Nervous system

Explain resting potential (7)

• Na+/K+ pump actively transports 3Na+ out of axon and 2 K+ in, which requires ATP

• Concentration gradient across membrane is established

• Membrane is partially permeable to K+ so they move out of axon by facilitated diffusion

• Membrane is relatively impermeable to N+, so they can’t move into the axon

• Uneven distraction of ions causes pd to be established across membrane

• Membrane is polarised

• Inside of axon has pd -70mV

Explain action potential (4)

• when the nerve is stimulated, the Na+ gated channels open

• Na+ rapidly move into area by facilitated diffusion

• If threshold is reached, the membrane becomes depolarised

• Inside of axon has pd +40mV

Explain repolarisation

• Na+ gated channel closes

• Na+/K+ pump actively transports Na+ back out of axon

• K+ gated channel opens

• Permeability of membrane to K+ increases, allowing them to diffuse out at a faster rate

• Membrane becomes hyper polarised

• Inside of axon has pd of -40mV

Oscilloscope trace of action potential

1) Resting potential

2) Action potential

3) Depolarisation

4) Hyperpolarisation

Sub threshold stimuli

• Do not cause enough Na+ gated channels to open to reach threshold

• No action potential is generated

Threshold stimulus

• causes all Na+ gated channels to open

• Reaches threshold

• Action potential is generated

All or nothing law

Stimulus causes or doesn’t cause an action potential

Intensity of stimulus above threshold is conveyed by…

Frequency of action potentials

Refractory period

• Na+ gated channels can’t be opened

• No action potential

• Occurs when membrane is hyperpolarised

Importance of refractory period

• prevents actions potentials from mergijg into eachother

• Limits frequency of action potentials

• Ensures that nerve impulse travels in one direction only

Propogation of a nerve impulse

1. when nerve is stimulated an action potential is established in one section of the axon

2. Inside of axon becomes positively charged

3. Localised electrical circuit is set up along axon membrane, causing Na+ gated channels to open in next section of axon

4. Na+ diffuses into axon, establishing action potential

5. Process repeats so each section of neurone is depolarised in sequence

3 factors affecting speed of propagation of a nerve impulse

1. temperature

2. Diameter of axon

3. Mylination

How does temperature affect speed of propagation (5)

• increase of temperature increases speed

• Provides more KE

• ions diffuse faster

• Rate of respiration increases so more ATP for Na+ / K+ pump

How does diameter of axon increase speed of propagation of nerve impulse (4)

• increase in diameter increases speed

• More cytoplasm, so more ions

• Increase in diameter, decrease longitudinal resistance along axon

• Localised circuits are longer

Saltatory Propagation

Action potentials jump from node to node

Nodes of Ranvier

• Unmyelinated gaps in myelinated neurones where action potentials can be established

How myelination increases speed of propagation of nerve impulses

• Schwann cells insulate axon

• Preventing movement of Na+ / K+

• Action potentials can only be established at nodes of renvier

• Action potential jumps from node to node (saltatory propagation)

Schwann cell

Myelinated cell of neurone

synapse

Junction / gap between neurones

Communication between two neurones is via

Chemicals called neurotransmitters

Transmitters we need to know

Noradrenaline

Acetylcholine (Ach)

A) presynaptic neurone

B) mitochondrian

C) vesicles containing neurotransmitter

D) pre synaptic membrane

E) post synaptic membrane

Space between post and pre synaptic neurone

Synaptic cleft

Describe synaptic transmission (8)

1. Action potential arrives at synaptic knob

2. This causes Ca2+ gated channels to open, Ca2+ rapidly enters the synaptic knob by facilitated diffusion

3. Synaptic vesicles move to and fuse with pre synaptic membrane

4. Ach is released into synaptic cleft by exocytosis

5. Ach diffuses across synapse to post synaptic membrane

6. Ach binds to complimentary shaped receptors on the post synaptic membrane

   the receptors are linked to Na+ gated channels, binding causes them to open

7. Na+ rapidly move into post synaptic neurone by facilitated diffusion

8. If the threshold is met, an action potential is established in the post synaptic neurone

Describe what happens in synaptic transmission once an action potential has been generated (5)

Ach binds to active sire of Achase

This hydrolyses the Ach into its components (choline and acetate)

Components diffuse back towards pre synaptic membrane

Products are taken back into pre synaptic neurone by endocytosis, and Ach is reformed inside vesicle

Energy, from the hydrolysis of ATP is required.

