3.6.1 STIMULI

• Organisms respond to changes in their internal + external environment to increase chances of survival and ensure conditions optimal for their metabolism.

  • External - animals move away from harmful environments. Plants can change growth to find more favourable conditions.
• Stimulus = any change in the internal or external environment, e.g. a change in temperature, light intensity, pressure.
• Taxes and kineses - simple responses maintaining mobile organism in a favourable environment.

  • Taxis — directional movement in response to a stimulus eg. woodlice move away from light (negative phototaxis).
  • Kinesis — non-directional (random) movement in response to stimulus. Intensity of stimulus affects response. Eg. In high humidity woodlice move slowly and turn less often, so they stay where they are. Air drier - move faster and turn more often, so enter a new area. 
• • Receptors detect stimuli — they can be cells, or proteins on cell surface membranes. 

  Many different types of receptors and are specific to one type of stimulus.

  • Effectors are cells that bring about a response to a stimulus, to produce an effect. Muscle cells and cells found in glands. 

  Receptors communicate with effectors via the nervous system or the hormonal system, sometimes using both.
• Neurones (nerve cells) -

  The nervous system is made up of a network of neurones.

  3 main types:

  • Sensory neurones - transmit electrical impulses from receptors to the central nervous system (CNS) — the brain and spinal cord.
  • Motor neurones - transmit electrical impulses from the CNS to effectors.
  • Relay neurones - transmit electrical impulses between sensory neurones and motor neurones.
• Nervous Communication

  • A stimulus is detected by receptor cells and an electrical impulse is sent along a sensory neurone. 
  • When an electrical impulse reaches the end of a neurone, chemicals called neurotransmitters take the information across the gap (synapse) to the next neurone, where another electrical impulse is generated. 
  • The CNS (the coordinator) processes the information and sends impulses along motor neurones to an effector.
• When an electrical impulse reaches the end of a neurone, chemical messengers called neurotransmitters are secreted directly onto cells (e.g. muscle cells) — so the nervous response is localised. 

  Neurotransmitters are quickly removed once they’ve done their job, so the response is short-lived. 

  Electrical impulses are really fast, so the response is usually rapid — this allows animals to react quickly to stimuli.
• A simple reflex is a rapid, involuntary response to a stimulus. 

  The pathway of communication goes through the spinal cord but not through conscious parts of the brain, so the response is automatic. 

  Information travels really fast from receptors to effectors - as no time spent deciding how to respond.

  Simple reflexes are protective — help organisms avoid damage to the body as response is rapid.
• The Reflex Arc - the pathway of neurones linking receptors to effectors in a simple reflex. 

  3 neurones involved — sensory, relay and motor.

  • Receptors detect stimulus (eg. thermoreceptors in skin detect heat of hot surface)
  • A sensory neurone carries impulse to CNS
  • A relay neurone in spinal cord (CNS) carries impulse to a motor neurone
  • Motor neurone carries impulse to effector (eg. muscle cells in biceps)
  • Effectors bring about response (eg. bicep muscle contracts, to move hand away from heat source) to prevent damage.
• If there’s a relay neurone involved in the simple reflex arc then it’s possible to override the reflex, e.g. brain could tell your hand to withstand the heat.
• Flowering plants increase their chances of survival by responding to changes in their environment.

  • Sense direction of light and grow towards it (maximise light absorption for PHS).
  • Sense gravity, so roots and shoots grow in the right direction.
  • Climbing plants have a sense of touch - can find things to climb and reach sunlight.
• A tropism is the response of a plant to a directional stimulus (a stimulus coming from a particular direction). Plants respond to stimuli by regulating their growth. 

  • Positive tropism = growth towards stimulus.
  • Negative tropism = growth away from stimulus.
• Phototropism is the growth of a plant in response to light. 

  • Shoots are positively phototropic and grow towards light. 
  • Roots are negatively phototropic and grow away from light.
• Gravitropism is the growth of a plant in response to gravity. 

  • Shoots are negatively gravitropic and grow upwards. 
  • Roots are positively gravitropic and grow downwards.
• Plants respond to directional stimuli using specific plant growth factors — chemicals that speed up or slow down plant growth.

  Produced in the growing regions of the plant (e.g. shoot and root tips) and move to other tissues, where they regulate growth in response to directional stimuli.
• Indoleacetic Acid (IAA) - A growth factor produced in the tips of shoots and roots in flowering plants. 

