Topic 8 - Sensory System

List and describe the three-step process of the sensory system

reception - sensory receptors in our sense organs (i.e. ears, skin and eyes) detect a physical stimulus (i.e. light waves, sound waves, or pressure)

transduction - sensory receptors convert the detected physical energy into an electrochemical signal (action potential)

transmission - newly created neural impulses travel along nerve fibers to the brain - information is sent to specific areas for processing

List and describe three types of sensory receptors based on structural receptor type

a neuron can have 3 different structural receptor type:

• non-encapsulated ending

  • dendrites are not surrounded by specialized connective tissue capsule

• encapsulated ending

  • sensory nerve ending are enclosed in connective tissue

• specialized receptor cell

  • has distinct structural components that interpret a specific type of stimulus

List and describe five types of sensory receptors based on the type of stimulus that activates them

mechanoreceptors - respond to distortion caused by pressure

nociceptors - senses harmful stimuli (i.e. tissue injury)

thermoreceptors - detect changes in temperature

chemoreceptors - detect specific molecules (i.e. relates to taste, smell, osmolarity, blood CO2 levels, blood glucose)

photoreceptors - responds to particular wavelengths of light

Identify and describe the functions of the four types of skin mechanoreceptors

o In addition, determine which category of sensory receptors (structural receptor type) is applicable to each type of skin mechanoreceptor

Merkel discs

• non-encapsulated ending

• discs are located in the epidermis

• facilitate detection of light touch or pressure, corners, edges, curvatures → provides information of an object’s shape + texture

Ruffini endings

• encapsulated ending

• endings are located in the deep dermis

• facilitate detection of sustained pressure, stretching, slipping, or sliding of objects across the skin surface

Meissner corpuscles

• encapsulated ending

• corpuscles are located in the upper dermis

• facilitate the detection of low-frequency vibrations

  • i.e. tapping, fluttering gentle pulsation, fine touch, and fine dynamic movement

Pacinian corpuscles

• encapsulated ending

• corpuscles located in deep part of the dermis + subcutaneous layer

• facilitate the detection oh high-frequency vibrations + deep pressure

  • i.e. buzzing, humming, tickling

List and describe the function of the two types of proprioceptors

o In addition, determine which category of sensory receptors (structural receptor type) is applicable to proprioceptors

specialized mechanoreceptors located in muscles, tendons, ligaments, and joints → monitors body’s position + movement (i.e. enables awareness of body posture + movement without visual input)

• muscle spindles - found within skeletal muscles

  • detects changes in muscle length + rate of stretch

• Golgi tendon organs - found at junction of muscle + its tendons

  • detects tension and force of muscle contraction

→ both muscle spindles + Golgi tendon organs have encapsulated nerve endings

List and describe the function of the two types of baroreceptors

o In addition, determine which category of sensory receptors (structural receptor type) is applicable to baroreceptors

specialized mechanoreceptors that monitor blood pressure by sensing the tension in blood vessels

• carotid sinus baroreceptors - found in carotid arteries

  • supply blood to the brain

  • extremely sensitive to rapid pressure changes

• aortic arch baroreceptors - found in arch of aorta

  • receptors are less sensitive than carotid sinus baroreceptors

  • responds in a similar manner to carotid sinus baroreceptors

→ carotid sinus + aortic arch baroreceptors have non-encapsulated nerve endings

Differentiate between fibers of the two types of nociceptors

o In addition, determine which category of sensory receptors (structural receptor type) is applicable to nociceptors

fast + sharp - immediate + localized pain mediated by A-delta fibers

• thin + myelinated fibers

• rapidly transmit signals to the brain

• enables a quick response to acute injury (damage to the body that occurs suddenly + unexpectedly)

slow + aching - prolonged + diffuse pain transmitted by C fibers

• unmyelinated fibers

• conduct signals more slowly

• provides a lingering awareness of injury

• promotes a protective behavior to aid healing

→ both A-delta fibers + C fibers are non-encapsulated

Differentiate between fibers of the two types of thermoreceptors

o In addition, determine which categories of sensory receptors (structural receptor type) are applicable to thermoreceptors

o List specific example of a cold thermoreceptor

o List specific example of a warm thermoreceptor

Thermal sensations are detected by thermoreceptors - specialized sensory receptors in the skin that respond to changes in temperature

cold - activated between 10-35 degrees Celsius

• example: Krause end bulb (encapsulated)

• nerve fibers that transmit signals for cold → A-delta fibers

warm - activated between 30-45 degrees Celsius

• example: Ruffini endings (encapsulated)

