2. neurons and synaptic transmission

neuron definition

• basic building blocks of the nervous system

• they are nerve cells that process and transmit messages through electrical and chemical signals, providing the nervous system with its primary means of communication

• vary in size from less than 1mm to up to 1m

• there are 100 billion in the human nervous system - 80% in the brain

neuron types

1. sensory neuron

2. relay neuron

3. motor neuron

neurons diagram

sensory neuron

• carry messages from the PNS to the CNS

• keep the brain informed about the external and internal environment information coming from the sense organs

• only transmit messages one way and cannot receive messages

• long dendrites, short axons

relay neuron

• connect sensory neurons to the motor and other relay neurons

• messages can be transmitted and received

• short dendrites, short axons

motor neuron

• connect the CNS to effectors such as muscles and glands

• transmit and receive messages

• control physical movements ie contraction of muscles

• short dendrites, long axons

neuron structure

components include

• cell body (or soma)

• dendrites

• axon

• myelin sheath

• nodes of ranvier

• terminal buttons

cell body

• includes a nucleus which contains the genetic material of the cell

dendrites

• branch like structures protruding from the cell body

• carry nerve impulses from neighbouring neurons towards the cell body

axon

• carries the impulses away from the cell body down the length of the neuron

myelin sheath

• fatty layer covering the axon

• protects it and speeds up electrical transmission of the impulse

nodes of ranvier

• if the myelin sheath was continuous, it would have the reverse effect and slow down the electrical impulse

• thus it is segmented by gaps called nodes of ranvier

• they speed up the transmission of the impulse by forcing it to jump across the gaps along the axon

terminal buttons

• at the end of the axon

• communicate with the next neuron in the chain across a gap known as the synapse

location of neurons

• cell bodies of motor neurons may be in the CNS but they have long axons which form part of the 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 visual system

electrical transmission of neurons

• when a neuron is in a resting state, the inside of the cell is negatively charged compared to the outside

• when it is activated by a stimulus, the inside becomes positively charged for a split second causing an action potential to occur - creates an electrical impulse that travels down the axon towards the end of the neuron

synaptic transmission definition

• process by which neighbouring neurons communicate with each other by sending chemical messages across the synapse that separates them

how does chemical transmission work

• neurons communicate with each other within groups known as neural networks

• each neuron is separated from the next by a tiny gap called a synapse

• signals WITHIN neurons are transmitted electrically

  signals BETWEEN neurons are are transmitted chemically across the synapse

• when the electrical impulse reaches the end of the neuron (presynaptic terminal) it triggers the release of neurotransmitter from tiny sacs called synaptic vesicles

synapse diagram

synaptic transmission steps

1. signal begins as electrical impulse (action potential) within presynaptic neuron

2. when it reaches axon end on pre-synaptic knob neurotransmitters released from vesicles

3. theyre released into the synaptic cleft as a chemical substance

4. diffuse across synaptic cleft and temporarily bind with receptor sites on postsynaptic membrane

5. stimulates postsynaptic neuron to generate another action potential that travels down its axon

6. neurotransmitters then destroyed by enzymes or recycled to prevent continued stimulation

7. re-absorbed neurotransmitter may be used again by presynaptic neuron

diagram

presynaptic knob

• a swelling at the end of the presynaptic neuron

• contains vesicles holding neurotransmitters

synaptic cleft

• space in between axon of one neuron and dendrites of another

• where electrical signals are turned into chemical messengers that can be detected by the next neuron

postsynaptic membrane

• membrane receives a signal from presynaptic neuron

neurotransmitter definition

• brain chemicals released from synaptic vessicles that relay signals across the synapse from one neuron to another

broad division of neurotransmitters

1. those that perform an excitatory function

2. those that perform an inhibitory function

how do neurotransmitters work

1. diffuses across the synapse

2. once it has crossed the gap, it is taken up by a postsynaptic receptor site on the dendrites of the next neuron

3. chemical message is converted back into an electrical impulse

4. the process of transmission begins again in this other neuron

direction of travel of neurotransmitters

• only one way

• because they are released from the presynaptic neuron terminal and recieved by the postsynaptic neuron at receptor sites

neurotransmitter types and examples

• several dozen types

• each has its own specific molecular structure that fits perfectly into a postsynaptic receptor site

• also have specialist functions - eg 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

• when a neurotransmitter increases the positive charge of the postsynaptic neuron - increases likelihood it will pass on the electrical impulse

• eg adrenaline

inhibition

• when a neurotransmitter increases the negative charge of the postsynaptic neuron - decreases likelihood it will pass on the electrical impulse

• eg seratonin

summation

• whether a postsynaptic neuron fires is decided by the process of summation

• 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