Signalling 2 Flashcards
What are the specialist cells used for electrical signalling?
Neurons
Define a nerve impulse:
What else can it be called?
Wave of altered charge across a nerve cell membrane that sweeps along the axon from cell body to termini
AKA: action potential, wave of depolarisation
What state is the neuron in at rest?
Membrane potential of -70mV maintained by ion pumps in plasma membrane (Na+/K+ pump moves 3 Na+ out for every 2 K+ in)
What occurs in Neuron when it is stimulated?
- Na+ ion channels open (ligand binding or mechanically gated channels open) causing cell to become more positive (DEPOLARISATION)
- Depolarisation causes K+ channels to open
- Na+ channels close after a while whilst K+ stays open
- K+ ions flow out of cell (REPOLARISATION)
- Slight overshoot of K+ leaving cell (HYPERPOLARISATION)
- K+ channels close, Na+ channels reset and return to resting potential
How does wave of depolarisation travel down axon?
Ions difuse down conc gradient
Change in membrane potential causes adjacent voltage-gated ion channels to open
Why does signal only travel one direction?
Refractory period: short period of time following action potential when new one cannot be inititated in small area of membrane
Absolute refractory period: Na+ channels cannot be activated and opened
Relative refractory period: Na channels work but start at hyperpolarisation makes it harder to reach threshold
How is a stronger signal sent?
Magnitude of action potential is same but increased frequency at which they are fired if there is a stronger stimulatory signal. =
What is the role of myelin?
Insulates neuron by blocking depolarisation. Current travels through myelinated stretches of neurone and depolarises only at the nodes of Ranvier
Increases speed of transmission
How is an actional potential initiated?
(clue - summation)
Summation of Excitory Pre-Synaptic Potentials (EPSPs) and Inhibitory Pre-Synaptic potentials (IPSPs)
If summation of all inputs leads to overall membrane potential reaching threshold potential of -55mV then Na+ channels open and AP is intitiated
Temporal summation: input occurs multiple time from same area
Spacial summation: inputs from different areas of cell
What are neurotransmitters?
Chemical signalling molecules that transmit a signal across a synapse.
Release when AP reaches pre-synaptic neuron termini, bind to receptors on dendrites to cause another AP
Different NT associated with different nervous system functions
Can be excitory or inhibitory
How are neurotransmitters ‘turned off’?
Reuptake of neurotranmitter or enzymes to degrade neurotransmitters in cleft
Chemical classification of neurotransmitters:
- Amino acids & derivatives: glutamate (main excitory neurotransmitter in CNS) or GABA (main inhibitory neurotransmitter in CNS)
- Catecholamines: derived from TYr, e.g. dopamine or serotonin
- Acetylcholine: neuromuscular juntions
- Peptides: e.g. substance P, endorphins
Use GABA as an example of a neurotransmitter:
- GABA released from pre-synaptic neuron into synapse
- GABA binds to GABA A receptor (an Cl- ion channel) causing a conformational change so ion channel opens
- Cl- ions flow into cell causing hyperpolarisation
- Hyperpolarisation means threshold potential is harder to reach so it has an inhibitory effect
Explain Acetylcholines action at a neuromuscular junction:
- ACh released from pre-synaptic bulb
- Diffuses across synaptic cleft
- ACh binds to nAChR receptor (a sodium ion channel receptor)
- Ligand gated ion channel opens and Na+ entres cell
- Na+ influx causes depolarisation of muscle cell membrane resulting in contraction
- Acetylcholinesterase in cleft removes ACh
How does signalling work at GAP junctions? Give examples:
Small pores between adjacent cells formed by connexin orteins. Ions carry charge from one cell to another as they are small enough to fit through.
Examples:
- Electrical synapses between neurons (although most are chemical)
- Cardiac myocytes: specialised cells allow electricle impulse to travel across heart tissues causing contraction. Charge flows between adjacent cells.
