Module 10 - The Sensory System: BIOL 2457 Anatomy & Physiology I

The Sensory System

Learning Objectives

• Describe the structural and functional classifications of sensory neurons.
• Explain the differences between general senses and special senses.
• Explain the role of the receptive field in dictating what kinds of stimuli each receptor type can detect.
• Explain how the brain interprets the modality, location, and duration of a stimulus.
• Compare slow-adapting (tonic) and fast-adapting receptors (phasic).
• Define “threshold” as it relates to pain and identify which special senses have high vs. low thresholds.
• Define pain threshold, pain tolerance, and the three types of pain.
• Identify the major anatomical structures and cells involved in olfaction and describe the functions of each.
• Describe how the process of olfaction occurs, including the brain region involved in processing olfactory information.
• Identify the major anatomical structures (papillae) and cells involved in gustation and describe the functions of each.
• Describe how taste sensations are detected, including the five taste sensations and the brain region involved in processing gustatory information.
• Identify the major anatomical structures of the eyeball and describe their functions.
• Differentiate between the visual accessory structures of the eyeball and describe the functions of each.
• Trace the pathway of light through the eye from the cornea to the cells of the retina to the occipital lobe of the brain.
• Explain how pupil size and lens accommodation influence vision.
• Describe the anatomical basis for myopia, hyperopia, presbyopia, and astigmatism.
• Compare the locations, functions, and pigments of rods and cones.
• Describe the location and the importance of the fovea centralis.
• Describe the general functions of each structure in the outer, middle, and internal ears.
• Describe the pathway of sound waves from the external environment, through the middle ear bones, through the fluids of the internal ear.
• Explain how hair cells translate vibrations into a neural signal, and trace the pathway of that auditory signal from the hair cells to the auditory cortex.
• Explain the difference between static & dynamic equilibrium.
• Identify the structures involved in the detection of static vs. dynamic equilibrium and describe the functions of each.
• Trace the pathway of equilibrium impulses from the hair cells in the internal ear to their final CNS destination.
• Identify the cranial nerves involved in each of the special senses.

Introduction to the Senses

• Some stimuli are somatic.
  • Touch
  • Vibration
  • Temperature
  • Itch
  • Pain
• Somatic stimuli are detected throughout the body.
• Some stimuli are detected by special sensory organs.
  • Light
  • Sound
  • Chemicals
• Special sensory stimuli are detected by structures in the head.
• Sensations are stimuli in the environment that are detected by the PNS.

Structural Classes of Sensory Receptors

• Sensory receptors can be classified based on their location:
  • Exteroreceptors detect stimuli in the external environment.
  • Interoreceptors detect stimuli in the internal environment.

Functional Classes of Sensory Receptors

• Sensory receptors can be classified based on the type of stimuli they detect
  • Photoreceptors detect light.
  • Nociceptors detect pain.
  • Thermoreceptors detect temperature.
  • Chemoreceptors detect chemicals.
  • Mechanoreceptors detect touch/pressure/stretching.

Types of Sensory Receptors

• Many stimuli are directly detected by the dendrites of sensory neurons.
• Some stimuli are indirectly detected by sensory neurons.
  • The stimulus activates a separate cell first.
  • Examples: vision, hearing, taste

Interpreting Stimuli

• The receptive field of a sensory receptor is the area that, when stimulated, will affect the activity of a neuron.
  • While this refers to the location of the stimulus, it is also influenced by the nature of the stimulus.

Interpreting Stimuli - Sensory Coding

• Sensory coding can be used to help the brain distinguish between different kinds of stimuli.
  • Each type of detected stimuli is sent to the cerebrum in a specific pathway.
  • Stimuli of multiple modalities may all be integrated at the same time to generate a better understanding of an experience.
  • Example: petting a cat

Interpreting Stimuli - Frequency Coding

• Frequency coding can be used to help the brain determine the intensity of a stimulus.
  • The number of signals from a single receptor can be used to determine intensity.
  • The number of activated receptors can be used to determine intensity.

