Neuro Exam #3

Structures of the Diencephalon

• Thalamus

• Hypothalamus

• Epithalamus

• Subthalamus

Thalamus receives info from…

• Basal ganglia

• Cerebellum

• ALL sensory systems EXCEPT olfactory

Function of the Thalamus

• Sensory integration

  • Receives, starts processing, & forwards sensory info to the cortex (- olfaction)

  • Affective link to sensations (agreeableness/ disagreeableness) including pain

  • This info goes to both primary & association cortices

• Motor integration

  • Receives & sends projections to/from basal ganglia & cerebellum

• Consciousness

  • Contributes to levels of consciousness, alertness, & attention through influences on the cortex from thalamocortical projections

Types of Thalamic Nuclei

1. Relay: convey info from sensory systems (- olfactory), basal ganglia, or cerebellum to the cerebral cortex

2. Association: process emotional & some memory info or integrate different types of sensation

3. Nonspecific: regulate consciousness, arousal, & attention

Functions of Thalamic Nuclei

1. Info from the thalamus travels to/from the cortex, specifically with association & intralaminar nuclei

2. Receive specific inputs from well-defined tracts

3. Project to (& receive fibers from) well-defined cortical areas related to specific functions

• can be sensory, motor, or limbic - but NOT a combo

Types of Sensory Relay Nuclei

1. Ventral posterolateral (VPL)

2. Ventral posteromedial (VPM)

3. Medial geniculate body/nucleus

4. Lateral geniculate body/nucleus

Ventral posterolateral (VPL)

• Gets input from medial lemniscus (dorsal columns) & spinothalamic tracts

• Cortical output to somatosensory cortex

• Carries sensory info from the body

Ventral posteromedial (VPM)

• Gets input from trigeminaothalamic tract

• Cortical output to somatosensory cortex

• Carries sensory info from the face

Medial geniculate body/nucleus

• Gets auditory input via inferior colliculus

• Projects to the primary auditory cortex

Lateral geniculate body/nucleus

• Gets visual input via the optic tract

• Projects to primary visual cortex

Types of Motor Relay Nuclei

1. Ventral anterior

2. Ventral lateral

Ventral anterior

• Gets info from the basal ganglia

• Projects to primary motor cortex

Ventral lateral

• Gets info from the cerebellum & basal ganglia

• Projects to primary motor cortex

Types of Association Nuclei

1. Anterior nucleus (declarative memory)

2. Lateral dorsal (declarative memory)

3. Medial group (emotion)

4. Pulvinar (sensory integration)

5. Lateral posterior (sensory integration)

• Do not receive input from a single source - instead receive multimodal inputs

• Project to association areas of the cortex - which integrate & process info from multiple systems

Anterior nucleus

• Gets info from mammillothalamic tract & hippocampus

• Projects to cingulate gyrus

• Declarative memory - facts (semantic) + events (episodic)

Lateral dorsal

• Gets info from hippocampus

• Projects to cingulate gyrus

• Declarative memory - facts (semantic) + events (episodic)

Medial group

• Gets info from prefrontal cortex, olfactory (cortical), & limbic system structures (i.e. amygdala)

• Projects to prefrontal cortex

• Important for emotions & memory

Pulvinar

• Gets info from parietal, temporal, & occipital lobes in addition to visual system

• Projects to visual association areas (in parietal, temporal, occipital lobes)

• Important for sensory integration

Lateral posterior

• Gets input from & projects to parietal, temporal, & occipital lobes

• Important for sensory integration & cognition

Types of Nonspecific Nuclei

Intralaminar & Reticular:

• Regulates consciousness, arousal, & attention

• Receives input from reticular systems & projects to multiple cortical areas

• Provides interconnections with other thalamic nuclei

Function of the Hypothalamus

• Aids in controlling hormones to maintain homeostasis

• Controls metabolism, reproduction, stress response, & urine production

• Essential for survival - as it integrates behavior with visceral functioning

Function of the Pituitary Gland

• Control most of the endocrine system

• Release of pituitary hormone = essential for survival

  • Integrates behaviors with visceral function

• Specifically target:

  • Adrenal cortex

  • Thyroid gland

  • Ovaries or testes

Hormones Released by Anterior Pituitary Gland

• Growth hormone

• Thyroid-stimulating hormone (TSH)

• Adrenocorticotropic hormone (ACTH)

• Luteinizing hormone

• Follicle-stimulating hormone

• Prolactin

Hormones Released by Posterior Pituitary Gland

• Antidiuretic hormone (ADH)

• Oxytocin

Function of the Epithalamus

• Houses the pineal gland - which helps regulate circadian rhythms & influence secretions of pituitary gland, adrenal & parathyroid glands, & islets of Langerhans

