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Muscle tone:
partial state of contraction of extrafusal fibers
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Who needs some muscle tone?
everyone!
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When is it most efficient to contract a muscle?
when it is already partially contracted
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Posture is a consequence of:
muscle tone
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Posture is a result of:
- constant responses to proprioception input from adjustment of mm to shifts in gravity
- adjust to have a steady platform on which to function
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What 3 primary factors determine muscle tone?
- intrinsic characteristics of extrafusal fibers (skeletal m cells)
- gravity pulling on skeletal mm and activating stretch reflexes (Ia and alpha motor neurons)
- gamma bias -finessing system of m tone
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An alpha motor neuron is the type of neuron that stimulates:
the skeletal mm and is under voluntary control
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Gamma motor neurons are involved in:
reflexes and adjusting tension on muscle spindles
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Muscle spindles are components of mm that:
tell the brain the state of contraction of the mm
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Gamma motor neurons are not under:
voluntary control
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Gamma motor neuron has effect on:
- alpha motor neurons
- Ex: helps w/ coordination and is what is activated when you are nodding off; increased stretch in neck activates gamma motor neuron which tells your brain that your head is moving, causing the reflex that contracts your neck mm and brings your head back up
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What is Gamma bias due to?
supraspinal descending pathways influencing gamma motor neurons (LMN)
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Where do the gamma bias influences (descending supraspinal input) originate from?
- cerebellum
- reticular formation of brainstem
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What does the cerebellum do?
maintain and influence muscle tone
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Where are the cell bodies of gamma motor neurons?
ventral horn of SC
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What do gamma motor neurons innervate?
contractile portion of intrafusal fibers
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Are gamma motor neurons bigger or smaller than alpha motor neurons?
smaller
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When do gamma motor neurons discharge?
spontaneously
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What do gamma motor neurons not do?
- not excited monosynaptically by Ia fibers (no peripheral input)
- do not respond to type II
- GTO (Ib) don't influence gamma motor neuron activity
- don't respond to peripheral input
- not involved with Renshaw cell activity
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What is the effect of gamma motor neurons?
to alter sensitivity of muscle spindle by altering length of intrafusal fiber and tension they exert
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What does gamma bias allow?
muscles to be reset
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When gamma motor neurons fire, they cause:
- skeletal mm aspect of intrafusal fiber to contract
- makes it easier for Ia fibers to fire b/c more sensitive to fire which increases muscle tone
- --this can also work in opposite manner, making it harder to fire Ia fiber; intrafusal fiber becomes slacked
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When does gamma bias prime skeletal muscle tone?
- when you anticipate something
- ex: waiting in the blocks as a sprinter
- ex: waiting the last few minutes of class (about to get up and leave)
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What are some clinical conditions of muscle tone?
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Hypotonia:
- decreased m tone
- decreased resistance to passive movement
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How do you get hypotonia?
- eliminating LMN (transected N); no reflex arc
- eliminate afferent sensory input, alpha motor neurons don't know what to do/when to contract
- lesions of cerebellum; doesn't send info down, or influence gamma bias, therefore no sensitivity
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Hypertonia:
- excessive m tone
- increased resistance to passive movement
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What are types of hypertonia?
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What is spasticity characterized by?
hyper-reflexia of DTR due to UMN damage
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Spasticity:
increased resistance to passive movement
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What is an example of spasticity?
clasp knife phenomenon
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Clasp Knife Phenomenon
an initial increase in resistance followed by a sudden disappearance of resistance (or all of a sudden relaxation)
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What can cause spasticity?
- traumatic brain injury
- heart attack (with ischemia)
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What characterizes rigidity?
increase in m tone in all mm, although strength and reflexes are not affected
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What are two expressions used to describe rigidity?
- lead pipe rigidity
- cog-wheel rigidity
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What is lead pipe rigidity?
- plastic
- uniform throughout ROM
- damage to basal ganglia
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What is cog-wheel rigidity?
- rigidity is a series of jerks during ROM
- seen in Parkinson's due to basal ganglia dysfunction
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What are the primary functions of the cerebellum?
- coordinates voluntary mm activity
- coordinates equilibrium activity
- influence mm tone (can have too much/too little)
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Damage the cerebellum then we will have what kind of problem?
coordination
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Does the cerebellum project directly to SC?
no, it doesn't directly tell LMN what to do
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Does the cerebellum initiate voluntary m movement?
no
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If you damage the cerebellum, you can still have movements b/c:
- it indirectly projects to SC
- you never see paralysis if you damage just the cerebellum, can cause problems though
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What information does the cerebellum need?
- position, state of contraction (m tone), and activity of m and joints (unconscious proprioceptive)
- equilibrium state of body (needs to know what inner ear is doing)
- info being sent via corticobulbar and corticospinal tracts to skeletal mm of body
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How does the cerebellum receive info about the position, state of contraction and activity of m and joints?
via anterior (enters cerebellum through superior peduncle) and posterior (inferior peduncle) spinocerebellar tracts
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How does the cerebellum receive info about equilibrium?