Interpreting Stimuli - Location

• The location of a stimulus is determined based on the parts of the receptive field that are activated.
• Locations are detected with varying levels of acuity (detail).
  • Areas with a large receptive field will have lower acuity.
  • Areas with a small receptive field will have higher acuity.

Interpreting Stimuli - Duration

• The duration of a stimulus can lead to adaptation of a receptor.
  • Adaptation means a receptor no longer responds to a stimulus.
• Rapid-adapting receptors are known as phasic receptors.
  • These receptors respond to a stimulus once, then ignore it until a new stimulus is received.
  • Example: wearing clothes
• Slow-adapting receptors are known as tonic receptors.
  • These receptors detect stimuli that need constant monitoring.
  • Example: blood pressure, pain levels

Sensory Perception

• Step 1 – Sensory receptor activation (sensation)
  • Information collected by the PNS is sent to the CNS.
• Step 2 – Circuit-level transport
  • 1st order neurons either process sensory information in reflexes, or pass it along to 2nd order neurons in the spinal cord.
  • 2nd order neurons carry the information to the thalamus via white matter tracts.
  • 3rd order neurons in the thalamus carry the information to the correct cerebral cortex location.
• Step 3 – Perception
  • The sensory areas of the cerebral cortex interpret the sensation.

Somatic (General) Senses

• Somatic sensory information is detected by multiple types of receptors.
• The somatic (general) senses are tactile sensations, temperature, proprioception, and pain.
• Receptor types based on their locations:
  • Exteroreceptors detect sensations from the skin.
  • Interoreceptors detect sensations from skeletal muscles, tendons, and joints.
• Receptor types based on the stimuli they detect:
  • Mechanoreceptors detect touch, pressure, and vibration.
  • Thermoreceptors detect temperature.
  • Proprioceptors detect stretching (of muscles & tendons).
  • Nociceptors detect pain.

Tactile Sensations

• Light touch sensations are detected by Meissner’s corpuscles, Merkel discs, and hair follicle receptors
  • These are rapidly-adapting receptors.
• Heavy touch sensations (pressure) are detected by Pacinian corpuscles, and Ruffini corpuscles
  • These are slowly-adapting receptors
  • These sensations require extended, deep deformations of the skin
• Vibration sensations are detected by Meissner’s corpuscles and Pacinian corpuscles
  • These are rapidly-adapting receptors
• Types of receptors involved:
  • Exteroreceptors
  • Mechanoreceptors

Tactile Mechanoreceptor Activation

• Mechanically-gated cation channel closed
  *Plasma membrane
  *Cytosol
  *Mechanically-gated cation channel open
  *Influx of $Na^+$ and $Ca^{2+}$ causes a depolarizing receptor potential

Temperature Sensations

• Types of receptors involved:
  • Exteroreceptors
  • Thermoreceptors
• The free nerve endings of the skin detect changes in temperature using transient receptor potential (TRP) channels.
  • Different channels open at different temperatures
  • Noxious cold & noxious hot stimuli register as pain
• Some TRP channels can also be opened by chemicals
  • Example: menthol and the cold channel
  • Example: capsaicin and the noxious heat channel

Thermoreceptor Activity

• [Diagram of thermoreceptor activity showing the response to different temperatures]

Pain Sensations

• Types of receptors involved:
  • Exteroreceptors
  • Interoceptors
  • Nociceptors
• Pain is a sensation designed to protect the body from tissue damage.
  • It is critical to health and survival.
• The pain threshold is the intensity needed for a stimulus to be considered painful.
  • This is low in children & the elderly
  • This is higher in adults
• Pain tolerance is the sensitivity of a person to the perception of pain.

Types of Pain

• Somatic pain is detected by nociceptors in the skin, skeletal muscles, and joints.
• Visceral pain is detected by nociceptors in the organs.
• Referred pain is visceral pain perceived as somatic pain.
  • The location where referred pain is perceived (felt) can indicate which organ(s) are in distress.

