Communication

1
Communication

• Unit 1
• Communication, Homeostasis and Energy

2
Test Yourself

• What effect does temperature change have on
  enzyme action?
• What other environmental factors inhibit the
  action of enzymes?
• List three changes to the external environment to
  which we might need to respond.
• What is the main role of the
• Heart
• The lungs
• The kidneys

3
Keep testing yourself

• What is meant by cell signalling?
• In what other process in the body is cell
  signalling particularly important?
• Explain the role of cell surface receptors in
  cell signalling.

4
Learning Outcomes

• Outline the need for communication systems within
  multicellular organisms, with reference to the
  need to respond to changes in the internal and
  external environment and to coordinate the
  activities of different organs.

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Key words

• Sensitivity
• Stimulus
• Internal communication
• Plants
• Animals
• Receptor
• Effector

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The need for a constant internal environment

• All living things need to maintain a certain
  limited set of conditions inside their cells.
• Why?

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The need for a constant internal environment

• Cellular activities rely on the action of enzymes
• Specific limited set of conditions
• Suitable temperature
• Suitable pH
• Aqueous environment
• No toxins / inhibitors

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External environments

• As the external environment changes it places
  stress on the living organism.
• The environmental change is a stimulus and the
  way in which the organism changes its behaviour
  or physiology is its response to the stress.

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Definitions

• Stimulus
• Any change in environment that causes a response
• Response
• A change in behaviour or physiology as a result
  of a change in the environment.

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Learning outcomes

• State that cells need to communicate with each
  other, which they do by a process called cell
  signalling.
• State that neuronal and hormonal systems are
  examples of cell signalling

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Internal Environments

• The internal environment of the cells in animals
  is tissue fluid.
• Activity of the cell alters its environment
• Use up substrates
• Produce products, some of which may be toxic
• Accumulation of excess waste acts as a stimulus
  to cause the removal of these wastes

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Maintaining internal environment

• Summary
• Composition of the tissue fluid is maintained by
  the blood
• Wastes accumulating in tissue fluid enter the
  blood
• Excretion prevents the accumulation of wastes in
  the blood
• Concentrations of all substances in the blood are
  monitored

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Coordination

• In a multicellular organism cells become
  differentiated (specialised) forming tissues and
  organs.
• A good communication system is required
• List the features of a good communication system

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Good communication system

• Whole body
• Cell communication
• Specific
• Rapid
• Short term and long term

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Cell signalling

• How cells communicate with each other
• The neuronal system and the hormonal system work
  by cell signalling.

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Learning Outcomes

• define the terms negative feedback, positive
  feedback and homeostasis
• explain the principles of homeostasis in terms of
  receptors, effectors and negative feedback

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Homeostasis

• Maintaining a constant internal environment
  despite external changes
• Examples
• Body temperature
• Blood glucose concentrations
• Blood salt concentration
• Water potential of blood
• Blood pressure
• Carbon dioxide concentration

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Negative feedback

• Reversal of any change in internal environment to
  return to an optimum steady state.

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Negative Feedback
Optimum condition
Change away from optimum
Receptor detects change
Return to optimum conditions
Communication system informs effector
Effector reacts to reverse change
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Negative feedback

• Structures required for pathway to work
• Sensory receptors
• Communication system
• Effector cells

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Examples of Negative feedback

• Control of room temperature
• Control of body temperature
• Control of blood glucose levels
• Control of body water concentration

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Positive Feedback

• Increases any change that is detected by
  receptors
• Does not lead to homeostasis

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Positive Feedback
Optimum condition
Change away from optimum
Receptor detects change
Communication system informs effector
Effector reacts to increase change
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Examples of positive feedback

• If core temperature drops too low
• Dilation of the cervix at the end of pregnancy

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Stretch and Challenge

• Enzyme action and temperature regulation
• As core body temperature rises the increase will
  affect the activity of enzymes. This can lead to
  heat exhaustion and even death.
• Describe the effect of increasing body
  temperature on enzyme action.
• Suggest what actually causes death as body
  temperature rises.

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Answers

• Temperature increase rate of enzyme action
  increases
• 10oC increase will double the rate of reaction
• Above 50oC enzymes denature rate of reaction
  falls quickly
• Death

27
Stretch and challenge

• The stress response
• The usual response to stress is to release the
  hormone adrenaline. This hormone has a wide
  range of target cells and prepares the body for
  activity. The activity may be to stay and fight
  or it may be to run away. The hormone is known
  as the fight or flight hormone.

