Physiology Flashcards
what do dendrites do
receive inputs from other neurones and convey graded eletrical signals passively to the soma
what does the soma contain
nucleus, ribosomes, mitochondria, endoplasmic reticulum
what is the axon hillock and initial segment
site of initiation of the all or non action potential
what is the role of the axon
conducts ouput signals as action potentials to the presynaptic terminal
what is the synapse
point of chemical communication between neurones
what type of neurones are: peripheral autonomic neurones
unipolar
what type of neurones are: dorsal root ganglions
pseudounipolar (one neurite that bifurcates)
what type of neurones are: retinal neurones
bipolar (2 neurites)
what type of neurones are: lower motor neurones
mulitpolar (3 or more neurites)
what is a neurite
process that arises from a soma
why do passive signals not spread far from their site of origin
as the diminish as they spread (leaky membranes) - action potentials is different, have constant amplitude and dont diminish
what is membrane potential change
as the current passes through axons it leaks into extracellular space creating a potential change
how does passive conduction affect action potential velocity
passive conduction is a factor in AP propagation
the further the local current spread the fast the AP conduction velocity
how is passive current spread (and therefore AP velocity) sped up
increase membrane resistance (myelination)
decrease axial resistance of axoplasm (increase axon diameter)
what cells myelinate
schwann cells in PNS
oligodendrocytes in the CNS
(both macroglia)
is conduction in myelinated axons faster or slower than unmyelinated axons of the same diameter
faster
what is saltatory conduction
the action potential jumps from one node of ranvier to the next
name two demyelinating disorders
mulitple sclerosis (CNS) guillian barre (PNS)
what are the steps of chemical neurotransmission
- uptake of precursor
- synthesis of transmitter
- storage of transmitter
- depolarisation by action potential
- Calcium influx
- calcium induces release of transmitter (exocytosis)
- receptor activation
- enzyme mediated inactivation of transmitter or re uptake of transmitter
what is the synaptic cleft
gap between pre and post synaptic membranes
what is in the synaptic cleft
fibrous extracellular protein
what holds the neurotransmitter in the synapse
vesicles in the pre synaptic terminal
what are the synaptic membrane differentiations
presynaptically- active zones around which vesicles cluster
postsynaptically- the postsynaptic density which contains neurotransmitter receptors
what are the morpholgical types of synapses
axodendritic
axosomatic
axoaxonic
(the location of the presynaptic terminal upon the post synaptic cell)
what is the most common CNS neurotransmitter for excitatory synapses
glutamate
what is the most common CNS neurotransmitter for inhibitory synapses
GABA or glycine
what is the inhibitory/ excitatory post synaptic potential
a local graded excitatory (depolarising) or inhibitory (hyperpolarising) response to transmitter (glutamate or GABA or glycine)
what type of neurotransmitters are glutamate, GABA and glycine
amino acid
what is synaptic integration
when either:
-many inputs converge upon a neurone to determine its output (spacial summation)
or
-when a single input modulates output by variation in action potential frequency of that input (temporal summation)
what type of neurotransmitter are dopamine, histamine, noradrenaline and serotonin
amines
what type of neurotransmitter are cholecystokinin dynorphin enkephalins neuropeptides somatostatin substance P TRH vasoactive intestinal polypeptide
peptides
what releases acetylecholine, amino acids and amines
synaptic vesicles
what releases peptides
secretory vesicles
what do Glutamate, GABA, glycine, acetylcholine, and 5-HT activate and mediate
ionotropic ligand gated ion channels
mediate fast neurotransmission
All, except glycine, can also activate metabotropic G-protein-coupled receptors. These mediate relatively slow neurotransmission
what is direct gating
when neurotransmitters act directly on the ion channel
done by ionotropic receptors
what is indirect gating
when neurotransmitters acts indirectly on the ion channel
mediated by activation of metabotropic receptors
what causes fast excitatory postsynaptic potentials
activation of nicotinic (ionotropic) ACh receptors. Channels conduct Na+ and K+
what causes slow excitatory postsynaptic potentials
activation of muscarinic (G protein- coupled) ACh receptors. ACh closes a K+ channel (M-type)
via what does glutamate have inhibitory effects
metabotrophic glutamate receptors
how can Ionotropic Glutamate receptors be classified
classified via their response to non-endogenous agonists that mimic glutamate
non-NMDA receptors bind the agonists kainate or AMPA controlling a channel permeable to Na+ and K+
NMDA receptor controls a channel permeable to Na+, Ca2+ and K+
what do non NMDA ionotropic receptors (AMPA and kainate)
do
mediate fast excitatory synaptic transmission in the CNS
what do NMDA ionotropic receptors do
contributes a slow component to the excitatory synaptic potential
what is the clinical relevance of NMDA receptors
Certain anaesthetic agents e.g ketamine and psychomimetric agents e.g. phencyclidine are selective blockers of NMDA-operated channels
how do metabotropoc glutamate work
don’t have an integral ion channel but exert their effect by activation of a second messenger cascade
Role is modulation of neurotransmission.
