Cells and control study guide

A reflex arc is a neural pathway that mediates a reflex action. These are involuntary, rapid responses to stimuli which protect the body from harm. The pathway involves three main types of neurons: sensory, relay, and motor neurons. When a sensory receptor detects a stimulus, it generates a nerve impulse that travels along a sensory neuron to the central nervous system (CNS), specifically the spinal cord. In the spinal cord, the sensory neuron synapses with a relay neuron. The relay neuron then transmits the impulse to a motor neuron. The motor neuron carries the impulse from the CNS to an effector organ, like a muscle or gland, which then produces the response (e.g., muscle contraction to withdraw a hand from a hot object). This entire process occurs without conscious thought, making it very fast.

Stem cells are undifferentiated cells that have the ability to differentiate into various specialised cell types. This unique property allows them to play crucial roles in growth, repair, and regeneration in living organisms. In animals, embryonic stem cells are pluripotent, meaning they can differentiate into any cell type of the body. They are found in early embryos and are essential for the development of a complete organism. Adult stem cells, found in various tissues, are multipotent or oligopotent, capable of differentiating into a limited range of cell types within a specific tissue to replace dead or damaged cells. For example, mesenchymal stem cells can differentiate into bone, cartilage, and fat cells, contributing to tissue maintenance and repair. In plants, meristematic cells, located in regions called meristems (such as root tips and shoot tips), function similarly to stem cells. They are totipotent, meaning they can differentiate into all plant cell types, and are responsible for the continuous growth of the plant throughout its life, forming new leaves, roots, and flowers.