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Nervous System: Part 1

Terms in this set (14)

1) Sensory Neurons (Afferent Neurons): Neurons responsible for converting external stimuli from the environment into corresponding internal stimuli (electrical/chemical signals). They are activated by sensory input, and send projections to other elements of the nervous system, ultimately conveying sensory information to the brain or spinal cord. Sensory neurons are unipolar and the cell body is located outside the main section (extension). In sensory neurons both the dendrite and the axon is myelinated.
2) Interneurons: Act as the "middle men" that form a connection between sensory neurons and motor neurons (ie. they create neural circuits enabling communication between sensory or motor neurons and the central nervous system). They are located in the CNS and operate locally, meaning that their axons connect only with nearby sensory and motor neurons. Interneurons make it possible for certain sensory signals to bypass the brain and go directly back through the motor neurons - thus preventing injuries and making very quick movements possible (reflex arcs). Interneurons are bipolar and the cell body is located in the middle of the cell (the axon and the dendrite are roughly the same length). In interneurons, long axons are myelinated.
3) Motor Neurons (Efferent Neurons): Neurons located in the central nervous system (cell body). They project their axons outside of the CNS to directly or indirectly control muscles. Motor neurons are multipolar, meaning each cell contains a single axon and multiple dendrites. They are the most common type of neurons. In this type of neuron, the dendrites branch out from the cell body - and as such, only the axon is myelinated.
Action Potential: The depolarization of the membrane to transmit an impulse. It is a large change that can travel long distances. They occur as the result of Na+ moving in and K+ moving out of the neuron. Triggering an action potential at one spot on a membrane causes another one right next to it. The action moves like a wave down the neuron until it reaches the end, where the message is passed to another nerve of effector via the synapse. The parts of the action potential are as follows:
1) Resting Potential: During the resting state, the inside of the cell is negatively charged. Voltage-gated Na+ and K+ channels present in the membrane are also closed - polarity is maintained by potassium and sodium channels which leak slightly.
2) Development of Graded Potential: Action potentials begin as graded potentials. Depolarization of a nearby membrane segment (say, a signal initiated by a tough) causes a few sodium channels to open. Sodium ions enter (because Na+ travels down its concentration gradient), depolarizing the cell slightly. During this phase, the voltage-gated channels remain closed.
3) Depolarization: The slight change in voltage as a result of the graded potential triggers the voltage-gated sodium channels in the membrane to open. This causes sodium to rush into the cell (down its concentration gradient). So many sodium ions enter that, at its peak, depolarization causes the inside of the cell to temporarily become positively charged. As the peak of an action potential is reached, voltage-gated potassium channels begin to open and Na+ channels begin to close - this process stops depolarization. NOTE: Depolarization is an "all or nothing response" and is only triggered when the small change in voltage is greater than the threshold (-55 mV).
4) Repolarization (Refractory Period): When the voltage-gated potassium channels open, positively charged potassium ions flood out of the cell and down their concentration and electrical gradients. This process restores the original resting potential (-70 mV) and also (coupled with the fact that Na+ channels are still inactive) causes a refractory period in which that section cannot become depolarized again. This period is important - as it stops impulses from travelling in both directions through the neuron at once.
1) The action potential reaches the axon bulb and Ca2+ ions diffuse into the axon bulb as a result.
2) These ions cause synaptic vesicles to fuse with the presynaptic membrane and release their neurotransmitters into the synaptic cleft (by exocytosis). Contractile filaments in the acon bulb help to pull the vesicles over to the edge of the cell.
3) The neurotransmitters bind to receptors on the postsynaptic cell membrane.
4a) The bound receptors induce a change in the postsynaptic cell (usually a graded potential - either hyperpolarizing/inhibitory or depolarising/excitatory). This signal (potential) travels away from the synapse and down the dendrite of the postsynaptic neuron.
4b) If the postsynaptic cell is not a neuron, the signal will alter the cell's activity in a different way. For example, if the postsynaptic cell is a gland cell, the signal could stimulate or inhibit hormone secretion and if it is a muscle cell, it could stimulate or inhibit contraction.
5) If an excitatory graded potential is big enough, and the threshold reached in the postsynaptic cell, the action potential will be initiated and the electrical signal will travel down the length of the axon.
6) An enzyme (eg acetylcholinerase) is released into the synaptic cleft to break down the neurotransmitters and prevent continuous stimulation of the postsynaptic cell. NOTE: A) some of these neurotransmitters are taken back up by the synapse as opposed to being destroyed. B) Synapses can only go in one direction, this is because the presynaptic cell only contains the neurotransmitters and the postsynaptic cells only have the receptors.