Neurons, synaptic transmission and neurotransmitters Flashcards
Structure of the neuron.
A nerve cell, the basic unit of the nervous system
-Cell body (soma) includes a nucleus containing the genetic material of the cell.
-Dendrites- branch-like structures sticking out of the cell body, carry impulses from other neuron’s to the cell body
-Axon- a tube like structure, carries the impulses away from the cell body down the length of the neuron.
-Myelin sheath - in many neurons, the axon is covered by a fatty layer formed from special cells (glial cells). This protects the axon and speeds up transmission from the impulse
-Nodes of ranvier - gaps In the myelin sheath increase speed by forcing the impulse to ‘jump’ across the gaps along the length of the axon
-Terminal buttons- at the end of axons, not physically connected from the next neuron, but involved in the communication across a gap known as the synapse
Types of neuron
(sensory, motor, relay)
Sensory neurons
-Carry messages from sensory receptors (e.g. touch and temperature- sensitive receptors in the skin) along nerves in the PNS to the CNS (long dendrites, short axons)
Motor neurons
-Carry messages from the CNS, along nerves in the PSN, to effectors such as muscles and glands (short dendrites, long axons)
Relay neurons
-Connect sensory and motor neurons together, ad also connect to other relay neurons (short dendrites, short axons, only found in the CNS)
functions of the neuron
When depolarisation passes a certain threshold an action potential is fired
When a Neuron is in a resting state, the inside of the cell is negatively charged relative to the outside. But when it is sufficiently activated by a stimulus. the inside of the neuron becomes positively charged for a split second.
This rapid switch from negative to positive is called depolarisation, which creates an electrical impulse (action potential, abbreviated as AP) from the axon to the terminal buttons
The AP is generated once depolarisation reaches a certain threshold and is always the same intensity, regardless of the size of the stimulus that generated it. If the threshold is not reached there is no AP, so an AP is an all-or-nothing event.
Myelinated Neurons can conduct up to 500 APs a second
Synaptic transmission
How neurons communicate with each other
-Neurons communicate within neural networks without physically touching (they are separated by a tiny gap called the synaptic cleft). This means signals within neurons are transmitted electrically, but signals between neurons are transmitted chemically across the synapse.
-When the AP reaches the end of the neuron (the presynaptic terminal button) it triggers the release if the neurotransmitter from the synaptic vesicles
-When neurotransmitter molecules cross the gap, they fit into matching receptor sites on the postsynaptic neuron. The chemical signal carried by the neurotransmitters is converted back into an electrical signal in the postsynaptic neuron, which may fire another AP.
-Neurotransmitter molecules that remain in the synapse are reabsorbed into the presynaptic neuron, usually broken down first ( a process called reuptake do the neurotransmitter can be reused)
Functions of neurotransmitters
Chemical that diffused across the synapse to the next neuron In the chain.
Several dozen neurotransmitters have been identified in the human brain each with its own molecular structure fitting perfectly into a specific type of postsynaptic receptor like a key box
Neurotransmitters have specific functions, e.g. acetylcholine is found where a Motor neuron meets a muscle, upon its release it causes the muscle to contract
Excitation and inhibition
The effect of neurotransmitters on neighbouring neurons
Excitation- dopamine causes excitation of the postsyaptc neuron by increasing its positive charge, making it more likely to fire.
Inhibition- Seretonin inhibits the postsynaptic neuron making the neuron more negatively charged and less likely to fire.
Summation
The excitatory and inhibitory influences from the neural network are summed
Firing of the postsynaptic neuron depends on summation ( of signals over time (temporal summation) or space (spacial summation)
If the net effect is inhibitory then the postsynaptic neuron is less likely to fire.
If the net effect is excitatory then the postsynaptic neuron is more likely to fire and, momentarily the inside of the postsynaptic neuron becomes positively charged, depolarisation takes place and the action potential travels down the dendrite towards the cell body then along the axon