Functions of synapse (4)

1. They are junctions

2. They ensure that the nerve impulse travels in one direction only (pre synaptic neurones have neurotransmitter vesicles, post synaptic neurones have receptors)

3. Prevents generation of actions potential in the post synaptic neurone

4. Allows summation

How do synapses stop the generation of an action potential in the post synaptic membrane?

• inhibitory synapses: binding of the neurotransmitter results in opening of ion channels

• that result in post synaptic membrane becoming hyperpolarised,

• Less likely that threshold is reached

• No action potential occurs

Summation definition and types

• definition: additive effects of several nerve impulses arriving at synapse

• Single nerve impulse only opens some Na+ gated ion channels, so no action potential

• Spatial summation: several pre synaptic neurones at one synapse, so several impulses arrive at synapse at the same time

• Temporal summation: one pre synaptic neurone. If high intensity stimulus, several impulses arrive at the synapse one after the other

2 types of drugs

Stimulants / agonists

Depressants / inhibitory / antagonists

Stimulants function (6)

• cause repeated stimulation of post synaptic neurone

• Increasing frequency of action potentials

• Neurotransmitter remains in synaptic cleft: inhibits Achase or prevents uptake of neurotransmitter

• Mimics neurotransmitter, binding to receptors and opening Na+ channels

• Decreasing threshold of post synaptic neurone

Depressants function (6)

• Prevent stimulation of post synaptic neurone

• decreasing or preventing action potentials

• Prevent entry of calcium 2+

• Prevent release of neurotransmitters

• Block neurotransmitter receptors

• hyperpolarise the post synaptic membrane

Receptors and transducers (3)

• detect stimuli (change in external or internal environment)

• Are specific to particular stimulus

• Transducers convert one form of energy into electrochemical energy in the form of a generator potential

Pacinian corpuscles (4)

• found in skin

• Sensitive to pressure

• Umyelinated end of sensory neurone, surrounded by concentric rings of connective tissue

• Axon membrane has stretch-mediated Na+ gated channels

Generator potential of pacinian corpuscles (5)

1. no pressure is applied:

• pacinian corpuscle is round, Na+ gated channels are closed, resting potential is established across membrane

1. Pressure is applied;

   • pacinian corpuscle is distorted, Na+ gated channels open

   • Na + diffuse into neurone

2. Membrane becomes depolarised

   • inside of neurone becomes positively charged, generator potential is created.

Retina contains..

Photoreceptors;

Rods and cones

Rods (8)

• more rods than cones

• Distributed evenly throughout retina but absent from fovea

• Contains light sensitive pigment rhodopsin

• Provides monochromatic vision

• Very sensitive, requires 1 photon of light to produce generator potential

• Functions at low light intensity

• Group of rod cells synapse with one bipolar neurone (retinal convergence)

• Low visual acuity / poorly resolved images

Cones (8)

• less cones than rods

• Present mainly in the fovea but absent from periphery

• Contains 3 types of iodipsin (photosensitive pigment); sensitive to red, blue, and green wavelengths of life

• Provides colour vision

• Less sensitive. Requires approx 100 photons of light to produce generator potential

• functions at high light intensity

• Each cone cell synapses individually with a single bipolar neurone

• Produces highly resolved images

Compare cones and rods (8)

Bipolar cell

Relay neurone

Fovea

Central pit of retinal muscle

Why are rods high sensitivity

• spatial summation / retinal convergence

low light intensity stimulus:

• several rod cells depolarise

• Due to spatial summation, it will cause the bipolar cell to depolarise

Why are cones low sensitivity

Temporal summation

• low intensity light will not generate action potential in the bipolar cells

• Requires high light intensity to bring about temporal summation to generate an action potential in bipolar cells, and ganglion cell to send impulse into the brain