M1(1) The Sensory System (Taste)

Sensory System

the part of the nervous system that detects internal and external stimuli through sensory receptors

• converts them into neural signals

• processes this information so the body can perceive and respond to its environment.

Sensory System Steps

1. Stimulus (e.g. flower scent)

2. Stimulation of a sensory receptor

3. Generation of nerve signals

4. Travel of signals

5. Interpretation of the signals in the brain

6. Perceiving of the stimulus (e.g. flower scent)

General Senses

Sensory systems that detect basic physical stimuli from many parts of the body rather than being limited to one specialized organ

• Touch

• Pain

• Temperature

• Proprioception

• Crude pressure

Two Ways to Classify Receptors

Receptors may be classified based on

1. Type of stimulus

2. Function / location

Type of Stimulus

Includes:

• Mechanoreceptors

• Thermoreceptors

• Nociceptors

• Chemoreceptors

Mechanoreceptors

Respond to changes in pressure or body movement

• e.g. pressure receptors in skin

Theremoreceptors

Respond to changes in temperature

• e.g. those in skin and internal organs

Nociceptors

Responds to damage (pain) or oxygen deprivation to tissues

• e.g. those in skin

Chemoreceptors

Responds to changes in the chemical concentration of substances

• e.g. taste buds, olfactory receptors

Photoreceptors

Responds to changes in light energy

• Located only in eye

Location and Function

Includes

• Proprioceptors

• Cutaneous receptors

• Pain receptors

Proprioceptors

Mechanoreceptors involved in reflex actions that maintain muscle tone

• helps us know the position of our limbs in space

• e.g. those that maintain body’s equilibrium and posture

Proprioceptor Types

Two forms:

• Muscle spindles

• Golgi tendon organs

Muscle Spindles

A type of proprioceptor that detects change in muscle length and speed of stretching

• “Over-stretching of muscle”

• Parallel to muscle fibers (inside muscle belly)

Knee-Jerk Reflex

1. Tapping of patellar tendon stretches quadriceps muscle

2. Muscle spindle detects stretching of muscle

3. Generation of action potential in associated sensory nerve (afferent)

4. In grey matter of spinal cord, sensory neuron synapses with motor neuron (efferent)

5. Motor signal to muscle

6. Muscle contraction

Golgi Tendon Organ

A type of proprioceptor that detects changes in muscle tension/force

• “Over-contraction of muscle”

• Located in tendon

Cutaneous Receptors

Sensory receptors located in the skin that detect touch / pressure

• Fine Touch Cutaneous Receptors

• Pressure Cutaneous Receptors

Fine Touch Cutaneous Receptors

• Meissner Corpuscles

• Merkel Disks

• Root Hair Plexus

Meissner Corpuscles

Cutaneous receptor sensitive to fine touch

• concentrated in the dermal papillary layer of hairless skin

Merkel Disks

Cutaneous receptor sensitive to fine touch

• found in the deepest epidermal layer

Root Hair Plexus

Cutaneous receptor sensitive to fine touch

• winds around the base of a hair follicle

Pressure Cutaneous Receptors

• Pacinian Corpuscles

• Ruffini Endings

• Krause End Bulbs

Pacinian Corpuscles

Cutaneous receptor sensitive to pressure

• lie deep inside dermis

Ruffini Endings

Cutaneous receptor sensitive to pressure

• found in the dermis and hypodermis

Krause End Bulbs

Cutaneous receptor sensitive to pressure

• located in the superficial layers of the dermis

Pain Receptors

Also known as nociceptors

• Located in skin and many internal organs

Pain Receptor Types

There are two types:

• Somatic Nociceptors

• Visceral Nociceptors

Somatic Nociceptors

Nociceptors sensitive to mechanical, thermal, electrical, or chemical damage

• e.g. those in the skin and skeletal muscles

Visceral Nociceptors

Nociceptors sensitive to excessive stretching of internal organs, oxygen deprivation, or chemicals released by damaged tissues’

• e.g. the pain sensation when stomach is too full

• e.g. crushing pain of a heart attack when blood supply to heart is reduced

Referred Pain

Pain that is felt in a different area of the body from where it actually originates

• Happens because some somatic nociceptors converge along the same nervous pathway

• Brain cannot distinguish the two

Referred Pain Example

Pain from the heart that occurs during a heart attack is often accompanied by referred pain the the left shoulder and arm

