bbs1004 case 4

The auricle is the shell-shaped projection surrounding the opening of the external acoustic meatus. The auricle is composed of elastic cartilage covered with thin skin and occasional hair. Its rim, the helix, is somewhat thicker, and its fleshy, dangling lobule lacks supporting cartilage. 

The external acoustic meatus is a short, curved tube (about 2.5 cm long by 0.6 cm wide) that extends from the auricle to the eardrum.

Sound waves entering the external acoustic meatus eventually hit the tympanic membrane, or eardrum, the boundary between the outer and middle ears. The eardrum is a thin, translucent, connective tissue membrane, covered by skin on its external face and by mucosa internally. Shaped like a flattened cone, its apex protrudes medially into the middle ear

the superior oval window and the inferior round window.

The mastoid antrum, a canal in the posterior wall of the tympanic cavity, allows it to communicate with mastoid air cells housed in the mastoid process.

The pharyngotympanic tube runs obliquely downward to link the middle ear cavity with the nasopharynx, and the mucosa of the middle ear is continuous with that lining the pharynx.

• the malleus
• the incus
• the stapes.

labyrinth or maze because of its shape

The bony labyrinth is a system of channels worming through the bone. 

The membranous labyrinth (vestibular apparatus) is a continuous series of membranous sacs and ducts contained within the bony labyrinth and (more or less) following its contour

perilymph

endolymph

• the vestibule
• the semicircular canals
• the cochlea.

It lies posterior to the cochlea, anterior to the semicircular canals, and flanks the middle ear medially. In its lateral wall is the oval window.

the saccule and utricle.  The smaller saccule is continuous with the membranous labyrinth extending anteriorly into the cochlea, whereas the utricle is continuous with the semicircular ducts extending into the semicircular canals posteriorly. 

maculae that respond to the pull of gravity and report on changes of head position.

The semicircular canals lie posterior and lateral to the vestibule, and each of these canals defines about two-thirds of a circle.

There is an anterior, a posterior, and a lateral semicircular canal in each internal ear. 

The anterior and posterior canals are oriented at right angles to each other in the vertical plane, whereas the lateral canal lies horizontally.

Snaking through each semicircular canal is a corresponding membranous semicircular duct, which communicates with the utricle anteriorly. Each of these ducts has an enlarged swelling at one end called an ampulla

equilibrium receptor region in ampulla. These receptors respond to angular (rotational) movements of the head.

It extends from the anterior part of the vestibule and coils for about 21⁄2 turns around a bony pillar called the modiolus. 

1. Gaze stabilisation
2. Balance
3. Spatial orientation

Proprioception, which is defined as the awareness of body movement and position in space, is mediated by muscle and joint sensory receptors called proprioceptors and may be either unconscious or conscious.

Sensory receptors, known as proprioceptors, are located in skeletal muscles, joint capsules, and ligaments. Proprioceptors monitor the position of our limbs in space, our movements, and the effort we exert in lifting objects. The input signal from proprioceptors goes to the CNS through sensory neurons.

1. muscle spindles
2. Golgi tendon organs
3. joint receptors.

Joint receptors are found in the capsules and ligaments around joints in the body. They are stimulated by mechanical distortion that accompanies changes in the relative positioning of bones linked by flexible joints. Sensory information from joint receptors is integrated primarily in the cerebellum.

Muscle spindles are stretch receptors that send information to the spinal cord and brain about muscle length and changes in muscle length. They are small, elongated structures scattered among and arranged parallel to the contractile extrafusal muscle fibers 

These receptors are found at the junction of tendons and muscle fibers, placing them in series with the muscle fibers. Golgi tendon organs respond primarily to muscle tension created during an isometric contraction and are relatively insensitive to muscle stretch. Golgi tendon reflexes cause relaxation, the opposite of the reflex contraction caused by muscle spindle reflexes.

the VOR works by sensing rotations of the head, it immediately commands a compensatory movement of the eyes in the opposite direction. The movement helps keep your line of sight tightly fixed on a visual target. Because the VOR is a reflex triggered by vestibular input rather than visual input, it works amazingly well even in the dark or when your eyes are closed.

