Physiology Flashcards
What are the two directions that impulses can move along an axon?
- anterograde is away from the soma
- retrograde is towards soma (used by viruses)
What are the different types of neurons?
- unipolar
- pseudounipolar: bifurcating axons
- bipolar: dendrites and axon
- multipolar: 1+ dendrites
Do passive signals or actions potentials decay?
- APs never decay
- passive signals decay due to leaky nerve cell membrane
How do you increase the passive current spread?
- increase radius to decrease resistance
- add myelin
What can axons synapse onto?
- dendrites
- soma
- another axon
What is the most frequent neurotransmitter for excitatory and inhibitory responses?
- excitatory: (cause depolarisation)
- inhibitory: GABA or glycine (cause hyperpolarization eg with Cl- influx)
What are the types of summation?
- spatial: many inputs at different places on the one soma
- temporal: many action potentials at the same time on the axon
What is an inotropic receptor?
- direct gating to the ACh channel
- FAST
What is a metabotropic receptor?
- ACh binding causes activation of a G protein which then causes K+ efflux
- SLOW
What are the three different channels that glutamate can bind to?
AMPA, kainate and NMDA
What things does the somatosensory system mediate?
- fine touch
- proprioception
- temperature
- pain
- itch
What are the three subdivisions of the somatosensory system?
- exteroceptive (information from surface of the body)
- proprioceptive (posture and movement)
- enteroceptive (internal state of the body)
What is the chain of neurons in a somatosensory impulse?
primary sensory afferent (in dorsal root ganglia or cranial ganglia) –> projection neuron (in brainstem or dorsal horn of spinal cord) –> projection neuron (in thalamic nuclei) –> somatosensory cortex
What do slow adapting sensory units do?
give continuous information to CNS about degree of stretch, force and position
What do fast adapting sensory units do?
detect a change in stimulus strength so the no. of impulses is proportional to the rate of change of the stimulus
What do very fast adapting sensory units do?
only response and make an AP for very fast movement
What is the receptive field or RF?
the area where the sensory unit can be excited for a particular neuron
What determines a high acuity in terms of RF and innervation?
small RF and high density of innervation
What determines a low acuity in terms of RF and innervation?
large RF and low density of innervation
What is the difference in the RF’s of Meissener’s and Pacinian corpuscles?
Meissener’s for touch = smaller RF
Pacinian for pressure = larger RF
What are the main two somatic sensory pathways?
- spinothalamic tract
- dorsal column medial lemniscal pathway
What are the functions of the spinothalamic tract?
pain, temperature, itch
What are the functions of the DCML?
discriminatory touch, pressure, vibration, weight discrimination, conscious proprioception
What are the two main tracts in the sensory dorsal columns?
- medial gracile tract (T6 and below)
- cuneate tract (T6 and above)
What is the arrangement of the sensory dorsal column?
lateral to medial is cervical, thoracic, lumbar and sacral
What is the main route for touch and conscious proprioception to the cerebral cortex?
dorsal column medial lemniscal pathway
What is the DCML sensory pathway?
1st neuron enters dorsal horn and branches to a synapse in horn (for spinal reflex) and long ascending axon –> via gracile or cuneate tract to either gracile nucleus or cuneate nucleus in the medulla –> next neuron crosses sides and ascends in medial lemniscus to the ventral posterior lateral nucleus of the thalamus
–> last neuron projects to the primary somatosensory cortex
What is lateral inhibition of neurons?
one active neuron inhibiting the neurons next to it to sharpen the stimulus perception
What is the pathway of the trigeminal sensory system?
three branches of CNV –> trigeminal ganglion –> synapse in the chief sensory nucleus or spinal nucleus –> fibres cross and go up to the VPM nucleus of the thalamus –> another neuron goes on to the cortex
What is the function of the posterior parietal cortex?
receives and integrates information from the somatosensory cortex and from other cortical areas eg visual, auditory etc
What do upper motor neurons do?
supply input to lower motor neurons to modulate their activity
What do LMNs do?
- receive information from UMNs
- receive information from proprioceptors and interneurons
- cause muscle contraction
What are the two types of LMNs?
- alpha motor neurons: innervate fibres that make force
- gamma motor neurons: innervate the muscle spindle (sensory)
What do the different motor neurons at different levels innervate?
- distal and proximal muscles = cervical and lumbar-sacral segments (enlargements)
- axial muscles = all levels
What is a motor neuron pool?
collection of alpha motor neurons that innervate a single muscle
What is the arrangement of LMNs in the ventral horn?
