biopsychology - neurons and synaptic transmission

Question 1

structure and function of neurons

Answer 1

• 100 billion neurons in the human nervous system, 80% located in brain
• transmit signals electrically and chemically to provide nervous system with primary means of communication

Question 2

types of neurons

Answer 2

sensory neurons - carry messages from PNS to CNS, have long dendrites and short axons, found in your skin, eyes and ears

relay neurons - connect sensory neurons to motors or other relays, have short dendrites and short axons

motor neurons - connects CNS to effectors such as muscles and glands, have short dendrites and long axons

Question 3

structure of a neuron / neuronal transmission

Answer 3

• vary in size from less than a mm to up to a metre long
• cell body includes nucleus (contains genetic material), branchlike structures called dendrites carry nerve impulses from neighbouring neurons to cell body
• axon carries impulses away from cell body down length of neuron, covered in fatty layer of myelin sheath which protects it and speeds up electrical transmission
• myelin sheath segmented by gaps called nodes of ranvier which speed up transmission of the impulse by forcing it to ‘jump’ across the gaps
• at the end of the axon are terminal buttons that communicate with the next neuron in the train across synapses

Question 4

location of neurons

Answer 4

• cell bodies of motor neurons are in CNS but long axons form part of PNS
• sensory neurons located in the PNS in clusters called ganglia
• relay neurons make up 97% of all neurons, most found in brain and visual system
• roughly 88 billion neurons in the human nervous system, 80% located in brain

Question 5

electrical transmission - firing of a neuron

Answer 5

• when neuron is in resting state, inside of cell is negatively charged compared to outside
• when neuron is activated by stimulus, inside of cell becomes positively charged for split second causing an action potential to occur
• creates an electrical impulse that travels down the axon towards the end of the neuron (action potential)

Question 6

synaptic transmission - chemical transmission

Answer 6

• neurons communicate with each other within groups called neural networks
• each neuron separated from next via tiny gap called synapse
• signals within neurons transmitted electrically whereas signals between neurons transmitted chemically
• when electrical signal reaches end of neuron, triggers release of neurotransmitter from tiny sacs called synaptic vesticles

Question 7

synaptic transmission - neurotransmitters

Answer 7

• chemicals that diffuse across synapse to next neuron in chain
• once it crosses the gap it is taken up by postsynaptic receptor site on dendrites of next neuron, chemical message is converted back into electrical impulse and process of transmission begins again in this other neuron
• direction of travel can only be one-way, as neurotransmitters must be received by postsynaptic neurons at the receptor site
• several dozen types of neurotransmitter have been identified in the brain, each has its own specific molecular structure which fits perfectly into a receptor site
• they also have specialist functions

Question 8

excitation and inhibition

Answer 8

• neurotransmittors have either excitatory or inhibitory effect on next neuron
• for example serotonin causes inhibition in receiving neuron, resulting in it becoming more negatively charged and less likely to fire
• on the other hand adrenaline causes excitation of the postsynaptic neuron by increasing its positive charge and making it more likely to fire

Question 9

summation

Answer 9

• whether a postsynaptic neuron fires is decided by process of summation
• excitatory and inhibitory processes are summed, if the net effect on the neuron is inhibitory then it is less likely to fire, if it is excitatory it is more likely to fire
• therefore the action potential of the postsynaptic neuron is only triggered if the sum of the excitatory and inhibitory signals at any one time reach the threshold