Neurophysiology, movement, reflexes, action potentials Flashcards
(68 cards)
1
Q
Central Nervous System (CNS)
A
brain and spinal cord
2
Q
Neurons
A
transmission, interpretation and regulation of impulses, small energy supply – most susceptible
CNS cells to injury
3
Q
Astrocytes
A
regulate extracellular neurotransmitter concentrations and fluid/electrolyte imbalances, repair
of injury in CNS and support and bundling of functionally related axons in the CNS
4
Q
Oligodendrocytes
A
– type of glial cell - axon myelination, possible neuronal cell body homeostasis in CNS
5
Q
Other cells in CNS
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ependymal, meningeal and endothelial cells, choroid plexus epithelial cells, microglia
6
Q
Peripheral Nervous System (PNS)
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spinal and cranial nerves carrying information to or from CNS
7
Q
3 main parts of PNS
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somatic, autonomic, enteric
8
Q
Schwann cells
A
– type of glial cells - myelin sheath formation in PNS (oligodendrocytes in CNS)
9
Q
Other cells in PNS
A
Other cells include fibroblasts and satellite glial cells
10
Q
What is axon degeneration of axons like in CNS compared to PNS
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CNS degeneration slower, with less
phagocytosis, PNS axons have capacity to fully regenerate
11
Q
neurogenesis
A
is formation of new neurons
12
Q
Neurons Main structure:
A
soma (body), dendrites (receive signals)
and axons (transmit signals
13
Q
Overall structure of Neurons
A
- Neurons lie close to each other but not in direct contact
with each other - Small gaps in between called synapses
- Nerve impulses are transmitted along the neuron and
across the synapse into the next neuron - Usually part of network or pathway with specific function
14
Q
Dendrites
A
receive signals – neuron may have many dendrites and each may branch to form a dendrite
“tree”with each ending able to receive electrical impulses - one neuron can therefore communicate with
thousands of other cells
15
Q
Soma
A
cell body, contains nucleus and other cell organelles
16
Q
Synapse
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– at every axon terminal – membrane junction
16
Q
Myelin sheath
A
– surrounds axon – lipid layer of insulation (electrical insulation)
17
Q
Axon
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– extension of cell body, transmits impulses. One axon per neuron but axon may branch at end,
creating multiple axon terminals
17
Q
Nodes of Ranvier
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gaps in myelin – signals skip between nodes, rapid transmission
18
Q
What cells are involved in production of myelin sheath
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Oligodendrocytes (CNS) or Schwann cells (PNS)
19
Q
Number of neurons in vertebrate nervous
system varies give numbers for mouse, human and whales/elephants
A
– ~ 100 million in mouse
– ~ 100 billion in human
– ~ 200 billion in whales and elephants
20
Q
Three main categories based on direction of nerve impulse carriage:
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sensory, motor and
interneurons
21
Q
Sensory neurons
A
– afferent - carry information from sensory receptors in tissues and organs to the
CNS, changing physical stimuli into electrical impulses (action potentials) – frequency may be altered
according to intensity of the stimulus
22
Q
Motor neurons
A
– efferent - carry information from the CNS to muscles and glands – change nerve
signals into action
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Interneurons
carry information between sensory and motor neurons
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Somatic efferent neurons
axons carry action potentials from CNS to synapses at skeletal muscles
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Visceral efferent neurons
carry action potentials to synapses with peripheral neurons controlling smooth
muscle, cardiac muscle and some glands
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Somatic afferent neurons
carry information (as action potentials) from receptors such as photoreceptors, auditory and
vestibular receptors of the inner ear, tactile receptors of the skin, skeletal muscular stretch
receptors
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Visceral afferent neurons
* carry action potentials generated from stretch receptors or chemoreceptors (eg oxygen,
carbon dioxide) located in visceral organs of chest and abdomen
* Most visceral afferent (and efferent) axons are part of the autonomic nervous system*
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autonomic nervous system
Autonomic nervous system comprises portions of PNS and CNS responsible for involuntary
control of smooth muscle, cardiac muscle, some gland and many physiological functions eg heart
rate, blood pressure, digestion
29
Six main regions of CNS
spinal cord, medulla oblongata, pons,
midbrain, diencephalon, telencephalon
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Spinal cord
receives sensory input from and supplies motor
output to truck and limbs
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Brainstem
medulla oblongata, pons, midbrain - same function
for face and head
32
How does the forebrain receive information and what does it do
Sensory information entering brainstem passed to forebrain –
sophisticated information processing
* Sensory information from spinal cord can also be relayed to
forebrain via brainstem
* Forebrain formulates sophisticated motor commands - output
sent to brainstem for movement of face and head, or to relay to
spinal cord for trunk and limb movement
* Forebrain can also send motor commands directly to spinal cord
33
What do we mean by ‘potential?’
