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(1)  Structure of the cochlea: three tubular canals

(a)The scala vestibuli and scala tympani contain perilymph, which has a high [Na+].

(b)The scala media contains endolymph, which has a high [K+].

■The scala media is bordered by the basilar membrane, which is the site of the organ of Corti.

(2)  Location and structure of the organ of Corti

■The organ of Corti is located on the basilar membrane.

■It contains the receptor cells (inner and outer hair cells) for auditory stimuli. Cilia protrude from the hair cells and are embedded in the tectorial membrane.

■Inner hair cells are arranged in single rows and are few in number.

■Outer hair cells are arranged in parallel rows and are greater in number than the inner hair cells.

■The spiral ganglion contains the cell bodies of the auditory nerve [cranial nerve (CN) VIII], which synapse on the hair cells.

3.  Steps in auditory transduction by the organ of Corti (see Figure 2.6)

■The cell bodies of hair cells contact the basilar membrane. The cilia of hair cells are embedded in the tectorial membrane.

a.  Sound waves cause vibration of the organ of Corti. Because the basilar membrane is more elastic than the tectorial membrane, vibration of the basilar membrane causes the hair cells to bend by a shearing force as they push against the tectorial membrane.

b.  Bending of the cilia causes changes in K+ conductance of the hair cell membrane.

Bending in one direction causes depolarization; bending in the other direction causes hyperpolarization­. The oscillating potential that results is the cochlear microphonic potential.

c.  The oscillating potential of the hair cells causes intermittent firing of the cochlear nerves.

4.  How sound is encoded

■The frequency that activates a particular hair cell depends on the location of the hair cell along the basilar membrane.

a.  The base of the basilar membrane (near the oval and round windows) is narrow and stiff. It responds best to high frequencies.

b.  The apex of the basilar membrane (near the helicotrema) is wide and compliant. It responds best to low frequencies.

5.  Central auditory pathways

■Fibers ascend through the lateral lemniscus to the inferior colliculus to the medial geniculate nucleus of the thalamus to the auditory cortex.

■Fibers may be crossed or uncrossed. As a result, a mixture of ascending auditory fibers represents both ears at all higher levels. Therefore, lesions of the cochlea of one ear cause unilateral deafness, but more central unilateral lesions do not.

■There is tonotopic representation of frequencies at all levels of the central auditory pathway.

■Discrimination of complex features (e.g., recognizing a patterned sequence) is a property of the cerebral cortex.

E.Vestibular system

■detects angular and linear acceleration of the head.

■Reflex adjustments of the head, eyes, and postural muscles provide a stable visual image and steady posture.

1.  Structure of the vestibular organ

a.  It is a membranous labyrinth consisting of three perpendicular semicircular canals, a utricle, and a saccule. The semicircular canals detect angular acceleration or rotation.

The utricle and saccule detect linear acceleration.

Rotation of head

Figure 2.7 The semicircular canals and vestibular transduction during counterclockwise rotation.

b.  The canals are filled with endolymph and are bathed in perilymph.

c.  The receptors are hair cells located at the end of each semicircular canal. Cilia on the hair cells are embedded in a gelatinous structure called the cupula. A single long cilium is called the kinocilium; smaller cilia are called stereocilia (Figure 2.7).

2.  Steps in vestibular transduction—angular acceleration (see Figure 2.7)

a.  During counterclockwise (left) rotation of the head, the horizontal semicircular canal and its attached cupula also rotate to the left. Initially, the cupula moves more quickly than the endolymph fluid. Thus, the cupula is dragged through the endolymph; as a result, the cilia on the hair cells bend.

b.  If the stereocilia are bent toward the kinocilium, the hair cell depolarizes (excitation). If the stereocilia are bent away from the kinocilium, the hair cell hyperpolarizes (inhibi-

tion). Therefore, during the initial counterclockwise (left) rotation, the left horizontal canal is excited and the right horizontal canal is inhibited.

c.  After several seconds, the endolymph “catches up” with the movement of the head and the cupula. The cilia return to their upright position and are no longer depolarized or hyperpolarized.

d.  When the head suddenly stops moving, the endolymph continues to move counterclockwise (left), dragging the cilia in the opposite direction. Therefore, if the hair cell was depolarized with the initial rotation, it now will hyperpolarize. If it was hyperpolarized initially, it now will depolarize. Therefore, when the head stops moving, the left horizontal canal will be inhibited and the right horizontal canal will be excited.

3.  Vestibular–ocular reflexes a.  Nystagmus

■An initial rotation of the head causes the eyes to move slowly in the opposite direction to maintain visual fixation. When the limit of eye movement is reached, the eyes rapidly snap back (nystagmus), then move slowly again.

■The direction of the nystagmus is defined as the direction of the fast (rapid eye) movement. Therefore, the nystagmus occurs in the same direction as the head rotation.

b.  Postrotatory nystagmus

■occurs in the opposite direction of the head rotation.

F.Olfaction

1.  Olfactory pathway a.  Receptor cells

■are located in the olfactory epithelium.

■are true neurons that conduct action potentials into the CNS.

■Basal cells of the olfactory epithelium are undifferentiated stem cells that continuously turn over and replace the olfactory receptor cells (neurons). These are the only neurons in the adult human that replace themselves.

b.  CN I (olfactory)

■carries information from the olfactory receptor cells to the olfactory bulb.

■The axons of the olfactory nerves are unmyelinated C fibers and are among the smallest and slowest in the nervous system.

■Olfactory epithelium is also innervated by CN V (trigeminal), which detects noxious or painful stimuli, such as ammonia.

