Neurons Flashcards
Key Features
1) There are 100 billion nerve cells (neurons) in human nervous system, 80% are located in the brain.
2) Transmit signals electrically & chemically to provide nervous system with its primary means of communication.
Neuron Membranes
A neuron’s outer surface is made up of a semipermeable membrane - Allows smaller molecules and molecules without an electrical charge to pass through it.
–> It also stops larger or highly charged molecules.
3 Types of Neurons
1) Sensory Neurons:
- Carry messages from sensory receptors (eyes, tongue and skin) to CNS
- Long dendrites & short axons
- Located in PNS in clusters called ganglia
2) Relay Neurons:
- Connect sensory neurons to motor or other relay neurons
- Short dendrites & short axons
- 97% of neurons are relay & most are in CNS.
3) Motor Neurons:
- Connect CNS to effectors e.g. muscles & glands
- Short dendrites & long axons
- Located in CNS but long axons are outside CNS & form part of PNS
Structure of Neuron
1) Cell body (soma):
- Includes a nucleus which contains genetic material of the cell.
2) Dendrites:
- Branchlike structures that protrude from cell body .
- Receive nerve impulses from other neurons.
3) Axon:
- Carries electrical impulse away from cell body & down the length of the neuron.
- Covered in fatty myelin sheath that protects axon.
- Gaps in axon are called nodes of Ranvier
–> speed up transmission of impulse.
- Axon terminal - end of axon containing synaptic vesicles filled with neurotransmitters.
Electrical Transmission (Firing of Neuron)
1) When a neuron is in a resting state, the inside of the cell is negatively charged compared to the outside.
2) When a neuron is activated, inside of the cell becomes positively charged for a split sec causing an action potential to occur.
3) This creates an electrical impulse that travels down the axon towards the end of the neuron.
Synaptic (Chemical) Transmission
1) Signals within neurons are transmitted electrically, but signals between neurons are transmitted chemically across the synapse - gap between one neuron to next.
2) When electrical impulse reaches end of the neuron (the presynaptic terminal) it triggers the release of neurotransmitter from synaptic vesicles.
3) Once a neurotransmitter crosses the gap, it is taken up by a postsynaptic receptor site on the next neuron.
4) This binding triggers an action potential and the chemical message is converted back into an electrical impulse & the process of electrical transmission begins.
Neurotransmitters
1) They are chemicals that diffuse across the synapse to the neuron in the chain.
2) Each has its own specific molecular structure that fits perfectly into a postsynaptic receptor site, like a lock & key.
3) Each has specific functions, e.g.
–> Serotonin affects mood & social behaviour which is why it has been implicated as a cause of depression.
Excitation & Inhibition
Neurotransmitters generally has either an excitatory or inhibitory effect on neighbouring neuron.
1) Adrenaline - generally excitatory, increasing positive charge of postsynaptic neuron –> causes an electrical charge in the membrane of that cell resulting in excitatory post synaptic potential (EPSP) making it more likely to fire.
2) Serotonin - generally inhibitory, increasing negative charge of postsynaptic neuron –> results in an inhibitory postsynaptic potential (IPSP) making it less likely to fire.
3) Dopamine - an unusual neurotransmitter as it is equally likely to have excitatory or inhibitory effects on postsynaptic neuron.
Summation
1) Excitatory & inhibitory influences are summed & must reach a certain threshold in order for action potential to be triggered.
2) If net effect of neurotransmitters is inhibitory then postsynaptic neuron is less likely to fire (i.e. no electrical signal is transmitted) - It is more likely to fire if net effect is excitatory.
