Physiology Flashcards
What does the somatosensory system mediate?
Fine discriminatory touch Joint & muscle position sense (proprioception) Temperature Pain Itch
Name the 3 divisions of the somatosensory system.
exteroceptive
proprioceptive
enteroceptive
Which division of the somatosensory system registers information from the surface of the body?
exteroceptive division
Which somatosensory division monitors postures and movement? Where are its receptors found?
proprioceptive
sensors in muscle, tendons and joints
Which division of the somatosensory system reports upon the internal state of the body?
enteroceptive division
- closely related to autonomic function
The primary sensory afferent neurone in the somatosensory pathway is normally found in the CNS.
T/F?
false - in PNS
cell body in either dorsal root ganglia or cranial ganglia
Where are the cell bodies of 2nd order somatosensory neurones found?
dorsal (post) horn of spinal cord or brainstem nuclei
Where are the cell bodies of 3rd order somatosensory neurones found?
thalamic nuclei
How is the receptor potential elicited?
stimulus (mechanical, thermal. chemical) opens cation-selective channels in peripheral terminal of primary sensory afferent neurone -> depolarising receptor potential
How is the amplitude of the receptor potential related to stimulus intensity?
graded and proportional to stimulus intensity
What triggers ‘all or none’ action potentials?
a supra-threshold receptor potential
The frequency of ‘all or none’ action potentials produced is proportional to the receptor potential amplitude.
T/F?
true
What happens when action potentials arrive at the central terminal?
the graded release of neurotransmitter on to the 2nd order neurones
What is the term used to describe how primary afferent neurones are especially tuned to respond to a specific type of energy?
the adequate stimulus
Name the 2 different threshold units in sensory receptors.
low threshold units
high threshold units
What types of threshold unit responds to fine discriminatory touch?
low threshold mechanoreceptors (LTMs)
What kind of threshold unit responds to cold-hot?
low threshold thermoreceptors
What is the response by low threshold units to increasing stimuli strength?
increased rate of firing of Low threshold mechanoreceptors (LTMs) + perception of increasing intensity
If the stimuli to low threshold mechanoreceptors (LTMs) is of extreme intensity, the qualitative character of the stimulus changes to emphasise the signal.
T/F?
False - the qualitative character (e.g. pressure) does not change (e.g. to pain), provided only LTMs are activated.
What type of threshold units are nociceptors?
high threshold (HT) units - respond to high (noxious, potentially damaging), nut not (normally) low intensity stimuli
What do thermal nociceptors respond to ?
high intensity to extreme degrees of heat (>45 or <10-15 degrees)
What do polymodal nociceptors receptors respond to ?
AT LEAST 2 of the HT unit category:
High intensity mechanical stimuli
Extremes of heat
Substances in tissue e.g. inflammatory-like response
What is adaptation?
determines whether sensory units change their firing rate in response to a stimulus of changing intensity, or fire continuously throughout a constant stimulus.
Slow adaptation (SA) or tonic/static response is used by which sort of receptors and why?
e.g. stretch receptors - continuous information to CNS - provides information about position, degree of stretch or force.
What are the characteristics of a slowly adapting response?
sensory units fire APs continuously throughout a constant stimulus and increase firing rate with increasing stimulus
Describe fast adapting responses.
the stimulus strength is detected and the number of impulses is proportional to rate of change of stimulus
What is an example of a very fast adapting sensory unit response?
vibration of different intensities e.g. Pacinian corpuscle
Which sensory receptors have the fasted conduction velocity?
proprioceptors of skeletal muscle (group A-alpha)
Which sensory receptors have the slowest conduction velocity?
group C - temperature, pain, itch
What is the receptive field (RF)?
the target territory from which a sensory unit can be excited
RF size varies greatly over the body and is inversely related to ….?
innervation density and sensory acuity
Areas with high density of innervation will have what size of RF and what type fo acuity?
small RF and high acuity
Areas with low density of innervation will have what size of RF and what type fo acuity?
large RF and low acuity
Which skin mechanoreceptors respond to touch and are abundant in areas of high 2-point discrimination?
meissner’s corpuscles
Which skin mechanoreceptors are deep in dermis and joint capsules and respond to deep pressure?
ruffini endings
What is an Iggo dome?
multiple Merkel cells grouped together and innervated by a single myelinated fibre
Describe Pacinian corpuscles.
unmyelinated ending of sensory nerve cell surrounded by a capsule - response to pressure.
