Nerve Impulses

Nerve Impulses

Nerve cells, also known as neurones, use nerve impulses to talk to each other and share information. Nerve impulses are like waves of electricity and chemicals that help make a signal when something happens. A nerve impulse makes changes in a neurone when something like pressure, heat or sound happens. This signal then goes along a long part of the neurone called an axon. The nerve impulse then goes to other neurones through a synapse or to muscle fibres by using something called a neuromuscular junction. A synapse is like a bridge between two or more neurones, and a neuromuscular junction is the gap between a neurone and a muscle fibre.

The structure of a neurone

The body has different types of neurons like sensory, motor, and interneurons. Here are some parts of neurons you should know:

Cell body: This is where the nucleus, rough endoplasmic reticulum (rER), and mitochondria are found.
Dendrites: These are like branches that come out of the cell body and make connections with other neurons. This is where receptors are usually found to start the nerve impulse. Myelin sheath and Schwann cells: Schwann cells cover the axon to make the myelin sheath. The myelin sheath helps the action potential move faster.
Nodes of Ranvier: These are the spaces between the Schwann cells.
Axon: This is like a long fiber that connects to the cell body at the axon hillock.
Axon terminals: These are the endings of the axon where action potentials are sent to other neurons.


The structure of a neurone

Interneurons are also called relay neurons because they connect sensory and motor neurons. Here are the main differences between sensory and motor neurons:

Sensory neuron: Carries signals from sensory organs to the CNS. Has only one extension from the cell body and a relatively short axon. Found in the skin, nose, ears, tongue, and eyes. Motor neuron: Carries signals from the CNS to muscles and glands. Has many dendrites and a relatively long axon. Found in glands and muscles.

Interneurons have a cell body in the middle of the axon and are found only in the CNS.

Factors affecting nerve impulse speed:

Myelination: This increases the speed of signal transmission, allowing the impulse to jump quickly between Nodes of Ranvier. Myelinated neurons can transmit impulses up to 150 m/s, while unmyelinated neurons only transmit at 0.5 to 10 m/s.

Temperature: Warmer temperature increases conduction speed by making the membrane more permeable to ions. Cold-blooded animals have slower conduction when the temperature is low.

Axon diameter: Larger axons conduct impulses faster than smaller ones due to the surface area to volume ratio. Overall, these factors affect how fast nerve impulses travel in the body.

Nerve impulse transmission

Neurons play a crucial role in sending and receiving signals to and from the CNS, which helps coordinate a response to a stimulus. Nerve impulses, which are responsible for this coordination, are transmitted as action potentials along the axon of a neuron. The main steps of an action potential include:

  1. Depolarization: When a neuron is stimulated, ion channels on its membrane open, allowing positively charged sodium ions to rush into the cell, making the inside of the cell more positive than the outside. This rapid influx of sodium ions causes the cell to depolarize.
  2. Repolarization: After depolarization, the neuron's ion channels close, and positively charged potassium ions begin to flow out of the cell, making the inside of the cell negative again. This process is called repolarization and restores the cell's resting potential.
  3. Hyperpolarization: Sometimes, after repolarization, the neuron's membrane potential becomes even more negative than its resting potential in a process called hyperpolarization. This occurs when potassium ion channels remain open for longer than necessary, causing the cell's membrane potential to become more negative than the resting potential.
  4. Resting phase: Once the hyperpolarization phase ends, the neuron returns to its resting, where the inside of the cell is charged and the outside is positively charged. The neuron is now ready to receive and transmit another action potential if it is stimulated again.
Phases of action potential transmission in neurones

The membrane potential refers to the difference in charge between the inside and outside of a neuron's plasma membrane. Because the membrane is non-polar, ions cannot flow through it freely, and ion channels are required to allow specific ions to pass through.

When a nerve impulse is triggered, certain proteins in the membrane are activated that allow ions to move across the membrane, leading to a change in membrane potential. The stimulus that initiates the nerve impulse causes a brief depolarization of the membrane potential, which triggers the opening of voltage-gated ion channels. These channels allow sodium ions to flow into the neuron, further depolarizing the membrane and triggering the action potential.

During the repolarization phase, potassium ion channels open, allowing potassium ions to flow out of the neuron, making the inside more negative again. This restores the resting membrane potential and allows the neuron to receive and transmit another action potential if stimulated again.

Overall, the movement of ions across the membrane is what causes the change in membrane potential, and different proteins and ion channels are activated during different phases of the action potential to allow the ions to flow in different directions.