  • Stimulates growth in shoots but high concentrations inhibit growth in roots.
  • Stimulates cells to elongate. 
  • Moved around the plant to control tropisms — moves by diffusion and active transport over short distances, and via the phloem over long distances. Results in different parts of the plant having different concentrations of IAA. Uneven distribution of IAA means there’s uneven growth of the plant.
• Phototropism

  IAA moves to more shaded side of shoots and roots = uneven growth.

  IAA conc increases on shaded side:

  • Shoots - cells elongate and shoot bends towards light
  • Roots - growth inhibited so shoot bends away from light
• Gravitropism

  IAA moves to underside (lower side) of roots and shoots = uneven growth.

  IAA conc increases on lower side:

  • Shoots - cells elongate so shoot grows upwards.
  • Roots - growth inhibited so root grows downwards.
• Receptors detect stimuli. They pass information about stimuli along the nervous pathway.

  • Specific — only detect one particular stimulus.
  • Many different types of receptor that each detect a different type of stimulus. 
  • Receptors in the nervous system convert the energy of the stimulus into the electrical energy used by neurones.
• The Resting Potential -

  When a nervous system receptor is in its resting state (not being stimulated), there’s a difference in charge between the inside and the outside of the cell — the inside is negatively charged relative to the outside (voltage/potential difference across the membrane). 

  • Resting potential = potential difference when a cell is at rest.
  • Generated by ion pumps and channels. 
• The Generator Potential -

  • When a stimulus is detected, the receptor cell membrane is excited and becomes more permeable, allowing more ions to move in and out of the cell — altering the potential difference. 
  • Generator potential = the change in potential difference due to a stimulus. 
  • A bigger stimulus excites the membrane more, causing a bigger movement of ions and a bigger change in potential difference — so a bigger generator potential is produced.
• The Action Potential

  If the generator potential is big enough it’ll trigger an action potential — an electrical impulse along a neurone.

  An action potential is only triggered if the generator potential reaches a certain level (the threshold level).

  • Action potentials are all one size, so stimulus strength measured by frequency of action potentials (no.  of action potentials triggered during a certain time). 
  • If the stimulus is too weak the generator potential won’t reach the threshold, so there’s no action potential.
• Pacinian Corpuscles

  • Mechanoreceptors — detect mechanical stimuli, e.g. pressure and vibrations. 
  • Found in the skin.
  • Contain the end of a sensory neurone (sensory nerve ending), which is wrapped in many layers of connective tissue (lamellae).
• • When a Pacinian corpuscle is stimulated, e.g. by a tap on the arm, the lamellae are deformed and press on the sensory nerve ending.
  • This causes the sensory neurone’s cell membrane to stretch, deforming the stretch-mediated sodium ion channels.
  • The channels open and sodium ions diffuse into the cell, creating a generator potential.
  • If the generator potential reaches the threshold, it triggers an action potential.
• Photoreceptors - Receptors in the eye that detect light. 

  Nerve impulses from photoreceptor cells carried from the retina to the brain by the optic nerve (a bundle of neurones). Where the optic nerve leaves the eye is the blind spot — no photoreceptor cells, so not sensitive to light.
• • Light enters the eye through the pupil and the amount of light that enters is controlled by the iris muscles. Light rays are focused by the lens onto the retina, which lines the inside of the eye. 
  • The retina contains the photoreceptor cells.
  • Fovea = area of the retina where there are lots of photoreceptors.
• How Photoreceptors Work

  • Light enters the eye, hits the photoreceptors and is absorbed by light-sensitive optical pigments. 
  • Light bleaches the pigments, causing a chemical change and altering the membrane permeability to sodium ions. 
  • A generator potential is created and if it reaches the threshold, a nerve impulse is sent along a bipolar neurone.
  • Bipolar neurones connect photoreceptors to the optic nerve, which takes impulses to the brain.
• 2 types of photoreceptor — rods and cones. 

  • Rods mainly found in the peripheral parts of the retina
  • Cones mainly found packed together in the fovea. 

  Contain different optical pigments making them sensitive to different wavelengths of light.