Found in dermis and sometimes in fascia
What is found at the border of dry skin and mucous membranes?
Krause end bulbs
What do two-point thresholds match the diameter of?
the corresponding receptive field
Describe the laminae found in grey matter.
10 distinct laminae of Rexed found from dorsal to ventral (I-X) horns
In which grey matter laminae are nociceptors found and which fibre class are they?
laminae I and II fibre class A-delta/C
In which grey matter laminae are LTMs found and which fibre class are they?
laminae III to VI fibre class A-beta
in which grey matter laminae are Proprioceptors found and which fibre type are they?
laminae VII to IX
A-alpha fibres
Sensory input to which dermatomes travels in the gracile tract (fasciculus gracilis)?
T6 and below (legs and lower trunk)
Sensory input to which dermatomes travels in the cuneate tract (fasciculus cuneatus)
input above T6 (arms and upper trunk)
describe contrast enhancement.
as information is conveyed from one neurone to the next in a sensory pathway, differences in the activity of adjacent neurones are amplified –> contrast enhancement
Describe lateral inhibition.
When one neurone is active, it inhibits activity of its neighbours via inhibitory interneurones & contrast enhancement = lateral inhibition
What effect does lateral inhibition have on stimulus perception?
it sharpens stimulus perception
What are Brodmann areas? Where does it get input from?
These areas are found and make up the S1 area immediately posterior to the central sulcus
BA 1, 2, 3a and 3b
input from VP thalamus
Which type of input does area Ba 3a receive?
proprioceptors (muscle spindles)
Which type of input does area BA 3b receive?
cutaneous - touch (texture, shape, stimulus size)
Which type of input does area BA 2 receive?
joint afferents, golgi tendon organs, deep tissues –> pressure & joint position; object perception
Which type of input does area BA 1 receive?
cutaneous - texture discrimination
A deficit in texture and shape discrimination would indicate a small lesion in which BA region?
3b
A loss of texture discrimination would indicate a small lesion in which BA region?
1
Impaired grasping and size and shape discrimination would indicate a small lesion in which BA lesion?
2
what does astereognosis mean?
impaired size and shape discrimination
How many cell layers does the somatosensory cortex have?
6 cell layers (I-VI) 1 - molecular 2 - external granular 3 - external pyramidal 4 - internal granular 5 - internal pyramidal 6 - multiform white matter
Thalamic inputs to SI terminate mainly on neurones within which layer of the somatosensory cortex?
layer 4 - internal granular
neglect syndrome is usually the result of damage to the …. cortex?
right parietal cortex - patients believe the left side of the world does not exist and may even disclaim the existence of the left side of their body.
Describe the hierarchy of motor control.
involves at least 3 levels, e.g. high, middle and low.
hierarchy from top (forebrain) to bottom (spinal cord).
Motor control is initiated by multiple sensory inputs and a need to move using internal mechanisms.
What is the function of high level in motor control hierarchy? Which structures are involved?
Function = strategy;
neocortical association areas; Basal ganglia
What is the function of the middle level in motor control hierarchy? Which structures are involved?
Function = tactics
Motor cortex, cerebellum
What is the function of low level in motor control hierarchy? Which structures are involved?
Function = execution
Brain stem, spinal cord
LMNs supply input to UMNs to modulate their activity.
T/F?
False - UMNs supply input to LMNs to modulate their activity.
Where do LMNs get input from and what is their role?
input: UMNs, proprioceptors, interneurones
They command muscle contraction and form the ‘final common pathway’.