what is mechanosensation
fine discriminatory touch (light touch, pressure, vibration, flutter and stretch)
what is nociception
pain
what is the exteroceptive division of somatosensation
(cutaneous senses) registers information from the surface of the body by numerous receptor types
what is the proprioceptive division of somatosensation
monitors posture and movement (sensors in muscle, tendons and joints)
what is the enteroceptive division of somatosensation
reports upon the internal state of the body and is closely related to autonomic function
how many neurones in series usually make up a somatosensory pathway- what and where are they
3
1st= primary sensory afferent (in either dorsal root ganglia or cranial ganglia)
2nd= projection neurone (dorsal horn of spinal cord or brainstem nuclei)
3rd= projection neurone in thalamic nuclei
somatosensory cortex
where in brain does proprioceptive input go to
cerebellum
what potential does a sensory stimulus create
stimulus (mechanical, thermal, or chemical) opens cation selective ion channels in peripheral terminal of primary sensory afferent eliciting a depolarising receptor potential
the amplitude of the receptor potential is graded and proportional to the stimulus intensity
what does a supra threshold receptor potential trigger
all or non action potentials conducted by the axon, at a frequency proportional to its amplitude
what is an all or none response
the strength of a response of a nerve cell or muscle fiber is not dependent upon the strength of the stimulus. If a stimulus is above a certain threshold, a nerve or muscle fiber will fire. Essentially, there will either be a full response or there will be no response at all for an individual neuron or muscle fiber
what do action potentials arriving at the central terminal of the 1st order neurone cause
a graded release of neurotransmitter on to second order neurones
what is the modality of a sensory unit
what type of stimulus excited it (the adequate stimulus)
what is the threshold of a sensory unit
what intensity of stimulus is required for excitation of the sensory receptor
what is the adaption rate of a sensory unit
does the sensory unit discharge action potentials continuously during the stimulus or does it respond preferentially to a changing stimulus
(whether they change their firing rate only in response to a stimulus of changing intensity, or fire continuously throughout a constant stimulus)
what is the conduction velocity of a sensory unit
how rapidly it conducts APs along its axon
what is the receptive field of a sensory unit
site and extent of its peripheral termination - can have a small/ large anatomical distribution
what is the sensory unit for pain
mechanical, thermal and polymodal nociceptors
what mediates discriminatory touch
low threshold mechanoreceptors
what are high threshold units
nociceptors: mechano thermal chemical polymodal
what are polymodal nociceptors
respond to at least 2 types of stimuli
what do slow adapting units respond to
provide continuous information to CNS the whole time there is a stimulus - provides information about position, degree of stretch or force
what do fast adapting units do
only produce APs proportional to the rate of change of the stimulus
do not constantly produce APs when stimulus is constant
detects changes in stimulus strength
what do very fast adapting units do
responds only to very fast movement (will not respond to slow changes in stimulus or a constant stimulus)
e.g. pacinian corpuscle (e.g. rapid vibration)
what do Aalpha axons do
proprioceptors of skeletal muscle
what do Abeta axons do
mechanoreceptors of skin
what do Adelta axons do
temperatures, pain
what do C axons do
temperature, pain and itch
how and why does the conduction velocity change between Aalpha, Abeta, Adelta and C axons
from Aalpha, Abeta, Adelta to C get less myelinated and therefore slower conduction velocity
what is the receptive field
the target territory from which a sensory unit can be excited
how is receptive field relative to innervation density
inversely proportionate
low density of innervation= large RF= high sensory acuity
what do afferent nerve fibres end in
either free nerve endings (partially naked)
or associated with specialised structures
what is spatial acuity
two point discrimination
where do you not get meissners copuscles
hairy skin
where are ruffini endings and what do they do
within dermis
pressure sensation
where are pacinian corpuscles and what do they do
semis and fascia
pressure
what do merkle discs, krause end bulbs, root hair plexuses and meissner corpuscles do
