Sensations that derive from mechanoreceptors include touch, tickle, pressure, joint position, muscle load, hearing, and equilibrium. The membranes of mechanoreceptors have ion channels that are opened by distortion of the membrane, resulting in graded receptor potentials that cause the cell to release neurotransmitter or to fire action potentials. Hair cells involved in hearing and equilibrium have stereocilia that are bent by pressure waves, resulting in receptor potentials.

Question 1

Different mechanosensor cell types enable responses to different aspects of touch such as sharpness, texture, pressure, vibration, and itch. Different rates of adaptation of touch receptors make it possible to discriminate between stimuli that are relevant and those that aren’t. Slowly adapting mechanosensors such as those in postural muscles provide continuous information. Rapidly adapting mechanosensors provide information about changing conditions and also improve spatial and temporal sensory ability.

Question 2

The activity of a muscle spindle stretch receptor increases the activity in the motor neuron to that muscle. The functional significance of this property of muscle spindle stretch receptors is that it enables continuous adjustments to changes in load, such as when you are holding a glass that is being filled. The activity in the Golgi tendon organ inhibits the activity in the muscle creating the stretch of that Golgi tendon organ. The functional significance is the prevention of damage to the muscle and tendons through the generation of too much tension, such as when you are attempting to pick up objects that are too heavy.

Question 3

Different frequencies of sound pressure waves cause different frequencies of flexion of the tympanic membrane, and these movements are transmitted and amplified by the ossicles of the middle ear into vibrations of the oval window membrane of the fluid-filled inner ear. Those vibrations create pressure waves in the fluid of the inner ear. That fluid surrounds the basilar membrane in the vestibular and tympanic canals. The membrane grades from thick at the proximal end of the canal to thin at the distal end. Pressure waves of different frequencies in the fluid of the tympanic canal cause the basilar membrane to vibrate in different locations. The hair cells are on the basilar membrane, and therefore different sets of hair cells are activated by pressure waves of different frequencies.