The Nervous System

The nervous sustem is made up of the central nervous system (CNS) and the peripheral nervous system (PNS).

CNS - made up of the brain and spinal cord, co-ordinates the senses and the body’s responses.

PNS - consists of neurones that carry impulses to and from the CNS.

Sensory neurone - conducts information inward, from sensory receptors to the CNS.

Relay (bipolar) neurone - connect motor and sensory neurones.

Motor neurone - conducts information outward, from CNS to effectors (muscle or gland). 

Neurone - a specialised cell made up of a long nerve fibres which carries the impulse. 

Nerves - bundles of nerve fibres.

Cell body - contains the cell nucleus, mitochondria and other organelles e.g. Nissi's granules. 

Axon - a nerve fibre that carries the impulse away from the cell body.

Dendron - a nerve fibre that carries the impulse towards the cell body.

Dendrites - slender finger-like processes which connect to neighbouring nerve cells.

The Schwann cell membrane wraps itself repeatedly around the nerve fibre forming a fatty layer. There are gaps between the Schwann cells called the nodes of Ranvier.

The myelin sheath is important because it:

• Protects the nerves from damage.
• Speeds up the transmission of the impulse.

The conduction velocity is always faster in a myelinated axon than in a non-myelinated axon of the same diameter.

• Ion channels present at the nodes of Ranvier allow the movement of Na+ and K+ ions across the membrane. An action potential can only be generated at these points in a myelinated neurone.
• Action potential moves from node to node - saltatory conduction
• Speeds up the impulse

potential difference (p.d.) - difference in electrical charge across the plasma membrane, measured in millivolts (mV).

resting potential - the normal, resting state of an axon. In this state the p.d. across the axon is –70 mV. The membrane is said to be polarised.

action potential -  when the p.d.across an axon is temporarily reversed. The p.d. changes to around +35 mV. The membrane is said to be depolarised.

• The p.d. across the membrane is created by the distribution of chraged ions.
• Na-K pumps in the membrane transport 3 Na+ ions OUT for every 2 K+ions IN - ACTIVE TRANSPORT
• The inside becomes more negative than the outside as more positive ions are pumped out. The membrane is more permeable to K+ ions
• K+ ions diffuse out of the axon - down the concentration gradient
• The inside of the axon has an overall negative charge due to the presence of organic anions that can't cross the membrane.
• An electrochemical gradient is produced, causing K+ ions to be attracted to the inside of the axon.
• K+ concentration force > electrochemical force. This results in an overall movement of K+ ions OUT of the axon, so the inside is more positive.
• p.d. across the axon is -70mV when there's no further net movement of K+. The axon is  polarised.This state is maintained until an impulse is present.

• Depolarisation - when the axon is stimulated, voltage-gated Na channels in the membrane open and Na+ ions flood in . The inside of the axon becomes more positive.
• Difference in charge increases to about  +35mV, affects neighbouring voltage-gated channels (VGCs), causing them to open, propagating the action pot. along the axon.
• Repolarisation - after about 0.5ms the Na VGCs close. the K channels open and K+ ions flood out.
• Net movement of ions causes the inside of the cell to become negative again.
• K channels remain open and there is a slight 'potassium overshoot', causing the inside of axon to become too negative - hyperpolarisation. Once the channels close the resting potential can be restored.

For an action potential to be generated, the stimulus must be greater than the threshold value.

A stimulus is below the threshold value if insufficient numbers of Na channels open, preventing full depolarisation of the axon.

Once this value is reached, the action potential generated is always the same size regardless of the strength of the stimulus - it's an 'all or nothing' response.

This is the time it takes for an area of the axon membrane to recover after an action potental. It limits the rate at which impulses flow along a fibre to 500-1000 each second and ensures that impulses flow in only one direction along nerves.

Depends on:

• the Na/K pump
• the membrane permeability to K+ ions

Absolute refractory period - after an action potential has been generated another impulse cannot be produced in the region, the Na+ ion channels are completely blocked. It lasts for 1 ms.

Relative refractory period - an action potential can only be initiated if the stimulus is stronger than the normal threshold value. Lasts for about 5 ms.