Lecture 21

Purdue BIO 203 Fall 2024

Sensory receptor

Any structure specialized to detect a stimulus

Sense organ

A structure that combines nervous tissues with other tissues that enhance its response to a certain type of stimulus

Exteroceptors

Sense stimuli external to the body

Interoceptors (visceroceptors)

Sense stimuli in the internal organs

Receptor potential

A type of local potential

Sensory adaptation

If the stimulus is prolonged then there is a decrease in neuron firing frequency, and so the body becomes less aware of the stimulus

Photoreceptors

Specialized cells in the retina of the eye that respond to light (rods and cones)

Mechanoreceptors

Respond to mechanical forces like pressure, touch, stretch and vibration

Thermoreceptors

Detect changes in temperature

Chemoreceptors

Detect chemical substances, essential for our sense of taste and smell

Nociceptors

Detect potentially harmful stimulants

Tactile receptors

Responsible for sensations of touch, pressure and vibration; located on the skin and other tissues

Proprioceptors

Provide information about the position and movement of our body parts. Located in muscles, tendons and joints

Baroreceptors

Monitor changes in blood pressure within blood vessels, as well as pressure change sin organs like the lungs, bladder and digestive track

Muscle spindles

Monitor muscle length and rate of stretch, ensuring smooth and coordinated muscle contractions

Golgi tendon organs

Located in the tendons; detect muscle tensions, helps regulate muscle force and prevents injury

Joint response

Found in joint capsules; provides information about joint position and movement; contributes to our sense of body awareness and balance

Special senses

• Vision, smell, hearing, taste and balance

• confined to the head

• sensory receptor on separate sensory cell

• possess dedicated organs or tissues for sensory detection

General senses

• receptors are widely dispersed throughout the body

• Unipolar sensory neuron

• thermoception, nociception, proprioception, tactile sensation

Unencapsulated Nerve Endings

• Simple, bare nerve endings without specialized connective tissue capsule

• widely distributed throughout the body

• Ex: free nerve endings, tactile discs, hair receptors

Encapsulated Nerve Endings

Characterized by nerve fiber endings encased in layers of glial cells or connective tissue

Encapsulation purposes

• protection of the nerve endings from mechanical damage

• modifications of the stimulus before it reaches the nerve ending

• enhancement of the receptors sensitivity to specific types of stimuli

Free nerve endings

• unspecialized

• Senses pain, heat, cold, crude touch

Merkel Disc (tactile disc)

• light touch

• plays a role in fine tactile discrimination

• Allows is to perceive texture and shape

• Sense steady pressure and texture, compression of the skin releases serotonin

lamellar corpuscle (pacinian)

• Found in skin, joints, internal organs

• high frequency vibration

• deep pressure

• rapid adapting receptors

Bulbous corpuscle (Ruffini)

• heavy sustained touch/pressure

• stretching of skin

• plays a role in proprioception and kinesthesia

Hair receptor

• highly sensitive to light touch

• encircles hair follicles

• senses hair movement

End bulb (Krause)

• located in mucous membranes

• temperature and touch

Tactile corpuscle (meissner’s)

• responds to flutter and stroking movements

• low frequency vibrations

• abundant in areas with high tactile sensitivity

• found in the fingertips and lips

Pain

Uncomfortable conscious perception of tissue injury or noxious stimulation

Nociceptive pain

• Stems from tissue injury (cuts, burns, chemical irritation) - tissue inflammation

• divided into somatic and visceral pain

• Pain that arises from actual or threatened damage to non-neural tissue and is due to the activation of nociceptors

Visceral pain

• Originates form internal organs within the thoracic, pelvic or abdominal regions

• vague and poorly localized sensations

• obstruction of capsular distention

• mucosal injury

Somatic pain

• Can be deep: bones, joint, muscles (skeletal system)

• Can be superficial: skin

Neuropathic pain

• pain is caused by a lesion or disease of the somatosensory nervous system

• burning, tingling, or “electrical” sensations

• peripheral neuropathy, stoke, multiple sclerosis, spinal cord injury

Nociplastic pain

• Chronic pain not caused by tissue or nerve damage

• Widespread pain

• fibromyalgia or IBS

Pro-inflammatory mediators: Bradykinin

A potent pain-inducing substance that sensitizes nociceptors

Pro-inflammatory mediators: Histamine

Increase in vascular permeability, contributing to inflammation and pain

Pro-inflammatory mediators: Serotonin

Enhances nociceptor sensitivity and promotes inflammation

Pro-inflammatory mediators: Prostaglandins

Sensitizes nociceptors to other pain-inducing substances

Pro-inflammatory mediators: Protons

Directly activates specific nociceptors, particularly those involved in muscle pain associated with lactic acid buildup

