A-level Biology/Mammalian Physiology and Behavior/Sense organs and the reception of stimuli

=Eye=

Structure
An image of the eye can be see to the below right:

Focussing
Light must be focussed onto the retina, the most sensitive section of this, and the only place where we can perceive colour, is the fovea. Light rays are travelling parallel until they reach the cornea, where they are refracted inwards as they pass through the eye. The cornea does most of the focussing, and the lens does fine adjustments to the refraction, so that rays from different distances can be brought to focus on the retina. The closer an object is, the more accommodation is required as the lens thickens and becomes more convex. This accommodation - the shape changing - is achieved by varying the tension of the suspensory ligaments which hold the lens in position.

This varying tension is occurs in a rather strange way - there is always pressure and tension inside the eye, and to stretch the lens - make it less spherical - the ciliary muscle relaxes, and thus the pressure pushes the sclera out wide, increasing tension on the sunspensory ligaments, flattening the lens. When the ciliary muscles are contracted, the diameter of the circle decreases and loosens the tension on suspensory ligaments, allowing the lens to become more spherical.

Retina
The retina is sensitive to light and energy that comes in the form of light rays is converted to nerve impulses that are carried in the optic nerve.

Structure

 * Contains rods/cones - photoreceptor cells
 * These cells are precisely arranged into a single layer
 * Pigmented epithelium in contact with this single layer
 * Also contains special neurones known as bi-polar and ganglion cells which convey information to the optic nerve
 * Light passes through these cell layers before it reaches the rods and cones, except in fovea
 * The blind spot - where the optic nerve leaves the brain, no photoreceptor cells.

Rods/Cones
Rods and cones have similar structures. The part of the cell facing the outside of the eye is the outer segment and the inner facing side is the inner segment. The end of the inner segment forms synapses with other cells in the retina. The outer segment contains many stages of membranes, formed from invaginations of the plasma membrane. In a rod cell the invaginations pinch off leaving a stage of unconnected discs, but in a cone cell they stay attached. These discs provide a large surface area of membrane that contains visual pigments, for reactions with light.

In a rod cell the pigment molecule consists of a protein known as opsin plus a compound known as retinal, and the molecule is known as rhodopsin. Pigments in a cone cell are similar except there are 3 different types of opsin, one for each of the primary colours of light. Rod cells are much more sensitive to light for reasons that will be explained later on.

Rods/Cones:

Bi-Polar/Ganglion Cells
Bipolar cells
 * Central body from which two sets of processes arise;
 * Those nearest to the rod/cone layer are short and branch into many endings which form synapses with a single cone or a number of rods.
 * The other process is longer and forms synapses with a ganglion cell.
 * Their role is to transfer information from rods/cones to ganglion cells.

Ganglion cells
 * Inner layer of retina
 * Numerous dendrites from synapses with bipolar cells
 * Location of action potential generation
 * Axons of ganglion cells make up the optic nerve

The neurotransmitter released by resting rod/cone cells reaches a bi-polar cell which can either increase or decrease the amount of neurotransmitter substance that it releases. Some ganglion cells fire more slowly, some more quickly in response to this change in the neurotransmitter substance it receives.

Rod Cells
A rod cell without light maintains a difference in electrical potential across its plasma membrane, but this is usually around -40mV (compared to the -60mV of a normal neurone) due to channels for both sodium and potassium to flow freely in and out. When light hits a rod cell, the retinal part of rhodopsin changes shape, from a carbon-tail-kinked 11-cis-retinal to an all-trans-retinal, which results in retinal no longer being able to bind with opsin. The rhodopsin molecule changes into an unstable form, causing those sodium/potassium channels to close and a greater potential difference -70mV builds up. The rod cell is now said to be hyperpolarised.

Cone Cells
Cone cells respond in a similar way, but cone cells require much more light before the cells hyperpolarize, and this is why rods are more sensitive to light (among other reasons). The three pigments are for red blue and green, and the brain judges colour by the intensity of signals from each type of cone.

Neurotransmitter
Rod/Cone cells are constantly releasing neurotransmitter - until light falls on them, when they stop

Visual Acuity
The reason we have lower visual acuity with rod cells is because several of them interface with one bipolar cell whereas cone cells interface with only one ganglion cell. This aids the detection of dim light as the small responses are pooled to produce a larger response.

