B6 - Reflexes, Stimuli, & the Nervous System

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  • B6 - Reflexes, Stimuli, & the Nervous System
    • Responding to change
      • When a room gets hotter, you sweat more
        • Stimulus = temperature increase
        • Triggered response = Increased sweating
    • Simple Reflex Actions
      • Simple reflex actions happen quickly as they're involuntary (automatic)
      • Simple animals rely on reflex actions for most of their behaviour so they always respond n the same way to a particular stimulus
        • E.g. woodlice move away from light
      • Reflex actions help animals to survive, including finding food & hiding from predators
      • A human baby shows newborn reflexes, including:
        • Grasping - tightly gripping a finger in the palm
        • Sucking a nipple or finger in the mouth
        • Stepping when a foot touches a flat surface
      • Adults also show reflex actions, for example:
        • The pupil reflex stops light damaging cells in the retina - in light, some muscles in the iris contract
        • If someone hits your leg just below the knee, your thigh muscles contract & your leg straightens - the knee jerk reflex
        • If you pick up something hot, you immediately drop it before it an burn & damage the skin (pain receptors)
    • Responding to Stimuli
      • Receptors: used to detect stimuli - these include:
        • Single cells, such as pain sensor cells in the skin
        • Cells in complex organs, for example, cells in the retina
      • Processing centres to receive information & co-ordinate responses
      • Effectors to respond to stimuli - these include:
        • Muscle cells, which contract to move part of the body
        • Glands, whose cells release chemical hormones
      • When multicellular organisms evolve, their bodies develop complex nervous & hormonal communication systems
      • Nerve impulses are electrical signals - they bring about fast, short - lived responses
      • Hormones travel in the blood. They cause slower, longer-lasting responses, for example:
        • The pancreas makes insulin to control blood sugar
        • Oestrogen helps to control the female menstrual cycle
    • The Nervous System
      • In the nervous system, neurons (nerve cells) link receptor cells to effector cells
      • Neurons transmit electrical impulses when stimulated
      • A neuron has an axon - a long extension of the cytoplasm surrounded by a cell membrane
        • Some axons are surrounded by a fatty sheath - this insulates the neurons from nearby cell, so electrical signals can pass along it quickly
      • In humans & other vertebrates, the central nervous system (CNS) is made up of the spinal cord & brain - coordinates an animals response to stimuli
      • In a simple reflex, impulses move from one part of the nervous system to the next in a reflex arc
      • The peripheral nervous system (PNS) links to the CNS to the rest of the body - its made up of sensory & motor nurons


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