Nervous System Physiology Quiz

Nervous System Physiology

• Study of nervous system functions
• Sensory Physiology, sensory receptors, and receptor potentials

Sensory Receptors

• List of receptor types
• Description of receptor potential
• Classification of fiber types
• Classification of somatic sensation
• Description of receptor functions
• Explanation of receptor adaptation

Somatic Sensations

• Senses classified as special senses (vision, hearing, smell, taste, equilibrium)
• Physiological classification of somatic senses: mechanoreceptive senses detect tactile & position sensations
  • Tactile sensations (touch, pressure, vibration, itch)
  • Position sensations (proprioception, equilibrium)
• Pain sense, nociception
• Other classifications:
  • Exteroreceptive sensations (from the surface of the body)
  • Proprioceptive sensations (physical state of the body: position, tendon & muscle sensations, pressure sensations from the bottom of the feet)
  • Visceral sensations (from viscera of the body)
  • Deep sensations (from deep tissues like fascia, muscles, & bone: deep pressure, pain, & vibration)

Types of Sensory Receptors

• Mechanoreceptors, detect tactile & position sensations (skin tactile sensibilities, expanded tips, Merkel's discs, spray endings, Ruffini's endings, encapsulated endings, Meissner's corpuscles, etc.)
• Thermoreceptors, detect temperature (cold receptors, warmth receptors)
• Nociceptors, detect pain (free nerve endings)
• Electromagnetic Receptors, detect light (rods & cones)
• Chemoreceptors, detect chemicals (taste receptors, olfactory epithelium, receptors of aortic and carotid bodies, receptors in hypothalamus)

Modality of Sensation: Labeled Line Principle

• Each principal type of sensation (pain, touch) is a modality of sensation
• Nerve fibers transmit only impulses
• Each nerve tract terminates at a specific point in the CNS
• Stimulus (e.g., electricity, overheating, crushing) is interpreted as the respective modality of sensation
• Phantom limb: sensation that an amputated part is still connected to the body (~70% of people), often felt in distorted & painful positions
• Mechanism of phantom limb sensations (irration & inflamed nerve endings called neuromas)
• Law of projection: local signals sent via a specific pathway are interpreted as pain in that location.
• Receptor recognizes specific stimuli through special forms of energy (intensity/amplitude)

Transduction of Sensory Stimuli

• Receptor potentials (different receptors can be excited by mechanical deformation, chemicals & electromagnetic radiation)
• Different processes: mechanical deformation, chemical application, and temperature changes alter permeability, electromagnetic radiation
• The receptor potential depends on stimulus amplitude/duration
• Transduction phase: stimulus causes change in local potential (receptor potential)
• The action potential phase: if the threshold is reached action potentials are transmitted.

Adaptation of Receptors

• Rapidly adapting (phasic): Meissner corpuscle end-organs, Pacinian corpuscle end-organs, hair end-organ
• Slowly adapting (tonic): Baroreceptors, chemoreceptors, muscle spindle, golgi tendon organ, Merkel cell end-organ, Ruffini corpuscle, joint capsule, & pain receptors

Encoding Stimulus Intensity

• CNS perceives stimulus intensity by changing frequency of impulses induced by receptor
• Amplitude of receptor potential increases as stimulus intensity increases, increasing the rate of action potentials in nerve fibers.

Classification of Fibers

• Nerve fiber diameter & conduction velocity affect signal transmission time
• Different fiber types (Aα, Aβ, Aγ, Aδ, C) have different functions & conduction velocities.
• A fibers (large myelinated fibers); B myelinated fibers, & C (small unmyelinated) - sensory fiber & postganglionic autonomic fiber

Somatic Sensations - Tactile and Position Senses

• Tactile sensations (touch, pressure and vibration) are detected by various receptors like Meissner corpuscles, Pacinian corpuscles, Ruffini's corpuscles, & Merkel's discs
• Hair end-organs adapt readily, detecting movement of objects and initial contact.
• Ruffini endings adapt slowly, signaling sustained pressure.
• Tactile location correlates to the number of receptors
• Types of sensory receptors: Tactile receptors, deep receptors near the joints, individual joint receptors

Receptors for Position Senses (Proprioception)

• Mechanoreceptors located in joints (e.g., Ruffini endings).
• Mechanoreceptors in the skin
• Muscle spindle receptors, Golgi tendon organs

Transmission of Tactile Signals

• Different fiber types (Aβ, Aδ, C) carry tactile signals with varying speeds, affecting the type of sensation perceived
• Location and intensity of touch are determined by rapidly-conducting fibers.