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Stretch and challenge

• The stress response
• When under stress women also release the hormone
  oxytocin. This results in a tendency to pacify
  or protect. It has been called the tend and
  befriend hormone. Oxytocin prompts a mother to
  protect her children.

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Stretch and challenge

• Suggest how these responses to adrenaline and
  oxytocin may have evolved.

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Learning Outcome

• describe the physiological and behavioural
  responses that maintain a constant core body
  temperature in ectotherms

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Maintaining body temperature

• Changes in body temperature affects the structure
  of proteins
• Endotherms
• Maintain body temperature within strict limits
• Independent of external temperature
• Ectotherms
• Body temperature fluctuates with external
  temperature

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Ectotherms

• Advantages
• Use less food in respiration
• Need less food
• Greater proportion energy used for growth

• Disadvantage
• less active in cooler temperatures
• May not be capable of activity in winter months

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Temperature regulation in ectotherms

• Increasing the heat exchange with their
  environment
• Expose body to sun
• Orientate body to sun
• Orientate body away from sun
• Hide in burrow
• Alter body shape
• Increase breathing movements

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Student Activity

• Design an A4 poster to summarise behavioural and
  physiological adaptions of ectotherms for
  temperature regulation.

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Stretch and challenge

• Temperature regulation in bee swarms
• Bees are ectothermic.
• However, it has been shown that the temperature
  of a bee swarm can be maintained accurately to
  within one degree of 35oC.
• This is achieved by bees moving to different
  parts of the swarm and by allowing passages for
  air flow through the swarm.

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Question

• Suggest how movement of bees within a swarm and
  air movement through the swarm can help to
  maintain the temperature of the swarm.

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Answer

• Bees in the centre of the swarm will be warmer
  than those on the outside.
• Warmer bees move towards the outer parts of the
  swarm while colder bees move toward the centre.
• This transfers heat from the centre to the outer
  parts of the swarm.

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Answer

• In hot weather the bees create more passages for
  air flow the passages are also wider
• Thus more air can pass through the swarm and
  carry heat away.
• In cooler weather there are fewer air passages
  and they are narrower.

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Quick Questions

• Why is it important to maintain body temperature?
• Make a list of 5 ectotherms
• Explain how basking on a hot rock in the sun can
  help an ectotherm to regulate its body
  temperature.

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Learning Outcome

• describe the physiological and behavioural
  responses that maintain a constant core body
  temperature in ectotherms and endotherms, with
  reference to peripheral temperature receptors,
  the hypothalamus and effectors in skin and
  muscles

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Endotherms

• Use internal sources of heat to maintain body
  temperature
• Many chemical reactions in the body are exergonic
• Endotherms also show behavioural and
  physiological adaptations

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Endotherm

• Advantages

• disadvantages

• Constant body temp.
• Activity possible even when cool
• Inhabit colder parts of planet

• Energy used up to maintain constant temp.
• More food required
• Less energy used in growth

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Physiological Adaptations
Too hot Too cold
Sweat glands in skin Secrete more sweat Less sweat secreted
Lungs, mouth and nose panting No panting
Hairs on skin Lie flat Raised
arterioles Vasodilation vasoconstriction
Liver cells Reduce rate of metabolism Increase rate of metabolism
Skeletal muscles Spontaneous contractions (shivering)
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Behavioural adaptations

• Too hot

• Too cold

• Move into shade
• Decrease exposed surface area
• Remain inactive / increase surface area

• Move into sunlight
• Increase exposed surface area
• Move about to generate heat in muscles
• Extreme cold roll into a ball to decrease
  surface area

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• Change in core temperature
• Thermoregulatory centre in hypothalamus detects
  change.
• Nervous and hormonal systems carry signals to
  skin, liver and muscles
• Fall in core temperature
• Rise in metabolic reactions
• Release more heat from exergonic reactions
• Release heat through muscle contractions
• Decrease loss of heat, temperature rises

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Control of body temperature
Skin temperature External Core Temperature
HYPOTHALAMUS Thermoregulatory centre

• TOO HOT
• Reduce metabolism
• Vasodilation
• increased sweating

• TOO COLD
• Shivering
• Increased metabolism
• Vasoconstriction
• Reduced sweating
• Skin hairs erected

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Detecting changes in body temperature

• Thermoregulatory centre in the Hypothalamus
• Monitors blood temperature
• Detects changes in core temperature
• Peripheral temperature receptors
• early warning system
• Detect changes in temperature of the extremities
• Sends signals to the brain to initiate
  behavioural mechanisms to maintain core
  temperature.

