Sensation
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Sensation. Senses: Means by which brain receives information about environment and body General: Distributed over large part of body Somatic: Touch, pressure, temperature, proprioception, pain Visceral: Internal organs and consist mostly of pain and pressure

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Sensation

Sensation

  • Senses: Means by which brain receives information about environment and body

    • General: Distributed over large part of body

      • Somatic: Touch, pressure, temperature, proprioception, pain

      • Visceral: Internal organs and consist mostly of pain and pressure

    • Special senses: Smell, taste, sight, hearing, balance

  • Sensation or perception: Conscious awareness of stimuli received by sensory receptors


Sensation1

Sensation

  • Sensation or perception is the conscious awareness of stimuli received by receptor.

  • Receptors transduce (change) different forms of energy into nerve impulses

  • Nerve impulses are conducted to the brain

    • Stimulus must initiate and action potential in the cerebral cortex

    • The brain interprets these impulses as sound or sight even though the impulses themselves are identical in nature.

      • The cerebral cortex screens the information and ignores most of what it receives – subconscious

  • Our senses act as energy filters that perceive a narrow range of energy.


Sensation requires

Sensation Requires:

  • A stimulus

  • Activation of a receptor,

  • Conduction of an action potential to a specific region of the CNS

  • Translation or interpretation of the signal.

  • Sensation or awareness of a stimuli occurs in the cerebral cortex.


General properties of receptors

General Properties of Receptors

  • A receptor is any structure specialized to detect a stimulus.

  • All receptors are transducers, changing stimulus energy into nerve energy.

  • Sensory receptors transmit four kinds of information:

    • Modality refers to the type of stimulus or sensation it produces (vision, hearing, taste, etc.).

    • Location is also indicated by which nerve fibers are firing.

      • Sensory projection is the ability of the brain to identify the site of stimulation.

    • Intensity of stimulous

    • Duration is encoded in the way nerve fibers change their firing frequencies over time.

      • Tonic vs Phasic receptor adaptation.


Sensation

Classification of Receptors


Types of sensory receptors

Types of Sensory Receptors

  • Classification by Stimulus Modality

    • Mechanoreceptors: Compression, bending, stretching of cells

    • Chemoreceptors: Smell and taste

    • Thermoreceptors: Temperature

    • Photoreceptors: Light as vision

    • Nociceptors: Pain

  • Classification by Origin of Stimuli

    • Exteroreceptors: Associated with skin

    • Visceroreceptors: Associated with organs

    • Proprioceptors: Associated with joints, tendons


Sensory nerve endings

Sensory Nerve Endings

  • Unencapsulated Nerve Endings

    • Free nerve endings: Cold receptors and warm

    • Merkel’s disk: Light touch, superficial pressure

    • Hair follicle receptor: Light touch, bending of hair


Sensory nerve endings in skin encapsulated nerve endings

Sensory Nerve Endings in SkinEncapsulated Nerve Endings

  • Pacinian corpuscle: Deep cutaneous pressure, vibration and proprioception

  • Meissner’s corpuscle: Two-point discrimination

  • Ruffini’s end organ: Continuous touch or pressure

  • Muscle spindle: Proprioception as to muscle stretch and control of muscle tone

  • Golgi tendon organ: Important in muscle contraction and tendon stretch proprioception


Two point discrimination

Two-Point Discrimination


Muscle spindle and golgi tendon organ

Muscle Spindle and Golgi Tendon Organ


Responses of sensory receptors

Responses of Sensory Receptors

  • Receptor: Interaction of stimulus with sensory receptor produces a local potential

    • Primary: Have axons that conduct action potential in response to receptor potential

    • Secondary: Have no axons and receptor potentials produced do not result in action potentials but cause release of neurotransmitters

  • Accommodation or adaptation: Decreased sensitivity to a continued stimulus

  • Proprioceptors

    • Tonic: Example is know where little finger is without looking

    • Phasic: Example is you know where hand is as it moves


Sensory nerve tracts

Sensory Nerve Tracts

  • Transmit action potentials from periphery to brain

  • Each pathway involved with specific modalities

  • First half of word indicates origin, second half indicates termination


Spinothalamic system

Spinothalamic System

  • Conveys cutaneous sensory information to brain

  • Unable to localize source of stimulus

  • Divisions

    • Lateral for pain and temperature

    • Anterior for light touch, pressure, tickle, itch


Dorsal column medial lemniscal system

Dorsal-Column/Medial-Lemniscal System

  • Carries sensations of

    • Two-point discrimination

    • Proprioception

    • Pressure

    • Vibration

  • Tracts

    • Fasciculus gracilis

    • Fasciculus cuneatus


Spinocerebellar system

Spinocerebellar System

  • Carry proprioceptive information to cerebellum

  • Actual movements can be monitored and compared to cerebral information representing intended movement

  • Tracts

    • Posterior

    • Anterior


Sensory areas of cerebral cortex

Sensory Areas of Cerebral Cortex


Somatic sensory cortex

Somatic Sensory Cortex


Sensation

Pain

  • Types

    • Referred: Sensation in one region of body that is not source of stimulus

    • Phantom: Occurs in people who have appendage amputated or structure removed as tooth

    • Chronic: Not a response to immediate direct tissue injury


Special senses

Special Senses

  • Olfaction

  • Taste

  • Visual system

  • Hearing and balance


Olfaction

Olfaction

  • Sense of smell

    • Olfactory epithelium

      • 10-20 million neurons

      • Bipolar neurons project through cribiform plate.

