Nerves

Types of Neurone:

• Sensory neurones: A neurone that transmits action potentials from the receptor to the central nervous system (CNS).
• Intermediate Neurone: A neurone in the CNS that transmits a nerve impulse between the sensory neurones and the motor neurones.
• Motor Neurone: A neurone that transmits action potentials from the CNS to a receptor.

Sensory Receptors:

• Are signal transducers.
• Convert the stimulus energy into a form of electrical energy called a nerve impulse.
• Are connected to sensory neurones, which send action potentials to the CNS.

• The axon of a neurone can be covered in the myelin sheath which is made up of Schwann cells that wrap themselves around the axon. These act as an insulator and allow the nerve impulses to move more quickly along the axon.
• In myelinated axons, the action potentials jump between gaps in the myelin sheath called the nodes of Ranvier. The jumping action is called saltatory conduction.
• In some organisms (for example, giant earthworms and squids) the axon has a much larger diameter, these allow nerve impulses to move much faster.

Resting Potential:

• At resting potential, the axon is polarised, meaning that it is more negative on the inside of the axon than the outside of the axon. The voltage across the membrane of the axon is -70mv.
• The resting potential is caused by the pumping of sodium ions out of the axon, and potassium ions being pumped into the axon by the sodium-potassium pump
• The resting potential is also caused by 100x more potassium ions leaking out of the axon through the potassium channel proteins, than sodium ions leakinig into the axon through sodium channel proteins. There is also a presence of negative ions inside the axon.

Depolarisation:

• This begins when the cell memrane of the receptor cell depolarises, because the receptor has detected a stimulus causing a generator potential.
• Depolarisation occurs when a few sodium voltage-gated channels open, and sodium ions flow into the axon down a chemical gradient.
• The positively charged ions are also attracted to the inside of the axon as it is negatively charged, they move down an electrical gradient, together the tow gradients are called the electrochemical gradient. 

All or Nothing Law:

• If enough sodium ions flow into the axon, it will depolarise enough to reach threshold potential. This will open many more sodium-voltage gated channels and more sodium ions will flood in until the voltage reaches +40mv. This means that the inside of the axon is now positive compared to the outside and has become an action potential/nerve impulse.
• If depolarisation doesn't reach the threshold potential, there will be no action potential, if it does reach threshold potential there will be one; this is the all or nothing law.
• The change in voltage causes sodium voltage-gated channels further along the axon to open, causing sodium ions to flood into the axon there, changing the voltage and creating an action potential there. This is how an action potential moves along an axon.

Repolarisation:

• The sodium voltage-gated channels close after 0.5ms, and the potassium voltage-gated channels open. This is called repolarisation as the inside of the axon once again becomes more negative compared to the outside of the axon.

Hyperpolarisation: 

• The potassium voltage-gated channels stay open for slightly too long, and the voltage across the membrane of the axon briefly drops below -70mv.
• This time is called the refractory period, and during this time another action potential cannot be started.
• The original -70mv is restored by the action of the sodium-potassium pump.

Action Potentials:

• Are all the same size, regardless of the strength of the stimulus.
• A stronger stimulus will produce a higher frequency of action potentials.
• If many neurones transmit an action potential at the same time, it is interpreted by the brain as a strong stimulus, this is called summation.
• The brain deduces the nature of the stimulus by the position of the receptor. 

Synapses are the junctions between two or more neurones, the gap between two neurones is called the synaptic cleft.

• The plasma membrane on the first neurone is called the presynaptic membrane.
• The plasma membrane on the second neurone is called the postsynaptic membrane.
• The presynaptic neurone contains small vesicles which contain many molecules of neurotransmitter. When an action potential reaches the end of the neurone, the calcium voltage gated channels open and calcium flows into the neurone, stimulating the movement of the vesicles towards the presynaptic membrane, where the neurotransmitter is released.
• The neurotransmitter diffuses across the synaptic cleft and binds to the specific receptors on the postsynaptic membrane.
• A cholinergic synapse uses acetylcholine as it's neurotransmitter.

Reflexes:

• Reflexes are fast, automatic, protect your body and prevent harm.
• They don't immediately go to the brain, the nerve impulse travels along the sensory neurone, through an intermediate neurone and then to a motor neurone.