C13 - Neurones

M5 - Communication and energy

features of receptors

• specific = to stimuli - light, temperature, pressure

• connect with sensory neurones and create a generator potential when stimulated

• e.g. pacinian corpuscle

mechanoreceptors

receptors that respond to pressure changes to establish a generator potential

the pacinian corpuscle

• mechanoreceptors in the skin

• rings of concentric connective tissue around a sensory neurone

• when not stimulated = at resting rate

pacinian corpuscle - resting potential

• at resting rate = more na+ ions outside the neurone than inside

• causes a potential difference of -70mv

• stretch mediated Na+ channels too narrow for movement

pacinian corpuscle - stimulation

• pressure applied = rings of connective tissue apply pressure on sensory neurone

• neurone has stretch mediated Na+ channels - they widen

• channels normally restrict movement of Na+

• pressure causes the channels to open

pacinian corpuscle - generator potential

• open Na+ channels = Na+ ions flood sensory neurone

• more Na+ ions inside the neurone than outside = depolarised

• charge inside neurone is more positive than outside

• changes potential difference

• creates a generator potential

pacinian corpuscle - action potential

• if generator potential reaches threshold (-50mv)

• action potential produced in sensory neurone

sensory neurones

carry nervous impulses from receptors to the CNS

• 1 dendron

• 1 axon

motor neurone

carry impulses from the CNS to effector organs

• 1 axon

• many short dendrites

relay neurones

intermediate neurones - receive impulses from sensory neurones and relay to motor neurones

• many short axons

• many short dendrons

neurone structure

• myelinated or non myelinated

• dendrons/dendrites

• axons

• cell body

dendrons

extensions from a cell body, divide into smaller branched = dendrites

• transmit impulses towards cell body

axons

single elongated nerve fibre that carries impulses away from the cell body

• can be very long

• cylindrical = narrow cytoplasm surrounded by membrane

cell body

where the nucleus of a neurone is located, surrounded by cytoplasm

• large amount of ER and mitochondria - for neurotransmitter production

myelinated motor neurone

• often myelinated in vertebrae

• Schwann cells wrapped around the neurone axon

• these form the myelin sheath

• nodes of ranvier

• myelin inc. speed of electrical impulse

nodes of ranvier

gaps between adjacent Schwann cells in the myelin sheath around the axon

potential difference

the difference in charge across a neurone membrane at resting state

• neurone more neg. charged as more positive ions are outside

sodium potassium pumps

• maintain resting potential

• 3 Na+ transported out neurone - 2 K+ transported in

• an active process

• leads to build up of positive ions outside cell

potassium ion channels

• K ion channels in neurone membrane - more permeable to K+

• when K+ are transported in neurones - can diffuse back out

• neurone membrane impermeable to Na+

• so ions can diffuse back into cell after being transported out

resting potential

the potential difference across a neurone membrane = -70mv

• maintained by K+ and Na+/K+ pumps

• neurone is said to be polarised

steps of an action potential

• stimulation

• depolarisation

• all or nothing

• repolarisation

• hyperpolarisation

• resting potential

action potential - 1. stimulation

• neurone stimulated

• Na+ ion channels open (voltage gated)

• Na+ ions flood into axon down gradient

• changes potential difference (axon is less negative)

action potential - 2. depolarisation

• if potential dif. inc. over threshold (-50mv)

• membrane depolarises

• more Na+ channels open, so more Na+ enter axon = pos. feedback

• sharp inc. in potential difference = to +30mv

• voltage gated Na+ close, voltage gated K+ open

action potential - 3. all or nothing

• depolarisation is an all or nothing response

• if threshold reached - depolarisation occurs (always the same change)

• if stimulus stronger than threshold - more action potentials more frequently (size won’t increase)

action potential - 4. depolarisation

• once membrane has depolarised to +30mv

• Na+ channels close, K+ channels open

• K+ leaves neurone down gradient - reduces charge = more negative axon

• potential difference becomes more negative = repolarisation

action potential - 5. hyperpolarisation

• period after repolarisation where pot. dif. becomes slightly more negative than resting potential - due to many K+ leaving axon