Functions of the Subthalamus

• Involved in movement regulation - apart of the basal ganglia circuit

• Facilitates basal ganglia output nuclei

• Found superior to substantia nigra of midbrain

Functions of Basal Ganglia

• Vital for normal motor functions:

  • Sequence movements

  • Regulate muscle tone & muscle force

  • Select & inhibit specific movements

• Involved with:

  • Cognitive functions

  • Behaviors

  • Emotions

Subcortical White Matter

• All white matter consists of myelinated axons

• Subcortical white matter fibers are classified into 3 categories:

  • Projection

  • Commissural

  • Association

Projection Fibers

• Convey signals from subcortical structures to the cerebral cortex, then to the spinal cord, brainstem, basal ganglia, & thalamus

• Thalamocortical projections relay somatosensory, visual, auditory, & motor info to the cerebral cortex

Commissural Fibers

• Connect homologous areas of the cerebral hemispheres

• Corpus callosum = largest group of commissural fibers, linking many areas of R + L hemispheres

Association Fibers

• Connect cortical regions within one hemisphere

• Short association fibers connect adjacent gyri

• Long fibers connect lobes within one hemisphere

Primary Visual Cortex

• Visual info travels to cortex via pathway from retina to lateral geniculate body of thalamus, then to PVC (primary visual cortex)

• Specialized individual neurons to distinguish between light & dark, various shapes, location of objects, & object movement

Secondary Sensory Areas

• Analyze sensory input from both thalamus & primary sensory cortex

• Contribute to analysis of one type of sensory info

• Neurons in secondary somatosensory area provide stereognosis by comparing somatosenation from current objects with memories of other objects

Lateropulsion

• Lesion in posterior thalamus - type of postural vertical disorder

• Client w hemiparesis (post thalamic stroke) uses stronger arm to push toward what they perceive is upright, when they are pushing past upright midline - resulting in increased chance of falling (toward weaker side)

• Pushing can occur in sitting or standing

• Very resistant to correction

Agnosia

• Caused by lesions in various areas

• Inability to recognize objects when using a specific sense

• Subtypes:

  • Astereognosis

  • Visual agnosia

  • Auditory agnosia

Astereognosis

• Inability to identify objects by touch & manipulation despite intact discriminative somatosensation

• Person would be able to describe object while being palpated, but not by touching or manipulating it

• Lesion in secondary somatosensory cortex

Visual Agnosia

• Inability to visually recognize objects - despite having intact vision

• Person can describe shape & size of an object using vision, but cannot identify the object visually

• Lesion in ventral visual system

• Prosopagnosia: highly specific type of visual agnosia - usually associated with bilateral damage to the inferior secondary visual areas

Auditory Agnosia

• Person deprived of sound recognition with destruction of secondary auditory cortex - though ability to perceive sound is spared

• Destruction of right auditory cortex interferes with interpretation of environmental sounds

• Destruction of left auditory cortex prevents understanding of speech

Vestibular System

• Vestibular info is essential for postural control & control of eye movements

• Vestibular apparatus contains sensory receptors that respond to head positioning relative to gravity & to head movements

• Info converted into neural signals conveyed by vestibular nerve to vestibular nuclei

Functions of Vestibular Nuclei

1. Sensory info about head movement & head position relative to gravity

2. Gaze stabilization (control of eye movements while head moves)

3. Postural adjustments

4. Autonomic function & consciousness

Vestibular Apparatus

• Consists of bony & membranous labyrinths & hair cells

  • Bending of hair cells determines frequency of signals conveyed by vestibular nerve

• Perilymph: fluid that separates bony & membranous labyrinths

• Endolymph: fluid that fills membranous labyrinths

Semicircular Canals

• Receptors detect rotational movement of the head by sensing the motion of endolymph

• Consist of 3 hollow rings - arranged perpendicular to one another

  • Each canal opens at both ends - into the utricle

  • Each canal has an ampulla that contains a crista:

    • Crista consists of supporting cells & hair cells

    • Hair cells embedded in gelatinous mass (cupola)

Semicircular Canal Function

• Each canal (in a pair) produces reciprocal signals:

  • Increased signals from one canal occur simultaneously with decreased signals from its partner

• Reciprocal signals = essential for normal vestibular function

• If signals are not reciprocal:

  • Difficulties with postural control, abnormal eye movements, & nausea

Otolithic Organs

Utricle & Saccule: membranous sacs within vestibular apparatus

• Not sensitive to rotation but respond to head position relative to gravity & to linear acceleration & deceleration

