control and coord

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• A Level Biology CIE
• Contents


  • 15.1 Control & Coordination in Mammals
  • 15.1.1 The Endocrine System
  • 15.1.2 The Nervous System
  • 15.1.3 Neurones
  • 15.1.4 Sensory Receptor Cells
  • 15.1.5 Sequence of Events Resulting in an Action Potential
  • 15.1.6 Transmission of Nerve Impulses
  • 15.1.7 Speed of Conduction of Impulses
  • 15.1.8 The Refractory Period
  • 15.1.9 Cholinergic Synapses
  • 15.1.10 Stimulating Contraction in Striated Muscle
  • 15.1.11 Ultrastructure of Striated Muscle
  • 15.1.12 Sliding Filament Model of Muscular Contraction
  • 15.2 Control & Coordination in Plants
  • 15.2.1 Electrical Communication in the Venus Flytrap
  • 15.2.2 The Role of Auxin in Elongation Growth
  • 15.2.3 The Role of Gibberellin in Germination of Barley

  
• Hormone


  A chemical substance produced by an endocrine gland and carried by the blood that transmits information from one part of the organism to another and brings about a change

  
• Endocrine glands


  • They produce and release hormones
  • They have a good blood supply to get hormones into the bloodstream quickly
• Hormone receptors


  • They are either found on the cell surface membrane, or inside cells
  • They have to be complementary to hormones for there to be an effect
• Peptide/small protein hormones


  • They are water-soluble and cannot cross the phospholipid bilayer of cell surface membranes
  • They bind to receptors on the cell surface membranes of their target cells, which activates second messengers to transfer the signal throughout the cytoplasm
• Steroid hormones


  • They are lipid-soluble and can cross the phospholipid bilayer
  • They bind to receptors in the cytoplasm or nucleus of their target cells
• Stimulating contraction in striated muscle

  1. Impulse arrives at neuromuscular junction

  2. Calcium ions diffuse into neurone

  3. Acetylcholine released from vesicles

  4. Acetylcholine binds to receptors on sarcolemma

  5. Sodium ion channels open, depolarising sarcolemma

  6. Action potential passes down T-tubules

  7. Calcium ions released from sarcoplasmic reticulum

  8. Calcium ions bind to troponin, changing position of troponin and tropomyosin

  9. Myosin-binding sites exposed on actin

  10. Sliding filament model of contraction begins

  
• Similarities between neuromuscular junction and cholinergic synapses, but neuromuscular junction is between a neurone and muscle, cholinergic synapse is between two neurones

  
• Striated muscle


  • Made up of muscle fibres
  • Each muscle fibre contains an organised arrangement of contractile proteins in the cytoplasm
  • Each muscle fibre is surrounded by a cell surface membrane
  • Each muscle fibre contains many nuclei

  
• Parts of a muscle fibre

  • Cell surface membrane = sarcolemma
  • Cytoplasm = sarcoplasm
  • Endoplasmic reticulum = sarcoplasmic reticulum (SR)

  
• Sarcolemma


  • Has many deep tube-like projections that fold in from its outer surface, known as transverse system tubules or T-tubules
  • These run close to the sarcoplasmic reticulum

  
• Sarcoplasm


  • Contains mitochondria and myofibrils
  • Mitochondria carry out aerobic respiration to generate ATP for muscle contraction
  • Myofibrils are bundles of actin and myosin filaments

  
• Sarcoplasmic reticulum


  • Contains protein pumps that transport calcium ions into the lumen

  
• Myofibrils


  • Located in the sarcoplasm
  • Made up of thick filaments of myosin and thin filaments of actin

  
• Parts of a myofibril

  • A-band
  • I-band
  • H-zone
  • M-line
  • Z-disc
• Thick filaments


  • Made of myosin molecules with globular heads pointing away from the M-line
• Thin filaments


  • Made of actin molecules linked together in two chains
  • Tropomyosin and troponin proteins are attached
• Sliding filament model of muscle contraction

  1. Action potential arrives at neuromuscular junction

  2. Calcium ions released from sarcoplasmic reticulum

  3. Calcium ions bind to troponin, changing position of troponin and tropomyosin

  4. Myosin-binding sites exposed on actin

  5. Myosin heads bind to actin, forming cross-bridges

  6. Myosin heads move, pulling actin filaments towards centre of sarcomere

  7. ATP hydrolysis provides energy for myosin heads to detach and rebind to new sites on actin

  8. Process repeats, shortening sarcomere

  
• Animations/videos can help visualise the sliding filament model
• Nervous system


