Neuroscience- Vestibular and proprioception- week 4

Brainstem

A compact, stalklike structure at the base of the cerebral hemispheres that serves as the conduit for all major sensory, motor, and cerebellar pathways while containing nuclei for consciousness, posture, and cardiorespiratory regulation.

CN I: Olfactory Nerve

Special somatic sensory, function is olfaction (smell).

CN II: Optic Nerve

Special somatic sensory, function is vision.

CN III: Oculomotor Nerve

Somatic motor for levator palpebrae superior and most extraocular muscles, Parasympathetic for pupil constriction and lens ciliary muscles.

CN IV: Trochlear Nerve

Somatic motor, function is eye movement (superior oblique muscle).

CN V: Trigeminal Nerve

General somatic sensory (touch, pain, temp, vibration for face and mouth), Branchial motor (muscles of mastication and tensor tympani).

CN VI: Abducens Nerve

Somatic motor, function is eye movement (lateral rectus).

CN VII: Facial Nerve

Branchial motor (muscles of facial expression, stapedius), Parasympathetic (lacrimal and salivary glands), Special visceral sensory (taste from anterior 2/3 of tongue), General somatic sensory (outer ear region).

CN VIII: Vestibulocochlear Nerve

Special somatic sensory, functions are hearing and vestibular (equilibrium) sensation.

CN IX: Glossopharyngeal Nerve

Branchial motor (stylopharyngeus), Parasympathetic (parotid gland), General somatic sensory (middle ear, pharynx, posterior tongue), Special visceral sensory (taste from posterior 1/3 of tongue).

CN X: Vagus Nerve

Branchial motor (pharyngeal/laryngeal muscles), Parasympathetic (heart, lungs, digestive tract), Special visceral sensory (taste from epiglottis), General visceral sensory (aortic arch chemoreceptors/baroreceptors).

CN XI: Spinal Accessory Nerve

Branchial motor, function is head turning (sternomastoid) and shoulder elevation (trapezius).

CN XII: Hypoglossal Nerve

Somatic motor, function is tongue movement.

Anosmia

The loss of the sense of smell, testing each nostril is important because unilateral loss is often unnoticed by patients and can be an early sign of subfrontal meningiomas or neurodegenerative diseases like Parkinson's and Alzheimer's.

Upper Motor Neuron (UMN) Lesion

Damage to descending motor pathways, signs include muscle weakness, increased tone (spasticity), and hyperreflexia (increased reflexes).

Lower Motor Neuron (LMN) Lesion

Damage to the anterior horn cells or peripheral nerves, signs include muscle weakness, atrophy, fasciculations, decreased tone (flaccidity), and hyporeflexia.

Bell's Palsy

An idiopathic LMN-type facial nerve disorder causing unilateral weakness of the entire half of the face, including the forehead, may include hyperacusis (loudness sensitivity) and decreased taste.

True Vertigo

A spinning sensation of movement indicating a lesion in the vestibular pathway (labyrinth, nerve, or central nuclei).

Orthostatic Measurements

Vital sign readings (BP/pulse) taken in supine vs. standing positions, exaggerated changes suggest lightheadedness caused by cardiovascular or autonomic dysfunction rather than a primary vestibular lesion.

Nystagmus

Rhythmic, involuntary eye oscillations, peripheral nystagmus is typically horizontal/rotatory and has a delay before onset, while central nystagmus can be vertical and starts immediately.

Peripheral Vestibular System Components

Semicircular canals (detect angular acceleration/rotation), Otolith organs—utricle and saccule (detect linear acceleration and head tilt relative to gravity).

Vestibular Ganglia of Scarpa

Location of the primary sensory neuron cell bodies for vestibular sensation.

Vestibular Nuclei

Four pairs of nuclei (superior, inferior, medial, lateral) on the floor of the fourth ventricle that receive input from CN VIII.

Vestibular Projections Pathway

Inner ear sensors → CN VIII → Vestibular nuclei → Ascending MLF (eye control) → Vestibulospinal tracts (posture) → Cerebellum (balance) → Thalamocortical path (conscious orientation).

Medial Longitudinal Fasciculus (MLF)

An ascending pathway connecting vestibular nuclei to eye movement nuclei (CN III, IV, VI) to stabilize vision during head movement.

Vestibulo-ocular Reflex (VOR)

A reflex that stabilizes images on the retina during head movement by moving the eyes in the opposite direction.

Lateral Vestibulospinal Tract

A descending pathway from the lateral vestibular nucleus to the entire cord to maintain balance and facilitate extensor tone.

Medial Vestibulospinal Tract (Descending MLF)

A descending pathway to the cervical spine that mediates the vestibulocervical reflex (VCR) for head and neck positioning.

Proprioception System Components

Muscle spindles, Golgi tendon organs, joint receptors, and the dorsal column-medial lemniscal pathway.

Monosynaptic Stretch Reflex

A rapid feedback arc where a muscle stretch activates muscle spindles, sending Ia afferent signals to the spinal cord to directly synapse on alpha motor neurons, causing muscle contraction.

Muscle Spindle

A specialized mechanoreceptor within the muscle that detects the amount and rate of muscle stretch.

Intrafusal vs. Extrafusal Muscle

Intrafusal fibers are inside the muscle spindle and innervated by gamma motor neurons, extrafusal fibers are the main muscle bulk responsible for force, innervated by alpha motor neurons.

Primary (Group Ia) Afferent Fiber

Large-diameter fibers that transmit rapid info about muscle stretch rate to the CNS.

Secondary (Group II) Afferent Fiber

Transmit info about the constant length of the muscle.

Golgi Tendon Organ (GTO)

Receptors located at muscle-tendon junctions that detect muscle tension, transmitted via Group Ib afferent fibers.

Alpha Motor Neurons

Large neurons in the ventral horn that innervate extrafusal muscle fibers to cause contraction.

Gamma Motor Neurons

Smaller neurons that innervate intrafusal muscle fibers to maintain spindle sensitivity.

Interneurons

Spinal cord neurons that can inhibit or facilitate signals between sensory and motor neurons, such as inhibiting an antagonist muscle during a reflex.

Muscle Tone

The internal state of muscle tension and its resistance to passive stretch.

Spasticity

A velocity-dependent increase in muscle tone and hyperreflexia resulting from UMN lesions, resistance increases as the speed of passive movement increases.

Rigidity

A non-velocity-dependent resistance to passive movement, often seen in basal ganglia disorders like Parkinson's.

Modified Ashworth Scale (MAS) Clinical Reasoning

The technique uses a quick stretch (one second) to specifically stimulate muscle spindles in the extrafusal muscle, in UMN lesions the loss of descending inhibition makes the alpha motor neurons hyperexcitable, causing a "catch" or resistance as the spindles trigger a contraction in response to the rapid movement.

Romberg Test

A balance screen where the patient stands with feet together, a failure (swaying/falling) only when eyes are closed indicates a proprioceptive deficit (sensory ataxia) involving the dorsal columns.

System Cooperation (Visual, Vestibular, Proprioception)

These three systems provide redundant information to allow for stable posture, oriented movement, and functional navigation, if one fails (e.g., proprioception), the body relies more heavily on the others (e.g., vision).

Ocular Dysmetria

A cerebellar sign where eye movements over- or undershoot the target.

Saccades

Rapid, voluntary eye movements to bring new targets into the field of view.

Coordination Screening

Includes appendicular tests (finger-to-nose, heel-to-shin) for cerebellar function and gait observation.