Transmission of Tactile Signals - Detection of Vibration

• Pacinian corpuscles detect higher-frequency vibrations.
• Meissner's corpuscles detect lower-frequency vibrations.

Transmission of Tactile Signals - Tickle & Itch

• Primarily transmitted by small, unmyelinated C fibers.

Dorsal Root, Ventral Root

• Afferent fibers carrying sensory signals enter the spinal cord through the dorsal root
• Sensory afferent fibers are the projections of dorsal root ganglia (DRG) cells.
• Ventral root contains efferent axons (motor neurons) and pre-ganglionic autonomic axons

Sensory Pathways for Transmitting Signals into CNS (Dorsal Column-Medial Lemniscal System)

• Nerve fibers: large & myelinated, ~30-110 m/sec
• Fine touch, tactile sensations, requiring high degrees of localization, intensity, and judgment of pressure.

A. Dorsal Column-Medial Lemniscal System

• Functions of afferent neurons stimulated by sensory stimuli in spinal cord:
  • Going to the brain in dorsal column
  • Function in local spinal cord reflexes
  • Participates in spinocerebellar pathways
• Soma of the first order neuron is located in DRG
• One peripheral branch functions like a dendrite; one central branch functions like an axon.
• This is a pathway for sensory information (pressure, fine touch, proprioception) from the trunk and limbs

B. The Anterolateral Pathway (Spinothalamic Tract)

• Characterized by slower transmission (8-40 m/sec)
• Less sensitive to spatial localization and intensity gradients
• Responsible for pain, temperature, crude touch and pressure senses

Function of the Thalamus in Somatic Sensation

• Primary relay station for sensory information coming into the cerebral cortex
• Although damage can reduce tactile sensibility, some crude sensibility returns.
• Pain sensation is affected moderately, and temperature sensation also affected
• The thalamus (and other lower centers) can discriminate tactile sensations

Cortical Control of Sensory Sensitivity (Corticofugal signals

• Corticofugal signals are transmitted from the cerebral cortex to lower sensory relay stations in the thalamus, medulla, & spinal cord
• Regulate the intensity of sensory input

Dermatomes

• Each spinal nerve innervates a segmental area of skin.
• Used to identify levels of spinal cord injury

Neural Lesions: Peripheral Lesions

• Dorsal root lesions prevent sensory signal transmission while ventral root lesions cause weakness in innervated muscles
• Affecting different parts of the body (pain, motor function)

Abnormalities Brown-Séquard Syndrome

• If the spinal cord is lesioned on one side, this syndrome occurs.
• All motor functions are blocked on the side of the transection.
• Pain, temperature, and crude touch sensations are lost on opposite side.
• The kinesthetic, position and vibration senses, discrete localization, and two- point discrimination are lost on same side.

Pain

• Pain is a protective mechanism activated by tissue damage.
• Nociceptors (free nerve endings) detect pain, responding to mechanical, thermal, or chemical stimuli.
• Chemical pain stimuli include extreme temperatures, trauma, & various chemicals.
• Chemicals that stimulate pain include bradykinin, serotonin, histamine, proteolytic enzymes, potassium ions, and acids
• If damage sustained. slow chronic pain begins after about 1s & increasing intensity
• Tissue destruction lead to prolonged unbearable suffering

Pain Sensation

• Nociceptors convert thermal, mechanical, and chemical stimuli into receptor potential using TRP channels
• TRPV1, TRPV2, TRPV3, TRPV4, & TRPA1 are types of TRP channels that detect variations in temperature, pressure, and chemicals

Pathways for Transmission of Pain Signals

• Fast pain signals: The neospinothalamic pathway - Fast, sharp, localized pain
• Slow pain signals: The paleospinothalamic pathway - Slow, burning, poorly localized
• Anterolateral system: contains both neospinothalamic and paleospinothalamic tracts; it transmits pain signals to different brain regions

Surgical Interruption of Pain Pathways

• Anterolateral cordotomy interrupts pain fibers but may not always be effective (issues with incomplete crossing)
• Cauterizing specific pain areas in the intralaminar nuclei of the thalamus can sometimes relieve severe pain, without impacting perception/awareness.
• Prefrontal lobotomies interrupt frontal lobe connections, which can help some but are now rarely performed.

