How often have you flinched when a friend or a sibling threatened you with a fake punch? Have you noticed how fast we react to the act of someone suddenly bringing their hand dangerously close to our face? This indicates that there is a fast flow of information between what your eyes saw and what the muscles of your body had to do to protect you. Do you know how this information flows?
Imagine electricity being transmitted via wires from the mainframe in your city to each and every outlet in your house. Something similar happens in your body when information flows through your body. The brain acts like a mainframe and the neurons act as wires and carry information in the form of electrical impulses from the brain to different parts of the body and vice versa. In this article we will discuss how impulses are transmitted through nerve fibres.
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A neuron is the structural and functional unit of the nervous system and messages are transmitted across different parts of the body as impulses through these neurons.
Fig: A neuron
Nerve impulse is a wave of depolarisation of the membrane of nerve fibres, that is, the axon of the neuron. The transmission of nerve impulse involve three events -
To begin with let us first discuss the mechanism of generation or a nerve impulse and then we will dive into the discussion on the conduction of nerve impulse.
The neural membrane of a resting nerve fibre has positive charge on its outer side and a negative charge on the inner side. This is called the resting membrane potential (RMP) The resting potential of the neural membrane is about -70mV. The neural membrane is polarised due to differential distribution of Na+ and K+ ions.
Na+ ions are the main extracellular cations which have the tendency to move inwards and K+ ions are the main intracellular cations present in the axoplasm which have a tendency to move outward. The K+ channels in the neuron membrane are always open and allow these ions to leave the cell whereas the gated Na+ channels remain closed and do not allow Na+ ions to enter. Thus, the neuron membrane is more permeable to K+ ions compared to Na+ ions. Even the Na+- K+ exchange pumps allow the exit of three Na+ ions in exchange for the entry of two K+ ions.
The inner surface of the neural membrane remains negative because more K+ ions leave the cell than Na+ ions entering the axoplasm. The axoplasm also contains Cl- ions and organic anions.
Fig: Resting membrane potential
On being stimulated by any stimulus of adequate strength, the permeability of the neural membrane for Na+ increases at the point of stimulus reception as the gated channels open. This results in Na+ ions rushing into the axoplasm of the neural fibre. Thus, at this particular part of the neuron, the inner surface of the neural membrane becomes positive and the outer surface becomes negative. This change in the distribution of electrical charges on the inner and outer surfaces of the neural membrane is known as the reversal of potential and results in depolarisation of the neural membrane. The depolarisation lasts for only a moment and occurs only at the point of stimulus.
The neural membrane adjacent to this depolarised point is still polarised and has a positively charged outer surface and negatively charged inner surface. This potential difference between two adjacent areas is called action potential.
The positive charge from the outer surface of the adjacent polarised area moves towards the negatively charged outer surface of the depolarised area. This results in depolarisation of the adjacent area which was earlier polarised and causes the membrane in this newly depolarised area to become more permeable to Na+ ions which rush from the outer side of the membrane into the axoplasm. This generates an action potential in this area while the originally depolarised area becomes polarised again. This occurs because an increase in the positive charge inside the axon prevents further entry of Na+ ions and permeability of the membrane decreases towards Na+ ions. The establishment of sodium channels helps to send Na+ ions to the outer surface of the neural membrane and hence it becomes positively charged and the inner surface becomes negatively charged. Thus, the resting potential of the membrane is restored and the depolarised region becomes polarised again. This process is known as repolarisation.
Thus, a wave of depolarisation or action potential moves from point to point along the length of the nerve fibre and is known as the nerve impulse.
The action potential that is set up at one end of the nerve fibre is conducted by the local circuits. During conduction of nerve impulse, negative charge on the outer surface of a depolarised area attracts the positive charge from the outer surface of the next polarised area while positive charge on the inner surface of the depolarised area is attracted by the negative charge on the inner surface of the next polarised area. So, the depolarised area becomes polarised and the next polarised area becomes depolarised and the action potential flows onward as a wave of depolarisation.
In nonmyelinated nerve fibres, the ionic changes are repeated over the membrane all along the length of the fibre. So, the action potential flows all along the membrane over the entire length of the fibre.
In myelinated nerve fibres, the myelin sheath acts as an insulating layer around the nerve fibres but it is discontinuous and is absent over the nodes of Ranvier. The myelinated regions of the nerve fibres cannot be depolarised. In such fibres, the ionic changes and the consequent depolarisation occurs only at the nodes of Ranvier which are free from the myelin sheath. Thus, the action potential jumps from one node to the next. This mode of transmission of the nerve impulse is known as saltatory conduction of nerve impulse.
