The Visual Fields

Physiology lec#44 (4-5-2011)

In this lecture we are going to continue talking about the visual fields, and then talk about the hearing process

The visual fields :

You have essentially two halves of the retina, a left half and a right half. Generally, the halves are referred to as atemporal half(next to your temple) and anasal half(next to your nose) the nasal retina sees the right half of the world, while the temporal retina sees the left half of the world.

Recall that the brain works on a crossed wires system. The left half of the brain controls the right side of the body, and vice versa. Therefore the left half of the brain is only interested in visual input from the right side of the world. To insure that the brain doesn't get nonessential information, the fibers from the retina sort themselves out to separate right hemi-field from left hemi-field. Specifically,fibers from the nasal retinas cross over at the optic chiasm- whereas the temporal retinas, already positioned to see the opposite side of the world, do not cross. Check slide number 7 to make it clearer.

Lesions :

The easiest way to demonstrate to yourself the consequences of lesions is to strike through a pathway, follow the fibers back to the retina, and see what was affected. Notice that there are lines and numbers drawn on the visual field diagram slide number 8. For each "cut", determine what parts of the patient's visual field will be affected. The way to record a loss of visual field is with two circles

(Heteronymous bitemporal hemi anopsia)

Lesion 4: if there is a complete damage in the optic radiation it will be similar to the optic tract destruction case . But in most cases there is a partial damage which causes quadrant anopsia

If the damage is in the upper part of the optic radiation then it will cause inferior quadrant anopsia.

if the damage is in the lower part of the optic radiation then it will cause superior quadrant anopsia .

Usually vision at the fovea is spared because there is such a large representation of the fovea in the cortex. The loss of vision is not a complete quadrant anopsia, but a notched quadrant anopsia. This phenomenon is calledmacularsparing.

Retinal Projections to Subcortical Regions:

• Superior colliculus (the visual reflex center): its function is to move the head in response to light , the tectospinal tract is involved.

Recall that the inferior colliculus is responsible for the head movement in response to hearing.

• Suprachiasmatic Nucleus of the hypothalamus:

Which control the circadian rhythms that synchronize various physiologic changes of the body with night and day (such as hormones secretion).

• Pretectal Nuclei:

Receives retinal inputs and sends axons to both Edinger-Westphal nuclei (nucleus of the third cranial nerve) It constitutes a centre of the pupil light reflex.The fibers synapse there and cross to the other side.

• Geniculate Nuclei :which is divided into

1-Ventral Lateral Geniculate that controls the behavioral functions of the body.

2-Dorsal Lateral Geniculate which is the major sub-cortical center relaying visual information to the primary visual cortex. As well as forming a “gate control “of information to the primary cortex. The fibers that come from the cortex back to this nucleus are corticofugal fibers which are of inhibitory function and involved in increasing the sensitivity.

Pretectum and Pupillary Light Reflex:

Shininga light into someone’s eye causes the pupil to narrow reflexively. The pretectum controls the action of the iris’ pupillary constriction via its projection to both EdingerWestphal nuclei, whose neurons send their axons to the ciliary ganglion via the third cranial nerve. The pretectum bilateral projections to the Edinger-Westphal nuclei ensure that both eyes react to light. Shining a light into the left eye elicits a direct pupillary reflex in the left eye and a consensual (indirect) pupillary reflex in the right eye. This light reflex informs us about someone’s visual pathways status.

For example, if the right occulomotor nerve is destroyed, flashing light at the right eye will elect an indirect reflex whereas there is no direct reflex. Or if the right optic nerve is destroyed neither direct nor indirect reflex occur while flashing light at the left eye will elect direct and indirect reflexes.

The Primary Visual Cortex:

The primary visual cortex is located at the occipital cortex and has a representation of the contralateral visual hemi-field. The primary visual cortex is a layered structure (6 layers), the fourth layer is the input layer.

The foveal region is mapped in its most posterior part, Because of the high density of receptors in the fovea; the visual cortex has an expanded representation of the fovea.

The Secondary Visual Cortex:

The secondary visual cortex receives signals secondarily; they are transmitted to these areas for analysis. It’s the area responsible for explaining and understanding what you see, thus if any damage happen to this area the result will be “ word blindness” you see but you don’t understand what you see while any injury in the primary visual area results in blindness.

The Autonomic Nervous System to the eye pupil:

-Theparasympatheticnervous supply causes constriction of the pupil, ormiosis, and is supplied bycranial nerveIII, theoculomotor nerve.

-Thesympatheticsupply to the pupil, which causes dilation of the pupil or mydriasis,originates from the cervical ganglion.

Auditory Sensation:

Thesound is a pressure waves that enter the ear

The amplitude is a measure of the intensity of the sound wave

Frequency is the number of cycles per second and ismeasured by hertz.

Audible sounds for young people are in the range of (20-20000) hertz. This range decreases in old people. Putting in mind that we hear sounds depending on both the frequency and intensity of the sound waves.

The decibel is the measure of the intensity of sound (pressure of the sound). The decibel is a logarithmic scale (to increase the range of intensity range).

Decibel (dB) = 10 log I/IR

I = intensity of sound, IR= reference intensity

Sound pressure level (SPL) unit is decibel:

SPL (dB) = 20 log P/PR

P= the sound pressure in N/m2 (N=Newton, m = meter).

PR= reference pressure the absolute threshold for human hearing.

The speed of the sound is much less than that of the light

The structure of the ear:

The ear pinna collects the sound and it’s very important. The oval window and the round window are both covered with a membranous structure. The round window plays an important role through equalizing the pressure of the inner ear because the middle ear is connected to the nasopharynx through the Eustachian canal.

Impedance (Resistance) matching:

The effective area of the tympanic membrane (eardrum) is about 17 times of that of the stapes. Thus, the force caused by a given sound pressure in the air acting on the area of the eardrum is concentrated through the ossicles onto the small area of the stapes, resulting in a pressure increase proportional to the ratio of the areas of the two structures. It also happens that the lever system formed by the middle ear bones gives another factor of about 1.3 in pressure increase.

The 17x of the tympanic membrane/stapes area ratio, multiplied by the 1.3x lever system factor, yields about a 22.1x increase in pressure, thus just about overcoming the air/liquid interface. We say that the middle ear matches the acoustic impedance between the air and the fluid, thus maximizing the sound waves pressure from the air to the fluid of the inner ear. The process is referred to asimpedance (resistance) matching.

The Attenuation Reflex of the middle ear muscles:

The tensor tympani(attached to the tympanic membrane ) and stapedius tensor (attached to the stapes) muscles in the middle ear contract reflexively in response to loud sounds. Both muscles increase the stiffness of the ossicular chain when they contract and thus reduce sound transmission depending on frequency. The reflexes are generally thought to be primarily a protective mechanism to shield the inner ear from damage due to intense sound. Also, this reflex attenuates sound when we speak.

The inner ear ( cochlea) :

Theinner earstructure called the cochlea is a snail-shell like structure divided into three fluid-filled scalas. Upper scala vistibuli , middle scala media and lower scala tympni .

The upper and lower scalas arefor the transmission of pressure and in the middle scala media is the sensitiveorgan of Corti, which detects pressure impulses and responds with electrical impulses which travel along theauditory nerveto thebrain.Theperilymphfluid(resembles the ECF rich in Na+) in the upper and lower scalas differs from theendolymphfluid (resembles the ICF rich in k+) in the media scala .

Done with hope by:

Nour Sadouq