5.1.3 Neuronal Communication Flashcards
(30 cards)
What is a transducer
Something that converts a stimulus into a nerve impulse
Types of sensory receptors and their stimulus
Mechanoreceptor (pressure and movement)
Chemoreceptor (chemicals)
Thermoreceptor (heat)
Photoreceptor (light)
Structure of a pacinian corpuscle
Layers of connective tissue seperated by gel
Neurone ending in centre
Capsule surrounding the corpuscle
Sensory neurone leads into it
How does a pacinian corpuscle work
Resting potential, stretch-mediated Na+ channels too narrow to allow Na+ in
When pressure is applied, corpuscle changes shape and membrane stretches, Na+ channels widen, Na+ diffuses in
Membrane becomes depolarised and generator potential is established
This creates an action potential that passes along the sensory neurone
Types of neurones
Sensory, relay, motor
Structure of sensory neurones
One axon and one dendron connected by a cell body
Structure of relay neurones
Many short axons and dendrons
Cell body in the centre
Structure of motor neurones
One long axon, many short dendrites
Cell body in centre of short dendrites
Structure of myelinated neurones
Schwann cells produce layers of membrane (phospholipid bilayer) around axon
Insulating layer
Nodes of Ranvier between Schwann cells
Role of sensory neurones
Transmit impulses from sensory receptor cell to relay neurone/motor neurone/brain
Role of relay neurones
Transmit impulses between neurones
Role of motor neurones
Transmit impulses from relay/sensory neurone to effector (muscle/gland)
Function of myelin sheaths
Speed up rate of electrical impulse along neurone
What is resting potential of a neurone
Potential difference across a neurones membrane when it is not transmitting an impulse
Outside is more positively charged than inside, membrane is polarised
= about -70 mV
How is resting potential created
- sodium-potassium pump (intrinsic protein) pumps 3 Na+ out of the axon and 2 K+ into the axon
- more Na+ outside axon cytoplasm and more K+ inside the cytoplasm
- Na+ diffuses into axon down electrochemical gradient
- K+ diffuses out axon down electrochemical gradient
- Na+ gated channels are mostly closed, but K+ are open so more positive ions outside than inside axon, creates resting potential of -70 mV
When does an action potential occur
A stimulus is detected by a sensory receptor, charge on axon membrane is reversed, potential difference across membrane changes rapidly, depolarisation (around +40 mV)
How does an action potential occur
- energy of stimulus triggers Na+ voltage-gated ion channels to open, membrane is more permeable, Na+ diffuses in down electrochemical gradient, less negative neurone
- more sodium ions actively transported channels open, more Na+ diffuses in, positive feedback
- p.d. Reaches +40 mV, voltage-gated Na+ channels close and voltage-gated K+ channels open
- membrane more permeable to K+, diffuses in down electrochemical gradient, reduces charge, inside more negative than outside
- hyperpolarisation: more K+ diffuse out, more negative than normal resting state
- voltage-gated K+ channels close, sodium-potassium pump causes Na+ to move out of cell, K+ moves in, axon returned to resting potential
How is an action potential transmitted along a neurone
- Depolarisation of one area of a neurone acts as a stimulus for the next region
- when Na+ is inside axon, attracted by negative charge ahead and concentration gradient so diffuse along
- repolarisation of one section, refractory period follows which ensures unidirectional action potentials, and that they don’t overlap
How does myelination increase rate of nervous transmission
Saltatory conduction
- depolarisation only occurs at nodes of ranvier
- action potential can then ‘jump’ to next node which is faster than wave of depolarisation along whole neurone as opening channels and ion moving takes time
- more energy efficient
What affects speed of action potential
Axon diameter: larger = faster transmission, less resistance to flow of ions
Temperature: higher = faster, ions move faster at higher temperatures (up until 40ºC)
what is the all-or-nothing principle
If level of stimulus is met (generator potential), a response will always be triggered
Same size action potential despite size of stimulus
No action potential if generator potential is not met
Structure of a synapse
Synaptic cleft: gap which seperates axon and dendrite of two different neurones
Pre-synaptic neurone: neurone where the impulse arrived from
Post-synaptic neurone: neurone that receives the neurotransmitter
Synaptic knob: swollen end of presynaptic neurone, many mitochondria and endoplasmic reticulum to manufacture neurotransmitters
Synaptic vesicles: vesicles containing neurotransmitters
Receptors: molecules which bind to neurotransmitters on postsynaptic membrane
Types of neurotransmitters
Excitatory: result in depolarisation of postsynaptic neurone, if threshold is met, action potential triggered (e.g. acetylcholine)
Inhibitory: result in hyperpolarisation of post-synaptic membrane, prevents action potential being triggered (e.g. GABA)
How are impulses transmitted across synapses
- action potential reaches end of presynaptic neurone
- depolarisation of presynaptic neurone causes Ca2+ channels to open
- Ca2+ diffuses in, vesicles containing neurotransmitters fuse with presynaptic membrane
- neurotransmitter released into synaptic cleft by exocytosis
- neurotransmitter diffuses across synaptic cleft, binds with receptors on postsynaptic membrane
- Na+ ligand-gated channels open, Na+ diffuses in and action potential is triggered