• prevents neurone being re stimulated instantly

• refractory period

• voltage gates K+ then close

• Na+/K+ pump = moves Na+ out cell, K+ in

action potential- 6. resting potential

• after refractory period

• K+ channels close

• membrane returns to resting potential

action potentials moving along a neurone - Na+ ions

• action potentials move along in a wave

• when its generated = more Na+ ions inside neurone than outside

• some Na+ diffuse sideways along the neurone axon

action potentials moving along a neurone - Na+ channels

• presence of Na+ changes potential difference further along membrane

• if it reaches threshold - Na+ channels further down open

• Na+ diffuses into neurone

• this section of the neurone becomes depolarised

action potentials moving along a neurone - wave of depolarisation

• Na+ diffuse all along the neurone

• takes place in neurone membrane

• creates a wave of depolarisation

action potential moving along a membrane - refractory period

• period of hyperpolarisation

• ion channels are recovering - cant stimulate an action potential immediately

• ensures wave of depolarisation only travels in 1 direction

factors that speed up nervous transmission

• myelination

• temperature

• axon diameter

speeding up transmission - axon diameter

• giant axons in giant squid

• allows a rapid escape response

• greater axon diameter = greater SA for ion movement across membrane

speeding up transmission - temperature

• inc temp = inc. kinetic energy

• ions have more kinetic energy

• move across membrane more rapidly

speeding up transmission - myelination

• Schwann cells wrap around axon - myelin sheath

• acts as electrical insulation = impermeable to ions

• depolarisation/action pot. cna’t occur at myelinated parts of the axon

• can only occur in gaps (nodes of ranvier)

• nerve impulse jumps from one node to the next

saltatory conduction

a method of rapid nervous transmission

• where impulses jump from one node of ranvier to the next, skipping myelinated sections

synapse

a junction between two neurones, or between a neurone and an effector

synaptic cleft

a gap between 2 cells at a synapse

• transmits action potentials across the synapse

presynaptic neurone

the neurone before the synapse

• AP reaches end of neurone

• transmitted across presynaptic membrane to postsynaptic membrane

synaptic knob

the end of the axon of the presynaptic neurone

• a swelling containing synaptic vesicles

• where nerve impulse is transmitted across synaptic cleft

• contains many mitochondria - energy needed to synthesise neurotransmitters

synaptic vesicles

vesicles containing neurotransmitters located in the synaptic knob

• when they fuse with the presynaptic membrane - neurontransmitters released into cleft

neurotransmitters

chemicals that allow an action potential to be transferred across a synapse

• realised from synaptic vesicle into synaptic cleft

• bind to specific receptors on postsynaptic membrane

postsynaptic membrane

the membrane of postsynaptic neurone/effector cells

• receptors on postsynaptic mem. have complimentary shape to neurotransmitters from synaptic knob

• neurotransmitters bind to receptors - continues action potential

• only receptors on post synaptic = ensures impulse moves in right direction

features of a synapse

• presynaptic membrane

• synaptic knob

• synaptic vesicles

• neurotransmitters

• synaptic cleft

• postsynaptic membrane

types of neurotransmitters

• excitatory

• inhibitory

inhibitory neurotransmitters

prevent an action potential being generated in the postsynaptic cell

inhibitory neurotransmitters - how they work

• when they bind to postsynaptic receptors - membrane is hyper polarised

• opens K+ channels

• prevents action potential

excitatory neurotransmitters

generate an action potential in the postsynaptic cell

excitatory neurotransmitters - how it works

• bind to postsynaptic receptors - membrane is depolarised

• establishes action potential

summation

the process where neurotransmitters from multiple neurones are summed together to produce a response

types of summation

• spacial

• temporal

spacial summation

when multiple presynaptic neurones from a junction with a single neurone

spacial summation - how it works

• each presynaptic neurone releases neurotransmitters

• many neurotransmitters bind to receptors on 1 postsynaptic memb.

• together the neurotransmitters can establish a generator potential

• this reaches threshold and generates an action potential

temporal summation

when multiple nerve impulses arrive at the same synaptic knob within a short period of time

temporal summation - how it works

• more neurotransmitters realised into the synaptic cleft

• more neurotransmitter available to bind to postsynaptic receptors

• together - neurotransmitters can establish a generator potential

• reaches a threshold and generates action pot.

neuromuscular junction

a synapse between a motor neurone and a muscle cell

• action potential transmitted using acetylcholine ( a neurotransmitter)

steps to impulse transmission with acetylcholine

• arrival at synaptic knob

• release of acetylcholine

• binding to receptors

• removal of acetylcholine

synapse transmission with acetylcholine - arrival to knob

• A pot. arrives at synaptic knob of motor neurone

• depolarises synaptic knob membrane

• voltage gated Ca2+ channels open

• Ca2+ ions diffuse into knob

synapse transmission with acetylcholine - release of ACh

• Ca2+ conc. inside knob increases

• synaptic vesicles move and fuse with presynaptic membrane

• acetylcholine is released into synaptic cleft - by exocytosis

synapse transmission with acetylcholine - binding to receptors

• ACh binds to receptors on postsynaptic mem. = nicotinic cholinergic receptors

• binding of ACh opens Na+ channels in postsynaptic muscle cell

• Na+ diffuses into muscle = depolarisation

• if reaches threshold - generates action potential

synapse transmission with acetylcholine - removal of ACh

• acetylcholinesterase (AChE) breaks down ACh in the synaptic cleft

• products of breakdown reabsorbed by presynaptic neuron

• used to resythesis more ACh

• ACh must be removed from receptors = stops action potential being continuously generated in postsynaptic

nicotinic cholinergic receptors

specific receptors in postsynaptic membranes that acetylcholine binds to

cholinergic synapses

a synapse that uses acetylcholine as a neurotransmitter

• between 2 neurones

cholinergic vs neuromuscular junctions - postsynaptic cell

• cholinergic = between 2 neurones

• neuromuscular = 1 motor neurone, 1 muscle cell

cholinergic vs neuromuscular junctions - number of receptors

• cholinergic = less receptors

• neuromuscular = more receptors

cholinergic vs neuromuscular junctions - response type

• cholinergic = inhibitory or excitatory response

• neuromuscular = always excitatory

cholinergic vs neuromuscular junctions - result of depolarisation

• cholinergic = results in action potential

• neuromuscular = results in muscle contraction

cholinergic vs neuromuscular junctions - acetylcholinesterase (AChE)