  • Consists of the central nervous system (brain and spinal cord) and the peripheral nervous system (all nerves in the body)
  • Allows us to make sense of our surroundings and respond to them, and to coordinate and regulate body functions

  
• Nerve impulses


  Electrical signals that pass along nerve cells called neurones

  
• Neurones

  Coordinate the activities of sensory receptors, decision-making centres, and effectors like muscles and glands
• Neurone structure


  • Have a long fibre called an axon
  • Axon is insulated by a fatty myelin sheath with uninsulated nodes of Ranvier
  • Cell body contains many dendrite extensions to connect to other neurones

  
• Types of neurones

  • Sensory
  • Relay (intermediate)
  • Motor
• Sensory, relay and motor neurones work together in a reflex arc to bring about a response to a stimulus
• Neurones

  The three types of neurone - the red line shows the direction of impulses

  
• Motor neurones

  • Have a large cell body at one end, that lies within the spinal cord or brain
  • Have a nucleus that is always in its cell body
  • Have many highly-branched dendrites that extend from the cell body, providing a large surface area for the axon terminals of other neurones

  
• Sensory neurones

  • Have the same basic structure as motor neurones, but have one long axon with a cell body that branches off in the middle of the axon - it may be near the source of stimuli or in a swelling of a spinal nerve known as a ganglion

  
• Reflex arc

  1. Sensory neurones, relay (intermediate) neurones and motor neurones work together to bring about a response to a stimulus

  2. A reflex arc is a pathway along which impulses are transmitted from a receptor to an effector without involving 'conscious' regions of the brain

  3. As it does not involve the brain, a reflex response is quicker than any other type of nervous response
• Simple reflex actions

  • Removing the hand rapidly from a sharp or hot object
  • Blinking
  • Focusing of the eye on an object
  • Controlling how much light enters the eye
• How sensory neurones, intermediate (relay) neurones and motor neurones work together to carry out a reflex action


  1. A pin (the stimulus) is detected by a pain receptor in the skin

  2. The sensory neurone sends electrical impulses to the spinal cord (the coordinator)

  3. Electrical impulses are passed on to relay neurone in the spinal cord

  4. The relay neurone connects to the motor neurone and passes the impulses on

  5. The motor neurone carries the impulses to the muscle in the leg (the effector)

  6. The impulses cause the muscle to contract and pull the leg up and away from the sharp object (the response)
• You may be asked to identify the different types of neurones in a diagram. It can be helpful to memorise the key differences between them – such as the location and size of the cell body.

  
• Receptor cell

  A cell that responds to a stimulus
• Receptor cells

  • Are transducers – they convert energy in one form (such as light, heat or sound) into energy in an electrical impulse within a sensory neurone
  • Are often found in sense organs (eg. light receptor cells are found in the eye)
  • Some are specialised cells that detect a specific type of stimulus and influence the electrical activity of a sensory neurone
  • Others are just the ends of the sensory neurones themselves
• What happens when receptor cells are stimulated

  1. They are depolarised

  2. If the stimulus is very weak, the cells are not sufficiently depolarised and the sensory neurone is not activated to send impulses

  3. If the stimulus is strong enough, the sensory neurone is activated and transmits impulses to the CNS
• Sequence of events resulting in an action potential in a sensory neurone (tasting salt as an example)

  1. Sodium ions diffuse through highly selective channel proteins in the cell surface membranes of the microvilli of the chemoreceptor cells

  2. This leads to depolarisation of the chemoreceptor cell membrane

  3. The increase in positive charge inside the cell is known as the receptor potential

  4. If there is sufficient stimulation by sodium ions and sufficient depolarisation of the membrane, the receptor potential becomes large enough to stimulate voltage-gated calcium ion channel proteins to open

  5. Calcium ions enter the cytoplasm of the chemoreceptor cell and stimulate exocytosis of vesicles containing neurotransmitter from the basal membrane of the chemoreceptor

  6. The neurotransmitter stimulates an action potential in the sensory neurone

  7. The sensory neurone then transmits an impulse to the brain