sense touch
what do free nerve endings sense
pain, temperatuere
how are low threshold mechanoreceptors classified
by their rates of adaption and size of receptive field
what are the low threshold mechanoreceptoes
free nerve endings, follicular nerve endings, merkel cell neurite complexes, encapsulated nerve endings (meissner corpuscles, ruffini endings, pacinian corpuscles)
does a pacinian or meissners corpuscle have a bigger receptive field
pacinian
which has a higher human detection threshold meissner or pacinain corpuscle
meissners
at what frequency are are pacinian corpuscles most sensitive
150- clinical test using a 128Hz tuning fork
what part of nerve is affected in shingles
dorsal root ganglion
what virus is shingles
varicella zoster
what class of fibres are nociceptors
Adelta and C
what class of fibres are low threshold mechanoreceptors
Abeta
what class of fibres are proprioceptors
Aalpha
what is the grey matter of the spinal cord divided into
10 distinct laminae of rexed
how does the dorsal medial lemniscal pathway ascend
1st. order neurone enters dorsal horn and branches forming: (i) synapses deep in the dorsal horn upon 2nd. order neurones (important in spinal reflexes) and (ii) a long ascending axon (via the dorsal column gracile, or cuneate tracts) synapsing in either the dorsal column gracile nucleus (GN) or cuneate nucleus (CN)
Axons of 2nd. cross collectively in the great sensory decussation and ascend in the medial lemniscus to the ventral posterior lateral (VPL) nucleus of the thalamus
3rd order primary somatosensory cortex via posterior internal capsule
what does the dorsal column medial leminscal pathway carry
info on discriminatory touch, pressure, vibration, conscious proprioception
how does the spinothalamic tract ascend
decussate as it enters spinal cord, goes to thalamus, cortex
what does the spinothalamic tract do
carry info on pain, thermosensation, crude touch, itch, tickle
what are the parts of the dorsal column of the spinal cord
medial gracile tract (info from T6 and below) and the lateral cuneate tract (above T6)
from lateral to medial carry info from cervical, thoracic, lumbar, sacral
what do the dorsal and ventral spinocerebella tracts do
convey unconscious proprioceptive information to the cerebellum
what is the major route by why touch and CONSCIOUS proprioception ascends to the cerebral cortex
dorsal column medial lemniscial pathway
what are the capabilities of the dorsal column medial lemniscal pathway
stereognosis (recognise object by touch) vibration detection fine touch two point discrimination conscious proprioception weight discrimination
what is contrast enhancement
when information is conveyed from one neurone to the next in a sensory pathway, differences in the activity of adjacent neurones are amplified
when one neurone is active it inhibits the activity of its neighbours via inhibitory interneurones= lateral inhibition
what is the point of lateral inhibition
sharpens stimulus perception
where are the soma of sensory neurones of the trigeminal nerves found
trigeminal sensory ganglion
what is the path of the trigeminal nerve to the brain
Central terminals of the trigeminal nerve synapse upon second order neurones in the chief sensory nucleus (general tactile stimuli), or spinal nucleus (pain, temperature information) which in turn decussate and project (via the trigeminal lemniscus) to the ventroposteriomedial (VPM) nucleus of the thalamus
Third order neurones relay information to the cortex via thalamocortical neurones
what makes up the central sulcus
brodmann areas 1 (texture discrimination), 2 (pressure and joint position) ,3a (propriocpetors) and 3b body position)
what parts of body on what parts of the somatosensory cortex
The toes are at the top of the post central gyrus with the tongue, pharynx and intra abdominal organs at the lower end, the hand separates the head from the face
how is the somatosensory cortex organised
is layered and columnar
are somatotopic maps constant
no are plastic
if area of sensation lost (amputated) area of somatosensory cortex will be utilized by other sensory inputs
if sensory input from an area increases the cortical representation of that area increases, relative that of inputs generating less activity
what does the posterior parietal cortex do
Receives and integrates information from central sulcus and other cortical areas (visual, auditory) and sub-cortical areas (thalamus)
Deciphers the deeper meaning of the information
what can damage to the posterior parietal cortex cause