  • Rods only give information in black and white (monochromatic vision).
  • Cones give information in colour (trichromatic vision).
  • 3 types of cones each containing a different optical pigment — red-sensitive, green-sensitive and blue-sensitive. When they’re stimulated in different proportions you see different colours.
• • Rods cells are very sensitive to light (work well in dim light). As many rods join to one bipolar neurone, so many weak generator potentials combine to reach the threshold and trigger an action potential.
  • Cones are less sensitive than rods (work best in bright light). As 1 cone joins to 1 bipolar neurone, so it takes more light to reach the threshold and trigger an action potential.
• Visual acuity = the ability to tell apart points that are close together.

  • Rods - low visual acuity because many rods join the same bipolar neurone, so light from two points close together can’t be told apart.
  • Cones - high visual acuity because cones are close together and 1 cone joins 1 bipolar neurone. When light from two points hits two cones, two action potentials (one from each cone) go to the brain — so you can distinguish two points that are close together as two separate points.
• The nervous system is split into 2 different systems:

  • Central nervous system (CNS) = brain + spinal cord
  • Peripheral nervous system =  the neurones that connect the CNS to the rest of the body.
• The peripheral nervous system has 2 different systems:

  • The somatic nervous system - controls conscious activities. 
  • The autonomic nervous system - controls unconscious activities. Split into:
  • The sympathetic nervous system - the ‘fight or flight’ system that gets the body ready for action. 
  • The parasympathetic system - the ‘rest and digest’ system that calms the body down. 

  The autonomic nervous system is involved in the control of heart rate.
• Cardiac muscle is ‘myogenic’ — it can contract and relax without receiving signals from nerves. The pattern of contractions controls the regular heartbeat.
• Control of Heart Beat 1

  • Sinoatrial node (SAN) (small mass of tissue in R atrium wall) sets heartbeat rhythm by sending out regular waves of electrical activity to the atrial walls. Causes the R + L atria to contract simultaneously. 
  • A band of non-conducting collagen tissue prevents waves of electrical activity from being passed directly from atria to ventricles. Waves transferred from SAN to atrioventricular node (AVN).
  • AVN responsible for passing the waves on to the bundle of His. Slight delay before AVN reacts, to ensure atria have emptied before ventricles contract. 
• Control of Heart Beat 2

  • The bundle of His is a group of muscle fibres responsible for conducting the waves of electrical activity between the ventricles to the apex (bottom) of the heart.
  • The bundle splits into finer muscle fibres in the R + L ventricle walls, called the Purkyne tissue. 
  • The Purkyne tissue carries the waves into the muscular walls of the R + L ventricles, causing them to contract simultaneously, from the bottom up.
• The SAN generates electrical impulses that cause the cardiac muscles to contract.

  The rate at which the SAN fires (i.e. heart rate) is unconsciously controlled by the medulla (part of the brain).
• Animals need to alter their heart rate to respond to internal stimuli, e.g. make sure the heart rate is high enough to supply body with enough oxygen. 

  • Internal stimuli detected by pressure and chemical receptors.
  • Electrical impulses from receptors sent to medulla along sensory neurones.
  • Medulla processes the information and sends impulses to the SAN along sympathetic or parasympathetic neurones.
• • Pressure receptors (baroreceptors) in the aorta and carotid arteries. Stimulated by high and low blood pressure.
  • Chemical receptors (chemoreceptors) in the aorta, the carotid arteries and medulla. Monitor oxygen level in the blood and also carbon dioxide and pH (indicators of O2 level).
• Control of Heart Rate in response to -

  High blood pressure/High blood O2, low CO2 or high blood pH levels:

  • Baroreceptors detect high blood pressure/Chemoreceptors detect chemical changes in the blood, and send impulses along sensory neurones to medulla, which sends impulses along parasympathetic neurones. 
  • These secrete acetylcholine - binds to receptors on the SAN. 
  • Causes heart rate to decrease (to reduce blood pressure back to normal/return O2, CO2 and pH levels back to normal).
• Control of Heart Rate in response to -

  Low blood pressure/Low blood O2 , high CO2 or low blood pH levels:

  • Baroreceptors detect low blood pressure/Chemoreceptors detect chemical changes in the blood, and send impulses along sensory neurones to medulla, which sends impulses along sympathetic neurones.
  • These secrete noradrenaline - binds to receptors on the SAN.
  • Causes heart rate to increase (to increase blood pressure back to normal/return O2, CO2 and pH levels back to normal).