C2.2 Neural signalling

C2.2.1 Neurons as cells within the nervous system that carry electrical impulses

Students should understand that cytoplasm and a nucleus form the cell body of a neuron, with elongated nerve fibres of varying length projecting from it. An axon is a long single fibre. Dendrites are multiple shorter fibres. Electrical impulses are conducted along these fibres

neurons

• cells within the nervous system that carry electrical impulses

• makes up the nervous system

dendrites - short branched fibres that convert chemical info into electrical signals

axons - an long single fibre that transmits electical signals to terminal regions for communication with other neurons

soma - a cell body containing the nucleus cytoplasm and organelles, where essential metabolic processes occur to mantain cell survival

in some nuerons, the axons may be surrounded by an insulating layer known as the myelin sheath

the myelin sheath improves the conduction speed of electrical impulses along the axon but it requires additional spance and energy myelin sheath

C2.2.2—Generation of the resting potential by pumping to establish and maintain concentration gradients of sodium and potassium ions

Students should understand how energy from ATP drives the pumping of sodium and potassium ions in opposite directions across the plasma membrane of neurons. They should understand the concept of a membrane polarization and a membrane potential and also reasons that the resting potential is negative

Resting potential:

• The voltage difference inside and outside the cell

• approx -70mV

• inside relatively more negative than outside

• electrochemical gradient

Maintained by

• Na/K pumps using ATP because it works against a concentration gradient

  • 3 Na+ gets moved out and 2 K+ gets in

• The membrane is more permeable to K+ than Na+ because of concentration gradients.

Pumped ions will leak back across the membrane through diffusion but it is slow.

C2.2.3 – Nerve impulses as action potentials that are propagated along nerve fibres

Action potential: rapid change in membrane potential

Nerve impulse: action potential that starts at the dendrites of a neuron and is propagated along the axon to the synapse of the neuron. Its an electrical signal involving the movement of postively charged ions.

Sodium channels will fail to open if the threshold is not met and the action potential will fail to be propagated. It is either a complete depolarisation or no depolarisation; “ an all for nothing “ process

Action potential has 2 phases

Depolarisation: negative to positive inside the neuron

Repolarisation: positive to negative inside the membrane

Depolarisation:

• Na+ channels open and ions diffuse in the neuron down a concentration gradient.

• inside more positive raising value to +30mV

Re-polarisation:

• Na+ channels close, K+ channels open

• K+ ions diffuse out of neuron down concentration gradient

• makes it negative to -80mV

Refractory period:

• after the nerve impulse

• Na/K pump restoring resting potential

steps of a nerve impulse

1. resting potential is -70mv

2. sodium potasium pumps maintain the resting potential when more sodium ions are outside and more potassium ion are inside

3. when a neuron is stimulated, sodium ion channels open

4. sodium ions diffuse in causing depolarisation (thats the signal)

5. a nerve impulse is sent as a wave of depolarisation along the membrane

6. potassium ion channels open

7. potassium ions diffuse out causing repolarisation

8. sodium potassium pumps re-establish the resting potential

Variation in the speed of nerve impulses

in humans the diameter is usually 1 micrometer with a speed of approximated 1 meter per second

in squid the diameter is typically 500 micrometer and has a speed of 25 meters per second

and the reasons why they have giant axons is so there’s a rapid response to danger but the downside is that they take more space and resources

C2.2.5 Synapses as junctions between neurons and effector cells

• Synapses are junctions between cells in the nervous system

• between neurons and receptors

• between muscle fibres and glands, otherwise known as effectors

• converts electrical signals to chemical signals at the synapse

• the gap is called the synaptic clef and it is fluid-filled so no elecric impulses can pass across

• signals can only cross the synapse in one direction

C2.2.6 Release of neurotransmitters from a presynaptic membrane

1. nerve impulse transmitted until the end of the neuron and presynaptic membrane

2. Depolarisation of presynaptic membrane

3. Ca2+ diffuse through channels in neuron

4. Ca2+ with neurotransmitters move to presynaptic membrane and fuse with docking proteins

5. Neurotransmitter is released via exocytosis

6. Neurotransmitters diffuse across synaptic gap

7. Neurotransmitters bind to receptors in postsynaptic membrane and opens ion channels

8. Ion diffusion across membrane which may trigger action potential

example of a neurotransmitter is acetylcholine

produced as the pre-synaptic neuron by combinding choline and acetyl

Acetylcholine is loaded into vesicles and then released into the synaptic cleft during synaptic transmission. Receptors for acetylcholine in the post-synaptic membrane have a binding site to which acetylcholine will bind. Acetylcholine remains bound only for a short time. During this time one action potential is initiated in the post-synaptic neuron. This happens because the enzyme acetylcholinesterase is present in the synaptic cleft and breaks down acetylcholine into choline and acetate. Choline is reabsorbed into the pre-synaptic neuron and here it is reconverted back into active neurotransmitter

Myelination

• made of the phospholid bilayer (fatty acid) and acts as an insulator

• Schwann cells lay down the layers of phospholipids

  its really important because they speeed up and can increase up to 100 meters per second

• myelin and insulation can be generated much much faster, improves reaction time

• disadvantage is that it does take up a lot of space