Cerebellum and Muscle Spindles

What do muscle spindles do?

Detect change in muscle length and velocity

What are the components of muscle spindles?

- Intrafusal fiber

- Afferent sensor endings

- Efferent motor ending

What are intrafusal fibers?

component of muscle spindles

- 3 types

ends are contractile, center region is non-contractile

What are the 3 types of intrafusal fibers?

- Dynamic nuclear bag

- Static nuclear bag

- Nuclear chain fibers

What are intrafusal fibers innervated by?

gamma motor neurons (efferents)

Type Ia and type II sensory fibers (afferents) provide sensory information back to where?

the spinal cord

Activities that stretch extrafusal fibers also stretch what and why?

intrafusal (muscle spindle) fibers due to them lying parallel or embedded into the skeletal fibers

How do intrafusal fibers fire?

tonically

When to intrafusal fibers increase there firing rate?

in response to stretch

What adjusts the sensitivity of muscle spindles?

gamma motor neurons

What are type Ia sensory axons called and what do they innervate?

primary receptors

innervate all 3 types of intrafusal fibers:

- dynamic nuclear bag

- static nuclear bag

- nuclear chain fibers

What are type II sensory axons called and what do they innervate?

secondary receptors

innervate 2 types of intrafusal fibers:

- static nuclear bag

- nuclear chain fibers

How does co-activation work during planned movements?

upper motor neurons usually stimulate alpha and gamma motor neurons simultaneously

activity of gamma motor neurons keeps muscle spindles under proper tension while muscles are shortened

What are GTOs? What do they do?

Golgi Tendon Organs

Provide information about tension in tendons

How do GTOs work?

Type Ib sensory fibers synapses with inhibitory interneuron which causes inhibition of muscle in which it lies

What does the GTO pathway also receive input from?

- Muscle spindles

- Cutaneous receptors

- Joints

- Descending pathways

In simple terms, what is the function of the cerebellum?

Feedback system for motor control

While the cerebellum is only 10% of the brain's volume it contains what?

50% of the total neurons in the CNS

The cerebellum's control is mostly...

ipsilateral

What should you think about when you think about the cerebellum?

Rule of 3s

- 3 Parts

- 3 Lobes

- 3 Functional divisions

- 3 Cellular layers

What are the functions of the cerebellum?

- Coordination

- Balance and Posture

- Motor Learning

- Sensory processing

- Cognition/memory

How do you define coordination?

muscles fire at the right time, in the right sequences and with the appropriate intensity to accomplish a fluid movement or action

How do you define how balance and posture work?

gets input related to where body is in space (proprioception), and changes in position (vestibular input)

How do you define motor learning?

practice or experience leads to permanent changes in the capability for producing skilled action. Complex function that includes several parts of the brain including the cerebellum

What happens with sensory processing when the cerebellum gets damaged?

when the cerebellum gets damaged it spares the primary sensory function but impairs ability to use sensory information to refine movement

How does a damaged cerebellum effect cognition/memory?

still under investigation but cerebellar damage linked to impaired control of emotional behavior

How does the cerebellum function as a comparator?

- part of a feedforward control system

- compares movement being performed to the movement that is desired

- Programs voluntary movement (especially skilled movement)

- Helps prepare the body for a particular motor act before the movement occurs

- Seems to "encode" the end point of movement

What is the cerebellar peduncle?

bundle of axons that connects the cerebellum to the brainstem

What are the 3 cerebellar peduncles?

- Superior

- Middle

- Inferior

What are the 2 systems to label the parts of the cerebellum?

- Anatomical description

- Functional description

How do you label the cerebellum using an anatomical description?

Two hemispheres + Vermis

3 Lobes:

- Anterior

- Posterior

- Flocculonodular

How do you label the cerebellum using a functional description of divisions?

- Vestibulocerebellum (flocculonodular lobe)

- Spinocerebellum (vermis and paravermis)

- Cerebrocerebellum (lateral portions of the hemispheres)

What is the vestibulocerebellum?

- Consists of the flocculus & nodulus

- Has connections with the vestibular system

What is the function of the vestibulocerebellum?

- Maintenance of balance, posture and equilibrium (via vestibulospinal and reticulospinal tracts)

- Control of eye movements

Damage to the vestibulocerebellum results in...

- Truncal ataxia

- Wide-based

- Staggering gait

- Nystagmus

What is nystagmus?

involuntary oscillating eye movements

What is the spinocerebellum?

Consists of 2 regions:

- Vermal

- Paravermal

Receives proprioceptive info from the spinal cord

What is the function of the spinocerebellum?

- Regulate posture and adjust limb movements

- Comparator

Damage to the spinocerebellum results in...