• Macula = hair cells enclosed by a gelatinous mass topped by calcium carbonate crystals (located within utricle & saccule)

• Otoliths = calcium carbonate crystals that are more dense than the surrounding fluid & gelatinous support

Otolithic Organ Function

• Changing head position tilts the macula

  • Otoconia weight displaces the gelatinous mass, bending the embedded hairs

• Bending hairs stimulates or inhibits the hair cells, depending on the direction of the bend - which determines the frequency of neurons firing in vestibular nerve

Vestibular Nerve Function

• Transmits info from semicircular canals & otolithic organs to vestibular nuclei in medulla & pons, & to flocculonodular node of cerebellum

• Peripheral part of vestibular system consists of vestibular apparatus & peripheral part of nerve

Use of Vestibular Information

• Semicircular canal info:

  • Stabilizes vision - keeps eyes on target when head turns

• Otolithic organ info:

  • Adjusts activity in lower motor neurons that Innervate postural muscles

Types of Vestibular Movement

1. Linear

2. Rotary

3. Orbital

4. Inversion

5. Sidelying

Linear movement

• Vertical: up + down

• Horizontal: forward + backward

Rotary movement

• Clockwise

• Counter-clockwise

• Variable nystagmus response based on plane of movement

Orbital movement

• Moving around an axis

Inversion movement

• Head below the heart

Sidelying movement

• Positional shifts in weight or motion

  • Changes in relation to gravity

Peripheral Vestibular System

• Receptor for vestibular system = vestibular labyrinth in inner ear

• Vestibular systems are essential for postural control & for coordination of movements - including eye movements

• Vestibular signals contribute to awareness of head orientation & to actively orienting the head & body relative to gravity & to movement

Central Vestibular System

• The effects of activating the central vestibular system can be demonstrated by rapidly rotating the head

• Simply spinning around elicits:

  • Altered postural control (leaning or falling)

  • Head orientation adjustment

  • Eye movement relfexes

  • Autonomic changes (nausea, vomiting)

  • Changes in consciousness (lightheadedness)

  • Altered conscious awareness of head orientation & head movement

Central Vestibular Structures

• 4 vestibular nuclei

• 6 pathways

• Vestibulocerebellum

• Vestibular cortex

Vestibular Nuclei

Located at the junction of the pons and medulla:

• Medial

• Lateral

• Superior

• Inferior

Vestibular System: 2 Roles in Motor Control

1. Gaze stabilization (requires VOR)

2. Postural adjustments: efferents from vestibular nuclei -

• Lateral vestibulospinal tract influences lower motor neurons to postural muscles in limbs and trunk

• Medial vestibulospinal tract conveys signals that adjust head position to upright via projections to cervical spinal cord

  • Project to areas that affect signals in corticospinal & reticulospinal pathways

Medial longitudinal fasciculus

Bilateral connections with the extraocular nuclei (cranial nerves 3, 4, & 6) and superior colliculus - influencing eye and head movements

Vestibulospinal tracts

Both medial and lateral - to lower motor neurons that influence posture

Vestibulocollic pathways

To the nucleus of the spinal accessory nerve (11) - influencing head position

Vestibulothalamocortical pathways

Providing conscious awareness of head position and movement and input to corticospinal tracts

Vestibulocerebellar pathways

To the vestibulocerebellum, which controls the magnitude of muscle responses to vestibular info (including gain of the vestibulo-ocular reflex)

Vestibuloreticular pathways

To reticular formation - influencing reticulospinal tracts and autonomic centers for nausea and vomitting

Vestibulo-Ocular Reflex (VOR)

• Eye reflex

• Helps keep your eyes on objects while the head & body are moving

• When your head or body moves one way, VOR makes your eyes move the other way

• Helps you see clearly & maintain your gaze & balance

Vestibular ataxia

• Gravity dependent

• Limb movements are normal in supine & ataxic while walking

Cerebellar ataxia

Ataxia remains the same regardless of position (sitting, standing, lying down)

Sensory ataxia

• Impaired vibratory & position sense

• Decreased or lost ankle reflexes

• Lack of nystagmus

• Lack of vertigo

Vestibular Disorders

• Most common symptom of vestibular system dysfunction is vertigo

• Vestibular disorders may also cause:

  • Pathologic nystagmus

  • Unsteadiness

  • Ataxia

  • Nausea

  • Vomitting

• To maintain orientation and control of posture, a person with a vestibular disorder may need to move slowly and devote conscious attention to staying upright

Vertigo

• Can be physiologic or pathologic

  • Pathologic vertigo occurs with both peripheral and central disorders; and arises from disturbance of spatial orientation in the vestibular cortex

