Structure & functions of sensory, relay & motor neurons

• Neurons are cells which make up nervous system and conduct electrical impulses
• Neurons are elongated with cell body extending into an axon on one side and dendrites on the other
• Neurons are covered in a complex cell membrane made up of several layers. The biochemical structure of this membrane allows it to conduct or transmit pulses of electrical activity known as action potential or electrical / nerve impulses
• Electrical impulses start from dendrites and travel through axon which turn it into action potentials
• All action potentials look the same but are identified by their frequency. 250-400 impulses are conducted along the neuron per second
• Neurons have fatty layers along the axon called myelin sheath. There are gaps between them called nodes of ranvier. Action potentials jump from gap to gap this is saltatory conduction and it results in faster transmission so faster information processing
• Sensory neurons carry information from sense organs to central nervous system. We have sensory receptors and information from them is carried by sensory neurons into spinal cord then brain
• Sensory neurons has long dendrites running from sensory receptor to cell body which is located close to spinal cord so axon runs into spinal cord where it will synapse onto relay neuron. It makes up sensory/afferent pathway)
• Relay/inter neurons interconnect sensory and motor pathways and different parts of CNS. Its only in the CNS and most of the brain is made up of relay neurons. They can have many interconnections since their dendrites are running in towards the cell body and they have one single output axon.
• Motor neurons carry commands from motor cortex of brain out of CNS to skeletal muscles allowing movement and it forms motor/effernet pathway. Final motor neurons has its cell body into the spinal cord and a long axon running out to the skeletal muscles.
• Between axon terminal and the next neurons is a tiny gap called synapse. Nerve impulses can't jump across them.
• Transmission of nerve impulses across synapse is chemical. Neurotransmitters are packets of chemicals stored within the axon or presynaptic terminal
• As nerve impulses travel down axon and reach axon terminal it Stimulates release of neurotransmitters into the synapse. Synaptic gap/cleft is very small so molecules can diffuse over to the postsynaptic membrane of the next neuron, they are detected by the synaptic receptors.
• Synaptic receptors are molecules with a particular structure which matches the structure of neurotransmitter molecule
• Neurotransmitter reaches postsynaptic membrane and binds to the receptor. Combination of neurotransmitter and receptor alters biochemical nature of postsynaptic membrane so nerve impulses are more likely to be triggered
• Neurotransmitter match/complement the post synaptic receptors at the post synaptic terminal like lock and key changing biochemical nature of post synaptic membrane
• The all or nothing rule states that for a nerve impulses to be triggered on the post synaptic membrane enough neurotransmitter molecules must be released from presynaptic terminal. When they combine with postsynaptic receptor, changes in postsynaptic membrane will cross a threshold and nerve impulses is triggered. Nerve impulse is then fired along the postsynaptic neuron
• To cross the synapse enough nerve impulses need to arrive at the presynaptic terminal in a short period of time to release enough neurotransmitters.
• Once neurotransmitters and postsynaptic receptors combine they can either be excitatory or inhibitory
• If combination of neurotransmitters and postsynaptic receptors makes a nerve impulse/action potential more likely to be triggered then the neurotransmitter is excitatory
• Neurotransmitters like dopamine and serotonin are excitatory neurotransmitters.
• GABA is a inhibitory neurotransmitter
• When GABA is released from presynaptic terminal and combines with postsynaptic GABA receptors it inhibits the postsynaptic membrane and makes action potentials less likely to occur