Cells and control study guide

Nerve impulse transmission involves a chain of specialised cells and junctions that enable rapid communication throughout the body. Sensory receptors detect stimuli, converting them into electrical signals. These signals are then transmitted as electrical impulses along sensory neurones. The sensory neurone's dendron carries impulses towards the cell body, and the axon carries them away from the cell body towards the central nervous system (CNS).

Within the CNS, relay neurones receive impulses from sensory neurones and transmit them to motor neurones. This allows for processing and coordination of responses. Both sensory and motor neurones often have a myelin sheath, a fatty layer that insulates the axon and significantly speeds up impulse transmission.

Motor neurones carry impulses from the CNS to effector organs, such as muscles or glands, to elicit a response. The junctions between neurones, or between a neurone and an effector, are called synapses. Here, electrical impulses cannot cross directly. Instead, neurotransmitters, chemical messengers stored in vesicles at the axon terminal, are released into the synaptic cleft. These neurotransmitters diffuse across the gap and bind to receptors on the post-synaptic membrane, triggering a new electrical impulse in the next neurone or stimulating the effector organ to respond. This chemical transmission ensures unidirectional flow of information.

The human eye is a complex organ, but various conditions can affect its ability to focus and perceive light correctly. These conditions are known as eye defects. Common defects include cataracts, which involve clouding of the lens, and refractive errors like long-sightedness (hyperopia) and short-sightedness (myopia). Long-sightedness is when light focuses behind the retina, making distant objects clear but near objects blurry. Short-sightedness causes light to focus in front of the retina, resulting in clear near vision but blurry distant vision. Colour blindness is a different type of defect, usually inherited, where certain cone cells in the retina do not function correctly, leading to difficulty distinguishing between specific colours, most commonly red and green.