5.1.3(a) - Sensory receptors as transducers
Sensory receptors are the input side of a mammalian nervous response. They detect particular forms of stimulus, convert that stimulus into an electrical signal, and allow the nervous system to receive information about changes inside or outside the body. In this lesson, the Pacinian corpuscle is the main example of how a receptor acts as a transducer.
Stimulus Specificity
A stimulus is a detectable change in the internal or external environment. It may be a change in pressure, temperature, light intensity, chemical concentration, or another condition that a mammal can detect.
Sensory Receptor
A sensory receptor is a specialised cell or nerve ending that detects a specific type of stimulus and initiates a nerve impulse when sufficiently stimulated.
Sensory receptors are not general-purpose detectors. Each receptor is most sensitive to a particular type of stimulus. This is called specificity. A pressure receptor is not equally good at detecting light; a photoreceptor is not equally good at detecting pressure.
| Stimulus type | Receptor type example | What is detected |
|---|---|---|
| Mechanical stimulus | Mechanoreceptor | Pressure, stretch or vibration |
| Temperature change | Thermoreceptor | Warming or cooling |
| Chemical stimulus | Chemoreceptor | A chemical concentration or chemical molecule |
| Light | Photoreceptor | Light energy |
The role of the receptor is to begin the conversion from stimulus information into a form the nervous system can use. The brain does not receive "pressure" or "temperature" directly. It receives patterns of nerve impulses that began at sensory receptors.
Transducer
A transducer converts one form of energy or stimulus information into another form. In sensory receptors, the output is an electrical signal in a neurone.
So a good explanation of a sensory receptor needs both ideas: what kind of stimulus it is specialised to detect, and how that stimulus is converted into nerve impulses.
Generator Potentials
The first electrical change produced by a sensory receptor is a generator potential. It is a local change in potential difference across the membrane of the receptor or sensory neurone ending. It is called "graded" because its size depends on the strength of the stimulus.
A weak stimulus may produce a small generator potential. If that generator potential does not reach the threshold, no nerve impulse is produced. A stronger stimulus can produce a larger generator potential. If threshold is reached, the sensory neurone produces nerve impulses.
Threshold
Threshold potential is the membrane potential at which depolarisation triggers an action potential in the sensory neurone.
Once a nerve impulse is produced, it is not made larger by a stronger stimulus. A nerve impulse is an all-or-nothing electrical event. Stronger stimuli are usually represented by a higher frequency of nerve impulses, not by larger individual impulses.
Interpreting stimulus strength
A pressure receptor is stimulated three times.
| Pressure stimulus | Generator potential | Nerve impulses |
|---|---|---|
| Very weak | Below threshold | None |
| Moderate | Reaches threshold | Low frequency |
| Strong | Well above threshold | Higher frequency |
The correct interpretation is that pressure strength affects the size of the generator potential and the frequency of impulses. It does not make each individual nerve impulse larger.
This is a common exam trap. If a question compares weak and strong stimulation, look for wording about generator potentials, threshold and impulse frequency.
Pacinian Corpuscle
The Pacinian corpuscle is a mammalian sensory receptor that responds to mechanical pressure and vibration. It is a type of mechanoreceptor.
Its structure suits its role. The sensory neurone ending is surrounded by many layers of connective tissue called lamellae. These layers form a capsule around the neurone ending. When pressure is applied, the capsule is deformed and the membrane of the sensory neurone ending is stretched.
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