Processing Pain Sensations

• Some painful stimuli are processed using spinal reflexes (like the flexor withdrawal reflex & the crossed extensor reflex).
  • The response is unconscious.

Processing Pain Sensations - Ascending Tracts

• Some painful stimuli are sent to the cerebrum using ascending tracts.
  • The response is conscious.
  • Primary Somatosensory Cortex - Conscious Awareness
  • Thalamus - Third-order Neuron
  • Limbic System - Emotional Response
  • Hypothalamus - Autonomic Response
  • Reticular Formation - Arousal Response
  • Skin - Nociceptor, First-order Neuron
  • Release of Substance P
  • Second-order Neuron
  • Spinal Cord
  • 1st order neurons release substance P

General Sensory Receptors Classified by Structure and Function

• Nonencapsulated
  • Free nerve endings of sensory neurons
    • Functional Classes: L: Exteroceptors, interoceptors, and proprioceptors. S: Thermoreceptors (warm and cool), chemoreceptors (itch, pH, etc.), mechanoreceptors (pressure), nociceptors (pain)
    • Body Location: Most body tissues; most dense in connective tissues (ligaments, tendons, dermis, joint capsules, periostea) and epithelia (epidermis, cornea, mucosae, and glands)
  • Modified free nerve endings: Epithelial tactile complexes (Merkel cells and discs)
    • Functional Classes: L: Exteroceptors. S: Mechanoreceptors (light pressure); slowly adapting
    • Body Location: Basal layer of epidermis
  • Hair follicle receptors
    • Functional Classes: L: Exteroceptors. S: Mechanoreceptors (hair deflection); rapidly adapting
    • Body Location: Surrounding hair follicles
• Encapsulated
  • Tactile (Meissner's) corpuscles
    • Functional Classes: L: Exteroceptors. S: Mechanoreceptors (light pressure, discriminative touch, vibration of low frequency); rapidly adapting
    • Body Location: Dermal papillae of hairless skin, particularly nipples, external genitalia, fingertips, soles of feet, eyelids
  • Lamellar (Pacinian) corpuscles
    • Functional Classes: L: Exteroceptors, interoceptors, and some proprioceptors. S: Mechanoreceptors (deep pressure, stretch, vibration of high frequency); rapidly adapting
    • Body Location: Dermis and hypodermis; periostea, mesentery, tendons, ligaments, joint capsules; most abundant on fingers, soles of feet, external genitalia, nipples
  • Bulbous corpuscles (Ruffini endings)
    • Functional Classes: L: Exteroceptors and proprioceptors. S: Mechanoreceptors (deep pressure and stretch); slowly or nonadapting
    • Body Location: Deep in dermis, hypodermis, and joint capsules

Olfaction

• Olfaction (smell) is the process of detecting chemicals in the environment
  • These chemicals are called odorants.
  • Odorants dissolve in the mucus on top of the olfactory epithelium.

Olfaction - Cells

• Olfactory receptor cells are bipolar neurons
  • Their olfactory cilia are covered in metabotropic receptors sensitive to specific odorants.
• Supporting cells function like neuroglia.
• Basal cells generate new olfactory receptor cells roughly every 2 months.
• Type of receptor involved: Chemoreceptors

Olfactory Processing

• Olfaction has a very low threshold.
  • It doesn’t take many odorants to smell something
• Olfaction is a rapidly-adapting sense.
  • Detection declines rapidly after 1 second
  • Detection stops after 1 minute
• Both conscious & unconscious processing of smells occurs.
  • Conscious processing: olfactory cortex (temporal lobe)
  • Unconscious processing: hippocampus, amygdala, hypothalamus, RAS

Gustation

• Gustation (taste) is the process of detecting chemicals in the environment
  • These chemicals are called tastants
  • Tastants dissolve in the saliva on top of the tongue
• Gustatory receptor cells are found inside the tastebuds of the tongue
  • They have microvilli covered in chemical receptors
  • Some receptors are ionotropic
  • Some receptors are metabotropic
• Basal cells generate new gustatory receptor cells roughly every 10 days
• Type of receptor involved: Chemoreceptors