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Stretch and challenge

• Should mountain rescue dogs carry brandy?
• In early part of the twentieth century St Bernard
  dogs were used for mountain rescues.
• Traditionally they carried a small container of
  brandy for the lost or injured climber to drink.
• Alcohol causes vasodilation.

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Question

• Explain why drinking brandy is not a good idea
  for someone who is lost or injured and exposed to
  cold weather.

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Answer

• If the climber is unable to find shelter, the low
  temperature could reduce the body temperature to
  the point where enzyme activity is severely
  reduced.
• Vasodilation caused by the alcohol in the brandy
  will increase the rate of heat loss from the
  body, because more blood carries heat from the
  bodys core to the surface where it can be lost.
• Hypothermia and death will happen sooner in a
  person who has drunk alcohol.

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Questions

• Explain why a shrew has to eat almost its own
  body mass each day, but an elephant eats less
  than one percent of its body mass each day.
• Suggest why the fairy penguin of Australia grows
  to about 25cm in height while the emperor penguin
  of Antarctica grows to a metre in height.

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Answers - shrew

• Shrew is very small with a large surface area to
  volume ratio.
• It loses heat through its skin
• A lot of food must be used to replace the heat
  lost
• Elephant is large with a small surface area to
  volume ratio
• Loses a smaller proportion of body heat.

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Answers - penguin

• Australia is warm penguins do not need to be
  large to maintain their body temperature
• Antarctica is very cold larger penguins have a
  smaller surface area to volume ratio so can
  maintain body temperature more easily.
• a huddle of penguins has a smaller surface area
  to volume ratio than a solitary penguin.

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Nervous Communication
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Learning Outcomes

• Outline the roles of sensory receptors in mammals
  in converting different forms of energy into
  nerve impulses.
• Describe, with the aid of diagrams, the structure
  and functions of sensory and motor neurones.

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Sensory Receptors

• Specialised cells that detect changes in
  surroundings
• Energy transducers
• Convert one form of energy to electrical energy
  of a nerve impulse
• Stimulus
• Change in energy levels in environment

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Sensory receptors

• Receptors
• Light sensitive cells
• Olfactory cells
• Taste buds
• Pressure receptors (pacinian corpuscles)
• Sound receptors
• Muscle spindles (proprioceptors)

• Energy changes detected
• Light intensity and wavelength
• Presence of volatile chemicals
• Presence of soluble chemicals
• Pressure on skin
• Vibrations in air
• Length of muscle fibres

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Neurones

• Function
• To transmit the action potential
• Structure
• Very long
• Maintain potential difference across cell
  membrane
• Gated ion channels in cell membrane
• Sodium/potassium pumps
• Myelin sheath / schwann cells / node of ranvier
• Cell body contains nucleus, mitochondria and
  ribosomes.

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Learning outcomes

• Describe and explain how the resting potential is
  established and maintained.
• Describe and explain how an action potential is
  generated.

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Membrane permeability

• Gated channel proteins specific to either sodium
  or potassium ions
• Increase permeability when open
• reduces permeability when closed
• Carrier proteins
• Active transport
• Sodium-potassium pump
• Transports more Na2 out of cell than K into
  cell.
• Result is that inside cell is more negatively
  charged than outside the cell
• Cell membrane is polarised.

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Na-K pump

• 3 Na leave the cell
• 2 K enter the cell
• Potential difference is created across the
  membrane

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Key
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1
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3
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• Summary of the sodium potassium pump!

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Learning Outcomes

• Describe and explain how an action potential is
  generated.
• Interpret graphs of the voltage changes taking
  place during the generation and transmission of
  an action potential.

