Physiology Flashcards
Define the following terms:
a) Resting potential
b) Action potential
c) Threshold
d) Hyperpolarization
e) Refractory period
a) Refers to the voltage across the membrane as a result of the differential distribution of ions across the membrane. Reflects the concentration and permeability of potassium, sodium and other ions that are distributed across the membrane.
b) Rapid changes to resting membrane potential of nerve cells (and receptors) that serve as signalling mechanism
c) The membrane potential above which, causes depolarisation and evoking of the action potential signal
d) Refers to the resting membrane potential becoming more negative, decreasing the excitability of neuron and decreased ability to generate action potential
e) The time in which a nerve cell is unable to fire an action potential
Define the following terms:
a) Receptive field
b) Label line
c) Sensitisation
a) Receptive field = area of skin where stimulus will excite a receptor
b) Label line = receptor and its associated afferent.
- Usually only responds to 1 type of stimulus -> blocking the generation of action potential in the label line will block the sensation
c) Sensitisation reflects the process seen after tissue damage that:
- Potentiates (increases) response of pain pathways to a given noxious stimuli
- Decreases threshold for excitation of pain pathway
State examples of mechanoreceptors (4)
o Pacinian Corpuscle
o Merkel cell
o Meissner corpuscle
o Ruffini ending
State the types of axons (nerve fibres) involved in sensations (Pain vs non-painful)
o Aβ = touch (non-painful) experience
o Aδ and C-fibre = pain
Compare conduction in myelinated and unmyelinated nerve fibers
o Wave of depolarisation can travel along the axon in 2 ways:
- Jumping from node to node (node of ranvier; known as saltatory conduction) -> helps to speed up conduction of action potential (much faster) for myelinated axons
- Straight down the axon to axon terminal (point to point) -> for unmyelinated axons (much slower)
- This explains why pain sensation is slower than touch sensation
Define synapse and describe its structure (Electrical synapse vs Chemical Synapse)
Synapse refer to the location at which transfer of signals between neurons occur via synaptic potentials
1) Electrical Synapse:
* Synaptic cleft is small and the current generated in the presynaptic neuron flows directly into the postsynaptic cell through specialized bridging channels called gap-junction channels.
2) Chemical Synapse:
* No structural continuity between pre- and postsynaptic elements. The two elements are separated by a space called synaptic cleft which is about 20-40nm.
* For transmission between the presynaptic and postsynaptic cell, an action potential in the presynaptic cell will release chemical transmitters in the cleft.
o Done via triggering opening of voltage gated Ca channel leading to fusing of synaptic vesicles containing neurotransmitter with the membrane resulting in release of neurotransmitter into the synaptic cleft
* The transmitter diffuses across the cleft to interact with specific receptors that either depolarize (called EPSP) or hyperpolarize (IPSP) the postsynaptic cell.
* The depolarization of postsynaptic membrane, if it reaches threshold membrane potential at trigger zone, leads to generation of action potential in the cell.
Whether is excitatory or inhibitory potential is dependent on the neurotransmitter released
Synapses are present at soma (inhibitory in nature; make neuron less excited) and dendrites (excitatory in nature)
State the features that desrcibe sensation and what they depend on (3)
1) Quality (reflect nature of stimulus)
Depends on label line
2) Intensity (strength of stimulus)
Depends on Frequency code + Population code
* Frequency code = number of action potentials per unit time
* Population code = number of receptors activated
3) Duration and location
Location depends on location of receptors activated and the part of brain (sensory homunculus) that processes the information
State the neurons involved in Somatosensory relay and the location where the relay lines cross the midline
1) First order neurons are afferents
2) Second order neuron:
* Pain pathway 2nd order neuron is in spinal cord
* Touch pathway 2nd order neuron is in medulla
3) Third order neuron: Thalamus
4) Fourth order neuron: Somatosensory cortex neuron
The relay crosses the midline in the thalamus for the touch pathway (Dorsal column pathway)
Relay crosses midline in the spinal cord for pain pathway (Spinothalamic pathway)
Intensity: the more intense the stimulus, the more the number of _____ and more intense will be the sensation
action potentials per unit time (frequency)
What are the key features of pain? (3)
o Source (location)
o Intensity
o Subjectivity in unpleasantness
State the definitions of hyperalgesia and allodynia
o Hyperalgesia = increased pain to a given noxious stimulus
o Allodynia = pain to a normally non-painful stimulus (e.g pain to a stimulus normally sensed as touch)
State the normal connection between touch and pain fibres and how touch allodynia may occur
o Aβ fibre contains a side branch that makes contact with an inhibitory neuron -> activation of Aβ leads to inhibition of pain pathway
o Contains another side branch that can excite spinothalamic neuron
o Potential changes resulting in touch allodynia:
Inhibitory Neuron become dysfunctional -> touch pathway can excite the spinothalamic pathway with no inhibition
Inhibitory Neuron switch to become excitatory neuron due to change in electrochemical gradient of [Cl-]
* Normally, GABA acts on spinothalamic tract neuron to open Cl channel -> Cl moves inside cell membrane
* In some chronic conditions, chloride electrochemical gradient is altered such that when GABA opens the Cl channel, Cl flows outward -> neuron depolarise more leading to excitation
Sensitisation of C-fibre after tissue damage leading to increased release of neurotransmitter that alters the properties of the spinothalamic tract -> Amplified excitatory input from side branch of Aβ fibre and C fibre
Explain the descending pathway of pain modulation as well as the gate theory
o MOA (descending pain modulation): excitation of prefrontal cortex (anterior frontal lobe) neurons -> those neurons activate other neurons directly and indirectly in the periaqueductal grey (PAG) to excite neurons in medulla (nucleus raphe magnus) which excites E (Encephalin) Interneuron (in spinal cord; dorsal horn) -> inhibit transfer of signal to spinothalamic neuron -> suppress pain
Encephalin = endogenous morphine
When morphine given, it will mimic effects of Encephalin in the spinal cord + excite neurons in PAG
o Gate Theory: Stimulation of large diameter touch afferents -> excite inhibitory interneuron that in turn decreases transmission of pain signal in spinal cord
Name the 3 limbic structures
o Thalamus
o Hippocampus
o Amygdala
State the role of basal ganglia
Output of the basal ganglia is important for the initiation of appropriate movement
