Neuro part 3

Cerebellum function

adjusts posture, coordinates movements

All functions here are nonconscious

Optimizes emotional, social, and cognitive functions

more densely packed with neurons than the cerebrum

Where does the cerebellum integrate info in order to achieve smooth movements?

1. From frontal lobes, about intended movements

2. From vestibular receptors and proprioceptors

3. About neural activity in brainstem motor areas and ventral horn cells in spinal cord

Does the cerebellum directly connect to LMN?

No, so it does not directly influence muscle activity

How does the cerebellum influence movement?

via synapses with upper motor neuron cell bodies in:

Motor cortex, premotor cortex, and brainstem

Severe damage to the cerebellum

does not interfere with sensory perception nor with muscle strength

Damage results in impaired coordination and postural control

Outer layer of cerebellum

Made of gray matter, has 3 layers

Outer and inner layers contain interneurons

Middle layer contains Purkinje cell bodies

2 types of axons that release excitatory transmitters transmit afferent signals coming into the cerebellum

1. Mossy fibers

2. Climbing fibers

Where is the cerebellum located?

Lies inferior to the occipital lobe

What attaches the cerebellar hemispheres to the posterior brainstem?

Attached by 3 large bundles of axons

1. Superior cerebellar peduncles- connect to midbrain

2. Middle cerebellar peduncles- connect to pons

3. Inferior cerebellar peduncles- connect to medulla

3 lobes within each cerebellar hemisphere

1. Anterior

2. Posterior

3. Flocculonodular

3 functional regions of cerebellum vertically

1. Midline vermis

2. Paravermis

3. Lateral hemisphere

Human movements can be categorized into 3 broad classes

1. Equilibrium

2. Gross movements of the limbs

3. Fine, distal voluntary movements

Cerebellum has specialized regions for controlling each of these aspects of movement

1. Vestibulocerebellum- posture and eye movements

2. Spinocerebellum- gross movement of limbs

3. Cerebrocerebellum

Vestibulocerebellum

Located in the flocculonodular lobe

Projects to vestibular nuclei: Influence postural control and control of eye movements

Spinocerebellum

Receives information about:

1. Movement commends from the cortex

2. Activity levels of spinal cord neurons

3. Movements or postural adjustments from proprioceptors

Information used to make anticipatory, corrective, and responsive adjustments to movements

Spinocerebellar Pathways and Tracts

Info in the spinal cord destined for cerebellum

2 pathways here- deliver high-accuracy information from peripheral receptors in muscles, tendons, and joints to the cerebellum

Info does not reach consciousness

High-accuracy pathways- ipsilateral

2 pathways:

1. Posterior spinocerebellar pathway

2. Cuneocerebellar pathway

Info from the periphery to cerebellar cortex

Posterior Spinocerebellar pathway (lower body)

Transmits proprioceptive information from the lower limb and the lower trunk

2nd order axons form this tract

Cuneocerebellar pathways (upper body)

Begins with primary afferents from proprioceptors in the neck, upper limb, and upper half of the trunk

2nd order neurons form this tract

Internal Feedback Tracts - contralateral

1. Anterior spinocerebellar tracts

2. Rostrospinocerebellar tracts

These monitor the activity of spinal interneurons and of descending motor signals from the cerebral cortex and brainstem

*Single neuron tracts

Anterior spinocerebellar tracts

Transmits information from the thoracolumbar gray matter

Axons decussate and ascend to the midbrain, contralateral

Rostrospinocerebellar Tract

Transmits information from the cervical spinal cord and T1

• To the ipsilateral cerebellum

• Enters the cerebellum via the inferior and superior cerebellar peduncles

Cerebrocerebellum

Named for its extensive, indirect connections with almost the entire cerebral cortex

Functions: coordination of voluntary movement, planning, timing, cognitive functions

Dentate nucleus

of cerebrocerebellum, involved in motor planning

Red nucleus

(in the midbrain) is part of a circuit that optimizes complex motor, cognitive, and emotion functions

Cognitive functions of cerebrocerebellum

Goal-directed behavior, language optimization, visuospatial function

Emotional functions of cerebrocerebellum

Processing, storage, retrieval of emotional memories; regulating emotional responses

