Nervous coordination

- Nerve impulse = A wave of electrical disturbance that travels along the membrane of an axon. This causes a temporary reversal of the electrical potential across the membrance.This reversal switches between 2 states, resting potential and action potential.

- Neurones transmit nerve impulses to effectors (muscles or glands) which respond to the stimuli.

- Sensory = transmit nerve impulses to the CNS

- Relay = link sensory neurones with motor neurones, they are within the spinal cord and brain.

- Motor = transmit nerve impulses from the CNS to the effectors for contraction (muscles) or secretion (glands).

- The diff neurones are arranged in a reflex arc.

- When there is no nerve impulse. The inside of the axon is more negatively charged than the outside by 65mV. Resting potential = -65mV.

- More +ve ions outside acon than inside 

- Axon membrane is POLARISED

To maintain resting potential...

- Na+ is actively transported out of the axon by sodium-potassium pump

- K+ actively transported in

- Creates a chemical gradient - more Na+ outside than in, more K+ inside than out

- Na+ diffuses back in K+ diffuses back out, most Na+ voltage gated gates are closed.

- K+ diffuses back out much quicker than Na+ diffuses back in 

- Equilibrium reached bc no net movement of ions 

- Action potential - When the charge inside the membrane becomes +40mV, caused by a stimulus causing the temporary reversal of charges across the axon membrane - Membrane is DEPOLARISED.

1) At resting potential, K+ voltage gated channels are open Na+ are closed

2) Energy of a stimulus causes some Na+ voltage gated channels to open

3) Na+ ions diffuse in to axon via electrochemical gradient, reverses potential diff across memb

4) As Na+ ions diffuse in more Na+ voltage gated channels open

5) Once action potential of +40mV had been reached Na+ ion channels close, K+ voltage gated channels open

6) K+ voltage gated channels open causes electrochemical gradient to reverse, more K+ channels open

7) More K+ ions diffuse out - axon repolarised

8) Outward flow of K+ ions causes a temporary overshoot of the axon - more neg than usual HYPERPOLARISATION

9) The K+ gates close, sodium potassium pump activated, sodium ions pumped out potassium pumped in

10) Resting potential of -65mV re estalished - axon REPOLARISED

- Cell body - Contains nucleus, RER -> produces proteins + neurotransmitters

- Dendrons - Small extensions of the cell body

- Dendrites - Dendrons divide into smaller extensions 

- Axon - Single long fibre that carries impulses away from the cell body

- Schwann cells - surround the axon, protect it give it electrical insulation. Carry out phagocytosis and play a role in nerve regeneration

- Myelin Sheath - Made of the membranes of schwann cells, surround the axon, membranes are rich in lipid myelin

- Nodes of Ranvier - The gaps between adjacent schwann cells, the gaps occur every 1-3mm along the axon and are 2-3um wide

If threshold value is not reached, no action potential occurs

1) Na+ ions diffuse into axon - enough must diffuse in to make potential difference -55mV

2) At -55mV Na+ voltage gated channels open and an action potential can be generated

3) If not enough Na+ enters to reach threshold value no action potential occurs

ALL OR NOTHING PRINCIPLE

Why an action potential can only travel in 1 Direction

- Temporarily after an action potential the section of membrance becomes HYPERPOLARISED so much K+ diffuses out potential diff becomes -90mV

- This is so far below threshold value that not enough Na+ could diffuse in to raise the p.diff to -55mV and cause an action potential - REFRACTORY PERIOD

THIS : 1) prevents the nerve impulses from travelling backwards and 2) keeps impulses seperate

1) Myelination - myelin = faster, acts as an electrical insulator prevents action potentials forming, they can only form at nodes of ranvier - SALTATORY CONDUCTION. Unmyelinated membrane is depolarised all the way along

2) Axon diameter - bigger diameter, faster nerve impulse travels - less leakage of ions therefore action potential reached more quickly in each section of the axon

3) Temperature - higher temp, faster nerve impulse travels - ions have more kinetic energy more movement diffuse faster. ALSO higher temp = higher rate of respiration (enzymes work faster) more ATP available for active transport of ions - TOO HIGH ENZYMES DENATURE

Functions

- synapses slow down action potentials

- a single impulse along one neurone to initiate new impulses in a number of different neurones at a synapse

Processes

- action potential transported down pre synaptic neurone

-calcium diffuses in causes vesicles containing neurotransmitters to fuse with membrane and release neurotransmitters into synaptic cleft

-neurotransmitters bind with their binding receptors on post syn membrane

-sodium channels open, sodium diffuses into post syn bulb -> neurone 

-action potential continued

Unidirectionality - the one way transmission of nerve impulses between neurones

Why?...

- Neurotransmitter is only stored and released in the pre synaptic bulb

- Receptors for neurotransmitter are only located on the post syn membrane

Summation - several action potentials at the same time in order to generate a response

Temporal summation - Several pre-synaptic neurones to the post synaptic bulb, together they release enough NT to trigger an APotential.

Spatial summation - APotentials occur repeatedly at short intervals in pre syn neuron, each causes release of small amounts of NT into synaptic cleft. Collectively they release enough NT to bind to post syn receptors so enough Na+ diffuses in. 

-presynaptic neurone releases a particular neurotransmitter into the synaptic cleft, it binds to receptors on chloride ion channels on the postsyn membrane

-chloride ion channels open and chloride ions diffuse into the postsyn neurone

-the neurotransmitter binding also causes K+ ion channels to open and K+ ions diffuse out and into synaptic cleft

-the gain of negative chloride ions and loss of positive potassium makes the membrane more negative -80mV

-the membrane is hyoerpolarised and an action potential is less likely to occur. 

Stimulate synapses 

1) mimic a NTand bind to receptor on post syn membrane

2) stimulate release of more NT

3) inhibit enzyme that breaks down NT - NT bound to receptor for longer channel open for longer

Inhibit synapses

1) Inhibit release of NT

2) Block receptors on Na+/K+ channels on post syn membrane - fewer impulses sent

Valium = inhibitory

SSRIs = reduces reuotake of serotonine so it stays in synaptic cleft for longer - binds to receptors for longer, more impulses