neurological disorders (e.g. agnosia (inability to process sensory information), astereognosia (cant identify objects by touch), hemispatial neglect syndrome) with simple sensory skills remaining intacta
where are upper and lower motor neurones
upper in brain
lowerin brainstem and ventral horn of the spinal cord
what is the relationship between upper and lower motor neurones
UMN supply input to LWN to modulate their activity
what do LWN recieve input from
upper motor neurones
proprioceptors
interneurons
what commands muscle contraction
lower motor neurones
what makes up lower motor neurones
alpha motor neurones- innervate the bulk of fibres within a muscle that generates a force
what do gamma motor neurones do
innervate a sensory organ within the muscles known as a muscle spindle
give examples of synergistic muscles
biceps brachii and brachialis
triceps brachii and anconeus
(ALSO as these muscles pairs oppose the other pairs actions they are agonists)
what do axial muscles do
control the movement of the trunk (posture)
where are proximal or girdle muscles
shoulder, elbow, pelvis and knee
how do axons of lower motor neurones leave the spinal cord
in central roots (or via cranial nerves)
what is a motor unit
an alpha motor neurone and all the muscle fibres it innervates- the smallest functional component of the motor system
what is a motor neurone pool
a collection of alpha motor neurones that innervate a single muscle
how do alpha motor neurones grade force of muscle contraction
frequency of action potential discharge of the alpha-MN (each causes a twitch)
the recruitment of additional synergistic motor units
where in spinal cord are the cell bodies of the lower motor neurones
ventral horn (ones going to axial muscles are medial to those going to distal muscles. flexors dorsal to extensors)
what are the three sources of input to an alpha motor neurone (LMN) that can regulate its activity
central terminals of dorsal root ganglion cells (whose axons innervate muscle fibres)
UMN in motor cortex and brainstem
spinal interneurones
what does muscle stength depend on
activation of muscle fibres
- firing rate of LMNs
- number of LMNs that are stimultaneously active
- coordination of the movement (antagonist, intergration/ control of reflexes)
force production by innervated muscles fibres
- fibre size (hypertrophy)
- fibre phenotype (fast or slow contracting muscle)
what needs to summate for a muscle to contract
action potentials- a single AP will cause a muscle fibre to twitch
how does the size of alpha MN (soma diameter) change with motor unit size
small motor units are innervated by small alpha MN and vise versa
how does the size of alpha MN (soma diameter) change with the muscle fibre type
α-MNs innervating fast type tend to be larger and have faster conducting axons than those of slow units
what are the two major types of muscle fibres and how do they differ
slow twitch and fast twitch
differ in how quickly myosin ATPse splits ATP to provide energy for cross bridge formation- reflected in time to develop peak tension
also express different myosin heavy chains
what are type I muscle fibres
slow oxidative
where do slow oxidative muscle fibres get their ATP from
oxidative phosphorylation
what do slow oxidative fibres do (type I)
slow contraction and relaxation
fatigue resistant
antigravity, sustained movement
why are slow oxidative fibres red
high myoglobin content
what are type II muscle fibres
fast types:
- type IIa
- type IIx (or IIb)
where do type II fibres get their ATP from
type IIa- oxidative phosphorylation
type IIx- glycolysis
what do type IIa muscle fibres do
fast contraction and relaxation
fatigue resistant
(red and reasonably well vascularised)
sustained locomotion
what do type IIx muscle fibres do
fast contraction
not fatigue resistant
(pale in colour and poorly vascularised)
burst power
how does the threshold and size of alpha MN change with the different muscle fibres
fast fatiguing (IIx) have large alpha MN and high threshold
type IIa have intermediate alpha MN and threshold
type I have small alpha MN and low threshold
how does the tension change between types of muscle fibres
IIX- very high
IIa- high
I-low
what is the henneman size principle
The susceptibility of an α-MN to discharge action potentials is a function of its size. Smaller α-MNs (part of slow motor units) have a lower threshold than larger ones (part of fatigue resistant, or fast fatiguing, motor units). Slow motor units are more easily activated and “trained” by any training that activates the muscle.