• cholinergic = found in synaptic cleft

• neuromuscular = stored in clefts in postsynaptic membrane

reasons for coordination

• coordinated cell functions and systems to operate effectively

• most systems cant work in isolation

what does cell signalling do

• transfers signals locally = e.g. between neurones at synapses

• transfer signals over large distances = e.g. hormones like ADH acting on kidneys

coordination in plants

• down have nervous system

• achieved with plant hormones

neurones

specialised nerve cells

• transmit electrical impulses rapidly around the body

• so the organism can respond to changes in environment

multiple sclerosis

neurological condition - affects brain and spinal chord

• autoimmune disease

• problems with muscle movement, balance, vision

multiple sclerosis - what happens

• immune system attacks healthy body cells

• causes thinning/loss of myelin sheath

• once advanced = breakdown of axons/neurones

mechanoreceptor

• stimulated by pressure and movement

• e.g. pacinian corpuscle, in skin

transducers

• all sensory receptors are transducers

• they convert a stimulus into a nerve impulse

chemoreceptor

chemical stimuli

• e.g. olfactory receptors (detects smells), in nose

thermoreceptors

detect heat

• e.g. end-bulbs of Krause, tongue

photoreceptors

detect light

• e.g. cone cell (dif. light wavelengths), in eye

pacinian corpuscle - found where

• in skin

• abundant in fingers and soles of feet

• in joints = so you know which joints are changing direction

pacinian corpuscle - structure

• end of sensory neurone is in centre of corpuscle

• surrounded by layer of connective tissue - each layer separated by gel

• Na+ channels within membrane

• neurone ending in corpuscle = has stretch mediated Na+ channel

• blood capillary around connective tissue layers, under capsule

propagation of action potentials

the nerve impulse is propagated from one end of a neuron to the other

• first region of depolarisation triggers the rest of the neurone

propagation of action potential - 1. depolarisation

• resting potential of axon

• more ions outside = more positive = membrane is polarised

• stimulus = influx of Na+ ions

• charge reverses - depolarises membrane

• creates action potential

propagation of action potential - 2. voltage gated channels

• influx of Na+ = opens voltage gated Na+ channels

• establishes local electrical circuits = extend slightly further down axon

• influx of Na+ in new region - causes depolarisation

• behind new region - VG Na+ channels close, VG K+ open

• K+ leaves axon down gradient

propagation of action potential - 3. repolarisation

• depolarisation continues down the axon

• due to K+ moving out in the earlier sections - membrane reploarises

• hyper polarises

• axon membrane returns to resting potential

oscilloscope

measures the presence and frequency of action potentials

transmission of impulses across synapses - 1. depolarisation

• action potential reaches end of presynaptic neurone

• depolarises presynaptic membrane

• Ca+ channels open - Ca+ diffuses into presynaptic knob

transmission of impulses across synapses - 2. vesicles

• synaptic vesicles fuse with presynaptic membrane

• neurotransmitter released into synaptic cleft - exocytosis

• neurotransmitter diffuses across synaptic cleft

• binds with specific receptor on postsynaptic membrane

transmission of impulses across synapses - 3. Na+ channels

• neurotransmitter causes Na+ channels to open

• Na+ diffuses into postsynaptic neurone

• triggers an action potential

• impulse propagated along postsynaptic neurone

transmission of impulses across synapses - 4. ACh

• any acetylcholine left in the cleft is broken down

• releases ACh from receptors

• products taken back to presynaptic knob

• ATP from mitochondria used to recombine products of ACh breakdown - stored in vesicles

role of synapses

• ensure unidirectional impulses

• allow impulse from 1 neuron to send impulses to multiple neurones - so a single stimulus creates a number of simultaneous reponses

• many neurones can feed into 1 synapse - different stimuli create single result

effects of drugs on synapses - stimulators

• mimic shape of neurotransmitters = can bind to receptors and trigger action potentials (nicotine)

• stimulate release of more neurotransmitters (amphetamines)

• inhibit neurotransmitter breakdown enzyme - loss of muscle control (nerve gas)

effects of drugs on synapses - nervous inhibitors

• blocks receptors = neurotransmitter cannot bind and active receptor, can cause paralysis (curare)

• binds to specific receptors and changes shape of receptor = can increase binding of neurotransmitter, increases activity (alcohol and GABA receptors