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Nervous system summary
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Summary of 2 pages for the course Anatomy at BCU (Physiology)
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Summary of Nervous systemThe nervous system controls, directly or indirectly, every part of our body, sometimes
through voluntary responses to sensory inputs, but often automatically, through the
autonomous nervous system (ANS) or spinal reflexes.
The central nervous system (CNS: brain and spinal cord) is the control center
communicating with the rest of the body (all organs) and the outside world through the
peripheral nervous system (PNS). The PNS contains afferent sensory neurons, providing
the input to the CNS and efferent motor neurons that provide an output to muscles and
organs to generate actions that keep us alive (and more). In all aspect the working of the
nervous system can be seen in the sequence: stimulus, sensor, afferent, CNS (controller),
efferent, effector, response.
ANS afferents control the activity of cardiac muscles, smooth muscles and glands. The ANS
task is to maintain vital physiological factors in the optimum range through feedback
systems: ensuring homeostasis. Sympathetic nerve activity prepares the body for fight and
flight, like increasing heart rate, but decreasing digestion. Parasympathetic nerve activity
has the opposite effect and favor processes important for rest and digest, like stimulating
digestion, but reducing hart rate. The ANS controls the activity of all organs through the
balance between the effects of sympathetic nerve activity, counteracted by the effect of the
parasympathetic nerves.
Voluntary actions involve various parts of the brain to take many aspects into account. Some
areas are dedicated to process simple sensory information, some more complicated
information. The frontal lobe makes a judgment based on all information available from
sensory input, memory and current emotions and activates appropriate motor output in areas
dedicated to movement. The CNS that weighs up all sensory information and experiences in
memory and decides on an action that is than executed by sending information through
efferent nerves to muscles or organs to provide a response.
Many simple responses to external challenges are by-passing the brain and use pre-wired
reflex loops in the spinal cord to provide a quick and effective response designed to
minimize harm. The brain is made aware of the challenge when the spinal response is already
provided. The brain can willfully inhibit reflexes if they would be deemed unhelpful.
The basic anatomy of neurons includes 1) dendrites: areal-like extensions that receive
synaptic inputs from thousands of other neurons, bringing electrical activity towards the 2)
cell body that weighs up all inputs and can generate an electrical impulse that propagates
down the 3) axon; a long thin extension, often splitting and ending in pre-synaptic
terminals that form synapses (junctions with a tiny synaptic cleft in-between) with the
dendrites of other neurons.
Pre-synaptic terminals contain vesicles filled with neurotransmitter, a signaling chemical.
When the presynaptic neuron sends an electrical impulse neurotransmitter is released and
binds to neurotransmitter-specific receptors (sensors), which causes a tiny electrical change
in the post-synaptic neuron. If big enough (coinciding neurotransmitter release many
synapses), the postsynaptic neuron starts an electrical impulse that propagates down its
axon to each synaptic terminal, where it will trigger neurotransmitter release, to start a similar
process in the next neuron. The neurotransmitter glutamate is excitatory; it increases the
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