Neurons and synaptic transmission Flashcards
What are neurons?
- there are 100 billion neurons in the human nervous system, 80% of which are located in the brain
- by transmitting signals electrically and chemically, these neurons provide the nervous system with its primary means of communication
What are the different types of neurones?
- motor neurons,
- sensory neurons
- relay neurons
What are sensory neurons?
- these carry messages from the PNS to the CNS
- they have long dendrites and short axons
What are relay neurons?
- these connect the sensory neurons to the motor neurons or other relay neurons
- they have short dendrites and short axons
What are motor neurons?
- these connect the CNS to effectors such as muscles and glands
- they have short dendrites and long axons
What is the structure of neurons?
-neurons vary in size from less than a millimetre to up to a metre long, but all share the same basic structure
-the cell body includes a nucleus, which contains the genetic material of the cell
-branch like structures called dendrites protrude from the cell body
-these carry nerve impulses from neighbouring neurons towards the cell body
The axon carries the impulses away from the cell body down the length of the neuron
-the axon is covered in a fatty layer of myelin sheath that protects the axon and speeds up electrical transmission of the impulse
-if the myelin sheath was continuous this would have the reverse effect and slow down the electrical impulse
-thus the myelin sheath is segmented by gaps called nodes of Fancier
-These speed up the transmission of the impulse by forcing it to jumps across the gaps along the axon
-finally at the end of the axon are terminal buttons that communicate with the next neuron in the chain across a gap known as the synapse
What are electric transmissions - firing neurons?
- when a neuron is in a resting state the inside of the cell is negatively charged compared to the outside
- when a neuron is activated by a stimulus, the inside of the cell becomes positively charged for a split second causing an action potential to occur
- this creates an electrical impulse that travels down the axon towards the end of the neuron
What is chemical transmission - synapses?
- neurons communicate with each other within groups known as neural networks
- each neuron is separated from the next by a synapse
- the synapse includes the space between them as well as the presynaptic terminal and postsynaptic receptor site
- signals within neurons are transmitted electrically; however signals between neurons are transmitted chemically by synaptic transmission
- when the electrical impulse reaches the end of the neuron it triggers the release of neurotransmitter from tiny sacs called synaptic vesicles
What are neurotransmitters?
- neurotransmitters are chemicals that diffuse across the synapse to the next neuron in the chain
- once the neurotransmitter crosses the gap, it is taken up by the postsynaptic receptor sites - in other words, the dendrites of the next neuron
- here, the chemical message is converted beck into an electrical impulse and the process of transmission begins again in this other neuron
- several dozen types of neurotransmitter have been identified in the brain as in the spinal cord and some glands
- each neurotransmitter has its own specific molecular structure that fits perfectly into a post-synaptic receptor site, similar to a lock and key
- neurotransmitters also have specialist functions
- for instance, acetylcholine is found at each point where a motor neuron meets a muscle, and upon its release it will cause muscles to contract
What is excitation?
- when a neurotransmitter, such as adrenaline, increases the positive charge of the postsynaptic neuron
- this increases the likelihood that the neuron will fire and pass in the electrical impulse
What is inhibition?
- when a neurotransmitter such as serotonin makes the charge of the postsynaptic neuron more negative
- this decreases the likelihood that the neuron will fire and pass on the electrical impulse
