.
Nerve Cell Function:
• Communication and coordination,
• Sensory nerves, which carries the message to brain.
• Allows us to react to a stimulus.
• They conduct electrical impulses away from the neuron's cell body.
• It carries messages from other neurons to a cell body.
Mechanism of Nerve Conduction:
When an action potential is produced in a polarized nerve membrane, local currents from the action potential spread to the adjacent membrane area and depolarize it to the threshold level, triggering another action potential and the process continues.
Thus, there are two essential components of action potential propagation-
(a) the flow of local currents, which depolarizes the adjacent membrane. This electro tonic conduction of membrane depolarization, however, tends to fade out very quickly.
(b) The triggering of a fresh action potential in the adjacent membrane.
The new action potential depolarizes the membrane maximally, and thereby restates the depolarization to its original magnitude of ~ 110 mV. Without the first conductive component, an action potential cannot be conducted at all. Without the second regenerative component, an action potential can be conducted only up to a limited distance beyond which it will fade out. Before the local currents fade out completely, a fresh action potential must be generated for the action potential to be propagated.
The conductive component is very fast and depends on the electrical characteristics of the membrane (called the cable properties of the membrane). The regenerative component is much slower. Frequent action potentials along the neuron tend to slow down the conduction velocity.
Mechanism of Synaptic Transmission:
The pre- and postsynaptic elements are separated by a 20 to 30 nm wide cleft called the synaptic cleft. The synaptic cleft contains enzymes that destroy the neurotransmitters released into the cleft. Synaptic transmission occurs through the following steps:
(1) The action potential arriving at the presynaptic nerve terminal depolarizes the presynaptic nerve terminal.
(2) The depolarization stimulates the influx of free Ca2+ into the nerve terminal by opening of voltage-gated Ca2+ channels.
(3) Ca2+ stimulates the sliding of synaptic vesicles along the presynaptic grid towards the presynaptic membrane, presumably by triggering the cross-bridge movements.
(4) The vesicles discharge their neurotransmitters into the synaptic cleft by exocytosis.
(5) The neurotransmitter released from the presynaptic terminal do not persist in the synaptic cleft for long as it is removed from these in one of the following ways:
(i) reuptake by the presynaptic terminal, which by far is the commonest mechanism. Exceptions are peptide neurotransmitter and acetylcholine;
(ii) rapid dissociation by enzymatic action e.g. acetylcholine is dissociated by acetyl-cholinesterase into acetyl CoA and choline and only the choline is taken up; and
(iv)diffusion away from the synaptic cleft.
STRUCTURE OF NEURON (= Nerve Cells):
A neuron is a structural and functional unit of the neural tissue and hence the neural system. Certain neurons may almost equal the length of body itself. Thus neurons with longer processes (projections) are the longest cells in the body. Human neural system has about 100 billion neurons.
Majority of the neurons occur in the brain. Fully formed neurons never divide and remain in interphase throughout life. Shortly after birth, new neurons do not develop. Certain neurons have flask shaped cytons and are called Purkinje cells, which occur in the cerebellum of the brain.
A neuron consists of main cell body and cytoplasmic processes arising from it.
(i) Cell body (= Cyton or Soma)
(ii) Dendrites (Dendrons)
(iii) Axon
(i) Cell body (= Cyton or Soma):
It varies in size and form. It may be up to 13.5 µm in diameter and may be irregular, spherical, oval, rounded, star-shaped or pyramidal. Like a typical cell it consists of cytoplasm, nucleus and cell membrane.
It has abundant cytoplasm, called neuroplasm and a relatively large spherical central nucleus with a distinct nucleolus. The cytoplasm has mitochondria, Golgi apparatus, rough endoplasmic reticulum, ribosomes, lysosomes, fat globules, pigment granules, neurofibrils, neurotubules and Nissl’s granules.
Presence of neurofibrils and Nissl’s granules is characteristic to all neurons. Neurofibrils play a role in the transmission of impulses. Neurotubules are in fact, microtubules which maintain the shape of the neuron.
The Nissl’s granules (also called Nissl’s bodies) are irregular masses of rough endoplasmic reticulum with numerous attached and free ribosomes and polysomes. The Nissl’s granules probably synthesize proteins for the cell.
