BISC 205 - Sensory System

Sensory system physiology

The study of how sensory systems receive, transduce, and process stimuli from environment to create conscious perception

involves sensory receptors converting physical or chemical stimuli into electrical signals, which are then transmitted through nerves to the CNS for interpretation

what are the five traditional senses?

• olfaction (smell)

• gustation (taste)

• somatosensation (touch)

• audition (hearing)

• vision (sight)

what is the three-step process in which sensory systems process stimuli?

1. reception - sensory receptors in our sense organs detect a physical stimulus

2. transduction - the sensory receptors convert the detected physical energy into an electrochemical signal

3. transmission - the newly created neural impulses travel along nerve fibers to the brain, where the information is send to specific areas for processing

What are the two main structural types of sensory receptors?

• encapsulated - sensory receptors whose nerve endings are surrounded by a connective tissue capsule

• unencapsulated - sensory receptors whose nerve endings are not surrounded by a connective tissue capsule

What are the different types of stimuli / receptors?

• mechanoreceptors

• nocireceptors

• thermoreceptors

• chemoreceptors

• photoreceptors

What are mechanoreceptors? What are the three categories?

responsible for detecting mechanical stimuli such as touch, pressure, and vibration

skin (cutaneous) receptors, propioreceptors, baroreceptors

What are the 4 types of skin receptors?

Merkel discs

Ruffini endings

Meissner corpuscle

Pacinian corpuscle

What are merkel discs?

unencapsulated endings

located in the epidermis

detects light touch/pressure, object’s shape and texture

What are ruffini endings?

encapsulated endings

located in the deep dermis

detects sustained pressure, stretch, slipping, or sliding of objects across the skin surface

What are meissner corpuscles?

encapsulated endings

located in the upper dermis

detects low-frequency vibrations (tapping or fluttering), fine touch, and fine dynamic skin movement

What are pacinian corpuscles?

encapsulated endings

located in the deep dermis

detects high-frequency vibrations (buzzing or humming), and deep pressure

What are propioreceptors?

• located in muscles, tendons, ligaments, and joints

• monitor the body’s position and movement

• encapsulated nerve endings

• muscle spindles: located within skeletal muscles and detect changes in muscle length and rate of stretch

• golgi tendon organs: located at the junction of muscle and its tendon, detects tension and force of muscle contraction

What are baroreceptors?

• monitor blood pressure by sensing tension in blood vessels

• unencapsulated nerve endings

• carotid sinus baroreceptors: found in carotid arteries, extremely sensitive to blood pressure changes

• aortic arch baroreceptors: found in arch of aorta, less sensitive than carotid sinus baroreceptors

What are nocireceptors?

• pain receptors

• respond to extreme heat, cold, mechanical injury, or chemical irritants

• found in skin, muscles, joints and internal organs

• unencapsulated

• two types: fast sharp , and slow aching

What are fast sharp nocireceptors?

immediate and localized pain is mediated by A-delta fibers, which are thin, myelinated fibers that rapidly transmit signals to the brain enabling a quick response to acute injury

What are slow aching nocireceptors?

Prolonged, diffuse pain is transmitted by C fibers, which are unmyelinated and conduct signals more slowly, providing a lingering awareness or injury and promoting protective behaviour to aid healing

What are thermoreceptors?

• respond to changes in temperature

• mostly unencapsulated, few encapsulated

• two main types: cold and hot

• when temperatures exceed normal thresholds, nocireceptors may be activated, leading to sensations of pain, such as the burning sensation from extreme heat or the stinging pain from extreme cold

Describe cold thermoreceptors.

• activated between 10^oC and 35^oC

• example: Krause end bulb (encapsulated)

• nerve fibers that transmit signals for cold = A delta fibers (myelinated)

Describe hot thermoreceptors.

• activated between 30^oC and 45^oC

• example: ruffini endings (encapsulated)

• nerve fibers that transmit signals for warmth = C fibers (unmyelinated)

What are chemoreceptors?

• detect chemical stimuli, plays a vital role in maintaining homeostasis

• two main types: central and peripheral

Describe central chemoreceptors.

• located in the brainstem

• unencapsulated

• detects changes in pH in cerebrospinal fluid, an indirect measure of blood CO2 levels

• excess H+ leads to low pH levels, which stimulates these receptors to help increase rate and depth of breathing (which helps eliminate excess CO2)

Describe peripheral chemoreceptors.

• located in carotid and aortic bodies

• encapsulated

• detects low O2 levels directly and high CO2 levels indirectly, leading to increased rate and depth of breathing

other chemoreceptors include:

osmoreceptors

gustatory receptors

olfactory receptors

What are osmoreceptors?

• located in hypothalamus

• unencapsulated

• sense changes in blood osmolarity to help regulate thirst and water balance

• activation of these receptors lead to two main response:

  • prompting you to drink water

  • stimulating of posterior pituitary gland to release ADH - targets kidney to reabsorb more water

What are gustatory receptors?

• not neurons, they are modified neuroepithelial cells

• unencapsulated afferent nerve endings

• these cells are clustered on taste buds (papillae)

What are olfactory receptors?

• unencapsulated

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

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

• electrical signals ultimately reach olfactory bulb of the brain along unmyelinated axons

What are photoreceptors?

• specialized cells in the retina that detect light and covert it into electrical signals for the brain to process as vision

• two main types: rods and cones

• both are specialized receptor cells

• rods and cones are unmyelinated - key feature that allows light to reach them unobstructed

• rods and cones use photopigment molecules (opsins) to absorb light, which initiates a chain reaction that enables sight

Describe rods.

• function in low-light conditions (scotopic vision), and motion detection, they do not detect colour

• roads are the primary photoreceptor cells for peripheral visual (found in large numbers around edges of retina)

• more numerous than cones

Describe cones

• function in bright light conditions (photopic vision); responsible for colour vision

• cones are the primary photoreceptor cells for central vision (densely packed in center of retina)

What are the 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

Red-green colour blindness

result from defect in L and M cones, whose genes are on the X chromosome (men are more affected)

Blue-yellow colour blindness

result from defect in S cones, whose gene is on chromosome 7 (men and women are affected equally)