Nerve impulse transmission mechanism

Nerve impulses can travel in two ways along the neurone's axon:

Saltatory conduction Continuous conduction

Saltatory conduction is a form of nerve impulse conduction that occurs in myelinated neurons, where the impulse jumps from one Node of Ranvier to the next. In contrast, continuous conduction occurs in unmyelinated axons, where the impulse travels along the whole length of the axon.

As you mentioned, saltatory conduction is more energy-efficient than continuous conduction because the presence of the myelin sheath and nodes of Ranvier allows the action potential to "jump" from one node to the next, rather than traveling the entire length of the axon. This means that only a small portion of the axon membrane needs to be depolarized, which requires less energy than depolarizing the entire axon membrane in continuous conduction.

Additionally, repolarization requires ATP, which is the energy currency of cells. In saltatory conduction, the nodes of Ranvier allow for a reduction in the amount of repolarization needed compared to continuous conduction, making it even more energy-efficient.

Overall, saltatory conduction is a more efficient form of nerve impulse conduction in myelinated neurons, as it requires less energy and speeds up the transmission of nerve impulses.

Transmission over the synapse

A synapse is the junction between two neurons, and it consists of the pre-synaptic membrane (the axon terminal site), the synaptic cleft (the gap between the neurons), and the post-synaptic membrane (the dendrite membrane of the receiving neuron).

To transmit information across the synapse, neurotransmitters are needed. These are chemical messengers that are produced in the neuron cell body and are released from the pre-synaptic membrane into the synaptic cleft. The neurotransmitters then diffuse across the synaptic cleft and bind to specific receptors on the post-synaptic membrane. the neurotransmitter binds to the receptor, it causes a change in the electrical potential of the post-synaptic cell, which can either depolarize or hyperpolarize it. If the change in potential is great enough, it can trigger an action potential in the post-synaptic neuron, allowing the nerve impulse to continue along the neural pathway. It's important to note that nerve impulse transmission across a synapse only occurs in one direction, from the pre-synaptic neuron to the post-synaptic neuron. This is because the neurotransmitters are released only from the pre-synaptic membrane and bind only to receptors on the post-synaptic membrane, meaning the nerve impulse cannot travel back to the originating neuron. Overall, synapses play a crucial role in allowing nerve impulses to be transmitted between neurons, and the use of neurotransmitters and specific receptors ensures that this process occurs accurately and efficiently.

Nerve impulses and the stimulus-response model

The process begins with the detection of a stimulus by a receptor, which is typically located on the dendrites of a neuron. If the stimulus reaches a certain threshold value, the receptor will transform it into a nerve impulse by letting certain ions flow in or out of the neuron.

The nerve impulse then travels to the central nervous system (CNS), which consists of the brain and spinal cord. The impulse travels by "jumping" from one neuron to the next, with the electrical charge being transmitted through the axon and transformed into a chemical change at the axon terminal through the release of neurotransmitters. This process repeats until the nerve impulse reaches the effector cells.

Once the nerve impulse reaches the CNS, it generates a response in the form of nerve impulses. These impulses are then transmitted to the effector cells, which actively respond to the stimulus by releasing substances (in the case of glands) or contracting or relaxing (in the case of muscles).

Overall, this process demonstrates the complex interplay between the molecular aspects of nervous impulses and the physiological responses that they produce in the body's effector cells. It highlights the intricate nature of the nervous system and its ability to respond rapidly and effectively to a wide range of stimuli.

Nerve Impulses - Key takeaways

Nerve impulses are waves of electrochemical changes across neurones that assist the formation of an action potential in response to a stimulus. Neurones send and receive impulses to and from the central nervous system. Factors affecting impulse conduction include temperature, axon diameter, and the myelination of neurones. The main phases of an action potential include depolarisation, repolarisation, hyperpolarisation, and the resting phase. Nerve impulses are transmitted over a synapse to other neurones via neurotransmitters.

Nerve Impulses

What is a nerve impulse?

A nerve impulse is a wave of electrical chemical changes across a neurone that assists in the formation of an action potential in response to a stimulus

How are nerve impulses transmitted?

Nerve impulses are transmitted from one neurone to another via neurotransmitters. Neurotransmitters released from the pre-synaptic membrane diffuse across the synaptic cleft and bind to receptors located on the post-synaptic membrane. 

Why do nerve impulses travel in one direction only?

Neurotransmitters are released only from the pre-synaptic membrane and receptors are located only on the post-synaptic membrane. Due to this organisation, the transmission of nerve impulses occurs in one direction only!

What are the four steps of a nerve impulse?

Depolarisation Repolarisation Hyperpolarisation Resting phase

What are the nerve cells called?

Nerve cells, also known as neurones, are the primary units of communication in the neurological system. Each neurone consists of a cell body, an axon and dendrites.

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