Centrioles, formerly believed to be absent in mature neurons, have been described in neurons and may be associated with the production and maintenance of microtubules. The cytoplasm immediately surrounding the nucleus is loaded with protein-synthetic machinery, and is called perikaryon.
Previously the name perikaryon was given to the cyton (cell body or soma). Ageing neurons contain a pigment lipofuscin (made up of residual bodies derived from lysosomes). Cyton is concerned with metabolic maintenance and growth.
(ii) Dendrites (Dendrons):
The short cytoplasmic processes of cell body which receives stimulus from other neurone are called dendrites.The dendrites conduct nerve impulses induced by stimuli towards the cell body.The dendrites at their origin from cell body are 5-10 µm in thickness but gradually their thickness decreases by profuse branching.
Function:
Dendrites receives impulses from axon of another neurone through synapse and conducts the impulse towards the cell body ,therefore it is called the receptive organ.
(iii) Axon:
The long cytoplasmic process of cell body which transmits impulse from cell body to other neurone is termed axon.Axon is considerably longer than dendrites.The axon arises from the cell body in a conical elevation called axon hillock ,which is devoid of nissl's granules.
The length of axon is variable and depends on the functional relationship of the neurone.The cytoplasm of axon known as axoplasm contains mitochondria ,neurofibrils but no nissl's granules.The membrane covering axon is called axolemma.Axon can give of branches, called collaters along its course and near the end it undergoes considerable branching into axon terminals or end brush ,the last part of which is enlarged to form end bulb.Axon is present in white matter of central nervous system and peripheral nervous system.
The nerve fibres or axon are covered by a lipid rich membrane called myelin sheath.The myelin sheath is formed by schwann cells and each schwann cell covers a part of the nerve fibre.The region where axon is not covered by myelin sheath is the junction of adjacent myelinated segments called node of ranvier.
There are two types of axon namely myelinated and non-myelinated. In myelinated nerve fibres Schwann cells form myelin sheath around the axon. The gaps between two adjacent myelin sheaths are called nodes of Ranvier.
Myelinated nerve fibres are found in cranial and spinal nerves. In non-myelinated nerve fibres Schwann cells do not form myelin sheath and are without nodes of Ranvier. They are commonly found in autonomous and somatic neural systems.
Function :
Axon transmits impulse from cell body of one neurone to dendron of another neurone through synapse.
Download PDF here
Types of Neurons:
Based on their structure and function, the neurons are classified into three:
1.Sensory Neurons
2.Motor Neurons
3.Interneurons or Connecting Neurons
1.Sensory Neurons:
The neurons that conduct impulses from the receptors or sense organs to the central nervous system are called the sensory neurons.
2.Motor Neurons:
The neurons that conduct impulses from the central nervous system to the effectors (muscles or glands) are called the motor neurons.
3.Interneurons or Connecting Neurons:
The neurons present in the brain that connect the sensory and the motor neurons are called interneurons or connecting neurons.
Types of Neurons on the Basis of Structure:
1. Non-Polar Neurons:
Each neuron has several branched processes (projections). These neurons are rare in vertebrates but occur in cnidarians (coelenterates) e.g., Hydra.
2. Unipolar Neurons:
The body has only one axon. It is found usually in the embryonic stage.
3. Pseudo unipolar Neurons:
A single process arises from the cyton and then divides into axon and dendrite. They are found in dorsal root ganglia of spinal nerves.
4. Bipolar Neurons:
Each bipolar neuron has one axon and one dendrite. They are present in the retina of eye.
5. Multipolar Neurons:These neurons have several dendrites and an axon. They are found in cerebral cortex.
Nerve Cell Function:
• Communication and coordination,
• Sensory nerves, which carries the message to brain.
• Allows us to react to a stimulus.
• They conduct electrical impulses away from the neuron's cell body.
• It carries messages from other neurons to a cell body.
Mechanism of Nerve Conduction:
When an action potential is produced in a polarized nerve membrane, local currents from the action potential spread to the adjacent membrane area and depolarize it to the threshold level, triggering another action potential and the process continues.
Thus, there are two essential components of action potential propagation-
(a) the flow of local currents, which depolarizes the adjacent membrane. This electro tonic conduction of membrane depolarization, however, tends to fade out very quickly.
(b) The triggering of a fresh action potential in the adjacent membrane.