- LMNs for the axial muscles are medial to the LMNs for the distal muscles
- LMNs for the flexors are dorsal to the LMNs for the extensors
What are the three things that regulate an alpha MN’s activity?
- central terminals of dorsal root ganglion cells
- UMNs in the brainstem and motor cortex
- spinal interneurons
What does muscle strength depend on?
- Activation of muscle fibres: firing rates of LMNs, no of active LMNs and coordination of the movement
- Force made by innervated fibres: fibre size and fibre phenotype
What causes sustained contraction of a muscle?
summation of muscle twitches (many Eps) in an alpha MN
What is the size of the motor unit relative to the size of the alpha motor neuron?
small motor units are innervated by small alpha motor neurons and vice versa
What is the scientific difference between slow and fast twitch fibres?
how quickly myosin ATPase splits ATP for cross bridge cycling
What are the features of slow (type 1) muscle fibres?
- ATP from oxidative phosphorylation
- slow
- fatigue resistant
- red due to myoglobin
- small alpha MN
What are the features of fast (types 2a and 2b) fibres?
- 2a: ATP from oxidative phosphorylation, red and fatigue resistant
- 2b: is ATP from glycolysis, fast and can fatigue, pale, large alpha MN
What is the difference in excitation of LMNs in relation to their size?
small LMNs are more easily excited than large LMNs so fine control of muscle force
What is the myotatic reflex?
when a skeletal muscle is pulled it pulls back because the change in length is registered by the muscle spindle eg knee jerk reflex
What is the reflex for muscle spindle fibres?
- monosynaptic reflex arc
- rapid
- extensor muscles
What is the Jendrassik manoeuvre and what does it prove?
- interlocking fingers and pulling accentuates the myotatic reflex
- proves that simple reflexes can be modulated by descending control
What are the intrafusal fibres in muscle spindle innervated by?
gamma motor neurons and stimulation causes contraction
What are the two types of intrafusal fibres?
- nuclear bag fibres
- chain fibres
Which intrafusal fibres are dynamic and which are static?
- nuclear bag fibres can be either dynamic (very sensitive to the rate of change) or static (more sensitive to the absolute length of the muscle)
- chain fibres are sensitive to the absolute length of the muscle (static gamma MNs)
What are the two types of fibres that innervate the intrafusal fibres?
- 1a = forms a primary annulospiral nerve around the centre of all of the fibres and are sensitive to the rate of change of stretch
- 2 = form flower spray endings and are sensitive to the absolute length
What does stimulation of the different types of gamma MN cause?
- static gamma MN fibres causes steady state to increase
- dynamic gamma MN enhances the dynamic response to stretch fibres
What are the uses for static and dynamic gamma MNs?
- static = slow and predictable movements
- dynamic = rapid and unpredictable muscle length changes
What do Golgi tendon organs do?
- at the junction of the muscle and tendon
- monitor changes in muscle tension and keep it in optimal range
- protect muscle from overload
What are Golgi tendon organs innervated by?
group 1b sensory afferents
What does the inverse myotatic reflex involve?
excitation of 1b from the golgi tendon organ –> excitatory synapse with an inhibitory neuron in the spinal cord –> inhibition of the alpha MN going to the muscle –> relaxation
What are proprioceptive axons?
- present in connective tissue of joints
eg free nerve endings, golgi-type endings, Paciniform endings and Ruffini endings
What are the three ways that proprioceptive information can arise?
- muscle spindles
- golgi tendon organs
- joint receptors
What do inhibitory interneurons do?
- mediate the inverse myotatic response (relaxation of the antagonist muscle)
- reciprocal inhibition between extensor and flexor muscles
What do excitatory interneurons do?
- flexor reflex (noxious stimulus causes limb to flex so excitatory interneurons cause the flexion)
- crossed extensor reflex (the same noxious stimulus causes opposite side to support body during reflex on other side)
What is the most important ion in the auditory system?
K+ which moves in through the hair cells and the channels open and close based on direction of the stereo cilia in relation to the kinocilium
Where do the auditory nerve axons synapse?
at the base of the hair cells in the inner ear
What does tectorial membrane vibration cause?
- hair cells bend
- neurotransmitter release (depending on direction of bend)
- K+ will enter the hair cells and this is caught in the nerve fibres
What do the different directions of the stereocilia mean?
- away from kinocilium = closed
- towards kinocilium = open channels
How are the K+ ions recycled in hearing?
through connective tissue and back into the scala media and damage to this pathway can cause deafness
How does different parts of the basilar membrane pick up different frequency sounds?
