How Are Nerve Impulses Transmitted

Signals are transmitted from neuron to neuron via an action potential, when the axon membrane rapidly depolarizes and repolarizes.

1. How Are Nerve Impulses Transmitted Across The Synapse
2. Nerve Impulse Pathway
3. How Are Nerve Impulses Transmitted Within A Neuron
4. Nerve Impulse Explained

Transmitting Nerve Impulses. The place where an axon terminal meets another cell is called a synapse. This is where the transmission of a nerve impulse to another cell occurs. The cell that sends the nerve impulse is called the presynaptic cell, and the cell that receives the nerve impulse is called the postsynaptic cell. Nerves: An illustration of the main nerves of the arm. A nerve is an enclosed, cable-like bundle of axons (the projections of neurons) in the peripheral nervous system (PNS). A nerve provides a structured pathway that supports the electrochemical nerve impulses transmitted along each of the axons.

Ginuwine same ol g download. Learning Objectives

• Explain the formation of the action potential in neurons

Key Points

• Action potentials are formed when a stimulus causes the cell membrane to depolarize past the threshold of excitation, causing all sodium ion channels to open.
• When the potassium ion channels are opened and sodium ion channels are closed, the cell membrane becomes hyperpolarized as potassium ions leave the cell; the cell cannot fire during this refractory period.
• The action potential travels down the axon as the membrane of the axon depolarizes and repolarizes.
• Myelin insulates the axon to prevent leakage of the current as it travels down the axon.
• Nodes of Ranvier are gaps in the myelin along the axons; they contain sodium and potassium ion channels, allowing the action potential to travel quickly down the axon by jumping from one node to the next.

Key Terms

• action potential: a short term change in the electrical potential that travels along a cell
• depolarization: a decrease in the difference in voltage between the inside and outside of the neuron
• hyperpolarize: to increase the polarity of something, especially the polarity across a biological membrane
• node of Ranvier: a small constriction in the myelin sheath of axons
• saltatory conduction: the process of regenerating the action potential at each node of Ranvier

Action Potential

A neuron can receive input from other neurons via a chemical called a neurotransmitter. If this input is strong enough, the neuron will pass the signal to downstream neurons. Transmission of a signal within a neuron (in one direction only, from dendrite to axon terminal) is carried out by the opening and closing of voltage-gated ion channels, which cause a brief reversal of the resting membrane potential to create an action potential. As an action potential travels down the axon, the polarity changes across the membrane. Once the signal reaches the axon terminal, it stimulates other neurons.

Depolarization and the Action Potential

When neurotransmitter molecules bind to receptors located on a neuron’s dendrites, voltage-gated ion channels open. At excitatory synapses, positive ions flood the interior of the neuron and depolarize the membrane, decreasing the difference in voltage between the inside and outside of the neuron. A stimulus from a sensory cell or another neuron depolarizes the target neuron to its threshold potential (-55 mV), and Na⁺ channels in the axon hillock open, starting an action potential. Once the sodium channels open, the neuron completely depolarizes to a membrane potential of about +40 mV. The action potential travels down the neuron as Na+ channels open.

Hyperpolarization and Return to Resting Potential

Action potentials are considered an “all-or nothing” event. Once the threshold potential is reached, the neuron completely depolarizes. As soon as depolarization is complete, the cell “resets” its membrane voltage back to the resting potential. The Na⁺ channels close, beginning the neuron’s refractory period. At the same time, voltage-gated K⁺ channels open, allowing K⁺ to leave the cell. As K⁺ ions leave the cell, the membrane potential once again becomes negative. The diffusion of K⁺ out of the cell hyperpolarizes the cell, making the membrane potential more negative than the cell’s normal resting potential. At this point, the sodium channels return to their resting state, ready to open again if the membrane potential again exceeds the threshold potential. Eventually, the extra K⁺ ions diffuse out of the cell through the potassium leakage channels, bringing the cell from its hyperpolarized state back to its resting membrane potential.

How Are Nerve Impulses Transmitted Across The Synapse

Myelin and Propagation of the Action Potential

For an action potential to communicate information to another neuron, it must travel along the axon and reach the axon terminals where it can initiate neurotransmitter release. The speed of conduction of an action potential along an axon is influenced by both the diameter of the axon and the axon’s resistance to current leak. Myelin acts as an insulator that prevents current from leaving the axon, increasing the speed of action potential conduction. Diseases like multiple sclerosis cause degeneration of the myelin, which slows action potential conduction because axon areas are no longer insulated so the current leaks.

A node of Ranvier is a natural gap in the myelin sheath along the axon. These unmyelinated spaces are about one micrometer long and contain voltage gated Na⁺ and K⁺ channels. The flow of ions through these channels, particularly the Na⁺ channels, regenerates the action potential over and over again along the axon. Action potential “jumps” from one node to the next in saltatory conduction. If nodes of Ranvier were not present along an axon, the action potential would propagate very slowly; Na⁺ and K⁺ channels would have to continuously regenerate action potentials at every point along the axon. Nodes of Ranvier also save energy for the neuron since the channels only need to be present at the nodes and not along the entire axon.

Synapse: the junction between the axon terminal of one neuron and dendrites, cell body or axon of another neuron is called synapse. The neuromuscular junction is also known as synapse.

Synaptic knob: the swelling terminal of axon or dendrites is known as synaptic knob.

Pre-synaptic neuron: the neuron carrying impulse toward synapse

Post synaptic neuron: the neuron which receive the impulse and carry the impulse away from the synapse.

Two theories have been put forward to explain the conduction of nerve impulse across the synapse. They are;

1. Electrical transmission theory
2. Chemical transmission theory

Electrical transmission theory:

Impulse transmission through synapse is accomplished by electric current. When the impulse reaches the pre synaptic knob, the impulse itself act as stimulus for the post synaptic neuron causing depolarization. Now the action potential generate in second neuron.

Chemical transmission theory:

Nerve impulse are conducted across the synapse with the help of chemical substances called neurotransmitter. The process of chemical transmission was discovered by Henry (1936).

Nerve Impulse Pathway

Mechanism

How Are Nerve Impulses Transmitted Within A Neuron

Figure: Nerve impulse transmission through Synapse Cubase le ai elements 9 license activation code. Cricket 2007 updated roster.

Nerve Impulse Explained

• When nerve impulse reaches the pre-synaptic knob, it depolarized the presynaptic membrane and causes the opening of voltage gated calcium channel.
• Diffusion of Ca++ ion in the presynaptic knob causes movement of synaptic vesicle to the surface of the knob. Synaptic vesicle carries the neurotransmitter.
• Synaptic vesicles then fused with the presynaptic membrane and get rupture to discharge its content ie. Neurotransmitter (Acetlcholine) into synaptic cleft.
• Synaptic vesicles then return to the cytoplasm of pre-synaptic knob for refilling.
• Some of the released neurotransmitter binds with the protein receptor present on the post synaptic membrane of another neuron and change the membrane potential.
• Other unbound neurotransmitter immediately get lost from the synaptic cleft.
• The depolarization of the post synaptic membrane opens the Sodium channel causing influx of Na+ ion. Thus causing depolarization and generate action potential. In this way, the impulse get transmitted to next neuron along the synapse.