The new action potential depolarizes the membrane maximally, and thereby restates the depolarization to its original magnitude of ~ 110 mV. Without the first conductive component, an action potential cannot be conducted at all. Without the second regenerative component, an action potential can be conducted only up to a limited distance beyond which it will fade out. Before the local currents fade out completely, a fresh action potential must be generated for the action potential to be propagated.
The conductive component is very fast and depends on the electrical characteristics of the membrane (called the cable properties of the membrane). The regenerative component is much slower. Frequent action potentials along the neuron tend to slow down the conduction velocity.
Mechanism of Synaptic Transmission:
The pre- and postsynaptic elements are separated by a 20 to 30 nm wide cleft called the synaptic cleft. The synaptic cleft contains enzymes that destroy the neurotransmitters released into the cleft. Synaptic transmission occurs through the following steps:
(1) The action potential arriving at the presynaptic nerve terminal depolarizes the presynaptic nerve terminal.
(2) The depolarization stimulates the influx of free Ca2+ into the nerve terminal by opening of voltage-gated Ca2+ channels.
(3) Ca2+ stimulates the sliding of synaptic vesicles along the presynaptic grid towards the presynaptic membrane, presumably by triggering the cross-bridge movements.
(4) The vesicles discharge their neurotransmitters into the synaptic cleft by exocytosis.
(5) The neurotransmitter released from the presynaptic terminal do not persist in the synaptic cleft for long as it is removed from these in one of the following ways:
(i) reuptake by the presynaptic terminal, which by far is the commonest mechanism. Exceptions are peptide neurotransmitter and acetylcholine;
(ii) rapid dissociation by enzymatic action e.g. acetylcholine is dissociated by acetyl-cholinesterase into acetyl CoA and choline and only the choline is taken up; and
(iv)diffusion away from the synaptic cleft.
A neuron is a structural and functional unit of the neural tissue and hence the neural system. Certain neurons may almost equal the length of body itself. Thus neurons with longer processes (projections) are the longest cells in the body. Human neural system has about 100 billion neurons.
Majority of the neurons occur in the brain. Fully formed neurons never divide and remain in interphase throughout life. Shortly after birth, new neurons do not develop. Certain neurons have flask shaped cytons and are called Purkinje cells, which occur in the cerebellum of the brain.
A neuron consists of main cell body and cytoplasmic processes arising from it.
(i) Cell body (= Cyton or Soma)
(ii) Dendrites (Dendrons)
(iii) Axon
(i) Cell body (= Cyton or Soma):
It varies in size and form. It may be up to 13.5 µm in diameter and may be irregular, spherical, oval, rounded, star-shaped or pyramidal. Like a typical cell it consists of cytoplasm, nucleus and cell membrane.
It has abundant cytoplasm, called neuroplasm and a relatively large spherical central nucleus with a distinct nucleolus. The cytoplasm has mitochondria, Golgi apparatus, rough endoplasmic reticulum, ribosomes, lysosomes, fat globules, pigment granules, neurofibrils, neurotubules and Nissl’s granules.
Presence of neurofibrils and Nissl’s granules is characteristic to all neurons. Neurofibrils play a role in the transmission of impulses. Neurotubules are in fact, microtubules which maintain the shape of the neuron.
The Nissl’s granules (also called Nissl’s bodies) are irregular masses of rough endoplasmic reticulum with numerous attached and free ribosomes and polysomes. The Nissl’s granules probably synthesize proteins for the cell.
Centrioles, formerly believed to be absent in mature neurons, have been described in neurons and may be associated with the production and maintenance of microtubules. The cytoplasm immediately surrounding the nucleus is loaded with protein-synthetic machinery, and is called perikaryon.
Previously the name perikaryon was given to the cyton (cell body or soma). Ageing neurons contain a pigment lipofuscin (made up of residual bodies derived from lysosomes). Cyton is concerned with metabolic maintenance and growth.
(ii) Dendrites (Dendrons):
The short cytoplasmic processes of cell body which receives stimulus from other neurone are called dendrites.The dendrites conduct nerve impulses induced by stimuli towards the cell body.The dendrites at their origin from cell body are 5-10 µm in thickness but gradually their thickness decreases by profuse branching.
Function:
Dendrites receives impulses from axon of another neurone through synapse and conducts the impulse towards the cell body ,therefore it is called the receptive organ.