The difference in electrical charge between
the inside and the outside of the neuron
(potential difference)
At rest, this is usually about -70mV i.e. the
inside of the cell is more negative than the
outside
34
What about an ‘action potential’?
An action potential is defined as a
sudden, fast, transitory, and
propagating change of
the resting membrane potential.
Happens due to a stimulus such as
pressure, temperature etc
Can also be caused by electrical
activity in adjoining neurons
35
Define Resting potential for neuron
The sodium concentration inside the neuron is low, and the potassium
concentration is high
* the opposite is true on the outside of the neuron
36
How is the resting potential generated?
Energy used by carrier molecules to transfer 3 NA out and 2 K in to the neuron (ions) This means Higher positive ion conc outside
NA cant diffuse back in as NA channels closed
Some of K can diffuse out and makes inside of neuron negative
Also negatively charged protein molecules inside that are too big to diffuse out so make it negative aswell
37
What initially happens when a stimulus first affects a neuron
A stimulus such as touch etc causes a type of
membrane channels to open resulting in a
small amount of Na+ to enter the neuron
(diffusion gradient)
This makes the inside of the neuron less
negative than the outside, so the membrane
potential increases to around -55mV
38
What happens when the membrane potential from a stimulus reaches -55mv (depolarisation definition)
When the membrane potential reaches
approx. -55mV, this causes voltage
dependent Na+ channels to open, and Na+
floods into the neuron
The inside of the neuron becomes less
negative and then more positive than the
outside, and the membrane potential reaches
approx. +30mV
This is called DEPOLARISATION
39
What happens when the action potential has taken affect and depolarisation has occured (repolarisation definition)
When the membrane potential reaches
approx. +30mV, voltage dependent K+
channels open (remember that there is a lot
of K+ inside the cell) and K+ floods out of the
neuron (diffusion gradient)
The inside of the neuron now becomes more
negative than the outside, and can actually
overshoot the resting potential
(hyperpolarisation)
This is called REPOLARISATION
40
What happens when the action potential has finished and potential restored/balance is being restored
The Na/K pump goes back to pumping sodium out
and potassium in
* The result of this is a return to the original distribution
of ions (low Na+ and high K+ inside and the opposite
outside)
* Until the balance of ions has been restored, no more
action potentials can be initiated
* This is called the REFRACTORY PERIOD
41
Propagation of action potentials
One of the things that initiates the influx of
Na+ that causes the initial depolarisation
of the membrane is a change in electrical
charge in an adjacent section of
membrane.
* When an action potential occurs, it
propagates along the axon by starting
depolarising the section next to it.
