OCR A Level Biology: Neuronal Communication

Role of neurones

To transmit electrical impulses rapidly around the body to allow the organism to respond to changes in internal and external environment

Parts of a general neurone

Cell body, Dendron, axon

Role of the cell body

To produce neurotransmitters

Structure of cell body

Nucleus, cytoplasm, lots of endoplasmic reticulum, mitochondria

Function of dendrons

To transmit electrical impulses towards the cell body

Function of axons

To transmit electrical impulses away from the cell body

Structure of axons

Cylindrical, narrow region of cytoplasm surrounded by plasma membrane

Types of neurone

Sensory, relay, motor

Structure of sensory neurones

One Dendron, one axon an cell body in the middle

Structure of relay neurones

Many short axons and dendrons

Structure of motor neurones

One axon, many short dendrites with a cell body at the top

Function of sensory neurones

To transmit impulses from a sensory receptor cell to a relay neurone, motor neurone or the brain

Function of relay neurones

To transmit impulses between neurones

Function of motor neurones

To transmit impulses from a relay or sensory neurone to an effector

what are Myelinated neurones

Neurones that have axons covered in myelin sheaths

What makes the myelin sheath in myelinated neurones?

Schwann cells grow around the axon multiple times, surrounding the axon with layers of membrane

Name for gap between Schwann cells

Node of Ranvier

Why nodes of Ranvier are useful?

Cause signal to jump which allows faster rate of transmission- Saltatory conduction

Why is the rate of transmission slower in non-myelinated neurones?

No nodes of Ranvier so no jumping, continuous transmission is much slower

Types of sensory receptors

Mechano, chemo, thermo, photo

Stimulus mechanoreceptors respond to

Pressure, movment

Example of mechanoreceptor

Pacinian corpuscle

Example of sense organ with mechanoreceptors

Skin

Example of chemoreceptor

Olfactory receptor

Example of thermoreceptor

peripheral receptors in the skin

Where do you find end bulbs of Krause?

Tongue

Shared features of sensory receptors

Specific to a single type of stimulus, transducers

Role of sensory receptors as transducers

Sensory receptors convert stimulus into a nerve impulse (Generator potential)

Structure of Pacinian Corpuscle

End of neurone surrounded by layers of connective tissue separated by layers of gel, sodium ion channels in membranes, stretch-mediated sodium channels

How Pacinian Corpuscles do transducing

Sodium ion channels too narrow in a normal state, resting potential present, corpuscle changes shape when pressure applied to the corpuscle, membranes stretch, channels widen, sodium ions diffuse in, membrane depolarises, generates generator potential, generator potential creates action potential

Resting potential

The potential difference across a neurone's membrane when it isn't transmitting an impulse

When there is a resting potential, where is there a more positive charge?

Outside the membrane

How resting potential develops

Sodium ions actively transported out of the axon and potassium ions actively transported in by sodium potassium pump, more sodium ions outside the membrane and more potassium ions inside the cytoplasm, sodium ions try to diffuse in and potassium ions try to diffuse out, gated sodium ion channels closed so sodium ions can't diffuse, potassium ions can move freely, more positive ions outside than inside

General value for resting potential

-70mV

Depolarisation

Change in potential difference across a membrane from negative to positive

How generator potential develops

Receptor cells respond to stimuli, gated sodium ion channels open, larger stimuli will open more channels, sodium ions diffuse into the axon, inside of neurone is less negative, change in potential difference across the membrane is a generator potential

How action potential develops

Generator potential reaches threshold, voltage gated Na+ channels open, lots of Na+ diffuse into the axon (Positive feedback), membrane depolarised, voltage gated Na+ channels close, voltage gated K+ channels open, K+ diffuse out of membrane and become depolarised, potential difference overshoots, membrane becomes hyper polarised, resting potential restored by sodium potassium pump, refractory period

Where is there positive feedback in action potentials?

The diffusion of sodium ions into the axon when doing a generator potential will open voltage-gated sodium ion channels so more sodium ions diffuse in

Threshold voltage value

-50mV

Potential difference across membrane when depolarised

+40mV

Name for phase after repolarisation

Refractory period

Role of refractory period

To allow cell to recover, to only allow action potentials to be transmitted in one direction

How an action potential is transmitted down a myelinated neurone

Depolarisation happens at the nodes of Ranvier, sodium ions pass through protein channels at the nodes, localised circuits between nodes, action potential jumps from one node to another

Technical name for transmitting an action potential down a myelinated neurone

Saltatory conduction

Benefits of saltatory conduction

Faster as fewer places where channels have to open, more energy efficient as less repolarisation so less ATP required

All-or-nothing principle

If a stimulus crosses a threshold value, a response will always be triggered. If it doesn't, no action potential will be triggered. Size of action potential not affected by the size of the stimulus

How does size of the stimulus affect action potentials?

Larger stimuli cause more action potentials to be generated in a given time, increasing frequency, increasing degree of response.

Parts of a synapse

Synaptic cleft, presynaptic neurone, postsynaptic neurone, synaptic knob, synaptic vesicles, neurotransmitter receptors

Approximate size of the synaptic cleft

20-30 nm

Organelles the synaptic knob contains

Mitochondria, large amounts of endoplasmic reticulum

Types of neurotransmitter

Excitatory, inhibitory

Excitatory neurotransmitters

Neurotransmitters that result in the depolarisation of the postsynaptic membrane

Inhibitory neurotransmitters

Neurotransmitters that result in the hyperpolarisation of the postsynaptic membrane

Example of excitatory neurotransmitter

Acetylcholine

Example of inhibitory neurotransmitter

GABA

How impulses are transmitted across a synapse

Action potential reaches end of presynaptic neurone, depolarisation causes calcium ion channels to open, calcium ions diffuse to knob, vesicles containing neurotransmitters fuse with membrane, released by exocytosis, diffuse over, bind with receptor on the membrane, sodium ion channels open, sodium ions diffuse into neurone, triggers action potential, propagated along the neurone

Why neurotransmitter must be removed

Prevents response from happening again, neurotransmitter can be recycled

Specifics of the structure of cholinergic synapses

Acetylcholine is the neurotransmitter, hydrolysed by acetylcholinesterase, breaks down to choline and ethanoic acid, reformation requires ATP

Role of synapses

Ensuring impulses are unidirectional, allow impulse from one neurone to be transmitted to a number of neurones, allow an impulse from a number of neurones to feed into one

Summation

When the amount of neurotransmitter builds up to reach the threshold to trigger an action potential

Types of summation

Spatial, temporal

Spatial summation

When a number of presynaptic neurones are connected to one postsynaptic neurone

Temporal summation

When a single presynaptic neurone releases neurotransmitter several times over a short period as a result of several action potentials