Motor units (LMNs and the muscle fibres that they innervate) are recruited in the order of their size (i.e. progressively increasing – small LMNs are more easily excited than large LMNs)
what does the activation of motor units in order of size and from type I first to IIa to IIx allow
a fine control of muscle force
what is the myotatic reflex
when a skeletal muscles is pulled it contracts
what does a muscle spindle to
senses change in length and rate of this change
contributes to non conscious proprioception
what makes up a muscle spindle
fibrous capsule
intrafusal muscle fibres
sensory afferents (Ia, myelinated, fast conducting, that innervates intrafusal muscle fibres
(btw extrafusal fibres generate force)
gamma motor neurone efferents that innervate intrafusal fibres
what are the steps of the myotatic reflex
(is a monosynaptic arc) stretch of muscle spindle activate Ia afferent excitatory synaptic transmission in spinal cord (mediated by release of glutamate) activation of alpha MN contraction of homonymous muscle
what spinal level does the biceps jerk test
C5-C6
what spinal level does the supinator (wrist) jerk test
C5-C6
what spinal level does the triceps jerk test
C7
what spinal level does the quadriceps (knee) jerk test
L3-4
what spinal level does the gastocnrmius (ankle) jerk test
S1
what can reinforce the knee jerk
jendrassik maneeuvre (pulling apart interlocked fingers)
what are intrafusal fibres made up of
a non contractile equatorial region innervated by Ia sensory neurones
contractile polar ends that receive efferent input from gamma MN with cell bodies in the ventral horn of spinal cord (these are driven by higher centres NOT the Ia fibres)
what happens to alpha and gamma MN during voluntary movement
are co activated so that intrafusal muscle fibres contract in parallel with the extrafusal fibres
what are the types of intrafusal muscle fibres
nuclear bag fibres: -bag 1/ dynamic (sensitive to rate of change of muscle length. innervated by dynamic gamma MN)
-bag 2/ static (sensitive to absolute length of muscle, innervated by static gamma MN)
chain fibres;
- sensitive to absolute length of muscles, innervated by static gamma MN
what types of afferent fibres innervate the intrafusal fibres
Ia (more sensitive to rate of change)
II (more sensitive to absolute length of intrafusal fibres)
both respond to stretch
rate of change= dynamic response
absolute length= steady state or static response
which type of gamma fibres are active when
In activities in which muscle length changes slowly and predictably only static γ-MNs are active
Dynamic γ-MNs are active during behaviours in which muscle length changes rapidly and unpredictably
what are golgi tendon organs
located at the junction of muscle and tendon
monitor changes in muscle tension
act to regulate muscle tension
what innervates the golgi tendon organs
group Ib sensory afferents
where do the Ib fibres go from the golgi tendon apparatus
enter spinal cord and synapse upon inhibitory interneurones which then synapse upon the alpha motor neurones of the homonymous muscles
(this forms the basis of the reverse myotatic reflex)
what are the components of the reverse myotatic reflex
an inhibitory interneurone between an Ib afferent and an alpha MN
gets info from golgi tendon organ
where are proprioceptive axons
in connective tissue of joints
can be fast or slow adapting for different movements
what is the role of proprioceptive axons
Respond to changes in angle, direction and velocity of movement of a joint. Also prevent excessive flexion, or extension
what are the three places proprioceptive information arises from
muscle spindles
golgi tendon organs
joint receptors
what 4 sources do spinal interneurones recieve input from
primary sensory axons
descending axons from the brain
collateral branches of LMNs
other interneurones
this input may be excitatory or inhibitory
the interneurones themselves may be excitatory or inhibitory
what is the role of interneurones
integrate incoming information to generate an output
what do inhibitory interneurones mediate
the inverse myotatic response
and reciprocal inhibition between extensor and flexor muscles
explanation (Myotatic reflex causes the homonymous extensor muscle (quadriceps) to contract, but for the leg to extend the antagonist flexor muscle (hamstring) must simultaneously relax
the Ia afferent from the muscle spindle extensor makes an excitatory monosynaptic contact with the α-MN innervating the homonymous muscle. Via a polysynaptic pathway involving an inhibitory interneurone, the Ia fibre also inhibits the α-MN supplying the flexor muscle)
what movement will initiate the myotactic reflex
voluntary contraction of an extensor will strecth an antagonist flexor- initiates reflex
how are descending pathways involved in the myotactic reflex
