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Nerves - Coggle Diagram
Nerves
Homeostasis
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muscular system
somatic motor neurons receive instructions from motor areas of brain and stimulate contraction of skeletal muscles
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cardiovascular system
centre in the medulla oblongata, provides nerve impulses to ANS that controls heart rate
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Ear
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Tympanum membrane
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3 layers
external cuticular- thin, hairless epidermis and dermis
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inner mucous- continuation of respiratory epithelium, it is a single layer of cuboidal cells with no cilia or goblet cells
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Inner ear
detects sound, head position and movement
fluid waves in cochlea push on membrane of cochlear duct and the hair cells bend and release neurotransmitter
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primary sensory neurons send info to medulla oblongata, the main pathway synapses in nuclei in midbrain and thalamus
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Sensors
head movement
vestibular apparatus- cristae are receptors for rotational acceleration and maculae are receptors for linear acceleration
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movement of the endolymph pushes on gelatinous cupula and activates receptor cells which bend to establish an action potential
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Synapse
sequence
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rapid removal of neurotransmitter (by enzymatic degradation, reuptake or autoreceptor)
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postsynaptic receptors
Types
metabotropic- G protein coupled receptors, relatively slow
ionotropic- ligand gated ion channels, very fast
The post synaptic membrane of motor end plate is highly folded adn AChRs are on the crests of the folds and voltage gated cation channels at the bottom
AChRs are gated channels that allows Na+ and K+ but since gradients favour Na+ net influx it causes depolarisation
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inhibitory
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A resting potential is around -60mV, but the equilibrium potential is also around -60mV so opening their channels does not hyperpolarise the membrane but does make it more difficult to reach threshold
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Summation
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For most neurons, summation takes place in the axon hillock where the cell membrane is not insulated by glia with many Na+ channels and is the place where all synaptic potentials spread to. If the resulting graded potential reaches the threshold it fires an AP but because post synaptic potentials decrease in strength as tehy spread fromthe site of the synapse, a synapse at the tip of the dendrite has less influence than one near hillock
Electrical synapses
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they are less common because they offer fewer means for integration of information since almost entirely excitatory and not suited for temporal summation
Neurophysiology
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spinal cord
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motor neurones pass into the ventral roots before uniting with the sensory axons to form the mixed nerve
transmission
Resting
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For neurons at rest, the membrane is mostly permeable to K+, since K+ concentration is higher in the cell it diffuses out but this leaves an unbalanced negative charge
Equilibrium is reached when tendency for K+ to diffuses out is countered by electric charge pulling K+ back in
The Na-K ATPase keeps K+ concentration of K+ inside greater than K+ outside adn Na+ inside less than outside
K+ channels are most open, the membrane potential at which net K+ diffusion out of the cell stops (equilibrium) is potassium equilibrium potential but this is less negative than resting potential
Depolarisation
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diffusion of sodium into the cell reduces the resting potential at that position which creates an excitatory post synaptic potential
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The membrane potential becomes even more negative when K+ channels open and the cell becomes hyperpolarized
Action
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If a neuron is sufficiently stimulated to cause slight depolarisation of cell body then that graded potential spreads by local current flow to the axon hillock, this causes sodium channels to open
when membraen is depolarised about 5-10mV above resting a threshold is reached and more sodium channels open, the rising phase of teh AP then stops in 1-2 ms
refractory period
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in resting conditions the inactivation is open and activation closed, depolarisation to threshold causes inactivation to close and activation to open (but activation changes faster)
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Inactivation remians closed for 1-2 ms before opening again, by the time the inactivation reopens the activation is closed and membrane is ready for another AP
Conduction
The magnitude of AP does not change over distance, this is because of the positive feedback mechanism that ensure AP always rises to maximum and the resulting depolarisation brings neighbouring areas to threshold
An AP is propagated in one direction away from the cell body and cannot reverse itself because of refractory period
APs are conducted faster in large diameter than in small diameter axons because the resistance to ionic current flow decreases as diameter increases
saltatory conduction-APs jump from node to node in myelinated axons, underneath the myelin sheaths there are no channels so APs cannot be propagated under it and it creates a local electric field inside the axon that spreads to the next node
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Macroglia support neurons and microglia are phagocytic cells, microglia are motile and are activated by foreign matter or damge to NS and are the first major immune defense mecahnsim in NS
astrocytes
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has small spaces called pervascular space, so fluid can leak out of incoming blood and then the astrocytes take up the fluid
they take up NT, can store glycan, release NT, aid in repair of neurones, and can signal changes in blood
Glia
Macroglia
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neurons and macroglia have a common origin in neural tube of early vertebrate embryo, the tube contains dividing neural stem cells which divide initially symmetrically then asymmetrical (one daughter cell remaining a stem cell and other daughter cell becomes a neural progenitor cell or macroglial progenitor cell
They include
Oligodendrocytes- these wrap around the axons of neurons covering them with concentric cell membrane layers
Schwann cells- provide same function for peripheral nerves that communicate between brain and spinal cord (myelin is wrapping produced by oligodendrocytes and Schwann cells)
Ependymall cells- line central fluid-filled chambers in the brain called ventricles and produce cerebrospinal fluid that bathes the brain and spinal cord
Astrocytes
contribute to blood-brain barrier(which protects brain from toxic chemicals) as the end-feet of their many projections that surround the brain's blood vessels. Astrocytes also may rid brain of metabolic wastes: in the bb barrier there is a perivascualr space between membranes of vessels and astrocyte end-feet that surround them (fluid leaks out of incoming vessels into this space and the astrocyte membranes have aquaporins that can take up fluid from the spaces and deliver it to wherever the astrocyte projects but the reverse can also happen) (this is the glymphatic system)
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They do not generate Aps but do release NTs that can alter neural activity (the contact of astrocyte with neurons has created the concept of tripartite synapse)
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They do not generate APs but do communicate as they are connected through electrical synapses and signals travelling cause changes in Ca2+ content in postsynaptic astrocyte
Microglia
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they are motile and are activate by foreign matter or neuron damge and are the first immune defense mechanism in the NS
Neurone structure
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cell body
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provides steady transport of materials (mitochondria, vesicles)
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Reflexes
Spinal Reflex
Grey matter is rich in neuronal cell bodies and white matter contains myelinated axons that conduct information up and down the spinal cord
spinal nerves extend from spinal cord at regular intervals on each side, each nerve has 2 roots: one connecting with dorsal horn og the grey matter and the other with the ventral horn
teh sensory axons in a spinal nerve enter the spinal cord through the dorsal root and the motor axon leve through teh ventral root
spinal knee-jerk reflex
The tap of the hammer on the knee stretches the tendon going over the knee , the tendon attaches the muscle of the uppre leg to bone in lower leg
stretching the tendon stetches the muscle fibres in upper leg and stretch receptors in that muscle transduce physical stimulus into APs
The APS are tehn conducted by a sensory neuron into dorsal horn which travels up to the ventral horn and synapses onto a motor neurone
The axon of motor neurone travels through ventral horn and extends to same muscle that was initially stretched causing contraction
Limb Movement
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coordination is achieved by an interneurone between sensory neuron nad motor neuron of antagonist muscle so the reciprocal inhibiton of antagonist muscles involves atleast 2 synapses
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