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Transmission of nerve impulses

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Tutor: Henry

Summary

Transmission of nerve impulses

In a nutshell

Nerve impulses are important for transferring information throughout the body. Neurones are responsible for the transport of information and have many features which enable them to carry out this important task.

Neurones

Definition

Neurones are electrical messenger cells that transmit information between different areas of the brain and from the brain to the other areas of the nervous system.

The structure of neurones enables electrical impulses to be carried along the neurone through a wave of depolarisation. Depolarisation is caused by the movement of ions across the neurone cell membrane. These include potassium ions $(K^+)$ and sodium ions $(Na^+)$ . The wave of depolarisation is known as an action potential.

Biology; Control systems; KS5 Year 12; Transmission of nerve impulses — 1. Nucleus, 2. Dendrite, 3. Cell body, 4. Axon, 5. Myelin sheath, 6. Node of Ranvier, 7. Axon terminal

Resting potential

Definition

The resting potential of the neurone is $-70\space mV$ . Millivolts ( $mV$ ) is the unit used to describe the potential of the neuronal membrane. The resting potential is achieved through the constant movement of potassium and sodium ions.

Ion movement

Ion movement is facilitated by the sodium-potassium pump and the potassium ion channel. The sodium-potassium pump transports two $K^+$ ions from outside the neurone into the cell and transports three $Na^+$ ions from inside the neurone to outside the cell.

The sodium-potassium pump is working against the concentration gradient and therefore requires ATP to actively transport the ions across the membrane. The movement of sodium ions outside the cell generates the electrochemical gradient achieving the $-70\space mV$ membrane potential. The potassium ion gradient is kept level through permeability of the membrane to potassium ions.

Action potentials

Action potentials allow messages to be transported down the neurone by depolarising the neuronal membrane. Sodium ion channels are crucial for depolarisation.

Step	Stage of action potential	Description
1.	Stimulation	Sodium ion channels open following stimulation. This allows them to move into the neurone cell down the electrochemical gradient. This creates a less negative environment inside the neurone.
2.	Depolarisation	Depolarisation can only be achieved when the stimulus excites enough sodium ion channels to open. Once enough sodium ion channels open a membrane potential of around $-55\space mV$ is achieved. This membrane potential is the threshold for initiating an action potential. Once the membrane potential reaches the threshold a cascade of sodium ion channels are opened triggering depolarisation of the neuronal membrane as more sodium ions diffuse quickly into the neurone.
3.	Repolarisation	After a certain amount of depolarisation the membrane potential reaches a level of $+30\space mV$ . This membrane potential causes sodium ion channels to close and potassium ion channels to open. This leads to diffusion of potassium ions out of the neurone across the cell membrane in the direction of the electrochemical gradient. The membrane potential becomes more negative. Hyperpolarisation takes place following repolarisation because potassium ion channels close slowly, therefore, the action potential becomes more negative than $-70\space mV$ .
4.	Achieving resting potential	The resting potential is restored by the sodium-potassium pump which utilises active transport to move potassium ions into the neurone and sodium ions out of the neurone.
5.	Refractory period	To prevent action potentials mixing up with one another, a refractory period is employed which keeps every action potential separate. Refractory periods are said to keep action potentials discrete. Once a segment of the neurone has reached the resting potential following stimulation the ion channels must recover and cannot open. This prevents another action potential being generated for a period of time.

Transmission of action potentials

Action potentials need to be delivered down the neurone in discrete "packages" to transmit information correctly.

Movement of action potentials

1.	To move the action potential along the neurone there is a cascade of depolarisation. Following stimulation, sodium ion channels open and sodium ions move into the neurone.
2.	The sodium ions in the cell move along the neurone altering the membrane potential in the next segment of the neurone triggering depolarisation in this segment.
3.	This process is repeated along the neurone and is how an action potential travels along the neurone sending electrical messages.

All or nothing

All or nothing describes the black and white nature of action potentials. There is either an action potential generated or there isn't. This is due to the threshold potential that needs to be reached to innervate an action potential.

At first glance it seems that for a large stimulation a large action potential will be generated and for a small stimulation a small action potential will be generated. However, every single action potential is exactly the same. Therefore, frequency of action potentials is used to translate between larger and smaller stimuli. Each stimulus can trigger more than one discrete action potential.

Speed of conductance

Definition

Speed of conductance refers to the speed at which an action potential can travel through a neurone. There are several factors that affect the speed of conductance.

Factor	Explanation
Myelin sheath	The myelin sheath acts as an insulator to speed up conductance. Schwann cells make up the myelin sheath in neurones in the peripheral nervous system. The Schwann cells cover the majority of the neurone, however, there are gaps where the membrane is exposed. The gaps have concentrated areas of sodium ion channels where depolarisation can take place.
Node of ranvier	The gaps in the myelin sheath are called the nodes of Ranvier. Depolarisation can only happen at the nodes. Sodium ions diffuse along the neurone to the nodes where the membrane is depolarised. The action potential "skips" from node to node. This is called saltatory conduction and is much faster than conduction without a myelinated neurone. Conduction without a myelin sheath is called passive, or continuous conduction.
Temperature	Temperature increases the speed of conductance by providing more energy to ions, this increases the speed of diffusion. Above $40\space \degree C$ the protein ion channels begin to denature so action potentials can no longer move along the neurone.
Diameter of axon	There is a positive correlation between the diameter of the axon and the speed of conduction. The smaller the diameter of the axon the greater the resistance to the movement of ions in the neurone.

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