Optimizing social behavior

Motor signs of cerebellar dysfunction

Abnormal motor execution that does not change with or without use of vision

Can affect: posture, automatic movements, eye movements, and voluntary movements

Ataxia: Movement disorder common to all lesions of the cerebellum

Unilateral cerebellar lesions cause ipsilateral impairments

Vestibulocerebellar Lesions

Cause: Nystagmus, Unsteadiness, Difficulty maintaining sitting and standing balance

Spinocerebellar lesions

Anterior lobe vermis lesions cause truncal ataxia- posture and steadiness

Anterior lobe paravermal area lesions cause: Dysarthria-slurred speech, Gait ataxia, and limb ataxia

Dysdiadochokinesia

Inability to rapidly alternate movements

For example, inability to rapidly pronate and supinate the forearm

Dysmetria

Inability to accurately move an intended distance

Manifestations with limb ataxia

1. Dysdiadochokinesia

2. Dysmetria

3. Action tremor

Compensation for limb ataxia: movement decomposition

Cerebrocerebellar Lesion Motor Effects

Interferes with coordination of fine finger movements

May cause dysarthria- speech disorder

Cerebellar ataxia

In this ataxia the following are normal:

• Vibration sense

• Passive proprioception

• Achilles deep tendon reflex

How to differentiate between somatosensory and cerebellar ataxia

movement coordination should be compared with eyes open and eyes closed

Cerebellar Cognitive Affective Syndrome

Visuospatial impairments- Deficits in copying, recalling visual images, judging distances

Language Deficits: word finding, brief responses, long latency before speaking, inability to understand metaphors and ambiguity

Goal-directed functions Deficits in planning and organizing activities

Emotions & behavior- flat affect; impulsiveness or disinhibited behavior

Autism spectrum disorders and ADHD

Developmental disorder

Decreased volume of both cerebellum and posterior vermis

Developmental dyslexia

Abnormalities in anterior and posterior cerebellar hemispheres

Acquired Disorders that Most Commonly affect the Cerebellum

Multiple sclerosis

Stroke

Tumor

Alcoholic degeneration

Compression of the inferior brainstem and cerebellum in the foramen magnum

Function of basal ganglia

Predict the effects of various actions, then make and execute action plans

Types of nuclei the basal ganglia include

Caudate

Putamen

Globus pallidus

Subthalamic nucleus (STN)

Substantia nigra

Caudate nuclei

C shape structure; located in the cerebrum

Putamen location

Located in the cerebrum

Globus pallidus location

Located in the cerebrum

Subthalamic nucleus (STN) location

Located in the diencephalon

Substantia nigra location

Located in the midbrain

Goal-directed behavior circuit

Evaluates information for making perceptual decisions, planning, and choosing actions in context

Head of caudate part of this

Not involved in controlling movements

Social behavior circuit

circuit that recognizes social cues, regulates self-control, and parses out relevant from irrelevant information

Emotion/motivation circuit

Integrates emotions with roles of other circuits

Partially responsible for the perception and experiences of emotions

Essential function is seeking rewards

Oculomotor circuit

Body of caudate is part of this circuit

makes decisions about spatial attention and eye movements

Determines whether to use fast eye movements to direct attention toward an object

Saccade

Rapid movement of the eyes

Reflexive Prosaccades

move eyes toward an object

Antisaccades

Move eyes away from object. Result from more complex interactions than the oculomotor circuit and require inhibition of prosaccade reflex

Motor circuit

Regulates muscle contraction, muscle force, multijoint movements, and sequencing of movements

Parts of the motor circuit

cerebral cortex motor areas, the substantia nigra compacta, putamen, external globus pallidus (eGP), STN, iGP, and motor areas of the thalamus

The basal ganglia has no direct output to

No direct output to LMNs

Stimulation of PPN

regulates contraction of postural and girdle muscles via the lateral reticulospinal neurons

Stimulation of the MLR

elicits rhythmical lower limb movements similar to walking or running

Disinhibition

involves at least two inhibitory neurons in series and a target neuron

effect of disinhibition is facilitation of the target neuron

3 pathways that process signals in the cortico-basal ganglia-thalamic circuit

Hyperdirect, Go, and No-Go Pathways

Normal movement requires activity in all 3 pathways

Hyperdirect (stop) pathway

powerful inhibition of the motor thalamus, thus stopping voluntary movement

Conveys powerful excitation from the cerebral cortex directly to the STN, the STN excites the iGP, and the iGP inhibits the motor thalamus