- Gait ataxia (as if drunk)

- Impaired heel-shin test

- Hypotonia

often seen in alcoholics (anterior lobe syndrome)

What is the cerebrocerebellum?

consists of lateral hemispheres

receives information from cerebral cortex

What is the function of the cerebrocerebellum?

planning movements of limbs, especially precise distal movements, timing

Damage to the cerebrocerebellum results in...

combination of deficits in coordination of voluntary movements

The cerebrocerebellum works in collaboration with...

motor planning areas and the basal ganglia

What does the cerebrocerebellum help decide?

when movement pattern is initiated and when terminated (coordination)

What specific symptoms and impairments would you see in a patient with damage to the cerebrocerebellum?

- Ataxia

- Decompositions (Asynergia)

- Intention tremor

- Dysmetria

- Dysdiadochokinesia

What is ataxia?

loss of coordination of movements

What is decomposition (Asynergia)?

loss of coordination of groups of muscles

Therefore, movements performed in a sequence of component parts rather than a single, smooth activity

What is an intention tremor?

a tremor which worsens as one reaches for a target

What is dysmetria?

inability to gauge distance

What is dysdiadochokinesia?

inability to perform rapid alternating movements

What is the cellular organization of the cerebellum?

- Mossy fibers

- Climbing fibers

- Granule cells

- Purkinje neurons

What are granule cells?

axons which bifurcate to form parallel fibers

What are purkinje neurons?

large neurons with extensive dendritic trees

What are the 3 layers of the cerebellar cortex?

- Molecular layer

- Purkinje cell layer

- Granular layer

What is contained within the molecular layer of the cerebellar cortex?

axons from granule cells and dendrites from purkinje cells

What is contained within the purkinje cell layer of the cerebellar cortex?

cell bodies of purkinje neurons

What is contained within the granule layer of the cerebellar cortex?

granule cells

What are the mossy and climbing fibers that are within the cerebellum?

Excitatory inputs the feed into the cerebellum

What information do mossy fibers transmit?

somatosensory, arousal and balance info

What information do climbing fibers transmit?

information related to movement errors

What do the granule cells do within the cerebellum?

Receive excitatory input from mossy fibers

- Axons bifurcate to form parallel fibers which form excitatory synapses with numerous Purkinje cells

What do the purkinje neurons do within the cerebellum?

it is the only output cell from the cerebellar cortex and they inhibit the deep cerebellar/vestibular nuclei

Where are the deep cerebellar nuclei embedded?

into white matter

How do the cells within the cerebellum work to help turn on and off the motor plan and refine movement?

- Input from mossy and climbing fibers excite nuclei, thus increasing firing rate

- Purkinje cells work to inhibit deep nuclei, thus decreasing firing rate

Excitation would work to "turn on" a motor plan and Inhibition would work to "turn off" a motor plan. Both of these working in tandem work to refine movement

Most of the tracts of the cerebellum are...

ascending

What information is carried by the posterior (dorsal) spinocerebeller tract?

info from muscle spindles and Golgi Tendon Organs (mostly trunk and LE)

What is the function of the posterior (dorsal) spinocerebeller tract?

unconscious proprioception from lower body

What information is carried by the cuneocerebellar tract?

info from muscle spindles and Golgi Tendon Organs (mostly neck and upper extremity)

What is the function of the cuneocerebellar tract?

unconscious proprioception from upper body

What information is carried by the anterior (ventral) spinocerebeller tract?

Golgi Tendon Organ's and cutaneous receptors

What is the function of the anterior (ventral) spinocerebeller tract?

more sensory input. theory is that this information is related to intended movement

What information is carried by the rostrospinocerebellar tract?

Not known - likely GTOs and cutaneous receptors

What is the function of the rostrospinocerebellar tract?

not certain... UE equivalent to the anterior spinocerebeller?

Descending cortical output is about 21 million fibers, what do they do?

1 million descend as the corticospinal tract to activate alpha motor neurons

The other 20 million are corticopontine fibers that synapse at the pontine nuclei and send information to the cerebellum

What inputs into the cerebellum?

- Vestibular, visual, auditory and somatosensory

- Spinal cord (spinocerebellar pathways)

- Almost all areas of the cerebral cortex

- Brainstem (pontine nuclei)

What should be taken into account when thinking about parallel pathways and the cerebellum?

somatosensory information from the periphery is constantly being sent to both the cerebellum and the primary somatosensory cortex

Signs and symptoms of cerebellar injuries include...

- Ataxia

- Decomposition (Asynergia)

- Intention Tremor

- Dysmetria

- Dysdiadochokinesia

- Dysarthria

- Hypotonicity

- Nystagmus

- Balance and equilibrium

What is dysarthria?

lack of normal flow and rhythm of speech and lack of ability to coordinate the muscles necessary to make phonemes

What would be the cause of hypotonicity in someone with cerebellar injury?

due to damage of the vermis and fastigal nuclei

Examples of types of etiologies that lead to cerebellar damage include?

- Stroke

- TBI (most often due to "shearing" force but also direct force)

- MS

- Tumors

- Hydrocephalus with herniation (especially of cerebellar tonsils)

- Arnold Chiari

- Cerebral palsy

- Huntington's disease

- Hereditary cerebellar ataxia (e.g., Friedreich's Ataxia)

- Toxicity and Alcoholism