(iii) Axon:
The long cytoplasmic process of cell body which transmits impulse from cell body to other neurone is termed axon.Axon is considerably longer than dendrites.The axon arises from the cell body in a conical elevation called axon hillock ,which is devoid of nissl's granules.
The length of axon is variable and depends on the functional relationship of the neurone.The cytoplasm of axon known as axoplasm contains mitochondria ,neurofibrils but no nissl's granules.The membrane covering axon is called axolemma.Axon can give of branches, called collaters along its course and near the end it undergoes considerable branching into axon terminals or end brush ,the last part of which is enlarged to form end bulb.Axon is present in white matter of central nervous system and peripheral nervous system.
The nerve fibres or axon are covered by a lipid rich membrane called myelin sheath.The myelin sheath is formed by schwann cells and each schwann cell covers a part of the nerve fibre.The region where axon is not covered by myelin sheath is the junction of adjacent myelinated segments called node of ranvier.
There are two types of axon namely myelinated and non-myelinated. In myelinated nerve fibres Schwann cells form myelin sheath around the axon. The gaps between two adjacent myelin sheaths are called nodes of Ranvier.
Myelinated nerve fibres are found in cranial and spinal nerves. In non-myelinated nerve fibres Schwann cells do not form myelin sheath and are without nodes of Ranvier. They are commonly found in autonomous and somatic neural systems.
Function :
Axon transmits impulse from cell body of one neurone to dendron of another neurone through synapse.
Types of Neurons:
Based on their structure and function, the neurons are classified into three:
1.Sensory Neurons
2.Motor Neurons
3.Interneurons or Connecting Neurons
1.Sensory Neurons:
The neurons that conduct impulses from the receptors or sense organs to the central nervous system are called the sensory neurons.
2.Motor Neurons:
The neurons that conduct impulses from the central nervous system to the effectors (muscles or glands) are called the motor neurons.
3.Interneurons or Connecting Neurons:
The neurons present in the brain that connect the sensory and the motor neurons are called interneurons or connecting neurons.
Types of Neurons on the Basis of Structure:
1. Non-Polar Neurons:
Each neuron has several branched processes (projections). These neurons are rare in vertebrates but occur in cnidarians (coelenterates) e.g., Hydra.
2. Unipolar Neurons:
The body has only one axon. It is found usually in the embryonic stage.
3. Pseudo unipolar Neurons:
A single process arises from the cyton and then divides into axon and dendrite. They are found in dorsal root ganglia of spinal nerves.
4. Bipolar Neurons:
Each bipolar neuron has one axon and one dendrite. They are present in the retina of eye.
5. Multipolar Neurons:These neurons have several dendrites and an axon. They are found in cerebral cortex.
Nerve Cell Function:
• Communication and coordination,
• Sensory nerves, which carries the message to brain.
• Allows us to react to a stimulus.
• They conduct electrical impulses away from the neuron's cell body.
• It carries messages from other neurons to a cell body.
Mechanism of Nerve Conduction:
When an action potential is produced in a polarized nerve membrane, local currents from the action potential spread to the adjacent membrane area and depolarize it to the threshold level, triggering another action potential and the process continues.
Thus, there are two essential components of action potential propagation-
(a) the flow of local currents, which depolarizes the adjacent membrane. This electro tonic conduction of membrane depolarization, however, tends to fade out very quickly.
(b) The triggering of a fresh action potential in the adjacent membrane.
The new action potential depolarizes the membrane maximally, and thereby restates the depolarization to its original magnitude of ~ 110 mV. Without the first conductive component, an action potential cannot be conducted at all. Without the second regenerative component, an action potential can be conducted only up to a limited distance beyond which it will fade out. Before the local currents fade out completely, a fresh action potential must be generated for the action potential to be propagated.
The conductive component is very fast and depends on the electrical characteristics of the membrane (called the cable properties of the membrane). The regenerative component is much slower. Frequent action potentials along the neuron tend to slow down the conduction velocity.
Mechanism of Synaptic Transmission:
The pre- and postsynaptic elements are separated by a 20 to 30 nm wide cleft called the synaptic cleft. The synaptic cleft contains enzymes that destroy the neurotransmitters released into the cleft. Synaptic transmission occurs through the following steps:
(1) The action potential arriving at the presynaptic nerve terminal depolarizes the presynaptic nerve terminal.