descending pathways that activate the alpha MN controlling the agonist muscles also via inhibitory interneurones inhibit the alpha MN supplying the antagonist muscles allowing unopposed movement
what do excitatory interneurones mediate
the flexor reflex and the crossed extensor reflex
what is the flexor reflex
when noxious stimuli causes the limb to flex by
- contraction of flexor muscles via EXCITATORY interneurones
- relaxation of extensor muscles via EXCITATORY AND INHIBITORY interneurones
what is the crossed extensor reflex
noxious stimuli causes the limb to extend by;
- contraction of extensor muscles via EXCITATORY interneurones
- relaxation of flexor muscles via EXCITATORY AND INHIBITORY INTERNEURONES
what do the flexor and crossed extensor reflexes have in common
both act to aviod noxious stimuli
both contract muscles by excitatory interneurones
both relax muscles by excitatory and inhibitory interneurones
how can a spinal central pattern generator command limb movement
by excitatory interneuones to display oscillatory or pacemaler activity (synchronicity of the flexor and extensor LMN circuits)
what structures control the strategy of a movement (its aim and how it is best achieved)
neocortical assocaition areas
basal ganglia
what structures control the tactics of a movement (what sequence of muscle contractions and relaxations will fulfil the strategic aim)
motor cortex and cerebellum
what structures control the execution of a movement (activation of motor and interneurone pools)
brain stem and spinal cord
what do the descending tracts arise from
cerebral cortex and the brain stem
what do the descending pathways do
control movement, muscle tone, spinal reflexes, spinal autonomic function, modulation of sensory trasmission to higher centre (gate keeper)
what controls the lateral descending pathways and what do they do
cerebral cortex
voluntary control of distal musculature- fine movements
lateral corticopsinal and rubrospinal tract
what controls the ventromedial descending pathways and what do they do
brainstem
posture and locomotion
(reticulospinal tracts, lateral vestibulospinal tract, tectospinal tract, ventral corticospinal tract)
what brodmans areas are responsible for the control of movement
4 and 6
what is the other name for the corticospinal tract
pyramidal
where are the cell bodies of the corticospinal tract found
in motor cortex (BA 4 and 6) and somatosensory areas of the parietal cortex
what is the path of the corticospinal tract
motor cortex
base of medulla (form medullary pyramid)
fibres cross at the pyramidal decussation (form lateral corticospinal tract)
remainder stay ipsilateral (and form the ventral corticospinal tract which cross more caudally)
terminate in dorsolateral region of the ventral horn (location of LMN and interneurones)
does the right or left hemisphere control the contra or ipsilateral side muscluclature via the corticospinal tract
contralateral
where are the cell bodies of the rubrospinal tract
red nucleus (receives input from the motor cortex and the cerebellum)
where do axons in the rubrospinal tract decussate
ventral tegmental decussation
where does the rubrospinal tract terminate
ventral horn
what muscles does the rubrospinal tract control
limb flexors
what do lesions of the lateral columns cause
loss of fractionated movements
slowing and impairment of accuracy of voluntary movements
where are the nuclei of the vestibulospinal tract
vestibular nuclei (medial and lateral)
where do the vestibular nuclei receive input from
CN VIII (vestibular nerve) from the vestibular labyrinth and cerebellum
what is the path of the vestibulospinal tract
axons from lateral vestibular nuclei descend ipsilaterally as the lateral vestibulo spinal tract to the lumbar spinal cord (this tract controls extensor MN of antigravity muscles)
axons form the medial vestublar nuclei descend as the medial vestibulospinal tract to the cervical spinal cord (control neck and back muscles guiding head movement)
where are the cell bodies of the tectospinal tract - where do they get input from
superior colliculis (aka optic tectum)
get input from retina and visual cortex
what is the path of the tectospinal tract
axons decussate in the dorsal tegmental decussation
descend to cervical spine (influence the muscles of the neck, upper trunk and shoulders)
where do the potine and medullary reticulospinal tracts arise from
reticular formation
what is the path and role of the pontine (medial) recticulospinal tract
descends ipsilaterally
enhances antigravity reflexes of the spinal cord
helps maintain posture- contracts extensors of the lower limbs
what is the path and role of the medullary (lateral) recticulospinal tract
descends bilaterally
opposes the action of the medial tract
releases antigravity muscles from reflex control
