Cells and control Revision Notes

Neurone Types and Their Functions

There are three main types of neurones involved in reflex arcs and nervous responses: sensory neurones, relay neurones, and motor neurones. Sensory neurones transmit impulses from receptors to the central nervous system (CNS). Relay neurones are found within the CNS and connect sensory neurones to motor neurones, allowing for processing. Motor neurones transmit impulses from the CNS to effectors (muscles or glands) to produce a response.

Neurone Structure and Myelin

Neurones consist of a cell body, dendrons, and an axon. Dendrons are short, branched extensions that receive impulses. The axon is a long extension that transmits impulses away from the cell body. Many axons are covered by a myelin sheath, an insulating layer that increases the speed of impulse transmission. Without myelin, impulses would travel much slower, impacting reaction times significantly.

Synaptic Transmission

Synapses are junctions where impulses are transmitted between neurones, or between a neurone and an effector. Electrical impulses cannot cross the synaptic gap (cleft). Instead, neurotransmitters are released from the pre-synaptic neurone into the cleft. These chemicals diffuse across and bind to receptors on the post-synaptic membrane, initiating a new impulse or stimulating a response. This ensures one-way flow of information.

What is a Reflex Arc?

A reflex arc is the neural pathway that controls a reflex action. It is an involuntary, rapid response to a stimulus, primarily serving a protective function. This pathway bypasses the brain for conscious decision-making, allowing for faster reactions.

Components of a Reflex Arc

The reflex arc consists of three key types of neurons: sensory, relay, and motor neurons. Sensory neurons detect stimuli and transmit impulses to the CNS. Relay neurons, found within the CNS (spinal cord), connect sensory and motor neurons. Motor neurons transmit impulses from the CNS to effector organs, such as muscles or glands, which then carry out the response.

Pathway of a Reflex Impulse

1. Stimulus Detection: A sensory receptor detects a stimulus.
2. Sensory Neuron: Transmits the nerve impulse from the receptor to the spinal cord.
3. Relay Neuron: In the spinal cord, it receives the impulse from the sensory neuron and passes it to a motor neuron.
4. Motor Neuron: Carries the impulse from the spinal cord to an effector (e.g., muscle).
5. Effector Response: The effector carries out the appropriate action (e.g., muscle contracts).

Cornea and Lens: Focusing Light

The cornea is the transparent frontal part of the eye, responsible for the majority of light refraction (bending light rays) to focus them onto the retina. It has a fixed curvature. The lens is situated behind the iris and pupil; it is a flexible, biconvex structure. Its curvature can change due to the action of ciliary muscles, allowing for accommodation – the adjustment of focus for objects at different distances. Both work together to ensure a clear image is projected onto the retina.

Iris: Light Regulation

The iris is the coloured, muscular part of the eye surrounding the pupil. It acts like the aperture of a camera. In bright light, the circular muscles of the iris contract, while the radial muscles relax, making the pupil smaller (constriction) to limit the amount of light entering. In dim light, the radial muscles contract, and the circular muscles relax, dilating the pupil to allow more light in. This reflex action protects the retina from damage and optimises vision in varying light conditions.

Rod and Cone Cells: Light Detection

The retina at the back of the eye contains two types of photoreceptor cells: rods and cones. Rod cells are far more numerous (around 120 million per eye) and highly sensitive to low light intensities, enabling scotopic (black-and-white) vision and peripheral vision. They are most concentrated around the periphery of the retina. Cone cells (around 6 million per eye) are less sensitive but responsible for photopic (colour) vision and high visual acuity. They are concentrated in the fovea, the central part of the retina. There are three types of cone cells, each sensitive to different wavelengths of light (red, green, and blue).

Cataracts

Cataracts are a condition where the lens of the eye becomes cloudy or opaque. This clouding prevents light from passing through clearly to the retina, leading to blurred vision, faded colours, and difficulty seeing at night. They commonly develop with age but can also be caused by injury, diabetes, or certain medications. Surgery to replace the cloudy lens with an artificial one is the primary treatment.

Refractive Errors: Myopia (Short-sightedness) and Hyperopia (Long-sightedness)

Myopia, or short-sightedness, occurs when the eye is too long or the cornea is too curved, causing light to focus in front of the retina. This results in distant objects appearing blurry, while near objects are clear. Hyperopia, or long-sightedness, occurs when the eye is too short or the cornea is not curved enough, causing light to focus behind the retina. This makes near objects appear blurry, while distant objects may be clear. Both conditions are corrected using corrective lenses (glasses or contact lenses) that adjust the focal point of light onto the retina, or through refractive surgery.

Colour Blindness

Colour blindness, also known as colour vision deficiency, is typically an inherited condition that affects an individual's ability to distinguish certain colours. It is most commonly caused by a mutation in the genes responsible for the light-sensitive cone cells in the retina, which are responsible for colour perception. The most common form is red-green colour blindness. There is currently no cure, but some people use special lenses or apps to help distinguish colours.