Topic 8 - Edexcel A (SNAB) A Level Biology Flashcards

• Organisms must respond to changes in their external and internal environments in order to survive
• Stimulus


  A change in the environment that is detected by specialised receptor cells
• Response to a stimulus

  1. Receptor cells detect stimulus

  2. Receptor cells send signals via nervous or hormonal system

  3. Signals sent to coordination centres in brain or spinal cord

  4. Signals sent to effectors (muscles or glands) to respond
• Receptors


  Cells that detect stimuli in the internal and external environment
• Components of the nervous system

  • Central nervous system (brain and spinal cord)
  • Peripheral nervous system (all nerves in the body)
• Nervous system


  • Allows detection of stimuli and coordination of responses
  • Transmits information as electrical impulses along nerve cells (neurones)
• Types of neurones


  • Sensory neurones
  • Relay neurones
  • Motor neurones
• Nerve impulse pathway


  1. Stimulus

  2. Receptor

  3. Sensory neurone

  4. Central nervous system

  5. Motor neurone

  6. Effector
• Hormones are chemical substances produced by endocrine glands and carried by the blood
• Hormones


  Chemical messengers that transmit information and bring about change by altering the activity of target organs
• Hormone action pathway


  1. Stimulus

  2. Receptor

  3. Hormone

  4. Effector
• Endocrine glands


  • Collectively known as the endocrine system
• Endocrine glands secrete hormones directly into the blood
• Endocrine glands can be stimulated to secrete hormones by another hormone or by a nerve impulse
• Neurones are specialised cells of the nervous system which carry electrical impulses around the body
• Neurones


  • Have a long fibre called an axon
  • Have a cell body containing the nucleus
  • Have nerve endings at the axon terminal to connect to other neurones
• Some neurones are myelinated, meaning their axon is insulated by a fatty layer called the myelin sheath
• Electrical impulses in myelinated neurones jump from one node of Ranvier to the next, speeding up transmission
• Sensory neurones


  • Have a cell body that branches off in the middle of the axon
  • Have dendrites attached to receptor cells
• Motor neurones


  • Have a large cell body at one end in the spinal cord or brain
  • Have many highly-branched dendrites
• Relay neurones


  • Are short neurones with axons and highly branched dendrites
• Response to a stimulus

  1. Receptor cells detect stimulus

  2. Sensory neurones transmit impulses to central nervous system

  3. Central nervous system coordinates response

  4. Motor neurones transmit impulses to effectors
• The diameter of the pupil in the eye is determined by two sets of muscles - circular muscles to constrict, and radial muscles to dilate
• Pupil response to bright light

  1. Bright light

  2. Light receptors in eyes

  3. Sensory neurones

  4. Central nervous system

  5. Motor neurones

  6. Circular muscles in iris contract
• Pupil response to low light

  1. Low light

  2. Light receptors in eyes

  3. Sensory neurones

  4. Central nervous system

  5. Motor neurones

  6. Radial muscles in iris contract
• Neurones transmit electrical impulses which travel along the neurone cell surface membrane from one end to the other
• Resting potential


  The negative electrical potential inside a resting neurone compared to the outside, usually around -70 mV
• Establishing and maintaining resting potential

  1. Active transport of sodium and potassium ions by sodium-potassium pumps

  2. Difference in membrane permeability to sodium and potassium ions
• Depolarisation


  Reversal of the electrical potential difference across the neurone membrane
• Action potential


  The reversal of resting potential that occurs to initiate a nerve impulse
• Resting membrane potential


  Around -70 millivolts (mV)
• Sodium-potassium pumps in the membrane of a resting neurone

  1. Generate a concentration gradient for sodium ions and potassium ions

  2. Together with the facilitated diffusion of potassium ions back out of the cell, generates a negative resting potential across the membrane
• Polarised


  When the resting potential is reached, the neurone membrane is said to be polarised
• To initiate a nerve impulse in a neurone

  The membrane needs to be depolarised
• Depolarisation


  The reversal of the electrical potential difference across the membrane
• Depolarisation of the membrane

  Occurs when an action potential is generated
• Action potential


  The potential electrical difference produced across the axon membrane when a neurone is stimulated
• Action potentials


  Involve the rapid movement of sodium ions and potassium ions across the membrane of the axon
• Voltage gated ion channels

  • They are closed when the membrane is at rest, but they are involved in the generation and transmission of action potentials
• When a neurone is stimulated

  1. A small number of sodium ion channels in the axon membrane open

  2. Sodium ions begin to move into the axon down their concentration gradient

  3. If the potential difference reaches around -55 mV, known as the threshold potential, more sodium ion channels open, leading to a further influx of sodium ions