Papillae of the Tongue

• Vallate papillae are the largest papillae of the tongue.
  • They form a “V” on the back of the tongue
  • Each contains 50 – 100 tastebuds
• Fungiform papillae are the most common papillae.
  • They cover the surface of the tongue
  • Each contains 3 – 5 tastebuds
• Foliate papillae are only seen in children.
  • They are located on the sides of the tongue
• Filiform papillae have NO tastebuds.
  • They have mechanoreceptors & nociceptors

Taste Sensations

• Sweet sensations are caused by organic substances
  • Examples: sugar, xylitol, sucralose, aspartame
  • High threshold for detection à a lot must be present
• Sour sensations are caused by acids
  • Examples: citric acid, acetic acid
• Salty sensations are caused by ionized salts
  • Examples: NaCl, CaCl2, KCl
  • High threshold for detection à a lot must be present
• Bitter sensations are caused by many different chemicals
  • Examples: caffeine, nicotine, cocaine, LSD, poisons
  • Lowest threshold for detection à very little must be present
• Umami (savory) sensations are caused by glutamate & aspartate
  • Examples: steak, aged parmesan, Asian foods, MSG

Gustatory Receptors

• Chemicals that register as salty or sour activate ionotropic receptors
• Chemicals that register as sweet, bitter, and umami activate metabotropic receptors
  *Regardless of the type of receptor used to detect each kind of tastants, all the gustatory information they collect is sent to the insula for processing.

Vision

• Vision is the process of detecting light waves.
  • Different colors of light have different wavelengths
  • Different retinal cells detect each wavelength
• ~70% of all sensory receptors in the body are found in the retina.
  • ~50% of the cerebral cortex is involved in visual processing
  • indicates vision is the most important special sense in humans
• Type of receptor involved: Photoreceptors

Accessory Structures of Vision

• Accessory structures are external eyeball structure that are NOT directly involved in vision.
• External Structures
  • Eyebrows: shade eyes from sunlight.
  • Eyelids: protects eyes from foreign objects; spreads moisturizing secretions across eyes.
  • Eyelashes: protects eyes from foreign object; triggers blinking reflex.
  • Lacrimal glands: produces lacrimal fluid.
  • Lacrimal ducts: transports fluid to eye.
  • Lacrimal puncta and Lacrimal canaliculi
  • Lacrimal sacs: fluid drains into this.
  • Nasolacrimal ducts: drains lacrimal sacs into nasal cavity.
  • Extrinsic Eye Muscles
    • Superior rectus
    • Inferior rectus
    • Lateral rectus
    • Medial rectus
    • Superior oblique
    • Inferior oblique

The Layers of the Eyeball

• The outermost layer of the eyeball is the fibrous tunic
  • The sclera = the white connective tissue
  • The cornea = the clear connective tissue
• The middle layer is the vascular tunic
  • The choroid = internal blood supply
  • The ciliary body = muscles to change lens shape
  • The iris = colored part of the eye
• The innermost layer is the neural tunic
  • The retina = membrane of rods and cones
  • The fovea centralis = location of highest visual acuity
  • The optic disc = blind spot

Fluids of the Eye

• The eyeball is divided into two fluid-filled cavities.
  • The anterior cavity includes everything in front of the lens
  • The posterior cavity includes everything behind the lens
• The anterior cavity is filled with aqueous humor.
  • This watery fluid is replaced after ~100 minutes.
  • Old fluid is drained through the Canal of Schlemm.
• The posterior cavity is filled with vitreous humor.
  • This thicker fluid includes proteins and sugars.

Path of Light Through the Eye

• Light enters the eye through the cornea
  • The cornea bends the light waves
• The light enters the pupil
  • This is the opening in the iris
• The light passes through the lens
  • This bends the light waves again
• The light is focused on the retina

Light Adjustments - Pupil Size

• When the circular muscles of the eyeball contract, the pupil gets smaller.
  • This is done when the light is bright.
  • This is done when the parasympathetic nervous system is activated.
• When the radial muscles of the eyeball contract, the pupil gets larger.
  • This is done when the light is dim.
  • This is done when the sympathetic nervous system is activated.

