Option E.2 – Perception of Stimuli
E.2.1 – Outline the diversity of stimuli that can be detected by human sensory receptors, including mechanoreceptors, chemoreceptors, thermoreceptors and photoreceptors
Within the body, there are a number of different, specialised sense cells that can detect and respond to stimuli. They are called receptors.
Sense cells will transfer the energy from the stimulus (such as sound energy, light energy, heat energy, etc) into an action potential to be sent to the rest of the nervous system.
For example, the sensory hair cells in the inner ear respond to sound waves and gravity. On the other hand, the rod and cone cells in the eye respond to light.
E.2.2 – Label a diagram of the structure of the human eye
The eye sockets provide protection for the eyes. Eyes receive information about the size, colour, shape and movement of other objects in the environment. To produce a three dimensional image, different information is received from each eye and then compiled.
E.2.3 – Annotate a diagram of the retina to show the cell types and the direction on which light moves
The retina can recognise wavelengths of light that are within the visible spectrum. It is made up of rod and cone cells. These cells are elongated, with the outer segment containing the light-sensitive pigment, and the inner segment containing the mitochondria.
There are more rod cells in the retina, and are present evenly across the retina, except at the fovea.
These are mainly concentrated around the fovea, where vision is the most accurate. Since the retina is inverted, light will reach the neurons that synapse with the rod and cone cells before reaching the outer segment of the rods or cones.
E.2.4 – Compare rod and cone cells
E.2.5 – Explain the processing of visual stimuli, including edge enhancement and contralateral processing
Since cone cells each have their own neuron, whilst the rod cells all feed into the same one, the amount of detail we can see is affected, or the resolution of our vision. When our cone cells are being used, or in bright light, we can see in greater detail. Each part of the image received by our brain is detected by a different cell, and the boundaries are not blurred. Conversely, in dim light, our vision has a lower resolution. This is because the rod cells all converge to a single neuron and the image is poorly resolved.
Although our vision becomes less detailed in dim light as a result of this arrangement, it does give us the advantage of being able to still see at these light levels. This is because the convergence of visual information makes us more sensitive to stimulus. The collective information from the rod cells allows for enough information to be pooled to create an action potential.
This takes place within the retina. It is demonstrated with the Hermann grid illusion. This illusion is created when we look at a grid of black squares on a white background, and have the impression that there are grey blobs at the intersections of the surrounding white lines. However, they do not remain if we look at them directly. Since the retina is circular, light will either fall directly in the centre, or around it. The ganglion cells of the optic nerve neurons respond differently depending on where they are located on the retina. Thos in the centre have increased activity, whilst those further from it have less. The lower activity from the outer cells is called lateral inhibition.
This is due to the optic chiasma, where the right brain processes information from the left visual field, and vice versa. Our perception of the world around us is processed in the brain based on the visual information we receive. This is illustrated by the abnormal perceptions of patients with brain lesions.
Our perception of visual stimuli is also affected by past experiences and expectations. Therefore, our vision is highly subjective and is not an entirely reliable sense.
E.2.6 – Label a diagram of the ear
E.2.7 – Explain how sound is perceived by the ear, including the roles of the eardrum, bones of the middle ear, oval and round windows, and the hair cells of the cochlea
Our ears are responsible for hearing and balance. Sound waves are channelled into the ear by the outer ear, or pinna.
This is a thin sheet of connective tissue. The sound waves reach the eardrum and cause it to vibrate. These vibrations are small movements towards and away from the middle ear. This allows for sound waves to be transmitted to the middle ear.
Bones of the Middle Ear
The middle ear is an air-filled space, and is the location of the smallest bones of the body. All of these bones are touching. Together they form a lever system, increasing the pressure on the oval window by 20 times. The first bone touches the eardrum, reducing the amplitude of the waves, but increasing their force as they are passed to the other bones until they reach the oval window. If particularly loud sounds are picked up, the muscles around the bones will reduce the vibrations to prevent damage.
This is also a membrane, and serves to pass the vibrations to the cochlea. The sound is magnified by the bones of the middle ear before it reaches the oval window. The third bone, called the stapes, or stirrup, touches it and passes on the vibrations. The cochlea is filled with fluid, with the other window, the round window, allowing for the fluid to vibrate.
Hair Cells in the Cochlea
The cochlea in located in the inner ear, and is a coiled, fluid-filled tube. It is divided into three compartments that are separated by membranes. The upper compartment touches the oval window, and the lower compartment touches the round window. The middle compartment is between the two canals, made up of the basilar membrane and an inflexible membrane.
The basilar membrane support the hair cells of the cochlea, located just below the inflexible membrane, with the hair touching the inflexible membrane. The pressure waves make the hair cells rub or pull against the inflexible membrane. The hair cells are connected to the auditory nerve, and their movement creates an action potential which can be passed down it.