Neuronal Communication Flashcards
What features are common to all sensory receptors
- Act as energy transducers which establish a generator potential
- Respond to specific stimuli
Describe the basic structure of the Pacinian corpuscle
Single nerve fibre surrounded by layers of connective tissue which are separated by viscous gel and contained by a capsule
Stretch-mediated Na+ channels on plasma membrane
Capillary runs along base layer of tissue
What stimulus does a Pacinian corpuscle respond to? How?
- Pressure deforms membrane, causing stretch mediated Na+ raises membrane to threshold
- If influx of Na+ raises membrane to threshold potential, a generator potential is produced
- Action potential moves along sensory neuron
Describe the features of all neurons
Cell body: Contains organelles & high proportion of RER
Dendrons: Branch into dendrites which carry impulses towards the cell body
Axon: long, unbranched fibres carries nerve impulses away from cell body
Describe the structure and function of a sensory neurone
Usually uni polar
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 sheath
Name 3 process Schwann cells are involved in
Electrical insulation
Phagocytosis
Nerve regeneration
Explain why myelinated axons conduct impulses faster than unmyelinated axons
Saltatory conduction: impulse jumps from one node of Ranvier to another. Depolarisation cannot occur where myelin sheath acts as an electrical insulator
So impulse does not travel along whole length
Where are myelinated and non-myelinated neurons found in the body
Myelinated: Most neurons in central and peripheral nervous systems e.g. those involved in spinal reflex
Non-myelinated: Group C nerve fibres involved in transmitting secondary pain
What is resting potential
Potential difference across a neuron membrane when not stimulated (-50V to -90V, usually about -70V in humans)
How is resting potential establish
- Membrane is more permeable to K+ than Na+
- Sodium-Potassium pump actively transports 3Na+ out of the cell and 2K+ into cell
Establishes electrochemical gradient: cell contents more negative than extracellular environment
Name the stages in creating an action potential
- Depolarisation
- Repolarisation
- Hyperpolarisation
- Return to resting potential
What happens during depolarisation
- Stimulus —-> facilitated diffusion of Na+ into cell down electrochemical gradient
- p.d. across membrane becomes more positive
- If membrane reaches threshold potential (-50mV) voltage-gated Na+ channels open (positive feedback mechanism)
- Significant influx of Na+ ions reverses p.d. to +40mV
What happens during repolarisation
- Voltage-gated Na+ channels close and voltage-gated K+ channels open
- Facilitated diffusion of K+ ions of the cell down their electrochemical gradient
- p.d. across membrane becomes more negative
What happens during hyperpolarisation
1.’Overshoot’ when K+ ions diffuse out = p.d. becomes more negative than resting potential
2. Refractory period: no stimulus is large enough to raise membrane potential to threshold
3. Voltage-gated K+ channels close 7 sodium-potassium pump re-establishes resting potential
Explain the importance of the refractory period
No action potential can be generated in the hyperpolarised section of the membrane
- Ensures unidirectional impulse
- Ensures discrete impulses
- Limits frequency of impulse transmission; larger stimuli have higher frequency
Why is the frequency of impulse transmission significant
Enables organism to distinguish size of stimulus although all action potentials have the same magnitude
Larger stimuli result in higher frequency of transmission since they overcome hyperpolarisation more quickly
What is the function of synapses
- Electrical impulse cannot cross junction
- Neurotransmitters send impulses between neurons / from neurons to effectors for excitatory or inhibitory response
- Summation of sub-threshold impulses
- New impulses can be initiated in several different neurons for multiple simultaneous responses
Describe the structure of a synapse
Presynaptic neuron end in synaptic knob: contains lots of mitochondria, endoplasmic reticulum & vesicles of neurotransmitter
Synaptic cleft: 20-30 nm gap between neurons
Postsynaptic neuron: has complimentary receptors to neurotransmitter (ligand-gated Na+ channels)
What happens in the presynaptic neuron when an action potential is transmitted between neurons
- Wave of depolarisation travels down presynaptic neuron, causing voltage gated Ca2+ channels to open
- Vesicles move towards and fuse with presynaptic membrane
3.Exocytosis of neurotransmitter into synaptic cleft
How do neurotransmitters travel across the synaptic cleft
Simple Diffusion
What happens in the postsynaptic neuron when an action potential is transmitted between neurons
- Neurotransmitter binds to specific receptor on postsynaptic membrane
- Ligand-gated Na+ channels open
- If influx of Na+ ions raises membrane to threshold potential, action potential is generated
What happens in an inhibitory synapse
1.Neurotransmitter binds to and open Cl- channels on postsynaptic membrane and triggers K+ channels to open
2. Cl- moves in and K+ moves out via facilitated diffusion
3. p.d. becomes more negative: hyperpolarisation so no action potential is generated
Define summation and name the two types
Neurotransmitter from several sub-threshold impulses accumulates to generate action potential
- Temporal summation
- Spatial summation
NB no summation at neuromuscular joints
What is the difference between temporal and spatial summation
Temporal : one presynaptic neuron releases neurotransmitter several times in quick succession
Spatial : Multiple presynaptic neurons release neurotransmitter
What are cholinergic synapses
Use acetylcholine as primary neurotransmitter.
Excitatory or inhibitory. Located at:
- Motor end plate (muscle contraction)
- Preganglionic neurons (excitation)
- Parasympathetic postganglionic neurons (inhibition e.g. of heart or breathing rate
What happens to acetylcholine from the synaptic cleft
- Hydrolysis into acetyl and choline by acetylcholinesterase (AChE)
- Acetyl and Choline diffuse back into presynaptic membrane
- ATP is used to reform acetylcholine for storage in vesicles.