    • Olfactory hairs

      • 10 – 20 Cilia per neuron.

      • Embedded in a mucous layer

      • Only neurons exposed to external environment

      • Replaced every 60 days.


Olfactory physiology

Olfactory Physiology

  • Process of Olfaction

    • Airborne chemicals are dissolved in the fluid covering the olfactory epithelium.

      • Chemicals must be volatile and water soluble.

    • Odor molecule binds with a specific receptor

    • G-protein coupled membrane receptor)

    • a second messenger is produced,

    • Sodium channels are opened in the membrane.

    • The cell is depolarized creating an axon potential.

  • Olfactory Discrimination

    • We can discriminate between ~10,000 different odors.

      • There are between 7-50 primary classes of odors

      • A characteristic fingerprint of the odor is used to identify the odor.

    • Olfactory receptors adapt quickly

    • Some odors can stimulate nociceptors in the trigeminal nerve.


Sensation

Olfactory Neuronal Pathways and the Cortex

  • Olfactory neurons project to the olfactory bulb.

  • Mitral cells project to the olfactory cortex.

  • Lateral olfactory area: conscious perception of smell

  • Medial olfactory area: visceral and emotional reactions

  • Intermediate olfactory area merges information from medial and lateral areas and projects back to olfactory bulb to modulate neuronal activity there.


Clinical considerations of olfaction

Clinical Considerations of Olfaction

  • Anosmia – inability to smell (1.2% of the population)

  • Ability to smell decreases with age.

  • 98-99% of people can smell banana, rose and cloves.

  • 35% of the population cannot smell androstenone (body odor).


Papillae and taste buds

Papillae and Taste Buds

  • Taste results from the action of chemicals on the taste buds found on papillae.

    • ~10,000 taste buds

  • Papillae Types

    • Circumvallate

    • Fungiform

    • Foliate

    • Filiform

  • Taste Bud Structure

    • Supporting cells - Form an exterior supporting capsule

    • Gustatory or taste cells contain gustatory villi or hairs with surface receptors and are replaced every 7 to 10 days.


Sensation

Physiology of Taste

  • Process of Taste

    • Molecules are dissolved in saliva.

    • Substance enters taste pore and attaches to chemoreceptor molecule

    • Depolarization of the taste cell.

    • Taste cells have no axons but release neurotransmitter

    • Neurotransmitter stimulates action potential in cells associated with the gustatory cells.


Sensation

Four Primary Taste Sensations Exist:

  • Salty

    • lateral anterior of tongue

    • The presence of Na+ is detected

  • Sweet

    • tip of the tongue

    • most organic molecules (particularly sugars) are sweet.

  • Sour

    • posterior lateral portion of the tongue

    • H+ are detected

    • all acids taste sour.

  • Bitter

    • most posterior central portion of the tongue

    • most sensitive

    • protective function - most poisons are bitter

  • Umami (Glutamate) may also be considered.


Actions of major tastants

Actions of Major Tastants


Neuronal pathways for taste

Neuronal Pathways for Taste


Sensation

Visual System


Anatomy of the eye

Fibrous tunic: Outer

Sclera: White outer layer, maintains shape, protects internal structures, provides muscle attachment point, continuous with cornea

Cornea: Avascular, transparent, allows light to enter eye and bends and refracts light

Vascular tunic: Middle

Iris: Controls light entering pupil; smooth muscle

Ciliary muscles: Control lens shape; smooth muscle

Retina: Inner

Contains neurons sensitive to light

Macula lutea or fovea centralis: Area of greatest visual acuity

Optic disc: Blind spot

Compartments

Anterior: Aqueous humor

Posterior: Vitreous humor

Lens

Held by suspensory ligaments attached to ciliary muscles

Transparent, biconvex

Anatomy of the Eye


Sensation

Compartments of the Eye

  • Posterior Compartment

    • Vitreous Humor

  • Anterior Compartment

    • Anterior Chamber

    • Posterior Chamber

    • Aqueous Humor

      • Produced by ciliary processes


Functions of the complete eye

Functions of the Complete Eye

  • Eye functions like a camera

  • Iris allows light into eye

  • Lens, cornea, humors focus light onto retina

  • Light striking retina is converted into action potentials relayed to brain


Light

Light

  • Focal point: Point where light rays converge and cross

    • The more spherical the lens the more the light is bent.