Receptor Potential

System by which sensory receptors start signal transmission

A small, local change in the electrical charge of a sensory receptor’s membrane in response to a stimulus

• If strong enough, can trigger an action potential in the sensory neuron

Receptor Potential Mechanism

Begins with a stimulus (e.g. light for receptors in the eye)

• Can be weak or strong (unlike action potentials that act on the all-or-nothing principle)

• Can add together

• Part of neurons or synapse with neurons that can create action potentials

Taste

• What: gustatory epithelial cells

• Stimulus: molecules of the food we eat

• Where: taste buds

Taste Buds

The overall sensory unit for taste

• Contains gustatory epithelial cells and supporting cells (e.g. basal epithelial cells)

Gustatory Epithelial Cell

The receptor cell for taste

Gustatory Epithelial Cell Structure

Gustatory hairs (microvilli) extend through taste pores into oral cavity

• bathed by saliva

Dendrites are coiled around the cell

• Pathway to the brain

Gustatory Epithelial Cell Types

• Type I Gustatory Epithelial Cell

• Type II Gustatory Epithelial Cell

• Type III Gustatory Epithelial Cell

Type I Gustatory Epithelial Cell

• Function mainly as supporting cells.

• Help regulate the taste bud environment.

• Do not form traditional synapses and do not release classical neurotransmitters.

Type II Gustatory Epithelial Cell

• Detect sweet, bitter, and umami tastes.

• Lack synaptic vesicles and do not form conventional synapses.

• Release ATP as a neurotransmitter through ion channels to activate sensory neurons.

Type III Gustatory Epithelial Cell

• Detect primarily sour (and some salty) tastes.

• Form traditional synapses with sensory nerve fibers.

• Release the neurotransmitter serotonin

Basal Epithelial Cell

Act as stem cells, dividing and differentiating into new gustatory epithelial cells

• Essential because taste receptors have a short lifespan (7-10 days)

Papillae

Small, raised projections on the surface of the tongue

• Ripping and moving food during chewing

Fungiform Papillae

• Mushroom-shaped.

• Scattered over the entire surface of the tongue

• Each papilla contains 1–5 taste buds.

Vallate Papill

• Largest and least numerous papillae.

• Contain many taste buds.

• Typically 8–12 papillae arranged in an inverted V at the back of the tongue.

Foliate Papillae

• Located on the lateral (side) edges of the tongue.

• Contain many taste buds in childhood.

• Decrease in number with age.

Filiform Papillae

• Do not contain taste buds

• Mainly help grip and move food

• Covers dorsal side of the tongue

Gustatory Pathway Afferent Fibers

Chorda tympani (branch of Facial Nerve, VII)

• Transmits impulses from taste receptors in the anterior 2/3rds of the tongue

Lingual branch (branch of Glossopharyngeal Nerve, IX)

• Transmits impulses from taste receptors in the posterior 1/3rd of the tongue

Vagus nerve (X)

• Transmits impulses from the few taste receptors in the epiglottis and lower pharynx

Gustatory Pathway Synapse

These afferent fibers synapse in the solitary nucleus of the medulla

• Impulses stream to the thalamus and proceed to the gustatory cortex in the insula

Basic Taste Sensations

1. Sweet

2. Sour

3. Salty

4. Bitter

5. Umami

Sweet

Detected from many organic substances, including sugars, saccharin, alcohols, some amino acids.

• Can also be triggered by certain lead salts (like in lead paint — toxic!).

Sour

Caused by acids, specifically the hydrogen ions (H⁺) in solution.

Salty

Caused by metal ions, mainly sodium ions.

• Table salt (NaCl) is the most common salty taste.

Bitter

Detected from alkaloids such as quinine, caffeine, nicotine, morphine, and strychnine.

• Also triggered by some non-alkaloid substances, e.g., aspirin.

Umami

Caused by amino acids like glutamate and aspartate.

• Responsible for:

  • “Beef taste” in steak

  • Tangy flavor of aged cheese

  • Flavor of monosodium glutamate (MSG)

Perception of Taste

The brain does not rely on just one taste receptor.

• It surveys the overall pattern of sensory input from all taste buds.

• The perceived taste is like a “weighted average” of the signals from sweet, sour, salty, bitter, and umami receptors.

Possible Sixth Taste

Evidence suggests humans can detect long-chain fatty acids with taste receptors.

• This may explain our preference for fatty foods.