Taste and Smell

Question 1

What are the five modalities of taste?

Answer 1

• Sweet
• Sour
• Salty
• Bitter
• Umami

Question 2

Describe the taste receptors.

Answer 2

• Surface of tongue covered in papillae, few mm in diameter
• Each papillae has several hundred taste buds (50-150 taste receptor cells)
• Taste buds are small groups of cells arranged like orange slices around a hollow pore. Some are support cells and some act as receptors for various chemicals in the food we eat.
• At the bottom of the taste buds are basal cells
• Taste receptor cells form synapses with the endings of gustatory afferent axons near the bottom of the taste bud. These carry sensory information to the CNS.
• When a taste receptor cell is activated by an appropriate chemical, its membrane potential changes, either hyperpolarising or depolarising.

Question 3

In general how do taste cells give signals to the brain?

Answer 3

• Electrical changes in taste cells that prompt signals to the brain are based on varying concentrations of charged atoms or ions.
• Taste cells, like neurons, normally have a net negative charge internally and a net positive charge externally.
• Tastants alter this state of affairs using various means to increase concentration of positive ions inside taste cells.

Question 4

Describe signal transduction at taste receptors.

Answer 4

• If cell is sufficiently depolarised it causes voltage gated Ca2+ channels to open
• Calcium enters the cytoplasm causing release of transmitter
• Transmitter binds to receptors on gustatory afferents
• If present in sufficient amounts this will cause action potentials to be propagated along the gustatory afferents and the signal is taken to the CNS.

Question 5

Describe signal transduction at salt receptors.

Answer 5

• Na+ enters through channels in the receptor cell membrane, depolarising the cell.
• This causes the voltage-gated Ca2+ channels to open
• Ca2+ enters the cytoplasm causing release of the transmitter which binds to receptors on gustatory afferents.
• This causes depolarisation of the postsynaptic gustatory afferent, and the generation of an action potential which is propagated along the gustatory afferent to the CNS.

Question 6

Describe the signal transduction of sour receptors.

Answer 6

• The presence of H+ ions closes K+ channels, blocking K+ from leaving.
• This causes the membrane to depolarise
• Depolarisation opens voltage gated Ca2+ channels triggering transmitter release.

Question 7

Describe the signal transduction at sweet receptors

Answer 7

• Sweet receptors have integral membrane proteins that bind natural sugars and artificial sweetener molecules.
• This activates a G-protein-coupled second-messenger pathway. The alpha subunit dissociates and activates adenylate cyclase.
• Adenylate cyclase catalyses the conversion of ATP to cAMP.
• cAMP binds and activates Protein Kinase A.
• This blocks potassium channels causing depolarisation.
• This causes voltage gated calcium ion channels to open so calcium ions enter the cell, triggering transmitter release.

Question 8

Describe the signal transduction at bitter receptors. (same for Umami)

Answer 8

1. Bitter substance binds to bitterness receptor
2. This activates a G-protein coupled second messenger pathway. The alpha subunit dissociates and activates phospholipase C.
3. Activates PLC causes the hydrolysis of PIP2 into IP3 and DAG.
4. IP3 binds to receptors on the endoplasmic reticulum. This are ligand gated calcium channels which open when IP3 is bound.
5. Calcium diffuses down a concentration gradient into the cytosol, triggering transmitter release.

Question 9

Describe olfactory cells.

Answer 9

The olfactory receptor neurons lie in the olfactory epithelium in the upper part of the nasal cavity. They are specialised afferent neurons that have a single, enlarged dendrite that extends to the surface of the epithelium. Several long cilia extend from the tip of the dendrite and lie along the surface of the olfactory epithelium where they are bathed in mucus. The cilia contain the receptor proteins (binding sites) for olfactory stimuli.

Question 10

Describe signal transduction at the olfactory receptors.

Answer 10

1. An odorant dissolves in the mucus covering the epithelium and binds to specific odorant receptors on the cilia. These are coupled to a G-protein.
2. This binding activates adenylate cyclase resulting in the formation of cAMP.
3. cAMP binds to specific ion channels which allow an influx of NA+ and Ca2+.
4. The influx of Ca2+ causes calcium-gated chloride channels to open causing the efflux of Cl-.
5. This causes depolarisation of the receptor cell.
6. If depolarised to its threshold, the receptor cell sends an action potential along its axon.
7. The axons of the olfactory receptor cells synapse in the brain structures known as olfactory bulbs. Axons from olfactory receptor cells that share a common receptor specificity synapse together on certain olfactory bulb neurons, so that specific odorant receptor cells activate only certain olfactory bulb neurons allowing the brain to determine which receptors have been activated.
8. Information passes from the olfactory bulbs to the olfactory cortex in the brain.

Question 11

Describe the stages of swallowing.

Answer 11

Swallowing involves both voluntary and involuntary reflex activity.

1. VOLUNTARY Oral phase: food is moistened with saliva and chewed forming a food bolus. The tongue pushes this to the back of the pharynx. This is under neural control of several areas of cerebral cortex.
2. INVOLUNTARY Pharyngeal phase:

This phase is under autonomic control of the swallowing centre located in the lower pons and medulla oblongata of the brainstem.

To ensure food enters the oesophagus:

     - The tongue blocks the oral cavity to prevent food going back into the mouth
     - The soft palate elevates and lodges against the back wall of the pharynx, preventing food from       entering the nasal cavity. 
     - The epiglottis covers the glottis to prevent food entering the trachea.
     - Respiration is inhibited

3. INVOLUNTARY Oesophageal phase:

Skeletal muscles surround the upper third of the oesophagus and smooth muscles the lower two-thirds forming the lower oesophageal sphincter.

• The upper oesophageal sphincter relaxes so that it opens. As soon as the food has passes, it closes, the glottis opens and breathing resumes.
• In the oesophagus, the food moves towards the stomach by a progressive wave of muscle contractions that compress the lumen and force food ahead. These waves of contraction are known as peristalsis.

Both skeletal and smooth muscles are involved in swallowing so the swallowing centre must direct efferent activity in both somatic nerves (to skeletal muscle) and autonomic nerves (to smooth muscle). Simultaneously afferent fibres from receptors in the oesophageal wall send information to the swallowing centre which can alter the efferent activity e.g. secondary peristalsis if a large food particle does not reach the stomach during the initial peristaltic wave.

Question 12

What is dysphagia and why does it occur?

Answer 12

Dysphagia is difficulty with swallowing. Swallowing involves the function of a number of cranial nerves. If these nerves malfunction for any reason (e.g. stroke) the condition can occur.