Pro-inflammatory mediators: Substance P and CGRP

Neuropeptides that play a role in the inflammatory response and the transmission of pain signals. Also induce vasodilation and the release of other inflammatory mediates

CGRP

Calcitonin Gene-Related Peptide

Axon reflex

One way the body responds to tissue damage and intimates the healing process

Endogenous molecules

• amplify or dampen pain

• contribute to the transmission of pain and make the nociceptors more sensitive

Papillae

• tiny projections on the surface of the tongue

• inside are embedded taste buds

Filiform papillae

Most numerous, don’t have taste buds, cone-shaped, keratinized projections that detect touch, temperature and pain

Tastant: Salty

Triggered by metal ions like sodium and potassium

Tastant: sour

Associated with acidic substances like lemons and vinegar

Tastant: sweet

linked to sugars and carbohydrates

Tastant: bitter

Associated with potentially harmful substances like spoiled foods and alkaloids

Tastant: Umami

“meaty” taste and induced by amino acids such as aspartic and glutamic

Gustatory receptor cell

• primarily chemosensory units within a taste bud

• possess microvilli (taste hairs) which project into the taste pore, facilitating direct interactions with tastants

• not true neurons

• neuroepithelial cells that renew every 7-10 days

Basal cells

• progenitor cells, exhibits stem-cell like properties, possess the capacity for both proliferation and differentiation into mature taste cells

• essential for maintaining the taste buds cellular composition and sensory function over time

Supporting cells (sustentacular cells)

• provide physical stability to the tase bud structure

• involved in maintaining the proper ionic environment within the taste bud

• provide metabolic support to taste receptor cells

Autonomic reflexes

Salivation, gagging, vomiting

Taste path: Taste buds

• First step

• upon tastant stimulation, the gustatory receptor cells release neurotransmitters, which activate adjacent sensory neurons

Taste path: cranial nerves

• second step

• Vagus, glossopharyngeal and facial nerves

Vagus nerve

Innervates taste buds in the pharynx, palate and epiglottis

Glossopharyngeal nerve

Innervates the posterior 1/3 of the tongue

Facial nerve

Innervates the anterior 2/3 of the tongue

Taste path

• third step

• cranial nerves convey gustatory information to the nucleus of the solitaire tract (NTS) in the medulla oblongata

Taste path: brain regions

• step four

• taste signals are sent to various brain regions: amygdala, hypothalamus and thalamus

Amygdala

Emotional processing and taste-related memory formation

Hypothalamus

Mediates autonomic reflexes

Thalamus

Relay center for projections to cortical regions

Taste path: cortex

• step 5

• primary gustatory cortex and orbitofrontal cortex

Primary gustatory cortex

Initial taste processing

Orbitofrontal cortex

Combines taste signals with inputs of smell and taste

Piriform cortex

Basic processing of odor information

Entorhinal cortex

• Processes olfactory information

• Contributes to odor discrimination and coding

• Serves as a link between primary olfactory areas and memory-related structures

Hippocampus

Memory formation, allows us to associate specific odors with path experiences

Secondary olfactory cortex

• Identifies and discriminates between different odors

• Also integrates olfactory information with inputs from taste and vision

Hypothalamus and Brainstem

Receives olfactory information and triggers autonomic responses that are associated with different scents

Orbitofrontal cortex

• Where we identify and discriminate among odors

• Integrates odor, taste and vision

Feedback loop benefits

• odor discrimination

• reduced background noise

• olfactory adaptation

• context sensitive perception

Granule cells

Can inhibit the mitral and tufted cells

Olfactory discrimination

Odors can change quality and significance under different conditions

Where is the gustatory cortex located?

Insula of the brain

Where is the olfactory mucosa or epithelium located and what does it contain?

• located in the roof on nasal cavity

• contains basal cells, supporting cells and olfactory receptor cells

Where are receptor proteins that detect odorants located?

Dendrites of olfactory receptor cells

The icons of the olfactory receptor cells bundle to form which cranial nerve?

Olfactory nerve (CN 1)

The olfactory nerve synapses with which neurons at the glomerulus?

Tufted and mitral cells in the olfactory bulb

What is a glomerulus, and where is it located?

Site in the olfactory bulb where icons of olfactory receptor cells containing receptors for the same odors synapse with tufted and mitral cells

The icons of which neurons form the olfactory tract?

Tufted and mitral cells

Where are the granule cells located and what is their function?

• Located in the olfactory bulb

• Release inhibitory neurotransmitter GABA-they inhibit mitral and tufted cells to relay olfactory information and enhance scent discrimination