* It can’t go backwards because of the
refractory period
42
All or nothing principle/action potential size compared to response size
* In a particular neuron, all the action potentials are the same size – they
either happen or they don’t
* The strength of the response, e.g. a muscle contraction, depends on
the frequency of the action potentials (and the number of neurons
activated)
* The role of action potentials in the nervous system is to convey
information between the periphery and the central nervous system
43
A neurotransmitter is defined as
a substance (e.g. norepinephrine, acetylcholine,
dopamine) that is released from the axon terminal
of a presynaptic neuron on excitation, and which
travels across the synaptic cleft to either excite or
inhibit the target cell
44
7 main categories of neurotransmitters
– amino acids
– amines
– catecholamines
– peptides
– opioids
– purines
– some atypical - gases, endogenous cannabinoids
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Neuromuscular junction
Specialised synapse where chemical transmission occurs
between motor neuron and skeletal muscle cell
46
What is the neurotransmitter at neuromuscular junction
Acetylcholine (ACh)
47
Describe the neurmuscular junction synapse
Each motor neuron synapses on several muscle fibres (cells) BUT
each muscle fibre is supplied by only one motor neuron
* As with other junctions, presynaptic and postsynaptic
components, and synaptic cleft in between
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One way transmission neurotransmission
ACh is only found on presynaptic
regions so transmission can only go from neuron TO muscle, not
the other way
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How is neurotransmitter released for neuromuscular junction
Action potential in presynaptic neuron triggers release of ACh
and in turn action potential in postsynaptic cell (muscle cell) and
ultimately contraction of the muscle cell
* Action potential is generated by Na+ K+ transfer with the
depolarisation leading to opening of voltage gated Ca2+
channels, increase in intracellular Ca2+ and subsequent ACh
release
50
What happens when ACh is released from neuromuscullar junction
ACh binds to postsynaptic ACh receptors: nicotinic acetylcholine receptors (also bind nicotine), causing
opening of Na+ channels and influx into cell, with depolarisation and action potential
* Action potential results in muscle cell contraction
* If many cells contract, muscle contracts and causes movement
51
What happens after Ach binds and has caused movement of the muscle at a neuromuscular junction
Brief binding of ACh, then broken down by acetylcholinesterase into acetyl and choline and recycled
* Acetylcholinesterase inhibitors prolong presence of ACh at NMJ
– Sometimes therapeutic purpose - Alzheimer’s (eg donepezil) , Myasthenia gravis (eg pyridostigmine)
– Sometimes agricultural/veterinary – organophosphate insecticides (have side effects in other species)
– Weapons - nerve agents eg Sarin gas (also an organophosphate)
* At presynaptic side some toxins (eg botulinum) can interfere with release of ACh from presynaptic vesicles
52
Cell types for sensory perception
– Free nerve endings with dendrites embedded in tissue eg pain and temperature receptors in dermis
– Neuron with endings encapsulated in connective tissue enhancing their sensitivity eg lamellated
corpuscles in dermis - pressure and touch
– Specialised receptors – distinct structural components which are adapted to interpret specific stimulus –
e.g. photoreceptors – light
53
Location sensory perception receptors types
Exteroceptor, Interoceptor, Proprioceptor
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Exteroceptor
located near stimulus in external environment eg somatosensory receptors in skin
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Interoceptor
stimuli from internal organs and tissues eg those detecting pressure in aorta
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Proprioceptor
located near moving part of body to interpret position as it moves
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Chemoreceptor
chemical stimuli eg taste and smell
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Osmoreceptors
concentration of solutes in body fluids
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Nociceptors
pain – can be chemical through release from damaged tissue, or other physical stimulus
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Mechanoreceptor
pressure, vibration, sound, body position
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Thermoreceptor
heat
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Baroreceptor
pressure in blood vessels
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Photoreceptor
light
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Somatosensation
touch – a broad category comprising pressure, vibration, tickle, itch, temperature,
pain, proprioception (awareness of position of body parts in space) and kinesthesia (awareness of position
and movements of joints and muscles)
65
Species differences of sensory perception examples
Eg humans and other vertebrates can only detect visible light but some insects eg bees can detect
ultraviolet, some reptiles can detect infra-red
– Some species have receptors humans lack eg magnetic receptors in migratory birds
– Some species have different colour vision compared to us
– Some have far more sensitive senses than humans – eg canine sense of hearing and smell far greater
than that of humans – sniffer dogs!
66
Sensory receptor-motor pathway
Sensory receptor detects stimulus
* Converts this into action potential in sensory afferent neuron of the PNS
* Travels to the CNS
* Processed in CNS (not always consciously)
* Travels to the skeletal muscle via PNS motor efferent neuron