Pain Suppression (Analgesia) System

• The brain has systems to suppress pain (analgesia).
• These systems involve the periaqueductal gray (PAG) and other brain stem areas.
• These areas release enkephalins and other peptides that inhibit pain signals
• The brain's opiate system - receptors for these naturally occurring neurotransmitters.

Projected Pain

• Pain felt in areas other than the tissue source of injury due to synapse in similar locations in the spinal cord causing a felt sensation.
• Neuralgia: sever pain attributed to irritated/damaged nerve.
• Examples: Trigeminal neuralgia, involving the trigeminal nerve in the face.

Referred Pain

• Visceral pain being felt in a different part of the body.
• Visceral pain fibers synapse in the spinal cord on the same 2nd-order neuron from a different part of the body
• This causes pain in the skin over the organ from similar signaling in the spinal cord, causing the sensation of pain in the skin instead of the organ

Visceral Pain Causes

• Ischemia (reduced blood flow)
• Chemical stimuli (e.g., from GI tract or organ damage)
• Spasms of hollow viscera
• Overdistention (severe swelling) of organs.

Poor Localization of Visceral Pain

• Difficulty localizing pain due to varied nerve connections and brain's lack of direct sensory organ awareness & lack of understanding of the different organs' differences between each other

Abnormalities of Pain: Hyperalgesia

• Excessive sensitivity to pain; can be primary (increased sensitivity in nociceptors themselves) or secondary (increased sensory transmission) due to tissue chemicals, etc.

Abnormalities of Pain: Herpes Zoster (Shingles)

• Painful rash caused by reactivation of latent herpes virus affecting segmental (dermatomal) regions of the body.

Headache of Intracranial Origin

• Brain tissue is immune to pain, but pressure, trauma, stretching, or damage to blood vessels & meninges elicits pain sensation.
• Several headaches of intracranial origin include inflammation, cerebrospinal fluid pressure drops, alcohol or constipation.

Headache — Migraine Headaches

• Mechanisms are not fully understood, but there seems to be a genetic predisposition to migraines.
• Various triggers such as smells, noise, light, sleep loss, caffeine, hormonal fluctuations, and medications can evoke them
• The prodrome, aura, pain, and postdrome phases are common features of a migraine attack and often can include nausea, vomiting, or sensory disturbances.

Headache — Theories of Mechanisms

• Vasospasm theory: changes in or constriction of blood vessels due to emotion /tension cause discomfort
• Migraine center theory: trigger factor increases the excitability of the cerebral cortex and activates the migraine center which is located in the brain stem.

Extracranial Headaches

• Muscle spasm
• Nasal and accessory nasal structures

Thermal Sensations

• Receptors (free nerve endings) with TRPM8 (stimulated by menthol) and TRPA1 are responsible for cold sensations
• TRPM8 activates below 25°C - feels cool and cold
• TRPA1 activates below 17°C - cold and freezing sensation occurs
• Cold receptors are predominantly type Aδ, but some are type C.
• Warm receptors are largely type C fibers (slow) in range 0.8 - 2m/sec

Motor Functions of the Spinal Cord — Cord Reflexes

• Study of spinal reflexes (involuntary, unlearned movements)

Organization of the Spinal Cord for Motor Functions

• Afferent neurons enter the spinal cord through dorsal roots or cranial nerves to the CNS
• Efferent fibers exit through ventral roots or cranial nerves; fibers are related to their nuclei (somatic, visceral)

Functional Anatomy of Spinal Cord

• Dorsal (posterior) roots are afferent; ventral (anterior) roots are efferent
• Dorsal root ganglia house cell bodies of sensory neurons
• Ventral horn cells house motor neurons

Functional Anatomy of Spinal Cord - Sources of Afferent Spinal Nerves

• Somatic sensory receptors
• Visceral sensory receptors

Functional Anatomy of Spinal Cord - Efferent Spinal Nerves

• Somatic, innervating skeletal muscles
• Visceral, innervating smooth muscles, cardiac muscles, and glands

Functional Anatomy of Spinal Cord - Gray Matter

• Integrative area for cord reflexes, initiating local segmental reflexes and also signal relay for higher CNS processing and response

Spinal Nerves

• There are 31 pairs of spinal nerves originating from the cord
• Each segment corresponds to a vertebra

