IGCSE Biology - Coordination, Response & Homeostasis

Mammalian Nervous System

• The human nervous system comprises:
  • Central nervous system (CNS): Brain and spinal cord.
  • Peripheral nervous system (PNS): All nerves in the body.
• Roles:
  • Enables understanding of surroundings and response.
  • Coordinates and regulates bodily functions.
• Information transmission:
  • Via nerve impulses: Electrical signals through nerve cells (neurons).
  • A bundle of neurons is called a nerve.

Types of Neurons

• Three main types:
  • Sensory: Transmit impulses from sense organs to the CNS.
  • Relay (intermediate): Found in the CNS, connecting sensory and motor neurons.
  • Motor: Carry impulses from the CNS to effectors (muscles or glands).
• Neuron structure includes a long fiber (axon).
  • Reduces impulse transfer time between cells.
  • Axon insulated by a fatty sheath with uninsulated nodes.
  • Electrical impulses jump from node to node.
• Cell body contains dendrites.
  • Connect to many neurons, receiving impulses and forming a communication network.
• Sensory neurons:
  • Long with a cell body branching off the middle of the axon.
• Relay neurons:
  • Short with a small cell body at one end and many dendrites.
• Motor neurons:
  • Long with a large cell body at one end and long dendrites.

The Reflex Arc

• Voluntary responses:
  • Conscious decisions initiate action, starting with the brain.
  • Example: Picking up a cup of coffee.
• Involuntary (reflex) responses:
  • Do not involve the brain as the coordinator; awareness occurs after completion.
  • Essential for basic survival and are rapid.
  • Voluntary responses take longer due to considering consequences.
• Reflex responses:
  • Automatic and rapid to stimuli like touching something sharp or hot.
  • Quicker than other nervous responses, minimizing body damage.
• Reflex Pathway:
  1. Stimulus (e.g., pin) detected by a receptor in the skin.
  2. Sensory neuron sends electrical impulses to the spinal cord.
  3. Impulse passed to a relay neuron in the spinal cord.
  4. Relay neuron connects to a motor neuron, passing the impulse.
  5. Motor neuron carries impulse to a muscle (effector) in the leg.
  6. Muscle contracts, pulling the foot away from the sharp object (response).
• Reflex actions are automatic, fast, and protective.

Synapses

• Junctions between neurons.
• Enable connection in the nervous system, directing nerve impulses along appropriate routes.
• Analogy: Railway points guiding trains.

Structure of a Synapse

• Neurons do not touch; junctions are called synapses.
• Electrical impulse travels along the first axon.
• Triggers release of neurotransmitters (chemical messengers) from vesicles at the presynaptic neuron.
• Neurotransmitters diffuse across the synaptic gap (cleft).
• Bind with receptor molecules on the postsynaptic membrane of the second neuron.
• Stimulates the second neuron to generate an electrical impulse.
• Neurotransmitters are then destroyed to prevent continuous stimulation.
• Impulses travel in one direction only.
• The synapse is the only place where drugs can act in the nervous system.

Sense Organs

• Receptors are specialized cells that detect environmental changes and stimulate electrical impulses.
• Sense organs contain receptors for specific stimuli.
• Stimulated receptor cells generate electrical impulses, passed to sensory neurons.
• Sensory neuron carries impulse to the CNS.
• Response is decided and impulse is passed to a motor neuron (via a relay neuron).
• Motor neuron carries impulse to effector (muscle or gland).
• Effector carries out the response.

The Eye

• Sense organ with receptor cells sensitive to light.
• Structures include:
  • Cornea
  • Iris
  • Lens
  • Retina
  • Optic nerve
• Cornea: Transparent covering that refracts light.
• Iris: Muscle controlling light entry to the pupil.
• Lens: Transparent disc that changes shape to focus light onto the retina.
• Retina: Layer of light receptor cells detecting light intensity and color.
• Optic nerve: Sensory neuron carrying electrical impulses to the brain.
• Pupil reflex: Controls light entering the eye by altering pupil diameter.
  • Dim light: Pupil dilates to allow more light in.
  • Bright light: Pupil constricts to prevent damage to the retina.
• Iris Muscles:
  • Circular muscles form circles around the pupil.
  • Radial muscles radiate outwards from the pupil.
  • Antagonistic action: one set contracts, the other relaxes.
  • Dim light: Radial muscles contract, circular muscles relax, pupil dilates.
  • Bright light: Radial muscles relax, circular muscles contract, pupil constricts.

Eye Accommodation

• Accommodation: The way the eye focuses on near or distant objects by changing lens shape.
  • Ciliary muscles and suspensory ligaments facilitate this change.
  • Near objects:
    • Ciliary muscles contract.
    • Suspensory ligaments loosen.
    • Lens becomes more rounded.
    • Light is refracted more.
  • Distant objects:
    • Ciliary muscles relax.
    • Suspensory ligaments tighten.
    • Lens becomes thinner.
    • Light is refracted less.
• Rods:
  • Detect light at low levels, important for night vision.
• Cones:
  • Detect light at three different wavelengths, enabling color vision.
• Distribution:
  • Rods are all over the retina, except the blind spot.
  • Cones are concentrated in the fovea.
• Fovea:
  • Enables sharp, colored images when light is focused effectively by the eye.

