I. Mechanoreceptors detect force and displacement
A. Simple hairs connected to a sensory neuron can sense displacement
B. Pressure and touch receptors in skin (e.g. Pacinian corpuscles for pressure, Ruffini’s corpuscles for skin stretch, and Merkel’s disks and free nerve endings for fine tactile)
C. Stretch receptors in artery walls or in muscles
1. Patellar reflex is an example of a proprioceptive reflex involving stretch receptors (Proprioception is a muscle sense that helps us with posture and balance.)
D. Hair cell receptors for body position and movement and sound
1. e.g. Lateral line systems can detect water movement due to hair cells called neuromasts arranged in a line along the sides of the body.
2. Cochlea in vertebrates detects sound – receptors are tiny hair cells functioning as mechanoreceptors.
II. Potentials in mechanoreceptors
A. Mechanoreceptors produce a graded potential called a receptor potential, that leads to the release of neurotransmitter in an associated afferent neuron resulting in a graded post-synaptic potential
B. Hair cells are mechanoreceptors
1. One long kinocilium (invertebrates often have more)
2. Several short projections called stereocilia which are actually microvilli and are arranged in decreasing size from the kinocilium end. The ends are connected to each other by tip links making the bundle stereocilia act as a unit
3. The stereocilia are polarized and respond to tiny movements and bend towards the kinocilium causing depolarization (K+ channels open) and releases more neurotransmitter and the afferent neuron depolarizes. Bending of the stereocilia away from the kinocilium results in hyperpolarization (K+ channels close) and less neurotransmitter and fewer APs.
Note: the fluid surrounding the hair cells is high in K+
A. Sound– Sound is energy transmitted through a gaseous, liquid or solid medium by setting up vibration of the medium molecules
1. Sounds consist of alternating pressure waves. High pressure (compression) and low pressure (rarefaction)
2. Loudness –determined by the magnitude of the waves
3. Pitch –determined by the frequency of the waves
B. Sound pathway
–External ear directs and amplifies sound
-Tympanic membrane that separates the external from middle ear vibrates in & out (pressure equalized on either side of tympanic membrane by auditory tube)
-Small ossicles in the Middle ear vibrate (malleus, incus, stapes) and amplify the sound wave
-Stapes pushes against the oval window which communicates with the Inner ear. All parts of the inner ear have an outer fluid-filled bony region (fluid is called perilymph) lined with an internal membranous region which is also filled with fluid (generally called endolymph)
-The cochlea is the bony region of the inner ear filled with fluid (perilymph) starting at the oval window as the scala vestibuli and continuing as the scala tympani which pushes against the round window.
-Inside of the cochlea is the membranous cochlear duct filled with endolymph; the base of the cochlear duct is the basilar membrane.
-Sitting on the basilar membrane is the Spiral Organ = Organ of Corti which bears hair cells which are mechanoreceptors for sound
C. Membrane potentials
1. When the cilia moves in one direction (towards kinocilium) it opens K + channels that close when it moves in the other direction (away from kinocilium).
2. The changing permeability to K + causes the membrane potentials to oscillate with the frequency of the sound
3. Shorter hairs closer to the oval window respond to high frequency sounds and those longer hairs nearer the apex, or helicotrema, respond to lower frequency sounds
IV. Equilibrium– balance & orientation of body in environment
A. Statocyst in many invertebrates – e.g. lobster has statocyst to detect orientation of body with respect to gravity – hollow, fluid filled cavities filled with sensory neurons and calcium carbonate crystals called statoliths.
B. Utricle and saccule within the vertebrate vestibule detect head position and linear acceleration/deceleration
Receptors are hair cells in the macula which are attached to a gelatinous membrane = otolithic membrane containing otoliths
C. Vertebrate semicircular canals contain semicircular ducts with a widened region (ampulla) containing the receptors for rotational acceleration/deceleration
1. Three perpendicular arches
2. Receptor cells are stereocilia/kinocilium within gelatinous material called the cupula
3. As rotation occurs, the fluid lags behind due to inertia resulting in bending of the stereocilia
4. Stereocilia bending towards the kinocilium leads to depolarization and bending away from the kinocilium leads to hyperpolarization.