(2) The depolarization stimulates the influx of free Ca2+ into the nerve terminal by opening of voltage-gated Ca2+ channels.
(3) Ca2+ stimulates the sliding of synaptic vesicles along the presynaptic grid towards the presynaptic membrane, presumably by triggering the cross-bridge movements.
(4) The vesicles discharge their neurotransmitters into the synaptic cleft by exocytosis.
(5) The neurotransmitter released from the presynaptic terminal do not persist in the synaptic cleft for long as it is removed from these in one of the following ways:
(i) reuptake by the presynaptic terminal, which by far is the commonest mechanism. Exceptions are peptide neurotransmitter and acetylcholine;
(ii) rapid dissociation by enzymatic action e.g. acetylcholine is dissociated by acetyl-cholinesterase into acetyl CoA and choline and only the choline is taken up; and
(iv)diffusion away from the synaptic cleft.
STRUCTURE OF NEURON (= Nerve Cells):
A neuron is a structural and functional unit of the neural tissue and hence the neural system. Certain neurons may almost equal the length of body itself. Thus neurons with longer processes (projections) are the longest cells in the body. Human neural system has about 100 billion neurons.
Majority of the neurons occur in the brain. Fully formed neurons never divide and remain in interphase throughout life. Shortly after birth, new neurons do not develop. Certain neurons have flask shaped cytons and are called Purkinje cells, which occur in the cerebellum of the brain.
A neuron consists of main cell body and cytoplasmic processes arising from it.
(i) Cell body (= Cyton or Soma)
(ii) Dendrites (Dendrons)
(iii) Axon
(i) Cell body (= Cyton or Soma):
It varies in size and form. It may be up to 13.5 µm in diameter and may be irregular, spherical, oval, rounded, star-shaped or pyramidal. Like a typical cell it consists of cytoplasm, nucleus and cell membrane.
It has abundant cytoplasm, called neuroplasm and a relatively large spherical central nucleus with a distinct nucleolus. The cytoplasm has mitochondria, Golgi apparatus, rough endoplasmic reticulum, ribosomes, lysosomes, fat globules, pigment granules, neurofibrils, neurotubules and Nissl’s granules.
Presence of neurofibrils and Nissl’s granules is characteristic to all neurons. Neurofibrils play a role in the transmission of impulses. Neurotubules are in fact, microtubules which maintain the shape of the neuron.
The Nissl’s granules (also called Nissl’s bodies) are irregular masses of rough endoplasmic reticulum with numerous attached and free ribosomes and polysomes. The Nissl’s granules probably synthesize proteins for the cell.
Centrioles, formerly believed to be absent in mature neurons, have been described in neurons and may be associated with the production and maintenance of microtubules. The cytoplasm immediately surrounding the nucleus is loaded with protein-synthetic machinery, and is called perikaryon.
Previously the name perikaryon was given to the cyton (cell body or soma). Ageing neurons contain a pigment lipofuscin (made up of residual bodies derived from lysosomes). Cyton is concerned with metabolic maintenance and growth.
(ii) Dendrites (Dendrons):
The short cytoplasmic processes of cell body which receives stimulus from other neurone are called dendrites.The dendrites conduct nerve impulses induced by stimuli towards the cell body.The dendrites at their origin from cell body are 5-10 µm in thickness but gradually their thickness decreases by profuse branching.
Function:
Dendrites receives impulses from axon of another neurone through synapse and conducts the impulse towards the cell body ,therefore it is called the receptive organ.
(iii) Axon:
The long cytoplasmic process of cell body which transmits impulse from cell body to other neurone is termed axon.Axon is considerably longer than dendrites.The axon arises from the cell body in a conical elevation called axon hillock ,which is devoid of nissl's granules.
The length of axon is variable and depends on the functional relationship of the neurone.The cytoplasm of axon known as axoplasm contains mitochondria ,neurofibrils but no nissl's granules.The membrane covering axon is called axolemma.Axon can give of branches, called collaters along its course and near the end it undergoes considerable branching into axon terminals or end brush ,the last part of which is enlarged to form end bulb.Axon is present in white matter of central nervous system and peripheral nervous system.
The nerve fibres or axon are covered by a lipid rich membrane called myelin sheath.The myelin sheath is formed by schwann cells and each schwann cell covers a part of the nerve fibre.The region where axon is not covered by myelin sheath is the junction of adjacent myelinated segments called node of ranvier.
