structure and function of neurons

general info

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

• Neurons are cells that make up the nervous system

• They send, receive and transmit messages

• Neurons transmit signals electronically and chemically and this provides the NS with its primary means of communication

• Neurons transmit information within and between neurons - when info has arrived at axon it travels via electrical signal known as action potential, to neighbouring neuron

sensory neuron function

• Found in receptor cells in PNS (located in clusters known as ganglia) and carry nerve impulses to the spinal cord and brain

• Nerve impulses are translated into sensations

• Not all sensory neurons reach the brain, as some stop at the spinal cord, allowing for quick reflex actions

relay neuron function

• Make up 97% of all neurons

• Found between sensory input and motor output/response

• Relay neurons are found in brain and spinal cord and allow sensory and motor neurons to communicate

motor neurons function

• Found in CNS and PNS and control muscle movements

  • Cell bodies may be in CNS but long axons are part of PNS

• When motor neurons are stimulated they release neurotransmitters that bind to the receptors on muscles to trigger a response

structural features of neurons

• Dendrites - receive signals from other neurons or from sensory receptor cells, typically connected to cell body which is referred to as the control centre of the neuron

• Cell body (soma) - core section of the neuron that provides energy to drive activities and contains the nucleus which houses the cell's genetic information

• Axon - long slender fibre that carries nerve impulses in form of electrical signal (action potential), impulse is carried away from cell body towards axon terminals

• Myelin sheath - most axons are surrounded by myelin sheath which insulates the axon so the electrical impulses travel faster along the axon

• Nodes of Ranvier - unmyelinated gaps through the axon which don't have myelin sheath, they speed up transmission by forcing it to jump across the gaps along the axon

• Axon terminals (terminal buttons) - connect neuron to other neurons or directly the organs, using process called synaptic transmission whereby chemical messages are passed across the synapse between neurons

differences between neurons

• Sensory neurons have myelin sheath to speed up transmission of action potential whereas relay neurons do not

• Sensory neurons are found in receptor cells in the ears, eyes etc and translate nerve impulses into sensations however relay neurons are found in between sensory and motor allowing them to communicate

• Sensory neuron transmits messages from PNS to CNS where messages reach brain and spinal cord whereas motor neurons transmit messages from CNS where they are located to muscles to trigger the response

reflex arc

1. Receptor in skin detects a stimulus

2. Sensory neuron sends electrical impulses to the relay neuron which is in the spinal cord of the CNS, relay neurons connect sensory neurons to motor neurons

3. Motor neuron sends electrical impulses to an effector

4. Effector will produce the response

electrical transmission

• When neuron is resting state, inside of the cell is negatively charged compared to outside

• Neuron is activated by stimulus, inside becomes positively charged for split second which causes the action potential to occur

• Creates electrical impulse that travels down the axon to the end of neuron

chemical transmission

• Communicate with each other within groups known as neural networks

• Neurons have gaps within them called synapses, signals within neurons are transmitted electrically

• Signals between neurons are transmitted chemically across the synapse

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

• Diffuse across the synapse

• Taken up by postsynaptic receptor site and on dendrites of next neuron

• Chemical message is converted back into an electrical impulse and process of transmission begins again in the other neuron (only one-way travel)

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

  • Have their own specialist functions - acetylcholine makes muscles contract

synaptic transmission

• Information is passed down the axon of the neuron as an electrical impulse known as action potential

• Once the action potential reaches the end of the axon it needs to be transferred to another neuron or tissue, by crossing the synaptic gap

• At the end of the neuron (axon terminal) are the synaptic vesicles which contain neurotransmitters

• Electrical impulse (nerve action potential) stimulates the release of the neurotransmitters from the synaptic vesicles (exocytosis)

• Neurotransmitters then carry the signal across the synaptic gap, bind to receptor sites on the postsynaptic cell which then becomes activated

• Once the receptors on the post-synaptic neuron have been activated, they either produce excitatory or inhibitory effects on the post-synaptic cell

  • Excitatory neurotransmitters (dopamine/adrenaline) make the neighbouring neuron more likely to fire an action potential by increasing the positive charge of the postsynaptic neuron

  • Inhibitory neurotransmitters (serotonin) make the neighbouring neuron less likely to fire an action potential by increasing the negative charge of the postsynaptic neuron

summation

• Whether a post synaptic neuron fires is based on process of summation, where excitatory and inhibitory influences are summed

  • These influences are from multiple neurons - as one neuron can connect to many others, receives multiple signals simultaneously

  • Combination of all the signals that determines whether or not the action potential is generated

• If net effect is inhibitory, post-synaptic neuron is less likely to fire (hyperpolarization)

  • Neurotransmitter GABBA causes inhibitory effect and negative charge in resulting neuron

• If net effect is excitatory, post-synaptic neuron is more likely to fire (polarization)

  • Acetylcholine is excitatory and increases positive charge in resulting neuron

• When electrical impulse is created it travels down the neuron

• An action potential of the post-synaptic neuron is only triggered if the sum of the excitatory signals at any one time reaches threshold and is greater than inhibitory signals

summation summed up lol

• When a neurotransmitter, crosses the synaptic gap it can have either an excitatory or an inhibitory effect on the postsynaptic neuron

• Excitation occurs when receptor stimulation results in an increase in the positive charge of the postsynaptic neuron and increases the likelihood of the neuron firing and passing on the electrical impulse - known as excitatory post-synaptic potential  (opposite for inhibition)

• Summation is addition of positive and negative post-synaptic potentials. Nerve cell can receive both EPSPs and IPSPs simultaneously, summed up and if net effect on the post synaptic neuron is inhibitory, neuron will be less likely to fire and other way round