Neurons and synaptic transmission Flashcards
what is a neuron
nerve cells that process and transmit messages through electrical and chemical signals
what are the three types of neurons
sensory neurons
relay neurons
motor neurons
sensory neurons
carry messages from PNS to the CNS. Tell brain about external and internal environment. They have long dendrites and short axons.
relay neurons
these connect the sensory neurons to the motor or other relay neurons. They have short dendrites and short axons.
motor neurons
these connect the CNS to effectors such as muscles and glands. They have short dendrites and long axons.
The structure of a neuron
The cell body includes a nucleus, which contains the genetic material of the cell.
Dendrites protrude from the cell body, they carry nerve impulses from neighbouring neurons towards the cell body.
Axon carries impulses away from the cell body. Axon covered in a fatty layer of myelin sheath that protects the axon and speeds up electrical transmission of the impulse.
The myelin sheath has gaps called Nodes of Ranvier which speed up the transmission of the impulse by forcing it to jump across the gaps along the axon.
Terminal buttons communicate with the next neuron in the chain across the synapse.
Location of sensory neurons
located outside of the CNS, in the PNS in clusters known as ganglia.
Location of relay neurons
make up 97% of all neurons. Found within the brain and the visual system.
Location of motor neurons
cell bodies of motor neurons may be in the CNS but they have long axons which form part of the PNS.
Electrical transmission – the firing of a neuron
In a neurons resting state, the inside of the cell is negatively charged compared to the outside.
When a neuron is activated by 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.
Synaptic transmission – Chemical transmission
Synaptic transmission is the process by which neighbouring neurons communicate with each other by sending chemical messages across the synapse that separates them.
When the electrical impulse reaches the presynaptic terminal, it triggers the release of neurotransmitters from synaptic vesicles.
Synaptic transmission – Neurotransmitters
Neurotransmitters are chemicals that diffuse across the synapse to the next neuron in the chain. Once it has crossed the synapse, it’s taken up by a postsynaptic receptor site on the dendrites of the next neuron. The chemical message is converted back into an electrical impulse.
Each neurotransmitter has its own specific molecular structure that fits perfectly into a postsynaptic receptor site.
Synaptic transmission – Excitation and inhibition
Neurotransmitters either have an excitatory or inhibitory effect on the neighbouring neuron.
Excitation – increases the positive charge of the postsynaptic neuron. This increases the likelihood that the postsynaptic neuron will pass on the electrical impulse. E.g. adrenaline.
Inhibition – increases the negative charge of the postsynaptic neuron. This decreases the likelihood that the postsynaptic neuron will pass on the electrical impulse. E.g. serotonin.
Synaptic transmission – Summation
Whether a postsynaptic neuron fires is decided by the process of summation. If the net effect on the postsynaptic neuron is inhibitory then the postsynaptic neuron is less likely to fire. If the net effect is excitatory, it is more likely to fire. Once the electrical impulse is created, it travels down the neuron.
The action potential of the postsynaptic neuron is only triggered if the sum of the excitatory and inhibitory signals at any one time reaches the threshold.