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Resting Potential

• Potential difference across the neurone cell
  membrane while the neurone is at rest
• Inside the cell is -60mv compared with outside
  the cell.
• Cell membrane is polarised

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Generating a nerve impulse

• The permeability of the cell membrane to sodium
  ions is increased
• Sodium ions move down a concentration gradient
  into the cell
• Creating a change in the potential difference
  across the membrane
• Inside the cell becomes less negative
• This is depolarisation

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Definitions

• Generator potential
• Small depolarisation caused by sodium ions
  entering the cell
• Action potential
• Depolarisation of the cell membrane
• Inside is more positive than the outside
• Potential difference 40mv
• Threshold potential
• Potential difference across membrane of -50mv

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Ionic movements in an action potential

1. Membrane is polarised at rest (-60mv)
2. Sodium ion channels open
3. Membrane depolarises (threshold value -50mv)
4. Voltage-gated sodium ion channels open and many
   sodium ions flood in
5. Potential difference across plasma membrane
   reaches 40mv

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Ionic movements in an action potential

1. Sodium ion channels close and potassium channels
   open
2. Potassium ions diffuse out of the cell, this is
   repolarisation
3. Hyperpolarisation the potential difference
   overshoots slightly
4. Resting potential restored

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Pupil ActivityLabel the diagram

• Resting potential K voltage-gated channels
  open, Na voltage-gated channels closed
• Hyperpolarisation and repolarisation
  sodium-potassium pumps restablish the resting
  potential

• Action potential established
• Repolarisation
• Sodium ions enter causing a greater influx of
  sodium ions (positive feedback)
• Na voltage-gated channels open

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Exam Question
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Summarygenerating an action potential

• Look at the animation
• For a narrated animation look at
  http//bcs.whfreeman.com/thelifewire/content/chp44
  /4402002.html

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Refractory period

• Allows the cell to recover after an action
  potential
• Ensures action potentials are only transmitted in
  one direction

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Summary of sensory reception

• Sensory receptors
• Are specific to a single type of stimulus
• Act as transducers
• Produce a generator potential
• Give and all or nothing response
• Become adapted

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Learning outcomes

• Describe and explain how an action potential is
  transmitted in a myelinated neurone, with
  reference to the roles of voltage-gated sodium
  ion and potassium ion channels.

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Transmission of an action potential

• Key ideas
• Local currents
• Voltage-gated sodium ion channels
• The myelin sheath
• Saltatory conduction

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Local Currents

• This is the movement of ions along the neurone
• During an action potential
• Sodium ion channels open
• Sodium ions diffuse across membrane
• Upsets balance of ionic concentrations
• Concentration sodium ions inside neurone rises
• Sodium ions diffuse sideways
• Movement of charged particles is a local current.

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Voltage-gated sodium ion channels

• These gates are operated by changes in the
  voltage across the membrane
• Movement of sodium ions alters the potential
  difference
• Depolarisation causes gates to open
• Sodium ions enter neurone at a point further
  along the membrane
• Action potential moves along the membrane

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Think

• Is this an example of positive or negative
  feedback
• Give reasons for your answer

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Saltatory conduction

• This speeds up the transmission of the action
  potential (up to 120ms-1)
• In a myelinated neurone
• Ionic exchanges can only occur at the nodes of
  Ranvier
• Local currents are elongated, sodium ions diffuse
  along neurone from one node of Ranvier to the
  next, a distance of 1 3 mm
• Action potential appears to jump from one node to
  the next

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Saltatory conduction
Transmission of an action potential
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Learning Outcomes

• Outline the significance of the frequency of
  impulse transmission.
• Compare and contrast the structure and function
  of myelinated and non-myelinated neurones.

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Information carried by action potentials

• Action potentials are always the same size
• Strength of stimulus
• Frequency of action potentials
• Strong stimulus will generate more frequent
  action potentials
• Brain interprets a stream of closely spaced
  action potentials as a strong stimulus
• A strong stimulus is likely to stimulate more
  neurones than a weak stimulus

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Information carried by action potentials

• Nature of stimulus
• Deduced by the position of the sensory neurone
  bringing the information

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Speed of conduction

• The wider the axon the faster the speed of
  transmission
• Myelin insulates axons, speeding up transmission
  of an action potential along them
• Myelinated neurones 100 120 ms-1
• Unmyelinated neurones 2 20 ms-1

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Pupil Activity

• Read through the handout on Multiple Sclerosis
• Complete the table
• Answer the question.

90
Learning Outcomes

• Describe, with the aid of diagrams, the structure
  of a cholinergic synapse.
• Outline the role of neurotransmitters in the
  transmission of action potentials.
• Outline the roles of synapses in the nervous
  system.

