5.0 Nervous System 3: Sensory

Question 1

Classify sensory receptors by structure and function

Answer 1

The five major groups of receptors are based on the type of stimulus to which they are most sensitive

Chemoreceptor:
receptor= free nerve endings
stimuli= organic molecules such as glucose for taste, fragrants for smell; oxygen, pH

Mechanoreceptor:
receptor= Merkel receptors, Ruffini corpuscles, Pacinian corpuscles, Meissner’s corpuscles muscle spindle
stimuli= pressure, vibration, gravity, sound

Photoreceptor:
receptor= rod and cone, photosensitive retinal ganglion cells (specialised receptor)
stimuli= photons of light

Thermoreceptor:
receptor= free nerve endings
stimuli= heat, cold

Nociceptor:
receptor= free nerve endings
stimuli= pain

Question 2

Explain what is meant by sensory field and describe the different types

Answer 2

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Question 3

Describe the somatosensory modalities

Answer 3

Each receptor is most sensitive to a particular type of stimulus. The modality of a stimulus is indicated by

which sensory neuron is activated
where the pathway of the activated neuron terminates in the brain

Question 4

Describe the sequence of events that leads from a stimulus to its perception

Answer 4

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Question 5

Outline how the CNS distinguishes the type of stimulus as determined by four properties

Answer 5

All sensory stimuli are converted to identical action potentials. The CNS distinguishes the type of stimulus as determined by four properties

its nature, or modality
its location
its intensity
its duration

Question 6

Distinguish between phasic and tonic receptors

Answer 6

Receptors fall into one of two classes of adaptation:

Tonic receptors; are slowly adapting receptors that fire rapidly when first activated, then slow and maintain their firing as long as the stimulus is present. Examples are receptors that continuously monitor the body e.g. pressure-sensitive baroreceptors and proprioceptors, and pain receptors (nociceptors).

Phasic receptors; are rapidly adapting receptors that fire when they first receive stimulus but cease to fire if the strength of the stimulus remains constant. Thus they are sensitive to the changes in a stimulus, e.g. chemoreceptors involved in sense of smell (e.g. when applying perfume, one can smell the fragrance initially but will adapt to the smell).

Question 7

Explain the concept of receptive field for various sensory systems

Answer 7

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Question 8

Explain how action potentials are generated in sensory neurons in response to stimuli

Answer 8

Each receptor type has an adequate stimulus, a particular form of energy to which it is most responsive. This allows the brain to perceive the stimulus accurately under normal conditions.

Regardless of the type of stimulus, when the stimulus has reached threshold for the receptor it opens and closes ion channels in the receptor membrane. In most cases this involves opening of a Na+ channel allowing a net influx of Na+ into the receptor, depolarising the membrane

This change in sensory receptor membrane potential is called a receptor potential