  • Reflection: light rays bounce off a non transparent object

  • Visible light: Portion of electromagnetic spectrum detected by human eye

    • The visible spectrum ranges form ~400 to 700 nm

  • Refraction: Bending of light

    • Divergence: Light striking a concave surface

    • Convergence: Light striking a convex surface

    • The cornea, aqueous humor, lens and vitreous humor all refract light.


Focus and accommodation

Focus and Accommodation

  • Focusing system of the eye creates a clear image on the retina.

  • Emmetropia: Normal resting condition of lens

  • Far vision: 20 feet + from eye.

  • Near vision: Closer than 20 feet

    • Accommodation

      • Occurs via changes in the shape of the lens.

    • Pupil constriction

      • Depth of focus

    • Convergence

  • The inverted image on the retina is detected by photoreceptors and passed via action potentials to the visual cortex.


The retina

The Retina

  • Pigmented retina

    • Single layer of pigmented cells (RPE)

  • Sensory retina

    • Three layers

      • Photoreceptor

      • Bipolar cell

      • Ganglion cell

    • Three layers separated by plexiform layers

      • Sensitivity vs. visual acuity.

  • Photoreceptors

    • Rods: Noncolor vision

    • Cones: Color vision


Sensory receptor cells

Sensory Receptor Cells

  • Photoreceptors

    • Bipolar cells that detect light.

    • Types:

      • Rods – noncolor, low illumination.

      • Cones – color vision, bright light.

    • Outer segment is made of ~700 folded membranes (discs) that contain photopigments.

    • Rhodopsin

      • Opsin

      • Retinal (Vitamin A derivative)

      • Coupled to a G protein


Rhodopsin cycle

Rhodopsin Cycle

  • Retinal in inactive cis configuration is attached inside opsion.

  • Light causes opsin to change shape causing activation of the G-protein. Na+ channels open and the cell hyperpolarizes.

  • Trans-retinal detaches from opsin.

  • Trans-retinal is converted to cis-retinal via ATP.

  • Cis-retinal reattaches to opsin in the dark configuration and the cell depolarizes.

Note: Light and Dark adaptation occur through the production or breakdown of rhodopsin.


Rod cell hyperpolarization

Rod Cell Hyperpolarization


Visual pathways

Visual Pathways


Eye disorders

Myopia: Nearsightedness

Focal point too near lens, image focused in front of retina

Hyperopia: Farsightedness

Image focused behind retina

Presbyopia

Degeneration of accommodation, corrected by reading glasses

Astigmatism: Cornea or lens not uniformly curved

Strabismus: Lack of parallelism of light paths through eyes

Retinal detachment

Can result in complete blindness

Glaucoma

Increased intraocular pressure by aqueous humor buildup

Cataract

Clouding of lens

Macular degeneration

Common in older people, loss in acute vision

Diabetes

Dysfunction of peripheral circulation

Eye Disorders


Inner ear

Inner Ear

  • Labyrinth

    • Bony

      • Cochlea: Hearing

      • Vestibule: Balance

      • Semicircular canals: Balance

    • Membranous

  • -Lymphs

    • Endolymph

      • In membranous labyrinth

    • Perilymph

      • Space between membranous and bony labyrinth


Structure of cochlea

Structure of Cochlea


Sensation

Hair Cell with 50-60 linked cilia


Auditory function

Auditory Function

  • Vibrations produce sound waves

    • Volume or loudness : Function of wave amplitude

    • Pitch: Function of wave frequency

    • Timbre: Resonance quality or overtones of sound


Effect of sound waves on cochlear structures

Effect of Sound Waves on Cochlear Structures


Cns pathways for hearing

CNS Pathways for Hearing


Balance

Static

Evaluates position of head relative to gravity

Detects linear acceleration and deceleration

Utricle and saccule

Maculae: Consist of hairs embedded in gelatinous mass containing otoliths

Kinetic

Evaluates movements of head

3 semicircular canals

Ampulla

Crista ampullaris

Cupula: endolymph moves when head moves

Balance


Structure of the macula

Structure of the Macula


Vestibule in maintaining balance

Vestibule in Maintaining Balance


Semicircular canals

Semicircular Canals


Cns pathways for balance

CNS Pathways for Balance


Ear disorders

Ear Disorders

  • Tinnitus

    • Ringing, clicking, whistling in ear due to disorders in middle or inner ear

  • Motion sickness

    • Dysfunctions caused by stimulation of semicircular canals during motion

  • Otitis Media

    • Infections in the middle ear

  • Earache

    • Results from otitis media, dental abscesses, TMJ pain


Effects of aging on the special senses

Effects of Aging on the Special Senses

  • Slight loss in ability to detect odors

  • Decreased sense of taste

  • Lenses of eyes lose flexibility

  • Development of cataracts, macular degeneration, glaucoma, diabetic retinopathy

  • Decline in visual acuity and color perception


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