• nerve fibers that transmit signals for warm → C fibers

when temperatures exceed normal thresholds → nociceptors may be activated, leading to sensations of pain i.e. burning or stinging of extreme temperatures

List and describe the functions of the different types of chemoreceptors (central, peripheral, osmoreceptor, gustatory receptor, and olfactory receptor)

o In addition, determine which category of sensory receptors (structural receptor type) is applicable to each main type of chemoreceptor

NOTE: Gustatory receptors are not categorized since they are not true neurons

→ (instead they are neuroepithelial cells)

two main types of chemoreceptors:

central - located in brainstem

• non-encapsulated

• detects changes in pH of cerebrospinal fluid - indirect measure of blood’s CO2 levels

• when CO2 diffuses into cerebrospinal fluid → forms carbonic acid → dissociates into H+ ions and bicarbonate ions

• excess H+ ions = low pH levels (acidic) → stimulates receptors to help increase depth and rate of breathing - helps eliminate excess CO2

peripheral - located in carotid + aortic bodies

• encapsulated

• directly monitors blood’s oxygen and CO2 levels

• detects low oxygen levels directly

• detects high carbon dioxide levels indirectly - senses drop in blood pH

• increases rate + depth of breathing

other chemoreceptors:

osmoreceptors - located in hypothalamus

• non-encapsulated

• senses changes in blood osmolarity (solute concentration)

• helps regulate thirst + water balance

• activation of receptors lead to 2 main responses:

  • prompts to drink more water

  • stimulates posterior pituitary gland → release antidiuretic hormone (ADH) - hormone results to more water absorption in the collecting duct of the kidney nephron

gustatory receptors

• not neurons - are modified neuroepithelial cells

• receptors cells are innervated (distributed) by non-encapsulated afferent nerve endings

• cells are clustered into barrel shaped sensory organs (taste bud)

olfactory receptors

• non-encapsulated

• process of detecting odorants + converting them into electrical signals in the brain

  • odor molecules detected by cilia of olfactory receptor neurons in nasal cavity’s olfactory epithelium

  • electrical signals reach olfactory bulb of brain along unmyelinated axons

    • axons are unmyelinated → axons are extremely short - not necessary to increase speed + do not need myelination

List and describe the functions of the two types of photoreceptors

o In addition, determine which category of sensory receptors (structural receptor type) is applicable to photoreceptors

o Differentiate between L, M, and S cones

§ State which cones are affected with respect to red-green color blindness and

blue-yellow color blindness

o Explain why these photoreceptor neurons are not myelinated

photoreceptors are specialized cells in the retina → detect light and convert it into electrical signals for the brain to process as vision

two main types of photoreceptors:

rods - function in low-light conditions (scotopic vision) + motion detection

• do not detect colour

• primary photoreceptor cells for peripheral vision → receptor cells are found in large numbers around edges of retina

• more numerous than cones

cones - function in bright light conditions (photopic vision)

• responsible for colour vision

• primary photoreceptors for central vision → densely packed in center of the retina

    three types of cones:

• L cones (long-wavelength)

  • most sensitive to red light

  • makes up 60% of total cone population

• M cones (medium-wavelength)

  • most sensitive to green light

  • makes up 30% of total cone population

• S cones (short-wavelength)

  • most sensitive to blue light

  • makes up 2-10% of total cone population

    genetic defects or damage to one or more types of cones → colour vision     deficiency

• red-green colour blindness

  • defect in L and M cones

  • genes are on the X chromosome (males are more affected, only has one X chromosome while females have 2)

• blue-yellow colour blindness

  • defect in S cones

  • gene is on chromosome 7 → both males and females are affected equally

→ rods + cones:

• not myelinated - allows light to reach them unobstructed

• are both specialized receptor cells

• use photopigment molecules (opsins) to absorb light → initiates chain reaction that enables sight

Explain why vision of mantis shrimp are better than human vision

• more colour receptors

  • more complex colour vision system → 12-16 photoreceptors (while humans only have 3)

• polarized light vision

  • can see different polarized light ( + circular polarization) → invisible to humans

  • helps detect prey (i.e. fish with shiny scales → camouflages to other animals)

• UV light perception

  • eyes are highly sensitive to ultraviolet light → excellent vision in the UV spectrum

• trinocular vision

  • each eye is divided into three sections

  • allows it to judge depth and distance

• independent eye movement

  • each eye can move + focus independently

  • allows the shrimp to scan a large field of view

• rapid processing in eye

  • processes visual information directly in their eyes (instead of brain)

  • allows for rapid colour recognition without the need to blend colours (like humans)