Structure and Function of Sensory, Relay and Motor Neurons Flashcards
Neurons:
the cells making up the nervous system, which conduct electrical impulses
Sensory neuron:
neuron carrying sensory information into the CNS from the body’s sensory receptors
Central nervous system:
major part of the nervous system, made up of the brain and the spinal cord
Relay neuron:
neuron within the CNS that interconnects different parts of the CNS
Motor neuron:
neuron carrying motor commands out from the CNS to the skeletal muscles of the body, allowing for movement
Synapse:
a tiny gap between two neurons, across which nerve impulses are passed
Neurotransmitters:
packets of chemicals stored within the axon; they enable nerve impulses to pass across the synapse to the postsynaptic neuron
What is a neuron?
-neurons are special cells in the nervous system that send electrical called nerve impulses
-they have a cell body, dendrites to receive signals, and an axon to send them
-these signals are called action potentials, travel from the dendrites, through the cell body, and along the axon
-they have the same basic properties in all neurons but can vary in sped and pattern, which carry information
-neurons can send signals up to 400 times per second and the brain’s complexity comes from its 100 billion neurons working together
Dendrite: function
receives the nerve impulses or signal from adjacent neurons
Cell body: function
contains the nucleus which contains the genetic material
Axon: function
carries impulse away from the cell body
Myelin sheath: function
cover the axon, and speeds up electrical transmission, also protects the axon
Nodes of Ranvier: function
gaps in the myelin sheath -> helps the impulse more as it jumps across gaps on the axon
Axon terminals: function
communicate with the next neuron across a synapse
Saltatory Conduction:
-in advanced animals such as us and mammals, many of the neuron making up the nervous system are covered in a fatty cover called the myelin sheath, which is not found in the nervous system in more primitive animals
-there are gaps in the sheath, known as the nodes of Ranvier, where the neuronal cell membrane is exposed
-action potentials have the property of being able to jump from gap to gap; this is known as saltatory conduction, and is many times faster than the standard continuous conduction along the neuronal axon
-faster transmission means faster information processing, and has led to the development of complex human cognitive abilities
Sensory, Relay, and Motor neurons:
-sensory neurons carry information from the sense organs into the CNS
-we have sensory receptors all over our skin, for touch, pain, pressure, etc.
-information from these receptors is carried by a sensory neuron into the spinal cord and then on to the brain
-from there the axon runs into the spinal cord where it will synapse onto a relay or interneuron
-the particular function of the relay neuron is to interconnect sensory and motor pathways, and also different parts of the CNS
-relay neurons lie entirely within the CNS, and much of the brain is made up of relay neurons
-motor neurons carry commands from the motor cortex of the brain out to the muscles of the skeleton
-the final motor neuron in the pathway has its cell body within the spinal cord, and a long axon running to the muscles of the skeleton
Synaptic Transmission:
-neurons are not physically connected to one another, between the axon terminal and the next neuron is a tiny gap, the synapse, which is visible only under an electron microscope
-although tiny, this gap presents an obstacle to the nerve impulse as it cannot automatically jump across
-transmission of nerve impulses across the synapse is chemical
-stored within the axon or presynaptic terminal are packets of chemicals known as neurotransmitters
-as nerve impulses travelling down the axon reach the axon terminal, they stimulate the release of neurotransmitter molecules into the synapse
-the synaptic gap is so small that the molecules can diffuse over to the postsynaptic membrane of the following neuron, located on this membrane are synaptic receptors
Synaptic receptors:
-synaptic receptors are specialised molecules that bind to neurotransmitter
-when neurotransmitters bind briefly to these receptors on the postsynaptic membrane, they change its biochemical state, increasing the likelihood of triggering a nerve impulse
-nerve impulses follow an “all or nothing” rule: they either happen or not
-to trigger an impulse, enough neurotransmitters must be released from the presynaptic terminal
-once they bind to the receptors and the threshold is crossed, the impulse travels along the postsynaptic neuron and continues through the next synapses
The purpose of the synapse:
-the synapse enables information processing in the nervous system
-for a signal to cross the synapse, enough neurotransmitters must be released in a short time
-if too few impulses arrive, the postsynaptic membrane won’t fire, and the information is lost
-each neuron can connect up to 1,000 others, creating a complex network essential for processing
-there are limited types of neurotransmitters (e.g. dopamine, serotonin) and receptors
-synapses are often defined by the neurotransmitter they use
-understanding their chemical nature helps explain how drugs affect behaviour, as some drugs alter neurotransmitter release or receptors interaction
Excitation:
synapses in the nervous system can be excitatory or inhibitory; excitatory synapses activate the postsynaptic neuron increasing neural activation in the central nervous system
Inhibition:
synapses in the nervous system can be excitatory or inhibitory; inhibitory synapses inhibit activity in the postsynaptic neuron, decreasing neural activation in the CNS; inhibitory circuits are vital to the balance between excitation and inhibition in the nervous system
Excitation and Inhibition:
-synapses can be excitatory or inhibitory based on the neurotransmitter’s effect on the postsynaptic receptor
-excitatory neurotransmitters, like dopamine and serotonin, make nerve impulses more likely, while inhibitory ones, like GABA, decrease the likelihood of an impulse
-GABA stabilises the postsynaptic membranes, inhibiting action potentials
-increased GABA activity reduces activity in other systems, like serotonin, leading to calming effect, anti-anxiety drugs such as benzodiazepines stimulate GABA release
-normal brain function depends on a balance between excitatory and inhibitory signals, imbalances such as excessive excitation or inhibition, may lead to conditions like epilepsy