As the nerve impulse reaches the axon terminals of the neuron, it is passed on to the dendrites of the next neuron through a point of no physical contact or a fine gap known as the synapse. This transmission is known as synaptic transmission. The neuron transmitting the impulse is known as the presynaptic neuron and the one receiving the impulse is known as the postsynaptic neuron. We will learn more about this under - Synapse and Synaptic transmission.
Q1. A nerve impulse travels along a myelinated nerve fibre at a speed that is 50 times faster than the nonmyelinated fibre. Why?
Answer: In myelinated nerve fibres, the nerve impulses do not have to run all along the fibre sd in nonmyelinated fibres. As the action potential travels by jumping from one node of Ranvier to the next in myelinated fibres the nerve impulses are conducted far more rapidly, around 50 times faster, through them than through non-myelinated fibres.
Q2. In the polarised state of the neural membrane
A. The inner surface of the membrane is negatively charged and the outer surface is positively charged.
B. The inner surface of the membrane is positively charged and the outer surface is negatively charged.
C. Both the inner and outer surface of the membrane are positively charged.
D. Both the inner and outer surface of the membrane are negatively charged.
Solution: In the resting state of a neuron, the neural membrane is said to be polarised. The neural membrane of a resting nerve fibre has positive charge on its outer side and a negative charge on the inner side. This is called the resting membrane potential (RMP) The resting potential of the neural membrane is about -70mV. Thus the correct option is a.
Q3. In a resting neuron, the outer surface of the neuron carries positive charge because
A. The membrane is more permeable to Na+ ions and allows them to move outside passively.
B. The gated sodium ion channels remain closed and do not allow Na+ ions to enter whereas potassium ion channels remain open and allow K+ ions to move out.
C. The gated potassium ion channels remain closed and do not allow K+ ions to enter whereas sodium ion channels remain open and allow Na+ ions to move out.
D. Both a and c
Solution: In a resting neuron, Na+ ions are the main extracellular cations which have the tendency to move inwards and K+ ions are the main intracellular cations present in the axoplasm which have a tendency to move outward. The K+ channels in the neuron membrane are always open and allow these ions to leave the cell whereas the gated Na+ channels remain closed and do not allow Na+ ions to enter. Thus, the neuron membrane is more permeable to K+ ions compared to Na+ ions. Even the Na+- K+ exchange pumps allow the exit of three Na+ ions in exchange for the entry of two K+ ions. Thus, the correct option is b.
Q4. Differentiate between the resting membrane potential and the action membrane potential.
Resting Membrane Potential
1. It is the potential difference across the membrane in the resting phase of a neuron.
1. It is the potential difference across the membrane in a stimulated neuron.
2. The interior of neuron is electronegative in relation to the extracellular fluid.
2. The interior of the neuron is electropositive.
3. At the resting potential, the neural membrane is more permeable to the K+ ions.
3. During resting potential, the neural membrane becomes more permeable to the Na+ ions due to opening of gated sodium ions channels.
4. Na+ - K+ ATPase pump maintains resting membrane potential
4. Na+ - K+ ATPase pump does not operate during the action potential.
Q1. What is the refractory period of a neuron?
Answer: The process of repolarisation of the entire neuron requires some time and during which the neuron cannot be stimulated again. During this period, also known as the refractory period, the neuron recovers from the previous impulse and gets ready for the next one.
Q2. What factors affect the speed of transmission of nerve impulses?
Answer: The speed of transmission of nerve impulses depends on three factors:
Answer: The long fibre-like process that develops from the cell body of a neuron is known as the axon or a nerve fibre.
A nerve is a bundle of several nerve fibres and and blood vessels enclosed within a connective tissue covering.
Q4. What are Pacinian Corpuscles?
Answer: The Pacinian corpuscles are receptors present deep in the dermis, that sense pressure. Each receptor is connected to a sensory neuron and when pressure on the skin changes its shape, the pressure sensitive sodium ion channels in the membrane of the sensory neuron open up and cause depolarisation of the membrane. As pressure increases, more channels open up leading tp generation of the action potential and the impulses travel through the sensory neuron.
Youtube link- https://youtu.be/gU22h1fl1gQ (10:29-17:40)