Ageing
With ageing comes two main problems - loss of visual acuity and cataracts. The loss of visual acuity is due to the loss of elasticity in their lenses, and so the lens cannot return to its correct shape after near focussing. Cataracts are the result of proteins denaturing and coagulating, resulting in that part of the lens becoming cloudy white and impairing vision. It can be fixed with glasses or surgery.

=Ear=

The ear is responsible for hearing and balance.

Structure
The ear can be divided into three parts, outer, middle and inner ear.

Outer ear:


 * Pinna - Collects sound waves and directs them towards tympanic membrane (ear drum).
 * Auditory canal - Sound travels along here
 * Tympanic membrane - Sound turns to vibrations here

Middle ear


 * Auditory ossicles - malleus (hammer), incus (anivil) and stapes (stirrup) - transmit vibrations from tympanic membrane to oval window membrane.
 * Eustaschian tube - air filled passage that maintains air pressure on both sides

Inner ear


 * Contains fluid
 * Embedded within the bones of the skull
 * Cochlea - contains receptor cells that convert energy in sound waves into a nerve impulse
 * Organ of Corti - the receptor cells inside the cochlear that runs along the basilar membrane throughout the length of the coiled cochlea. When the oval window vibrates, the vibrations are transmitted through the fluid in the cochlea and cause these receptor cells to vibrate.
 * The receptor cells also have stereocilia on their upper surface which are embedded in another membrane, and are in close contact with the neurones that form the cochlear nerve.
 * Three semi-circular canals, with two chambers called utriculus and sacculus. All full of fluid and contain hair cells. This is the area responsible for balance, and impulses pass from it to the vestibular nerve, which joins with the cochlear nerve, forming the auditory nerve.

Hearing
The above section described how vibrations in the air are transmitted to the hair cells which then vibrate. These hair cells were previously maintaining a resting potential across their plasma membranes, same as neurones, and when they vibrate they are depolarised, causing them to release neurotransmitter. This in turn depolarises the endings of nerve cells in the cochlear nerve. This creates action potentials which are then carried into the brain.

Loudness
The brain determines the loudness of a sound by the amplitude vibration which is converted into an increased rate of action potentials. Also, some cells only fire if there is an extremely loud sound.

Frequency
The frequency (pitch) of a sound is detected by the brain knowing which neurones are conducting nerve impulses into it. The hair cells in the basilar membrane closest to the oval window are receptive to high frequency sounds whilst the furthest away are for low-frequency sounds.

Direction
The direction a sound is coming from is determined by the loudness and the timing of the sound arriving in each ear.

Balance
Our sense of balance is provided by the receptor cells in the semicircular canals, the utriculus and the sacculus. The receptor cells synapse with the sensory endings of the vestibular nerve. In both semicircular canals, there is a patch of cells called a macula, and this is covered with a gelatinous layer containing crystals of calcium carbonate called otoliths. Each macula also contains numerous hair cells which have stiff stereocilia projecting from their upper surfaces, and the ends of the cilia are embedded in the gelatin layer, forming synapses with the neurones of the vestibular nerve.

The crystals of calcium carbonate, the otoliths, are heavy and are pulled downwards by gravity, and they pull the hairs of the hair cells with them, depolarising the ones they pull with them, changing the pattern of action potentials transmitted along the vestibular nerve. The macula in the two semicircular canals are arranged differently providing a 3D image to the brain of how the head is positioned. The hair cells in the utriculus are in the horizontal plane, sensing the orientation of the head when you are upright, whereas those in the sacculus are in the vertical plane, sensing the orientation of the head when you are lying down.

The semicircular canals are filled with viscous fluid and have a swelling at the end called an ampulla. In each ampulla there is a gelatinous structure called a cupula with stereocilia from hair cells embedded in it. When the head moves the fluid stays where it is (inertia) and so collects in the ampulla. This bends the cupula to one side, pulling the cilia and causing depolarisation of the hair cells. The three ampullae are orientated differently so at least two will detect a movement, regardless of direction.