neurones and synaptic transmission Flashcards
(9 cards)
Intro
• Over 100 Billion neurons in the human nervous system
80% in the brain
• Electrical and chemical transmission
• Primary means of communication for the nervous system
Types of neuron
Neuron The basic building blocks of the nervous system, neurons are nerve cells that process and transmit messages through electrical and chemical signals.
Sensory neurons These carry messages from the PNS (peripheral nervous system) to the CNS (central nervous system). 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.
-look at camera role
structure of neuron
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 (or soma) includes a nucleus, which contains the genetic material of the cell. Branchlike 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 Ranvier. These speed up the transmission of the impulse by forcing it to jump’ 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
location of neurones
The cell bodies of motor neurons may be in the central nervous system (CNS) but they have long axons which form part of the peripheral nervous system (PNS). Sensory neurons are located outside of the CNS, in the PNS in clusters known as ganglia. Relay neurons make up 97% of all neurons and most are found within the brain and the visual system.
electrical transmission-the firing of a neuron
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.
Chemical transmission
Neurons communicate with each other within groups known as neural
networks. Each neuron is separated from the next by an extremely tiny gap called the synapse. Signals within neurons are transmitted electrically.
However, signals between neurons are transmitted chemically across the synapse.
When the electrical impulse reaches the end of the neuron (the presynaptic terminal) it triggers the release of neurotransmitter from tiny sacs called synaptic vesicles.
neurotransmitters
Neurotransmitters are chemicals that diffuse across the synapse to the next neuron in the chain. Once a neurotransmitter crosses the gap, it is taken up by a postsynaptic receptor site on the dendrites of the next neuron (axons take signals to the synapse, dendrites take signals away). Here, the chemical message is converted back into an electrical impulse and the process of transmission begins again in this other neuron.
It is worth noting that the direction of travel can only be one-way. This is because neurotransmitters are released from the presynaptic neuron terminal and received by the postsynaptic neuron (at the receptor sites).
Several dozen types of neurotransmitter have been identified in the brain (as well as in the spinal cord and some glands). Each neurotransmitter has its own specific molecular structure that fits perfectly into a postsynaptic receptor site, similar to a lock and a key. Neurotransmitters also have specialist functions. For instance, acetylcholine (ACh) is found at each point where a motor neuron meets a muscle, and upon its release, it will cause muscles to contract.
excitation and inhibition
Neurotransmitters have either an excitatory or inhibitory effect on the neighbouring neuron. For instance, the neurotransmitter serotonin causes inhibition in the receiving neuron, resulting in the neuron becoming more negatively charged and less likely to fire.
In contrast, adrenaline (an element of the stress response which is both a hormone and a neurotransmitter) causes excitation of the postsynaptic neuron by increasing its positive charg and making it more likely to fire.
summation
Whether a postsynaptic neuron fires is decided by the process of summation. The excitatory and inhibitory influences are summed: 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, i.e. the inside of the postsynaptic neuron momentarily becomes positively charged. Once the electrical impulse is created it travels down the neuron.
Therefore, 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.