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The synapse
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The Cholinergic Synapse

• A synapse is a junction between two or more
  neurones.
• A synapse which uses acetylcholine as a
  neurotransmitter is called a cholinergic synapse.

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The presynaptic membrane

• The synaptic knob (bulb) is a swelling at the
  end of the presynaptic membrane. It contains
• Many mitochondria
• Smooth endoplasmic reticulum
• Vesicles containing acetylcholine
• Voltage-gated calcium ion channels in the
  membrane

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Transmission across the synaptic cleft

1. An action potential arrives
2. Calcium ion channels open
3. Vesicles containing acetylcholine move to the
   presynaptic membrane.
4. Vesicles fuse with the presynaptic membrane and
   release acetylcholine into the synaptic cleft.

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Transmission across the synaptic cleft
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Transmission across the synaptic cleft

1. Acetylcholine diffuse across the synaptic cleft
   to the postsynaptic membrane
2. Acetylcholine binds to receptors in postsynaptic
   membrane
3. Sodium ion channels open the membrane is
   depolarised and an action potential is produced.

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Transmission across the synaptic cleft
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Transmission across the synapse

• On the worksheet
• Label the diagram of the synapse
• Sort out the sentences into the correct order

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Acetylcholinesterase

• Acetylcholinesterase is an enzyme found in the
  synaptic cleft
• It hydrolyses acetylcholine into ethanoic acid
  and choline
• Choline is taken back to the presynaptic membrane
  to reform Acetylcholine

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Functions of a synapse

• Transmit information between neurones
• Are unidirectional
• Act as junctions
• Filter out low level stimuli
• Summation
• Amplification of low level signals
• Acclimatisation
• Prevent overstimulation and fatigue
• Memory and learning

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Synoptic Question

• The cytoplasm in the synaptic knob has a high
  proportion of certain organelles. These include
  smooth endoplasmic reticulum, mitochondria and
  vesicles. Each organelle has a specific role to
  play in the functioning of the cell.
• Describe the role of each of these organelles and
  explain why they are found in relatively large
  numbers in the synaptic knob.

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SAQ

• Describe the roles of
• Sodium ion channels
• Potassium ion channels
• Calcium ion channels
• In the transmission of information along and
  between neurones

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Past Paper Exam Questions

• 20 marks 20 minutes
• You should be able to complete this prep in 20
  minutes
• Papers taken from OCR June 05 06

104
SAQ

• Compare the structure of a motor neurone to that
  of the typical animal cell. How does the
  specialised structure of a neurone relate to its
  function?

105
Learning Outcomes

• Define the terms endocrine gland, exocrine gland,
  hormone and target tissue.
• Explain the meaning of the terms first messenger
  and second messenger, with reference to
  adrenaline and cyclic AMP (cAMP).
• Describe the functions of the adrenal glands.

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Definitions

• Endocrine Gland
• Secretes its product directly into the blood or
  lymph.
• Exocrine gland
• Secretes its product into a duct to take the
  secretions to the site of action.

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Hormones and target cells

• A hormone
• Is a protein or steroid molecule which acts as a
  chemical messenger
• Causes a specific response in target cells
• Target cells
• Possess a specific receptor on cell surface
  membrane complementary to the hormone

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First and second messengers
First Messenger
Endocrine cell

1. Protein hormone secreted from a cell in an
   endocrine organ

1. Hormone circulates in body fluids

1. Hormone binds to receptor on the plasma membrane
   of a target cell

target cell
Second Messenger
target of second messenger inside the cell

1. Activation of a second messenger inside the cell

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Adrenaline and cyclic AMP

• Adrenaline is released by the adrenal glands
• Binds to glycoprotein receptors on the plasma
  membrane of target cells
• The enzyme adenyl cyclase becomes active
• Concentration of cAMP in the cell increases
• cAMP activates the first of a cascade of enzymes

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• The last enzyme in the cascade is kinase
• Kinase binds to glycogen phosphorylase
• This catalyses the breakdown of Glycogen into
  glucose in the liver cells

111
Mechanisms of hormone action
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The adrenal glands

• Adrenal Cortex
• Uses cholesterol to produce steroids
• Glucocorticoids stimulate the synthesis of
  glycogen in the liver
• Mineralocorticoids increase the uptake of Na in
  the gut and raise blood pressure
• Adrenal Medulla
• Secretes Adrenaline in response to stress
• Preparing the body to fight or take flight