Lens Adjustments - Accommodation

• The lens can change its shape to focus light waves on the retina → accommodation
• (b) The lens flattens for distant vision.
  • Sympathetic input relaxes the ciliary muscle. This tightens the ciliary zonule and flattens the lens.
• (c) The lens bulges for close vision.
  • Parasympathetic input contracts the ciliary muscle. This loosens the ciliary zonule and allows the lens to bulge.

Myopia

• Focal point is in front of retina.
• Eyeball too long
• Corrected with Concave lens

Hyperopia

• Focal point is behind retina.
• Eyeball too short
• Corrected with Convex lens

Presbyopia

• With age, the muscles of the ciliary body weaken, reducing its ability to adjust the shape of the lens.
• The reduced ability to adjust the shape of the lens decreases its power of accommodation (which normally allows objects to be seen clearly, regardless of distance).
• The image is focused behind the retina.

Astigmatism

• Cornea - oval shape
• Multiple focal points

Cells of the Retina

• The sensory receptors of the retina are rods and cones.
• Rods are highly sensitive photoreceptors
  • they work in low light
  • Rods use the rhodopsin protein
  • detect in black & white
• Cones are less sensitive photoreceptors
  • they work in bright light
  • Cones use photopsin proteins
  • detect in color

Rods and Cones

• Because rods only have one pigment (rhodopsin), they are either activated or not.
  • This leads to black & white vision.
• Cones have three photopsin pigments.
  • The level of activation of each type of cone is integrated in the visual cortex of the cerebrum.

Rods and Cones - Distribution

• Cones are most numerous in the center of the retina.
  • This region is called the fovea centralis.
• Table 15.1 Comparison of Rods and Cones
  • RODS
    • Noncolor vision (one visual pigment)
    • High sensitivity; function in dim light
    • Mostly in peripheral retina
    • More numerous (20 rods for every cone)
  • CONES
    • Color vision (three visual pigments)
    • Low sensitivity; function in bright light
    • Mostly in central retina
    • Less numerous

Path of Light Through the Retina

• Light can only be detected by rods & cones
• Before light reaches these cells, it passes through ganglion cells & bipolar cells
• Photoreceptors then send information to bipolar cells which transmit to ganglion cells.
  • Axons of ganglion cells collect at the optic disc and leave the eye as the optic nerve

The Fovea Centralis

• The fovea centralis is the part of the eyeball with the highest visual acuity
• Why is vision the clearest here?
  • This is the location light naturally focuses in the eye
  • The density of photoreceptors (cones, in particular) is highest here
  • There are no bipolar cells or ganglion cells blocking the cone from light waves

Processing Visual Sensations

• Visual signals in the retina: photoreceptors à bipolar cells à ganglion cells
• Ganglion cells extend axons into the cerebrum as the optic nerve (CN II).
• Information is routed through the thalamus to the primary visual cortex in the occipital lobe.

Hearing

• The process of hearing involves detecting sound waves in the environment
  • The external ear collects sound waves
  • The middle ear intensifies them
  • The internal ear detects them
• The tympanic membrane divides the external ear from the middle ear
• The oval window divides the middle ear from the internal ear
• Type of receptor involved: Mechanoreceptors

The Cochlea

• The cochlea is the part of the internal ear that detects sound waves
• The cochlea is made of 3 fluid-filled tubes.
  • Scala vestibuli à sound waves from the oval window go here
  • Scala media (cochlear duct) à sound waves are detected here
  • Scala tympani à sound waves go to the round window from here

Process of Hearing

• Sound waves vibrate the tympanic membrane.
• Auditory ossicles vibrate. Pressure is amplified.
• Pressure waves created by the stapes pushing on the oval window move through fluid in the scala vestibuli.
• Sounds with frequencies below the hearing range travel through the helicotrema and do not excite hair cells
• Sounds in the hearing range go through the cochlear duct, vibrating the basilar membrane and deflecting hairs on inner hair cells.

