Cells and control Study Guide

The nervous system transmits information as electrical impulses. This process begins with sensory receptors, specialised cells or nerve endings that detect stimuli from the internal or external environment (e.g., light, sound, touch, temperature, chemicals). Upon stimulation, sensory receptors generate an electrical impulse. This impulse travels along a sensory neurone, which has a long dendron extending from the receptor to the cell body, typically located just outside the central nervous system (CNS), and an axon carrying the impulse into the CNS. Within the CNS, these impulses can be transferred to relay neurones (also known as interneurones). Relay neurones connect sensory neurones to motor neurones, or integrate information within the CNS. Motor neurones carry impulses away from the CNS to effector organs, such as muscles or glands. Their cell bodies are in the CNS, and they have a long axon extending to the effector. The transmission of an impulse along a neurone is an electrical event, but transfer between neurones, or between a neurone and an effector, occurs at specialised junctions called synapses. At a synapse, the electrical impulse arriving at the presynaptic neurone's axon terminal triggers the release of chemical messengers called neurotransmitters into the synaptic cleft. These neurotransmitters diffuse across the gap and bind to specific receptor proteins on the postsynaptic membrane of the next neurone (or effector cell), generating a new electrical impulse or response. The myelin sheath, a fatty layer wrapped around the axons and dendrons of many neurones, insulates them and speeds up the conduction of electrical impulses by saltatory conduction."

Cell differentiation is the process by which a less specialised cell becomes a more specialised cell type. In multicellular organisms, all cells originate from a single zygote through repeated cell divisions (mitosis). Initially, these cells are undifferentiated, meaning they are totipotent or pluripotent and can develop into various cell types. As an organism develops, cells undergo differentiation, where specific genes are activated or deactivated, leading to changes in the cell's structure and function. This specialisation is crucial because it allows different cell types to perform specific roles, contributing to the efficient functioning of tissues, organs, and organ systems. For example, muscle cells develop contractile proteins to facilitate movement, nerve cells form long axons to transmit electrical signals, and red blood cells lose their nucleus to maximise oxygen transport. Without differentiation, a complex organism capable of diverse functions could not develop or survive.