neurons + synaptic transmission

• the neurons are cells that are specialised to carry neural information throughout the body, neurons can be 1 of 3 types = sensory neurons, relay neurons or motor neurons
• neurons typically consist of a cell body, dendrites and an axon, dendrites at one end of the neuron receive signals from other neurons or from sensory receptors
• dendrites are connected to the cell body, the control centre of the neuron, from the cell body the impulse is carried along the axon where it terminates at the axon terminal
• in many nerves including those in the brain and spinal cord, there is an insulating layer that forms around the axon, the myelin sheath, this allows nerve impulses to transmit more rapidly along the axon
• if the myelin sheath is damaged impulses slow down, the length of a neuron can vary from a few millimetres up to 1 metre
• sensory neurons: carry nerve impulses from sensory receptors (e.g. receptors for vision, taste, touch) to the spinal cord and the brain
• sensory neurons: sensory receptors are found in various locations in the body e.g. in the eyes, ears, tongue and skin, sensory neurons convert information from these sensory receptors into neural impulses
• sensory neurons: when these impulses reach the brain they are translated into sensations of e.g. visual input, heat, pain etc so that the organism can react appropriately
• sensory neurons: not all sensory information travels as far as the brain w/ some neurons terminating in the spinal cord, this allows reflex actions to occur quickly w/o delay of sending impulses to the brain
• relay neurons: most neurons are neither sensory nor motor, but lie somewhere between the sensory input and the motor output
• relay neurons: relay neurons allow sensory and motor neurons to communicate w/ each other, these relay neurons (or interneurons) lie wholly within the brain and spinal cord
• motor neurons: the term motor neuron refers to neurons which conduct signals from the CNS to effector organs such as muscles, their cell bodies may be in the CNS but they have long axons which form part of the PNS
• motor neurons: motor neurons form synapses w/ muscles and control their contractions
• motor neurons: when stimulated the motor neuron releases neurotransmitters that bind to receptors on the muscle and triggers a response which leads to muscle movement
• motor neurons: when the axon of a motor neuron fires the muscle with which it has formed synapses w/ contracts
• motor neurons: the strength of the muscle contraction depends on the rate of firing of the axons of motor neurons that control it, muscle relaxation is caused by inhibition of the motor neuron
• synaptic transmission: once an action potential has arrived at the terminal button at the end of the axon, it needs to be transferred to another neuron or to tissue
• synaptic transmission: to achieve this it must cross a gap between the presynaptic neuron and the postsynaptic neuron, this area is known as the synapse which includes the end of the presynaptic neuron, the membrane of the post synaptic neuron and the gap in between
• synaptic transmission: the physical gap between the pre- and postsynaptic cell membranes is known as the synaptic gap
• synaptic transmission: at the end of the axon of the nerve cell are a number of sacs known as synaptic vesicles, these vesicles contain the chemical messengers that assist in the transfer of the impulse, the neurotransmitters
• synaptic transmission: as the action potential reaches the synaptic vesicles it causes them to release their contents through a process called exocytosis
• synaptic transmission: the released neurotransmitter diffuses across the gap between the pre- and the postsynaptic cell where it binds to specialised receptors on the surface of the cell that recognise it and are activated by that particular neurotransmitter
• synaptic transmission: once they have been activated the receptor molecules produce either excitatory or inhibitory effects on the post synaptic neuron
• synaptic transmission: this whole process of synaptic transmission takes only a fraction of a second w/ the effects terminated at most synapses by a process called 're-uptake', the neurotransmitter is taken up again by the presynaptic neuron where it is stored and made available for later release (a sort of recycling programme)
• synaptic transmission: how quickly the presynaptic neuron takes back the neurotransmitter from the synaptic cleft determines how prolonged its effects will be
• synaptic transmission: the quicker it is taken back the shorter the effects on the postsynaptic neuron, some antidepressant drugs prolong the action of the neurotransmitter by inhibiting this re-uptake process leaving the neurotransmitter in the synapse for longer
• synaptic transmission: neurotransmitters can also be 'turned off' after they have stimulated the postsynaptic neuron, this takes place through the action of enzymes produced by the body which make the neurotransmitters ineffective
• excitatory and inhibitory neurotransmitters: neurotransmitters are the chemical messengers that carry signals across the synaptic gap to the receptor site on the postsynaptic cell
• excitatory and inhibitory neurotransmitters: neurotransmitters can be classified as either excitatory or inhibitory in their action, excitatory neurotransmitters such as acetylcholilne and noradrenaline are the nervous systems 'on switches'
• excitatory and inhibitory neurotransmitters: these increase the likelihood that an excitatory signal is sent to the post synaptic cell, which is then more likely to fire
• excitatory and inhibitory neurotransmitters: inhibitory neurotransmitters such as serotonin and GABA are the nervous systems 'off switches' in that they decrease the likelihood of that neuron firing
• excitatory and inhibitory neurotransmitters: inhibitory neurotransmitters are generally responsible for calming the mind and body, inducing sleep and filtering out unnecessary excitatory signals
• excitatory and inhibitory neurotransmitters: an excitatory neurotransmitter binding w/ a postsynaptic receptor causes an electrical change in the membrane of that cell, resulting in an excitatory post-synaptic potential (EPSP) meaning that the postsynaptic cell is more likely to fire
• excitatory and inhibitory neurotransmitters: an inhibitory neurotransmitter binding w/ a postsynaptic receptor results in an inhibitory postsynaptic potential (IPSP) making it less likely that the cell will fire
• excitatory and inhibitory neurotransmitters: the nerve cell can receive both EPSPs and IPSPs at the same time, the likelihood of the cell firing is therefore determined by adding up the excitatory and the inhibitory synaptic input
• excitatory and inhibitory neurotransmitters: the net result of this calculation (known as summation) determines whether or not the cell fires, the strength of an EPSP can be increased in 2 ways
• excitatory and inhibitory neurotransmitters: in spatial summation a large number of EPSPs are generated at many different synapses on the same post synaptic neuron at the same time
• excitatory and inhibitory neurotransmitters: in temporal summation a large number of EPSPs are generated at the same synapse by a series of high-frequency action potentials on the presynaptic neuron
• excitatory and inhibitory neurotransmitters: the rate at which a particular cell fires is determined by what goes on in the synapses, if excitatory synapses are more active the cell fires at a high rate, if inhibitory synapses are more active the cell fires at a much lower rate if at all