Go pathway

activation disinhibits the motor thalamus and thus facilitates specific movements

also called Direct Pathway

No-Go Pathway

Also begins in putamen, in different cells than the Go pathway

The end of this pathway activity is suppression of unwanted movements

also called the Indirect Pathway

3 activities that the motor circuit regulates

1. Voluntary muscle activity

2. Postural and girdle muscle activity

3. Walking

Voluntary muscle activity

Via motor thalamus to:

• Corticospinal tracts

• Corticobrainstem tracts

• Corticopontine tracts

Postural and girdle muscle activity

Via PPN to reticulospinal tracts

Walking

Basal ganglia: Via MLR to starting, maintaining, and stopping stepping movements, via PPN to adjust muscle tone in postural muscles

Cerebral cortex provides goal orientation and control of ankle movements

What does the basal ganglia tonically inhibit?

inhibit the motor thalamus, PPN, and MLR

Hypokinetic

too little movement; loss of normal function

Hyperkinetic

excessive movement; gain of function not normally present

Parkinson’s

disease impacting basal ganglia, mainly hypokinetic

Basal ganglia

group of interconnected nuclei located in the cerebrum and midbrain

Basal ganglia 3 cortico-basal ganglia-thalamic nonmotor circuits

Goal-directed behavior circuit

Social behavior circuit

Emotion/motivation circuit

Feedforward

refers to the anticipatory use of sensory information to prepare for movement

Feedback

refers to the use of sensory information during or after movement to make corrections either to the ongoing movement or to future movements

information about the state of the system

In the absence of vision, reaching depends on

Somatosensation and proprioception to locate objects

Loss of somatosensation in individuals with complete deafferentation disrupts positioning of limbs

Sensory contributions for movement control

Vision

Proprioceptive Info

Vestibular Info

3 fundamental types of movement

Posture- brainstem

Locomotor- brainstem and spinal regions

Reaching/grasping- cerebral cortex

All regions of the nervous system contribute to each type of movement

Orientation

Orientation is the adjustment of the body and head to vertical

Balance

the ability to maintain the center of mass relative to the base of support

How is posture control achieved?

by central commands to the LMNs; the central output is adjusted to the environmental context by sensory input

3 senses used to orient to the world

1. Somatosensation

2. Vision

3. Vestibular

Somatosensation and posture

Provides information about weight bearing and the relative positions of body parts

Vision and posture

Provides information about movement and cues for judging upright

Vestibular and posture

Informs a person about head position relative to gravity and about head movement

Stepping pattern generators

control pattern of lower limb muscle activation during walking or running

Grasping

coordinated with activity of the eyes, head, proximal upper limb, and trunk; orientation and postural preparation are integral to the movement

When an object is contacted, grip force adjusts quickly, indicating feedforward control

After an object is grasped, somatosensory information corrects any error in grip force

Effect of Sensory Loss on Motor Control

Loss of visual feedforward impairs movements; must depend on somatosensation rather than visual guidance

Parietal lobe damage

interrupts the action stream, preventing accurate reaching and shaping of the hand

Loss of proprioception

Prevents the coordination of even simple multijoint movements. During fast movements, this causes movement decomposition

Peripheral nerves

Include all neural structures:

○ Distal to spinal nerves

○ Axons of cranial nerves outside the skull

Consist of parallel bundles of axons surrounded by 3 connective tissue sheaths

Endoneurium

Separates individual axons

Perineurium

Surrounds fascicles

Epineurium

Encloses the entire nerve trunk

Signs of peripheral nerve lesions

Paresis or paralysis

Sensory loss

Abnormal sensation

Muscle atrophy

Somatic peripheral nerves

are usually mixed and include sensory, autonomic, and motor axons

Cutaneous branches

supply the skin and subcutaneous tissues

Muscular branches

supply the muscles, tendons, and joints

How are peripheral axons classified?

classified according to speed of conduction and diameter