Neuronal Communication Flashcards
What are sensory receptors?
Specialized cells that detect specific stimuli
Act as energy transducers which establish a generator potential
Describe the basic structure of a Pacinian Corpuscle
Single nerve fibre surrounded by layers of connective tissue contained by a capsule
Stretch-mediated Na+ channels on membrane
Capillary runs along the base layer of tissues
What stimulus does a Pacinian corpuscle respond to? How?
Pressure deforms membrane, causing Na+ channels to open
If influx of Na+ raises membrane to threshold potential, a generator potential is produced
Action potential moves along sensory neurone
Describe the features of all neurons
Cell body - contains organelles and high proportion of RER
Dendrons - branch into dendrites which carry impulses towards the cell body
Axon - long, unbranched fibre carries nerve impulses away from cell body
Describe the structure and function of a sensory neuron
Usually unipolar
Transmits impulses from receptors to CNS
Describe the structure and function of a relay neuron
Usually bipolar
Transmits impulses between neurons
Describe the structure and function of a motor neuron
Usually multipolar
Transmits impulses from relay neurons in the CNS to effectors
Describe the additional features of a myelinated neuron
Schwann cells - wrap around axon many times
Myelin sheath - made from myelin-rich membranes of Schwann cells
Nodes of Ranvier - very short gaps between neighbouring Schwann cells where there is no myelin
Explain why myelinated axons conduct impulses faster than unmyelinated axons
Saltatory conduction - impulses jump between nodes of Ranvier.
Faster than continuous depolarisation
impulse does not travel along whole axon length
How does axon diameter affect transmission speed?
Larger diameter means less resistance to ion flow so wave of depolarization travels faster
What is resting potential?
Potential difference across neuron membrane when not stimulated
-70mV
How is resting potential established?
Na/K pumps - 3:2
Na+:K+
Some voltage gated K+ channels open, so some K+ leaks out
Voltage gated Na+ channels closed
Organic anions inside axon
Name the stages in generating an action potential
Depolarisation
Repolarisation
Hyperpolarisation
Refractory period
What happens during depolarisation?
Facilitated diffusion of Na+ into cell
Potential difference across membrane becomes more positive
If membrane reaches threshold potential, voltage-gated Na+ channels open
Potential difference reaches +30mV
What happens during repolarisation?
Voltage-gated Na+ channels close and voltage-gated K+ channels open
Facilitated diffusion of K+ ions out of cell
Potential difference across membrane becomes more negative
What happens during hyperpolarisation?
Excess of K+ ions leaves the axon, dropping the p.d below the resting level -70mV
Explain the importance of the refractory period?
No action potential can be generated in hyperpolarized sections of membrane
Limits frequency of impulses transmission; larger stimuli have higher frequency
What is summation in synapses?
Spatial summation - lots of presynaptic neuron converge on a single postsynaptic neuron
Temporal summation - single neuron fires action potentials in quickly
Describe the structure of a synapse
Presynaptic neuron ends in synaptic bulb - contains lots of mitochondria, ER and vesicles of neurotransmitter
Synaptic cleft - gap between neurons
Postsynaptic neuron - has complementary receptors to neurotransmitters
What happens in the presynaptic neuron when an action potential is transmitted between neuron
Wave of depolarisation travels down presynaptic neuron causes voltage-gated Ca2+ channels to open
Vesicles move towards and fuse with the membrane of the presynaptic bulb
exocytosis of neurotransmitters into the synaptic cleft
How do neurotransmitters cross the synaptic cleft?
simple diffusion
What happens in the postsynaptic neuron when an action potential is transmitted between neurons?
Neurotransmitters bind to specific complementary receptors of postsynaptic membrane
Ligand-gated Na+ channels open
If influx of Na+ ions raises membrane to threshold potential, an action potential is generated
What happens in inhibitory synapse?
Neurotransmitter binds to and opens Cl- channels on postsynaptic membrane and triggers K+ channels to open
Cl- moves in and K+ moves out via facilitated diffusion
potential difference becomes more negative - hyperpolarisation so no actionpotential
Define summation and name the 2 types
neurotransmitter from several sub-threshold impulses accumulates to generate action potential
Temporal summation
Spatial summation
What are cholinergic synapses?
Use acetylcholine as primary neurotransmitter. Exitatory or inhibitory.
What is the difference between temporal and spatial summation?
Temporal - one presynaptic neuron releases neurotransmitters serveral times in quick succession
Spatial - multiple presynaptic neurons release neurotransmitters
What happens to acetylcholine from the synaptic cleft
Hydrolysis into acetyl and choline by acetylcholinesterase AChE
Ethanoic acid and choline diffuse back into presynaptic membrane
ATP used to reform acetylcholine for storage in vesicles
How does temperature affect transmission speed?
Higher temp increases diffusion of ions so faster depolarization and transmission