There are two types of axon namely myelinated and non-myelinated. In myelinated nerve fibres Schwann cells form myelin sheath around the axon. The gaps between two adjacent myelin sheaths are called nodes of Ranvier.
Myelinated nerve fibres are found in cranial and spinal nerves. In non-myelinated nerve fibres Schwann cells do not form myelin sheath and are without nodes of Ranvier. They are commonly found in autonomous and somatic neural systems.
Function :
Axon transmits impulse from cell body of one neurone to dendron of another neurone through synapse.
Download PDF here
Types of Neurons:
Based on their structure and function, the neurons are classified into three:
1.Sensory Neurons
2.Motor Neurons
3.Interneurons or Connecting Neurons
1.Sensory Neurons:
The neurons that conduct impulses from the receptors or sense organs to the central nervous system are called the sensory neurons.
2.Motor Neurons:
The neurons that conduct impulses from the central nervous system to the effectors (muscles or glands) are called the motor neurons.
3.Interneurons or Connecting Neurons:
The neurons present in the brain that connect the sensory and the motor neurons are called interneurons or connecting neurons.
Types of Neurons on the Basis of Structure:
1. Non-Polar Neurons:
Each neuron has several branched processes (projections). These neurons are rare in vertebrates but occur in cnidarians (coelenterates) e.g., Hydra.
2. Unipolar Neurons:
The body has only one axon. It is found usually in the embryonic stage.
3. Pseudo unipolar Neurons:
A single process arises from the cyton and then divides into axon and dendrite. They are found in dorsal root ganglia of spinal nerves.
4. Bipolar Neurons:
Each bipolar neuron has one axon and one dendrite. They are present in the retina of eye.
5. Multipolar Neurons:These neurons have several dendrites and an axon. They are found in cerebral cortex.
Nerve Cell Function:
• Communication and coordination,
• Sensory nerves, which carries the message to brain.
• Allows us to react to a stimulus.
• They conduct electrical impulses away from the neuron's cell body.
• It carries messages from other neurons to a cell body.
Mechanism of Nerve Conduction:
When an action potential is produced in a polarized nerve membrane, local currents from the action potential spread to the adjacent membrane area and depolarize it to the threshold level, triggering another action potential and the process continues.
Thus, there are two essential components of action potential propagation-
(a) the flow of local currents, which depolarizes the adjacent membrane. This electro tonic conduction of membrane depolarization, however, tends to fade out very quickly.
(b) The triggering of a fresh action potential in the adjacent membrane.
The new action potential depolarizes the membrane maximally, and thereby restates the depolarization to its original magnitude of ~ 110 mV. Without the first conductive component, an action potential cannot be conducted at all. Without the second regenerative component, an action potential can be conducted only up to a limited distance beyond which it will fade out. Before the local currents fade out completely, a fresh action potential must be generated for the action potential to be propagated.
The conductive component is very fast and depends on the electrical characteristics of the membrane (called the cable properties of the membrane). The regenerative component is much slower. Frequent action potentials along the neuron tend to slow down the conduction velocity.
Mechanism of Synaptic Transmission:
The pre- and postsynaptic elements are separated by a 20 to 30 nm wide cleft called the synaptic cleft. The synaptic cleft contains enzymes that destroy the neurotransmitters released into the cleft. Synaptic transmission occurs through the following steps:
(1) The action potential arriving at the presynaptic nerve terminal depolarizes the presynaptic nerve terminal.
(2) The depolarization stimulates the influx of free Ca2+ into the nerve terminal by opening of voltage-gated Ca2+ channels.
(3) Ca2+ stimulates the sliding of synaptic vesicles along the presynaptic grid towards the presynaptic membrane, presumably by triggering the cross-bridge movements.
(4) The vesicles discharge their neurotransmitters into the synaptic cleft by exocytosis.
(5) The neurotransmitter released from the presynaptic terminal do not persist in the synaptic cleft for long as it is removed from these in one of the following ways:
(i) reuptake by the presynaptic terminal, which by far is the commonest mechanism. Exceptions are peptide neurotransmitter and acetylcholine;
(ii) rapid dissociation by enzymatic action e.g. acetylcholine is dissociated by acetyl-cholinesterase into acetyl CoA and choline and only the choline is taken up; and
(iv)diffusion away from the synaptic cleft.
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