Hormones in Humans

• Hormone: Chemical substance produced by a gland and carried by the blood.
• Alters the activity of specific target organs.
• Glands producing hormones constitute the endocrine system.

Transport

• Endocrine glands have a good blood supply.
• Hormones enter the bloodstream to reach target organs.

Mechanism

• Hormones affect cells with target receptors.
• Receptors are complementary to hormones.
• Liver regulates hormone levels in the blood.

Comparison of Nervous & Hormonal Control

Glucagon

• Blood glucose levels are controlled by insulin and glucagon (negative feedback).
• Both hormones are made in the pancreas.
• Insulin:
  • Produced when blood glucose rises.
  • Stimulates liver and muscle cells to convert glucose into glycogen.
• Glucagon:
  • Produced when blood glucose falls.
  • Stimulates liver and muscle cells to convert glycogen into glucose.

Adrenaline

• 'Fight or flight' hormone produced in dangerous situations.
• Increases blood glucose concentration.
• Increases pulse and breathing rate.
• Diverts blood flow to muscles.
• Dilates pupils.

Additional Effects of Adrenaline

• Increasing the concentration of glucose in the blood.
• Increasing heart rate.

Homeostasis: Definition

• Maintenance of a constant internal environment.
• Conditions (temperature, blood pressure, water concentration, glucose concentration) are kept within set limits.
• Ensures reactions in body cells function properly.
• Deviation from normal can lead to death without intervention.

Role of Insulin

• Secreted when blood glucose levels are high.
• Converts excess glucose into glycogen in the liver and muscles.
• Decreases blood glucose concentration.
• Glycogen is converted back to glucose when blood glucose levels have dipped.

Negative Feedback

• Conditions change from the ideal or set point and returns conditions to this set point.
• If the level of something rises, control systems reduce it again.
• If the level of something falls, control systems raise it again.

Blood Glucose Control

• Blood glucose levels are controlled by insulin and glucagon (negative feedback).
• Both hormones are made in the pancreas.
• Insulin:
  • Produced when blood glucose rises.
  • Stimulates liver and muscle cells to convert excess glucose into glycogen.
• Glucagon:
  • Produced when blood glucose falls.
  • Stimulates liver and muscle cells to convert stored glycogen into glucose.

Type 1 Diabetes

• Blood glucose levels are not able to be regulated as the insulin-secreting cells in the pancreas are not able to produce insulin.
• Blood glucose levels are often far too high.
• It can be treated by injecting insulin.
• Symptoms of diabetes include extreme thirst, weakness or tiredness, blurred vision, weight loss and loss of consciousness in extreme cases.

Homeostasis: Temperature Control

• Control of body temperature is a homeostatic mechanism.
• Humans maintain temperature around 37°C for optimal enzyme function.
• High temperature denatures enzymes.

Skin

• Brain contains receptors sensitive to blood temperature.
• Skin has temperature receptors, sending impulses to the brain.
• Brain sends impulses to effectors in the skin for temperature maintenance.
• Fatty tissue under the dermis insulates to prevent heat loss.

Responses to changes in temperature

• Vasoconstriction: Blood flow in capillaries slows down.
• Vasodilation: Blood flow in capillaries increases.

Tropisms

• Plants respond to environmental changes (stimuli) for survival (e.g., light, water, gravity).
• Responses are slower than animals.
• Growth towards (positive) or away (negative) from a stimulus.

Examples

• Shoots: positive phototropic, negative gravitropic.
• Roots: negative phototropic, positive gravitropic.

Investigating Phototropisms

• Seedlings in A grow towards the light source.
• Seedlings in B grow straight up.
• Seedlings in C grow straight up looking for light and the plant becomes tall and slender with yellowing leaves due to the lack of light.

Investigating Gravitropisms

• all radicles (roots) have grown downwards (positive gravitropic response) regardless of which way they were initially facing (horizontal, up or down) and all plumules (shoots) have grown upwards (negative gravitropic response).
• all radicles and all plumules have all grown neither up nor down but straight outwards in whichever direction they were placed.

Auxins: Chemical Control of Tropisms

• Growth hormone produced in the tips.
• Stimulates the cells behind the tip to elongate (get larger); the more auxin there is, the faster they will elongate and grow.
• If light shines all around the tip, auxin is distributed evenly throughout and the cells in the meristem grow at the same rate.
• When light shines on the shoot predominantly from one side though, the auxin produced in the tip concentrates on the shaded side, making the cells on that side elongate and grow faster than the cells on the sunny side.
• Positive phototropism in plant shoots
• Gravity modifies the distribution of auxin so that it accumulates on the lower side of the shoot.
• the role of auxin can be tested using seedlings placed in a box that has a slit on one side, only allowing light in from one direction.
• the darker side of the stems grow longer and faster than the cells in the light.
• In roots, higher concentrations of auxin results in a lower rate of cell elongation.