Nervous system

Nerve net (hydra)

Have no CNS

ganglion cells provide connections in all direction

axons non myelinated

slow conduction speed

What does the nervous system do

Detect changes in body and local environment

Produce a response

2 types of nervous system + structure

CNS - Brain + Spinal cord

Protected by meninges (protective membrane)

PNS - other neurones

2 divisions of the PNS + function

Somatic nervous system - voluntary actions

Autonomic nervous system (heart rate + breathing)

Sensory neurone

Carries impulse form receptor cells to co-ordinator

Relay neurone

Receive form sensory and relay to motor

Motor neurone

From coordinator to effector to bring a response

Dendrite

Carry impulse towards cell body

Axon

Transmits away from cell body

Schwann cells

Surround neurones and insulate

Myelin sheath

Made of many Schwann cells - acts as electrical insulator

Nodes of Ranvier

Areas on axon where myelin sheath is missing

Cell body

Contains nucleus + other cell organelles

Describe a reflex arc

Sensory neurone enter spinal cord by dorsal root (back)

neurones synapses signal through relay neurone in grey matter

Motor neurone travels via ventral root (front)

What are the 4 types of axon transport proteins

Na+ / K+ pump

Voltage gated Na+ and K+ channels

K+ always open channels

Sodium / potassium pump

Uses active transport to move 3 Na+ out for every 2 K+ in

Movement of ions at resting potential

-70 due to 3Na+ out and 2K+ in actively

K+ always open channel allows some out

Resting potential + charge

Potential difference between inside and out of membrane when impulse is not being conducted

Due to the movement of ions the membrane is polarised and rests at -70mV

Action potential part 1 - Depolarisation

Stimulus causes opening of Na+ channel

Ions diffuse in across electrochemical + concentration gradient

If enough diffuse in and raise to -55mV all Na+ channels open and rapid diffusion of ions cause an action potential of +40mV

Action potential part 2 - Repolarisation

When PD reaches 40mV - Na+ close and K+ open

K+ rapidly diffuse out down concentration and electrochemical gradient

Action potential part 3 - Hyperpolarisation

Due to rapid diffusion out of K+ ions - axon becomes more negative at 90mV

K+ channels then close and Na+/K+ pump restarts and membrane become polarised again

All or nothing law

To cause depolarisation, stimulus must exceed threshold value

The action potential size is constant (40mV)

Refractory period

After action potential, Na+ voltage-gated Chanels are inactivated for a short time

Benefits of the refractory period

Prevents action potential being generated in opposite direction - second being generated too close to first

Why does an action potential travel further in a mylenated axon

Saltatory conduction

Depolarisation only happens at the nodes of ranvier rather than the whole axon

Why is saltatory conduction more efficient

Less places to depolarise / reprise requiring less ATP for active transport

Factors affecting speed of impulse

Mylenation

Diameter of axon - less resistance

Temperature movement of substances is quicker + ATP production quicker

Label A-E

A - Voltage gated Ca2+ channel

B - synaptic cleft

C - Ligand gated Na+ channel

D - Pre-synaptic bulb

E - Neurotransmitter vesicles contining acetylcholine

Describe the process that happens when an impulse reaches a synapse

Ca2+ channels open

Causes vesicles to move and fuse with membrane

Releases neurotransmitter to diffuse across the cleft

Binds to complimentary receptors and causes ligand-gated Na+ channels to open

This depolarises the post synaptic membrane and causes an action potential

Acetylcholinesterase

Hydrolyses acetylcholine into choline and ethanoic acid

ATP is then used to reform the into neurotransmitter in vesicles

Function of synapses

Transmit impulses between neurones

Make sure impulse travels in one direction only

Filter out low level stimuli

Temporal summation

Only be stimulated if there are frequent action potentials

Spatial summaiton

Only stimulated if multiple presynaptic neurones are present

Agonist - Synapse drugs

Binds to receptor and activates it

Antagonist - synapse drugs

Binds to receptor but doesn’t activate (similar to competitive inhibitor)

Organphosphorous insecticides

Inhibit acetylcholinesterase

Causes repeated firing of action potentials across posy synaptic membrane

Label A-G

Dendrite

Node of ranvier

Axon terminal

Schwann cell - insulation

myelin sheath - insulation

axon - transmits impulse away form cell body

Nucleus in cell body

Local circuits

When the channels adjacent are stimulated to depolarise the next part of the axon