Process of Hearing - Fluids and Hair Cells

• Perilymph is the fluid found in the scala vestibuli and tympani.
• Endolymph is the fluid found in the cochlear duct.
• The spiral organ within the scala media has hair cells that detect the movement of the basilar membrane.

Process of Hearing - Hair Cell Activation

*At rest, A few channels are open; cell slightly depolarized
*Tip links tighten, opening mechanically gated ion channels. More cations enter; cell depolarizes. + Neurotransmitter release. Action potentials in cochlear nerve.
*Tip links loosen, closing mechanically gated ion channels. 2 No cations enter; cell hyperpolarizes. + Neurotransmitter release. Action potentials in cochlear nerve.

Processing Auditory Sensations

• The hair cells of the cochlea send information to the brainstem using the vestibulocochlear nerve (CN VIII).
• Information is routed through the thalamus to the primary auditory cortex in the temporal lobe.

Equilibrium

• Equilibrium is the process of balancing
• Static equilibrium helps you to maintain balance as it relates to gravity
  • example: keeping your head up while driving
• Dynamic equilibrium monitors changes in your head’s rotation
  • example: shaking head "no"
• Type of receptor involved: Mechanoreceptors

Static Equilibrium

• The saccule and the utricle detect static equilibrium.
• The macula is the sensory receptor inside the utricle and the saccule.
  • The cells in the macula are called hair cells.
  • They have stereocilia on their upper surface.
  • Otoliths (calcium carbonate crystals) bump these stereocilia as the head moves.
  • opens mechanically-gated ion channels

Static Equilibrium Mechanism

• Head Upright - Steady stream of action potentials in vestibular nerve
• Head tilted forward
  • Hairs bend toward kinocilium
  • Hair cell depolarizes
  • Nerve fiber excited - Action potentials in vestibular nerve
• Head tilted backwards
  • Hairs bend away from kinocilium
  • Hair cell hyperpolarizes
  • Nerve fiber inhibited - Action potentials in vestibular nerve

Dynamic Equilibrium

• The semicircular canals detect dynamic equilibrium.
• The crista is the sensory receptor inside the semicircular canals.
  • The crista is made of hair cells.
  • On top of the hair cells is a gelatinous membrane called the cupula.
  • As the head rotates, fluid in the semicircular canals moves slowly
  • Movement of the fluid bends the cupula
  • opens mechanically-gated ion channels

Dynamic Equilibrium Mechanism

• At rest, the cupula stands upright.
• During rotational acceleration, endolymph moves inside the semicircular canals in the direction opposite the rotation (it lags behind due to inertia). Endolymph flow bends the cupula and excites the hair cells.
• As rotational movement slows, endolymph keeps moving in the direction of rotation. Endolymph flow bends the cupula in the opposite direction from acceleration and inhibits the hair cells.

Processing Equilibrium Sensations

• The hair cells of the utricle, saccule, and semicircular canals send information to the brainstem using the vestibulocochlear nerve (CN VIII).
• Information is routed to various locations.
  • thalamus
  • insula cerebral cortex
  • spinal cord
  • cranial nerves

Eye Disorder Comparison

• Auricle: the auricle
• Endolymph: the endolymph in the cochlear duct
• External acoustic meatus: the external acoustic meatus
• Hair cells: the hair cells in the spiral organ
• Ossicles: the ossicles
• Oval window: the oval window
• Perilymph: the perilymph in scala vestibuli
• Tympanic membrane: the tympanic membrane

Equilibrium - Fill in the Blank

• I use otoliths to detect equilibrium.
• I use hair cells to detect equilibrium.
• I use a cupula to detect equilibrium.
• I use the saccule & utricle to detect equilibrium.
• I help you stay balanced during rotational movements.
• I help you keep your head up in class.
• I use the semicircular canals to detect equilibrium.