• People may falsely perceive movement of themselves or their surroundings when they are experiencing vertigo

Peripheral Vestibular Disorders

• Typically cause recurring periods of vertigo, accompanied by moderate to severe nausea

• Nystagmus almost always accompanies peripheral vertigo

• Certain drugs may also cause peripheral vestibular damage

Benign Paroxysmal Positional Vertigo (BPPV)

Inner ear disorder that cause the acute onset of vertigo and nystagmus are -

• Benign: not malignant

• Paroxysmal: has a sudden onset of a symptom or disease

• Positional: denotes head position at the provoking stimulus

In BPPV, rapid change of head position results in vertigo and nystagmus:

• Symptoms subside in less than 2 minutes, even if the provoking head position is sustained

Activities that frequently provoke BPPV

• Getting in or out of bed

• Bending over to look under a bed

• Reaching up to retrieve something from a high shelf

• Turning over in bed

Most common cause of BPPV: Canalithiasis -

• Caused by the displacement of otoconia from the macula into a semicircular canal

BPPV may occur spontaneously in elderly people

Treatment: restore otoliths to their correct position generally via Epley Maneuver (requires advanced training in vestibular rehab)

Vestibular Neuritis

• Inflammation of vestibular nerve - typically caused by a virus

• Symptoms include:

  • Unsteadiness

  • Spontaneous nystagmus

  • Nausea

  • Severe vertigo (up to 3 days)

• Hearing is unaffected

• Caloric testing shows decreased or absent response on involved side

• During acute phase - medication may be used to suppress nausea, vertigo, & vomitting

Meniere’s Disease

• Causes sensation of fullness in the ear, and the following:

  • Tinnitus

  • Severe acute vertigo

  • Nausea

  • Vomitting

  • Hearing loss

• Associated with abnormal fluid pressure in the inner ear (findings unclear for cause vs. effect)

• Typically affects only one ear

• Often diagnosed in ages 40s-50s

• Drugs suppressing vertigo are useful during acute attacks

Bilateral Lesions of the Vestibular Nerve

• Interfere with reflexive eye movements in response to head movement

• Initial complaint is usually Oscillopsia:

  • Subjective sensation of visual objects bouncing when the head is moving

    • World seems to bounce up & down as the person walks - as normal reflexive adjustments for head movements are decreased

• Over time, the nervous system adapts to the change - people report less difficulty w disorienting movements of the visual field

Postural Vertical

• Alignment of the body relative to gravity

• Perceived by signals from the otolith organs that are conveyed to the posterior thalamus & then to the primary vestibular cortex

• Misalignment can cause postural vertical disorders

Postural Vertical Disorders

• Person misperceives postural vertical

• Misaligning body relative to gravity

• Strong resistance toward passive correction of the body alignment

1. Lateropulsion: pushing syndrome

2. Retropulsion: backward disequilibrium

3. Anteropulsion: forward disequilibrium

Central Vestibular Disorders

• Caused by damage to the vestibular nuclei and/or connections within the brain

• Typically produce milder symptoms > peripheral disorders

• Most commonly the result of:

  • Ischemia or tumors in the brainstem / cerebellar region

  • Cerebellar degeneration

  • Multiple sclerosis

  • Arnold-Chiari malformation

Lesions of the Vestibulothalamocortical Pathway

• Create abnormal perception of vertical w/o vertigo

• No dizziness occurs bc the signals in vestibular nuclei are symmetrical

• People with lesions that affect the vestibular system superior to the vestibular nuclei may experience:

  • Head tilt

  • Misidentification of vertical and/or lateropulsion

Hypo-Sensitivity (Vestibular Processing Dysfunction)

• Under-responsive

• Client seeks high levels of vestibular input in attempt to meet their sensory threshold

• Examples:

  • Sensory-seeking

  • Constantly changing head position

Hyper-Sensitivity (Vestibular Processing Dysfunction)

• Over-responsive

• Client avoids or becomes easily overwhelmed by vestibular input (low sensory threshold)

• Examples:

  • Motion sickness

  • Gravitational insecurity

  • Avoids / becomes distressed with movement activities and/or change in head position

Age-Related Vestibular Issues

• Vestibular system begins to decline significantly after the age of 40

• Functional impairment = increased fall risk

• Rehab role - system has the potential to be strengthened throughout the lifespan

Motor System

• Highly organized / interconnected

• Movement can be reflexive or intentional - initiated in response to external stimuli or internally driven

  • Even intentional movement may not be fully conscious

  • Majority of movement is automatic - not a lot of thought put into it

• Can be anticipatory or reactive

Structures of the Motor System

• Cortex

• Basal ganglia

• Cerebellum

• Brainstem

• Spinal cord

• Striated muscle

Anticipatory Movement

• Makes use of “feedforward” system

• Taking cues from surroundings & considering the demands of the task about to be undertaken - we can make small or large adjustments in our movements