Spinal Cord Ascending & Descending Tracts

• Different tracts in the spinal cord have different functions relaying sensory or motor info.
• Sensory: Pain, temperature, touch, vibration, proprioception are relayed in numerous pathways via the spinal cord
• Motor: Voluntary and postural movements, use pyramidal and extrapyramidal tracts

Spinal Cord, Neurons

• Include sensory relay neurons, motor neurons (alpha and gamma), and interneurons
• Motor neurons innervate skeletal muscles
• Interneurons are the most numerous in the spinal cord, facilitating complex integration

Interneurons Multisegmental Connections

• Provide pathways for multisegmental reflexes through branching from one cord segment to another

Interneurons Renshaw Cell Inhibitory System

• Axonal collaterals of anterior motor neurons synapse with Renshaw cells
• Renshaw cells inhibit adjacent motor neurons, producing subtle or greater regulation over motor patterns

Locomotor Pattern Generators in Spinal Cord

• Neural circuit localizing in the spinal cord that produces coordinated, sequential movements for locomotion

Reflexes

• Involuntary, unlearned motor responses to stimuli
• Components of a reflex arc include receptors, afferent pathways, integrating centers, efferent pathways, and effectors.
• Classification of reflexes include monosynaptic, disynaptic, and polysynaptic.
• Examples include the stretch reflex, Golgi tendon reflex & flexor-withdrawal reflexes

Monosynaptic Reflexes Stretch Reflex

• Stretch of the muscle triggers a contraction to resist and oppose sudden stretching,
• Muscle spindle and Golgi tendon organ involved
• The knee-jerk reflex is a typical example (monosynaptic pathway).
• Gamma motor neurons adjust muscle spindle tension, contributing to reflex dynamics

Muscle Sensory Receptors, Muscle Spindles & Golgi Tendon Organs

• Muscle Control; the importance of continuous feedback needed to fine tune movements and regulate muscle tension.
• Muscle spindle - a type of proprioceptor, sensitive to muscle length and rate of change of length
• Golgi tendon organ - a type of proprioceptor, sensitive to muscle tension and rate of change of tension; important for protecting muscles from overexertion

Muscle Spindle

• Extrafusal muscle fibers
• Intrafusal muscle fibers
• Motor end plates
• Sensory endings (Ia & II fibers).
• Gamma motor neurons

Muscle Spindle Sensory Endings

• Primary ending (Ia fibers) - middle of the muscle spindle
• Secondary ending (II fibers) - sides of the muscle spindle

Muscle Spindle Sensory Endings - Stimulation

Muscle Stretch Reflex

Muscle Spindle in Voluntary Motor Activity

• Co-activation of alpha and gamma motor neurons
• Maintaining precise muscle posture
• Prevent jerky, oscillatory movements

Brain Areas for Control of the Gamma Motor System

• Bulboreticular facilitatory area plays a crucial role in activating gamma neurons for postural control.
• Other brain regions, including the cerebellum, basal ganglia, and cerebral cortex, contribute to the control of gamma motor neuron activity.

Clinical Applications of the Stretch Reflex

• Assessing the state of spinal cord and motor control centers
• Observing the appropriate reflexes for age
• Observing and understanding some clinical abnormalities in spinal cord and/or brain stem

Monosynaptic Reflexes and Disorders

• Hyperactive reflexes suggest interruption of inhibitory pathways (e.g., lesions in brain motor areas)
• Hypoactive reflexes suggest a problem in any part of the reflex arc pathway; there may be a problem with the muscle spindle, afferent nerve fibers to the quadriceps muscle.

Clinical Applications of Clonus

• Assessing the stretch reflex, looking for oscillations with rapid stretching, a sign of facilitated impulses

Stretch Reflex in Clinics: Babinski Reflex

• Normal plantar response is the toes fanning downward
• Abnormal is the toes fanning upward, suggesting pyramidal pathway problems

Golgi Tendon Organ

• Detects muscle tension & inhibiting contraction of the same muscle to prevent overexertion or tearing of the muscle.

Flexor Reflex & Withdrawal Reflexes

• Painful stimulus trigger this reflex to withdraw from the painful stimulus
• This can happen on the same or opposite side because the brain is trying to stabilize the posture of the body

Reciprocal Inhibition & Reciprocal Innervation

• When one muscle group is excited, the antagonist muscle group is inhibited, allowing coordinated movement

Role of the Motor Cortex & Brain Stem

• Motor Function Control (Motor cortex responsible for the initiation and planning, and brainstem responsible for more complex or fine-tuned movements).