- width and the stiffness of the basilar membrane change
- hair cells are affected differently by different frequencies
- near oval window = higher frequency sounds
What are the roles of the inner and outer hair cells?
- inner hair cells = afferent signals
- outer hair cells = efferent inputs as they amplify membrane vibration
What do cochlear amplifiers do?
contract and expand to change the length of the outer hair cell
What does the frequency of sound directly correspond to?
how many APs are created
How do the impulses travel to the brain after the cochlear nucleus?
- mixing of stimuli from the two ears
- tonotopy is maintained all the way to the auditory cortex
What is linear and rotational movement detected by?
- Linear movement = otolith organs
- Rotational movement = semicircular canals
How are impulses created from the cupula?
- fluid displaces the cupula
- causes crista movement
- determines glutamate release
What happens to the contralateral ear when there is an increased in firing in one ear?
decrease in contralateral ear
What are the three main vestibular reflexes?
- vestibulo-ocular (eyes move to keep gaze fixed when head moves)
- vestibulo-colic (keeps head still when walking)
- vestibular-spinal (adjusts posture in rapid movements)
What is the dark current in relation to vision?
photoreceptors generate a current in the absence of light with a sodium influx so there is depolarisation in the dark
What do the different types of bipolar cells do in response to glutamate in vision?
some depolarise and some hyperpolarise
What does lateral inhibition in the retina cause?
center-surround organisation
How do visual impulses get from the retina to the brain?
retina –> lateral geniculate nucleus –> primary visual cortex
What is the simplified motor control pathway?
neocortical association areas and basal ganglia –> motor cortex and cerebellum –> brain stem and spinal cord
What are the two important divisions of the motor pathways?
- lateral eg corticospinal and rubrospinal
- ventromedial eg vestibulospinal, tectospinal and pontine/medullary reticulospinal
What is the difference between lateral and ventromedial pathways?
- Lateral pathways: under cerebral cortex control, voluntary control of distal muscles esp fine movements
- Ventromedial pathways: under brain stem control, posture and locomotion
What does the rubrospinal tract do?
- cell bodies in red nucleus
- decussation at ventral tegmental decussation
- control over LMNs of limb flexor muscles
What compensates when there is damage to the corticospinal tract?
rubrospinal tract can compensate (otherwise used less)
What does the tectospinal tract do?
- cell bodies in the superior colliculus
- gets visual/sensory and auditory information
- axons decussate in the dorsal tegmental decussation
What do the pontine/medullary reticulospinal tracts do?
- come from reticular formation
- pontine = medial and descends ipsilaterally to do standing posture
- medullary = lateral and descends bilaterally to oppose medial tract
What are the three forms of pain?
- nociceptive (adaptive, immediate protective response)
- inflammatory (adaptive, assists in healing)
- pathological (maladaptive, no purpose)
What are the differences between Adelta and C fibres?
- Adelta- fibres are mechanical/thermal nociceptors which respond quickly to stimuli to mediate fast pain
- C- fibres respond to all stimuli and mediate slow pain
What is the pathway from stimulus to pain perception?
stimulus onto free nerve ending → axon of nociceptor → via dorsal root ganglion → dorsal horn → second order projection neuron → spinothalamic and spinoreticulothalamic tract
What is special about peptidergic polymodal nociceptors?
subset of C-fibres and they have both
- afferent (transmit nociceptive info to CNS in dorsal horn)
- efferent (release pro-inflammatory mediators to contribute to neurogenic inflammation) functions
What is the process of neurogenic inflammation?
peptides released from free nerve ending due to tissue damage or inflammatory mediators → vasodilation, release of histamines and sensitisation of nociceptors → hyperalgesia and allodynia
How do visceral afferents get to the dorsal horn?
from nociceptors they then follow sympathetic pathways
How does referred pain work?
- brain can interpret the nociceptive information from the viscera as coming from the skin
- some visceral and skin afferents converge on the same spinothalamic neurons
What is the gate control theory?
- Abeta fibre activity > C/Adeltaf fibre = spinal gate is closed and the pain is not perceived
- if the reverse is true then the pain is perceived
What are the two major nociceptive tracts?
spinothalamic tract and the spinoreticular tract
What are the features of REM sleep?
- fast EEG activity which is similar to wakefulness
- muscle atonia
- narrative dreams
- happens at the end of the night
What are the features of non-REM sleep?
- slow EEG waves
- body is relaxed with reduced HR and BP
- non-narrative images dreams
- start of night