• Axons from the left horizontal canal innervate the left vestibular nucleus, which sends excitatory axons to the contralateral (right) cranial nerve VI nucleus. Motor axons from the abducens nucleus in turn excite the lateral rectus muscle of the right eye. Another excitatory projection from the abducens crosses the midline, back to the left side, and ascends to excite the left cranial nerve III nucleus (oculomotor nucleus), which excites the right medial rectus muscle of the left eye.
• Both eyes are turning right. However, to further ensure speedy operation, the left medial rectus muscle also gets excited via a projection from the vestibular nucleus directly to the left oculomotor nucleus. Speed is also maximized by activating inhibitory connections to the muscles that oppose this movement (the lateral rectus and medial rectus, in this case). To respond to head rotations in any direction, the complete VOR circuit includes similar connections between the right horizontal canal, the other semicircular canals, and the other extraocular muscles that control eye movements.

maculae and cristae ampullares

The maculae are sensory receptor organs that monitor the position of the head in space. In doing so, they play a key role in controlling posture. They respond to linear acceleration, changes in straight-line speed and direction, but not rotation.

stereocilia plus one kinocilium that project into a gel-like mass.

a jelly-like mass studded with tiny stones (calcium carbonate crystals) called otoliths

In the utricle, the macula is horizontal, and the hairs are vertically oriented when the head is upright. For this reason, the utricular maculae respond best to acceleration in the horizontal plane and tilting the head to the side, because vertical movements do not displace their horizontal otolith membrane.

In the saccule, on the other hand, the macula is nearly vertical, and the hairs protrude horizontally into the otolith membrane. The saccular maculae respond best to vertical movements, such as the sudden acceleration of an elevator.

When your head starts or stops moving in a linear direction, inertia causes the otolith membrane to slide backward or forward like a greased plate over the hair cells, bending the hairs. The hair cells release neurotransmitter continuously but movement of their hairs modifies the amount they release.

When the hairs bend toward the kinocilium, the hair cells depolarize, stepping up their pace of neurotransmitter release, and more impulses travels up the vestibular nerve to the brain.

When the hairs bend away from the kinocilium, the receptors hyperpolarize and release less neurotransmitter, generating fewer impulses.

The receptor for rotational acceleration, called the crista ampullaris or simply crista, is a minute elevation in the ampulla of each semicircular canal.

Each crista is composed of supporting cells and hair cells whose structure and function are basically the same as the hair cells of the cochlea and maculae. In this case, the gelled mass is an ampullary cupula, which resembles a pointed cap. The cupula is a delicate, loosely organized network of gelatinous strands that radiate outward to contact the “hairs” of each hair cell. Dendrites of vestibular nerve fibers encircle the base of the hair cells.

The cristae respond to changes in the velocity of rotational movements of the head. Because of its inertia, the endolymph in the semicircular ducts moves briefly in the direction opposite the body’s rotation, deforming the crista in the duct. As the hairs bend, the hair cells depolarize and impulses reach the brain at a faster rate. Bending the cilia in the opposite direction causes hyperpolarization and generates fewer impulses

Vestibular nystagmus is a complex of rather strange eye movements that occurs during and immediately after rotation.

formed by the lateral hemispheres, its involved in planning movements and learning. It also regulates coordination of muscle activation and is important in visually guided movements. input from the cerebral cortex

comprised of the vermis and intermediate zone of the cerebellar hemispheres. It’s involved in regulating body movements by allowing for error correction. It also receives proprioceptive information.

involved in controlling balance and ocular reflexes, mainly fixation on a target. It receives input from the vestibular system and sends output back to the vestibular nuclei. The motor loop through the lateral cerebellum.

The cerebellum is important for making postural adjustments in order to maintain balance. Through input from vestibular receptors and proprioceptors. It modulates commands to motor neurons to compensate for shifts in body position or changes in load upon muscles.

Alcohol can affect balance by altering the viscosity of the endolymph within the otolithic membrane 

Endolymph becomes thinner →  the hair cells can move more easily→  more signals are sent to the brain→ exaggerated and overcompensated movements of the body (more sensitive to linear acceleration)

• cerebellum detects difference between an intended movement and actual movement (coordination of movement)--> alcohol can disrupt this function impaired coordination, slowed reflexes and impaired balance
• alcoholic cerebellar degeneration preferentially affects the anterior lobe
• interfere with the feedback circuits involving the vestibulocerebellum or/ and spinocerebellum and their connections with the fastigial nucleus and its output pathway to the vestibular nuclei, reticular formation and the spinal cord

for balance needed: vision, proprioception and the vestibular system

at least two of the three senses are needed to maintain balance while standing

both senses have to work properly

takes away the vision to see whether proprioception is weakened/influenced (by alcohol).

with a loss of balance the test is positive

cerebral cortex- limbic system -cerebellum -hypothalamus - pituitary gland - medulla