- via vestibulocerebellar tract (vestibular nuclei [90% in pons, 10% in medulla] enters cerebellum through infoerior peduncle to pons)
- transmits unconscious proprioception
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How does the cerebellum receive info being sent via corticobulbar and corticospinal tracts to skeletal mm of body?
via inferior and middle cerebellar peduncles
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Characteristics of cerebellar control of body:
- each hemisphere controls info on ipsilateral side (R hemi controls R body)
- clinically, cerebellar symptoms will be ipsilateral (b/c cerebellum has several double decussation patterns)
- cerebellum doesn't initiate voluntary movement (initiation comes from corticospinal/corticobulbar tracts --no paralysis in cerebellar injury)
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How many functional lobes of the cerebellum are there?
3
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If you have damage to the cerebellar lobes, the damage will:
always be on the same side
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Ipsilateral ataxia on the R side of your body is due to:
R sided posterior lobe damage
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What are the lobes of the cerebellum?
- anterior
- posterior
- flocculonodular
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What is another name for the anterior cerebellar lobe?
spinocerebellum paleocerebellum
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What does the anterior cerebellar lobe do?
maintains m tone, posture, gross voluntary movement, and gait
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Is the anterior cerebellar lobe bilateral or unilateral?
bilateral structure
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What does the posterior cerebellar lobe do?
coordination of fine, voluntary movement
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What are other names for the posterior cerebellar lobe?
middle/pontocerebellum/neocerebellum
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What is the function of the flocculonodular cerebellar lobe?
maintenance of equilibrium
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What is another name for the flocculonodular cerebellar lobe?
archicerebellum
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What are some classical cerebellar symptoms?
- general ataxia
- hypotonia
- nystagmus
- (may be attributed to R/L side and to a specific lobe)
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General ataxia:
abnormality in muscular coordination leading to abnormality of voluntary movement
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What are the symptoms of general ataxia?
- mm contract weakly and irregulary
- intentional tremor/terminal tremor (may occur when approaching a target --closer you get more you shake)
- unsteady or drunken gait
- feet spaced far apart to stabilize and get a broader BOS
- lean or lurch to affected side
- dysmetria
- dyssynergia
- dysdiadokinesia
- dysarthria
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Which cerebellar lobe is responsible for mm contract weakly and irregularly with general ataxia?
anterior
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Which cerebellar lobe is responsible for intentional tremor/terminal tremor with general ataxia?
posterior lobe
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Which lobe is responsible for unsteady or drunken gait with general ataxia?
anterior
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Which lobe is responsible for feet spaced far apart to stabilize with general ataxia?
anterior
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Which lobe is responsible for lean or lurch to affected side with general ataxia?
anterior
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Dysmetria:
inability to stop a m movement at a desired point (finessed dysfunction)
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Which lobe is responsible for dysmetria with general ataxia?
posterior
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dyssynergia:
voluntary movements are jerky and tremor like
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Which lobe is responsible for dyssynergia with general ataxia?
posterior
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Dysdiadokinesia:
- inability to perform rapid, alternating movements
- i.e., rapid pronation/supination of forearm quickly
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Which cerebellar lobe is reponsible for dysdiadokinesia with general ataxia?
posterior
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Dysarthria:
- slurred or hesitant type of speech; scanning speech
- give old people time to respond to your question. They can comprehend (unless there's a cognitive problem) the question, just takes more time, esp. as you age
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Which cerebellar lobe is responsible for dysarthria with general ataxia?
posterior
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Hypotonia:
loss of m tone
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Which cerebellar lobe is responsible for hypotonia?
anterior
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Hypotonia with DTR:
pendular swinging after DTR is diagnostic for cerebellar dysfunction
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Pendular knee jerk:
after knee jerk --swings through normal sport for a while (this is abnormal and indicates cerebellar dysfunction)
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What does hypotonia cause?
diminished resistance to passive movement b/c mm tone has been reset at a lower level
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Is the cerebellum damaged?
no influence to gamma bias, therefore hypotonia occurs b/c little sensitivity
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Nystagmus:
- rhythmic oscillation of eyes
- ataxia of the eyes due to influence of cerebellum on the extrinsic eye mm
- rapid and slow tracking phase
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Which cerebellar lobe is responsible for nystagmus?
flocculonodular lobe
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What is the rapid phase of nystagmus called?
saccades
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What is the slow phase of nystagmus called?
slow tracking component
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How is nystagmus named?
named in the direction of the saccades
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What should you rule out before diagnosing nystagmus (a cerebellar dysfunction)?
proprioceptive, vestibular nuclei, or ear problems
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