Lecture 22: Sensory

Question 1

Types of sensory receptors

Answer 1

Mechanoreceptors-pressure, vibration, cell stretch, balance, sound, muscle length and tension, joint position and movement
Chemoreceptors: chemicals bind to specific receptors on cell membrane, eg CO2, pH, various organic and inorganic molecules. Thermoreceptors operate in similar way except receptor on membrane responds to temperature.

Question 2

Sensory transduction:

Answer 2

The sensory receptor here will be specialised nerve endings

1. Stimulus opens ion channels, depolarising membrane, producing receptor (generator) potential
2. Receptor (generator) potentials are graded potentials. If afferent nerve is sufficiently depolarised, action potentials generated, propagating to CNS
3. Here receptor cell separate from afferent nerve
4. Stimulus changes membrane potential of receptor, opening or closing Ca2+ channels, increasing or decreasing Ca2+ concentration inside cell
5. Triggers or inhibits release of chemical transmitter
6. Transmitter signals receptor on afferent nerve

Question 3

Describe the 3 major sensory systems present in vertebrates

Answer 3

1. Somatosensory system: (senses external environment)
   - mechanoreceptors that detect touch
   - mechanoreceptors in muscles, tendons and joints detect position
   - receptors in skin that detect temperature and noxious stimuli
2. Visceral sensory system (senses internal environment)
   - mechanoreceptors detect eg blood pressure
   - chemoreceptors detect pH, O2
3. Special sensory systems (senses external environment)
   - photoreceptors (vision)
   - mechanoreceptors (hearing, balance, and equilibrium)
   - chemoreceptors (taste and smell)

Question 4

Explain what is meant by ‘modality’ and ‘adequate stimulus’
Intensity
Duration of stimulus

Answer 4

When stimulus activates sensory receptor, nervous system is able to identify:
Modality: specific form of energy that a sensory receptor responds to eg eye has photoreceptors that normally detect light
Adequate stimulus: modality to which a receptor best responds to
Location:
Intensity: amplitude of receptor potential, AP rate in afferent neuron and recruitment

Question 5

Explain how the type, location, intensity, and duration of a stimulus are identified or coded by the nervous system.
When stimulus activates sensory receptor, nervous system able to identify:
Modality (receptor type and signal pathway)
Location (receptive fields)
Intensity (amplitude of receptor potential, AP rate in afferent neuron and recruitment)
Duration of stimulus (slow vs very fast)
Explain the above

Answer 5

1.Modality: receptor type
-each type of sensory receptor responds only to a specific form of energy, or modality eg eye has photoreceptors- detect only light
-the modality to which receptors responds best is called adequate stimulus
-pathway for each modality terminates in specific area of the brain. If in cerebral cortex we are consciously aware of it
2. Stimulus location: receptive fields
Identified by:
-location of receptive fields stimulated
-neural pathways activated
In skin, stimulus localisation is enhanced by:
-Smaller receptive fields
-greater overlap of receptive fields of different afferent nerves
3. Intensity:
-more intense stimuli produce receptor potentials of greater amplitude
-AP rate and burst duration proportional to stimulus intensity and duration refer to slide 21
-stimuli of increasing intensity activate or recruit greater numbers of receptors
-recruitment may be within single sensory unit, or by stimulus of additional units
4. Stimulus duration: tonic receptors
-most receptor adapt to a stimulus
-with constant stimulus intensity, decrease in:
-magnitude of receptor potential
-AP rate in afferent neuron
-tonic receptors adapt slowly
-suited to signalling prolonged stimuli
Eg tension receptors in tendons, and stretch receptors in skin
-phasic receptors adapt rapidly. Suited to detecting dynamic qualities of mechanical stimuli. Eg pacinian corpuscles in skin

Question 6

What are the sensory receptors on the body surface?

Answer 6

Mechanoreceptors: detect various forms of chemical energy, including pressure, vibration, touch, stretch
Thermoreceptors: detect temperature
Nociceptors: detect tissue-damaging (noxious) stimuli
Signals from these receptors are conveyed by the thalamus and relayed to the cerebral cortex, were the particular sensation is perceived.

Question 7

Refer to table on slide 27 and 28 and learn it

Answer 7

Ya

Question 8

Temperature receptors:

Answer 8

• Free nerve endings, mainly in skin, lining of oral cavity and surface of tongue
• Activation of receptor opens in ion channels in cell membrane
• Allows ions to enter cell, cool, warm and hot
• cold receptors are more superficial and in greater numbers that hot receptors

Question 9

Thermoreceptors in snakes

Answer 9

• some snakes have highly sensitive thermoreceptors

- enables them to locate prey in total darkness

Question 10

Nociceptors on body surface

Answer 10

• Free nerve ending receptors that respond to tissue damaging stimuli
• in brain, signals received as pain
• activated by high intensity mechanical and thermal stimuli
• most also respond to chemical stimuli, including inflammatory chemicals released from damaged tissue
• subtypes have differing sensitivities and response thresholds to each of the 3 modalities

Question 11

Proprioceptors in muscles, tendons and joints

Answer 11

-mechanoreceptors in muscles, tendons and joints
-detect changes in muscle length, changes in muscle tension and position of joints
-signals from these receptors reach cerebral cortex region of brain, were conscious perception occurs
-signals also travel to:
A) spinal cord for spinal reflexes
B) unconscious area of the brain (cerebellum)

Question 12

Explain the spinal reflex: knee jerk

In steps

Answer 12

1. Hammer tap stretches tendon, which stretches sensory receptor in leg extensor muscle
2. Sensory neuron synapses with and excites motor neuron in the spinal cord
3. Sensory neuron also excites spinal interneuron
4. Interneuron synapse inhibits motor neuron to flexor muscle
5. Motor neuron conducts action potential to synapse on extensor muscle fibres, causing contraction
6. Flexor muscles relaxes because the activity of its motor neurons has been inhibited
7. Leg extends