What is the role of alpha-motor neurones (a-MNs)?
innervate bulks of fibres within muscle that generate force.
What do gamma-MNs do?
innervate sensory organ with muscle aka. muscle spindle
Do axons of LMNs exit the spinal cord via dorsal or ventral roots?
ventral roots (or via cranial nerves)
Where in the spinal cord would you find a greater number of motor neurones?
cervical enlargement (C3-T1) and lumbar enlargement (L1-3)
Describe the distinct distribution of LMN soma in the ventral horn.
LMNs to axial muscles are MEDIAL to those innervating distal muscles.
LMNs to flexors are DORSAL to those supplying extensors.
Name the 3 sources of input regulating the activity of an alpha-MN.
Central terminals of DRG cells whose axons innervate muscle spindles.
UMNs in the motor cortex & brainstem.
Spinal interneurones.
alpha-MN + all the skeletal muscle fibres it innervates –> ?
motor unit
Muscle contraction results from…?
Individual and combined action of motor units which must be co-ordinated.
The collection of alpha-MNs that innervate a single muscle = ?
motor neurone pool
What are the 2 principle mechanisms which grade the force of muscle contraction by alpha-MN?
Frequency of AP discharge of the a-MN
The recruitment of additional, synergistic, motor units.
Activation of muscle fibres depends on ?
The firing rates of the LMNs involved; The number of LMNs that are simultaneously active; The co-ordination of the movement.
Fibre size and phenotype are important factors in determining ?
force production by innervated muscle fibres
What is the result of a single AP in an alpha-MN?
causes muscle fibre to twitch
How do slow twitch and fast twitch skeletal muscle fibres differ?
In how quickly myosin ATPase splits ATP to provide energy for cross bridge cycling.
They also express different myosin heavy chains (MHC)
Time to develop peak tension is a reflection of…
myosin ATPase activity
Describe slow-oxidative (type 1 ) fibres.
ATP from oxidative phosphorylation
Slow contraction and relaxation
Fatigue resistant
Describe type IIa (fast oxidative) fibres.
ATP from oxidative phosphorylation
Fast contraction and relaxation
Fatigue resistant
Describe type IIb (fast glycolytic) fibres.
ATP from glycolysis (anaerobic)
Fast contraction - not fatigue resistant (not much ATP produced)
What colour are each type of skeletal muscle fibres?
type 1 and type IIa = red
type IIb = pale
Fast fatiguing motor units are connected with which type of muscle fibres?
IIb/IIx
= burst power
Fatigue resistant motor units are connected with which type of muscle fibre?
type IIa
= sustained locomotion
Slow motor units are connected with which muscle fibre type?
type 1 fibres
= antigravity, sustained movement
Describe the Henneman Size Principle.
The susceptibility of an a-MN to discharge action potentials is a function of its size.
i.e. slow (& small) motor units are more easily activated - have a lower threshold than larger ones.
What does recruitment of alpha motor neurones by size allow for?
the fine and graded development of muscle force - motor units are recruited in order of their size.
This type of activation allows for fine control of muscle force across a wide range of tensions developed.
What is the myotonic reflex?
when a skeletal muscle is pulled, it pulls back.
What does the muscle spindle register?
change in length (& rate of change)
What does a muscle spindle consist of?
A fibrous capsule
Intrafusal muscle fibres
Sensory afferents - innervate the intra-fusal fibres (Ia class)
Gamma motor neurone efferents - innervate infra-fugal fibres
How can the myotonic reflex be reinforced?
the Jendrassik manoeuvre - ask patient to interlock fingers and try pull hands apart when instructed. tap tendon immediately - the reflex is usually exaggerated.
During voluntary movement, alpha and gamma MNs are normally co-activated.
T/F?
True -
so that the intra-fusal muscle fibres contract in parallel with the extrafusal fibres.
Maintains the sensitivity of the spindle to stop it from ‘going slack’ when the extrafusal fibres contract