• Allows a person to move quickly since the motor plan has already been created & implemented before the action occurs

• Examples:

  • Postural adjustments

  • Catching a ball

Reactive Movement

• Movement generated in response to sensory input

• Movement can be reflexive or voluntary

• Examples:

  • Error correction (when input about the actual movement does not match up with the motor plan) such as lifting a water bottle that was just emptied

Reflexive Movement

• Involuntary, short-lasting movement

• Occur once an external stimulus reaches a threshold

  • Stereotyped (always the same): simple or complex

• Can be controlled / modulated by higher cortical levels

• Examples:

  • Patellar or biceps tendon reflex

  • Withdrawal reflex

Voluntary Movement

Unique in 4 ways:

1. Involves a decision to act / goal-directed (can be generated completely within the person OR in response to external stimuli)

2. It is learned

3. It is under conscious control (esp. during the motor learning process)

4. Makes use of other types of movement such as reflexes, reciprocal innervation (agonist flexes, antagonist extends), automatic postural adjustments

Synergy

• Groups of muscles will act together to support movements

• Individual movements become linked and will act together

• Sometimes they complete the same actions (firing)

• Sometimes they complete different actions (some firing, some not)

• Examples:

  • Making a fist while keeping the wrist straight - requires groups of muscles to fire together (flexors=fingers, extensors=wrist)

Interneuron

• Intermediate neurons that allow communication between neurons

• Only found in the CNS (brain + spinal cord)

• Very important for a number of functions, including reflexes, modulating activities, etc.

Lower Motor Neuron

• Neurons going to muscle that will be activated

• Termination is at a neuromuscular junction

• Cell bodies in spinal cord (CNS) & axons that go to ipsilateral groups of striated muscle fibers outside the CNS

• Examples:

  • Motor component of spinal nerves that leave the ventral root; cell bodies are in the anterior horn of the spinal cord

  • Cranial nerves that innervate muscles; cell bodies are in the cranial nerve nuclei in the brainstem (CNS)

Alpha Motor Neurons

• Large cell bodies & large, myelinated axons

• Synapses at the neuromuscular junction of extrafusal skeletal muscle

• The LMN that will cause muscle contraction & create movement

• Most common LMN

Gamma Motor Neurons

• Medium-sized, myelinated axons

• Synapse w the intrafusal fibers in the muscle spindle

• Important for maintaining sensitivity of the muscle spindle to stretch

• Important component of the stretch reflex

Alpha-Gamma LMN Coactivation

• Cell bodies within ventral horn, axons leave spinal cord via ventral roots

• Alpha & gamma motor neurons function together most of the time - to maintain muscle spindle length (so that it’s sensitive to stretch)

• Occurs bc upper motor neurons & interneurons that have connections to alpha motor neurons have collaterals that go to gamma motor neurons too

Influences on LMN

• Alpha LMN will receive influences from many different sources - some can be excitatory & some can be inhibitory

• The action of the LMN in this case will be summative

• Inputs may come from:

  • Cortex

  • Brainstem

  • Somatosensory afferents

Neuromuscular Junction

• Synapse between the LMN & the striated muscle

  • LMN will synapse on multiple muscle fibers - as a result, an action potential traveling along one LMN will cause multiple muscle fibers to contract

• The neuromuscular junction is a chemical synapse using the neurotransmitter ACh

• That means - the only thing LMN can do is cause the muscles to fire

  • Inhibiting the LMN stops muscle from firing

• ACh needs to be inactivated rapidly to allow the muscle to be ready for further action (AChE breaks neurotransmitter down)

Motor Unit

• Single alpha MN & the group of muscle fibers it innervates

• Fundamental element of the muscle

• Multiple motor units that act together within a striated muscle = motor pool

Types of Motor Units

1. Slow-twitch

2. Fast-twitch

• Type is dependent upon the neurons innervating the fibers

• Speed of contraction in response to electrical shock

• Fire at different times for different reasons - serving different purposes

Slow-Twitch Fibers

• Relatively few contractile filaments

• Can produce small amounts of tension for long periods of time

• Innervated by small alpha LMN with small axons

• Important for maintaining posture/position in quiet standing

• Often recruited first - before faster twitch fibers

Fast-Twitch Fibers

• Contract in brief, powerful twitches and relay on anaerobic catabolism

• Use a lot of energy - fatigable

• Innervated by largest alpha LMN

• Provide force for strenuous behavior like jumping