■The olfactory nerves pass through the cribriform plate on their way to the olfactory bulb. Fractures of the cribriform plate sever input to the olfactory bulb and reduce (hyposmia) or eliminate (anosmia) the sense of smell. The response to ammonia, however, will be intact after fracture of the cribriform plate because this response is carried on CN V.

c.  Mitral cells in the olfactory bulb

■are second-order neurons.

■Output of the mitral cells forms the olfactory tract, which projects to the prepiriform cortex.

2.  Steps in transduction in the olfactory receptor neurons

a.  Odorant molecules bind to specific olfactory receptor proteins located on cilia of the olfactory receptor cells.

b.  When the receptors are activated, they activate G proteins (Golf), which in turn activate adenylate cyclase.

c.  There is an increase in intracellular cAMP that opens Na+ channels in the olfactory receptor membrane and produces a depolarizing receptor potential.

d.  The receptor potential depolarizes the initial segment of the axon to threshold, and action potentials are generated and propagated.

G.Taste

1.  Taste pathways

a.  Taste receptor cells line the taste buds that are located on specialized papillae. The

receptor cells are covered with microvilli, which increase the surface area for binding taste chemicals. In contrast to olfactory receptor cells, taste receptors are not neurons.

b.  The anterior two-thirds of the tongue

■has fungiform papillae.

■detects salty, sweet, and umami sensations.

■is innervated by CN VII (chorda tympani).

c.  The posterior one-third of the tongue

■has circumvallate and foliate papillae.

■detects sour and bitter sensations.

48Brs Physiology

■is innervated by cn IX (glossopharyngeal).

■The back of the throat and the epiglottis are innervated by cn X.

d.CN VII, CN IX, and CN X enter the medulla, ascend in the solitary tract, and terminate on second-order taste neurons in the solitary nucleus. They project, primarily ipsilaterally, to the ventral posteromedial nucleus of the thalamus and, finally, to the taste cortex.

2.steps in taste transduction

■ taste chemicals (sour, sweet, salty, bitter, and umami) bind to taste receptors on the microvilli and produce a depolarizing receptor potential in the receptor cell.

III. motor systems

A.motor unit

■consists of a single motoneuron and the muscle fibers that it innervates. For fine control (e.g., muscles of the eye), a single motoneuron innervates only a few muscle fibers. For larger movements (e.g., postural muscles), a single motoneuron may innervate thousands of muscle fibers.

■The motoneuron pool is the group of motoneurons that innervates fibers within the same muscle.

■The force of muscle contraction is graded by recruitment of additional motor units (size principle). The size principle states that as additional motor units are recruited, more motoneurons are involved and more tension is generated.

1.small motoneurons

■innervate a few muscle fibers.

■have the lowest thresholds and, therefore, fire first.

■generate the smallest force.

2.Large motoneurons

■innervate many muscle fibers.

■have the highest thresholds and, therefore, fire last.

■generate the largest force.

B.muscle sensor

1.types of muscle sensors (see Table 2.5)

a. muscle spindles (groups Ia and II afferents) are arranged in parallel with extrafusal fibers. They detect both static and dynamic changes in muscle length.

b. golgi tendon organs (group Ib afferents) are arranged in series with extrafusal muscle fibers. They detect muscle tension.

c. Pacinian corpuscles (group II afferents) are distributed throughout muscle. They detect vibration.

d. Free nerve endings (groups III and IV afferents) detect noxious stimuli.

2.types of muscle fibers a. extrafusal fibers

■make up the bulk of muscle.

■are innervated by a-motoneurons.

■provide the force for muscle contraction.

b.Intrafusal fibers

■are smaller than extrafusal muscle fibers.

■are innervated by g-motoneurons.

■are encapsulated in sheaths to form muscle spindles.

■run in parallel with extrafusal fibers, but not for the entire length of the muscle.

■are too small to generate significant force.

3.  Muscle spindles

■are distributed throughout muscle.

■consist of small, encapsulated intrafusal fibers connected in parallel with large (forcegenerating) extrafusal fibers.

■The finer the movement required, the greater the number of muscle spindles in a muscle.

a.  Types of intrafusal fibers in muscle spindles (Figure 2.8)

(1)  Nuclear bag fibers

■detect the rate of change in muscle length (fast, dynamic changes).

■are innervated by group Ia afferents.

■have nuclei collected in a central “bag” region.

(2)  Nuclear chain fibers

■detect static changes in muscle length.

■are innervated by group II afferents.

■are more numerous than nuclear bag fibers.

■have nuclei arranged in rows.

b.  How the muscle spindle works (see Figure 2.8)

■ Muscle spindle reflexes oppose (correct for) increases in muscle length (stretch).

(1)  Sensory information about muscle length is received by group Ia (velocity) and group II (static) afferent fibers.

(2)  When a muscle is stretched (lengthened), the muscle spindle is also stretched, stimulating group Ia and group II afferent fibers.

(3)  Stimulation of group Ia afferents stimulates α-motoneurons in the spinal cord. This stimulation in turn causes contraction and shortening of the muscle. Thus, the original stretch is opposed and muscle length is maintained.

c.  Function of g-motoneurons

■innervate intrafusal muscle fibers.

■adjust the sensitivity of the muscle spindle so that it will respond appropriately during muscle contraction.

■a-Motoneurons and g-motoneurons are coactivated so that muscle spindles remain sensitive to changes in muscle length during contraction.

Nuclear bag fiber

Plate ending

Nuclear chain fiber

Figure 2.8 Organization of the muscle spindle. (Modified with permission from Matthews PBC. Muscle spindles and their motor control. Physiol Rev 1964;44:232.)