Cranial Nerves

• Olfactory (I) - smell
• Optic (II) - vision
• Oculomotor (III) - eye movement, pupil size, lens shape - Parasympathetic (accommodation, near vision)
• Trochlear (IV) - superior oblique muscle
• Trigeminal (V) - facial sensation, chewing, jaw muscles - 3 divisions (Ophthalmic, Maxillary, and Mandibular) sensory and motor functions
• Abducens (VI) - lateral rectus muscle, eye movements
• Facial (VII) - facial expression, taste (anterior 2/3 of tongue), secretions (tears, saliva)
• Vestibulocochlear (VIII) - hearing & balance
• Glossopharyngeal (IX) - taste (posterior 1/3 of tongue), swallowing, and salivary gland control
• Vagus (X) - parasympathetic function for organ regulation in thorax & abdomen
• Accessory (XI)- head & neck movement (trapezius & sternocleidomastoid muscles)
• Hypoglossal (XII) - tongue movements

Cerebellum

• Its function: Motor coordination, timing, and balance
• Divisions: Posterior lobe, Anterior lobe, & Flocculonodular lobe
• Deep nuclei: Dentate, Interposed, & Fastigial
• Many pathways input into the cerebellum for fine-tuning of movement including cortical and brainstem inputs

Cerebellum - Input Pathways

• Cortico-ponto-cerebellar pathway - from motor, premotor & somatosensory cortical areas
• Olivocerebellar tract - from inferior olive to cerebellum
• Vestibulo-cerebellar fibers - from vestibular apparatus to cerebellar nuclei.
• Reticulo-cerebellar fibers - from reticular formation to vermis
• Other peripheral pathways transmit signals that provide sensory info. about muscle length, tension, and movement to cerebellum

Cerebellum - Output Pathways

• Output of cerebellum through projections to deep cerebellar nuclei, and back through numerous pathways to various parts of the brain

Cerebellum - Functions (in different lobes)

• Vestibulocerebellum - balance and eye movements
• Spinocerebellum - muscle coordination and movement; receives information from proprioceptors and sensory receptors
• Cerebrocerebellum - planning and executing complex movements, based on higher cortical input; receives information from cortex; involved in timing & extramotor abilities

Cerebellum - Clinical Abnormalities

• Dysmetria - overshooting movement targets
• Ataxia - uncoordinated movements
• Intension tremor - wavering movement towards target, caused by movement overshoots
• Nystagmus - involuntary, rhythmic eye movements & suggests a problem with flocculonodular lobe
• Hypotonia - reduced muscle tone.

Basal Ganglia & Motor Control

Basal Ganglia - Structure

• Caudate nucleus, Putamen, Globus pallidus (internal & external segments), Substantia nigra (compacta & reticulata), & Subthalamic nucleus (STN).

Basal Ganglia - Circuitry

• Putamen circuit - involved in learned & subconscious movements (e.g. writing, hammering nails, etc.)
• Caudate circuit - involved in initiating and planning complex &/or instinctive movements (e.g. choosing one action over another, etc.).

Basal Ganglia - Pathways

• Two major pathways; direct & indirect.
• Direct pathway is used for initiating movement; indirect for inhibiting incorrect & inappropriate movements.
• Dopamine is critical in modulating these pathways to regulate motor function smoothly.

Basal Ganglia - Diseases

• Parkinson’s disease: decreased dopamine production → difficulty initiating and controlling movements (characteristic symptom: tremor).
• Huntington’s chorea: inappropriate, uncontrollable movements, caused by loss of many GABAergic neurons.

Stroke

• Interruption of blood flow in a specific artery to a region in the brain.
• Causes: rupture of a blood vessel or thrombosis (blockage due to a clot)
• Types: hemorrhagic (rupture) or ischemic (blockage)

Upper Motor Neuron Lesions

• Lesion above anterior horn cell: Spastic paralysis, hyperactive deep tendon reflexes; Babinski sign

Lower Motor Neuron Lesions

• Lesion in the anterior horn cell/nerve to specific muscle: hypotonicity (reduced muscle tone) &/or atrophy (muscle loss).

Test your knowledge on the physiology of the nervous system, focusing on sensory receptors and somatic sensations. This quiz will cover various types of sensory receptors, their functions, and the classification of sensations. Understand how the nervous system processes different sensory information.