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Adrenaline

• The role of adrenaline is to prepare the body for
  action, list as many of the effects of adrenaline
  as you can, and explain how the effect prepares
  the body for action.
• Relax smooth muscle in bronchioles
• Increase stroke volume of the heart
• Increase heart rate
• Cause general vasoconstriction
• Stimulates breakdown of glycogen
• Dilates the pupils
• Increase mental awareness
• Inhibit the action of the gut

114
Learning outcomes

• Describe, with the aid of diagrams and
  photographs, the histology of the pancreas, and
  outline its role as an endocrine and exocrine
  gland.
• Explain how blood glucose concentration is
  regulated, with reference to insulin, glucagon
  and the liver.

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The pancreas

• The islets of langerhans are patches of endocrine
  tissue scattered throughout the exocrine tissue
  of the pancreas
• Islets make up 15 of the pancreas
• A-cells secrete glucagon
• B-cells secrete insulin
• These hormones help to regulate blood glucose
  concentrations

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Section through the pancreas
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The pancreas

• Islets of Langerhans
• Groups of cells which carry out the endocrine
  functions
• Alpha cells (a cells)
• Secrete glucagon which stimulates glycogen ?
  glucose
• Beta cells (ß cells)
• Secrete insulin which stimulates glucose ?
  glycogen
• These two types of cells work antagonistically

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Control of blood glucose

• Blood glucose concentration in a healthy human 80
  120mg/100cm3
• A decrease in blood glucose
• Cells may run out of blood glucose for
  respiration
• An increase in blood glucose
• May upset the normal behaviour of cells
• Blood glucose levels never remain constant they
  oscillate above and below a required level due to
  the time delay between the change and the onset
  of corrective actions.

120
Blood glucose levels will rise due to the
following-
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Glucagon and Insulin

• Glucagon leads of activation of enzymes to
• Convert glycogen to glucose
• Increase the rate of gluconeogenesis
• Insulin
• Rate of respiration increases
• Rate of conversion glucose to glycogen increases
• Rate at which glucose is converted to fat and
  stored in adipose tissue increases

122
Learning Outcomes

• Compare and contrast the causes of Type 1
  (insulin-dependent) and Type 2 (non-insulin-depend
  ent) diabetes mellitus.
• Discuss the use of insulin produced by
  genetically modified bacteria, and the potential
  use of stem cells, to treat diabetes mellitus.

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Diabetes mellitus

• Type I diabetes
• Insulin-dependent diabetes or juvenile onset
  diabetes
• Beta cells do not make insulin
• Type II diabetes
• Non-insulin dependent diabetes
• Insulin is produced, but target cells do not
  respond to it adequately

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Diabetes

• Hyperglycaemia
• High blood glucose levels
• Associated with ketoacidosis
• Hypoglycaemia
• Low blood glucose levels

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Risk Factors / causes

• Type 1 Insulin dependent diabetes
• Viral infection
• Autoimmune response
• ? Genetic?
• Type 2 non-insulin dependent diabetes
• Obesity
• Genetic link family history
• A diet high in sugars
• Asian or afro-Caribbean origin
• Apple shaped
• BMI gt 27

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Treating Diabetes

• There is no cure
• Type 1
• Patients monitor blood glucose levels, take
  insulin injections
• Most common form of insulin is now GM insulin
• Type II
• Well-controlled diet / weight loss diet

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Stem Cell Therapy

• Stem Cell
• An undifferentiated cell capable of cell division
  and forming specialised cells
• Transplant stem cells into a pancreas that has no
  functioning beta cells
• Persuade these cells to form new beta cells that
  can secrete insulin.
• Use stem cells to produce white blood cells that
  do not attack the beta cells in the pancreas

128
Learning Outcomes

• Outline the hormonal and nervous mechanisms
  involved in the control of heart rate in humans.

129
Control of Heart Rate

• Beating of the heart is myogenic
• Each contraction is initiate by the sino-atrial
  node
• Information can be transferred through the body
  and to the SAN by nerves and hormones to increase
  the pace set by the SAN.

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Nervous control of heartbeat

• SAN receives nerve impulses along two nerves
• Vagus nerve (parasympathetic nerve)
• Slows down the rate of the SAN
• Sympathetic nerve
• Speeds up heart rate
• Both these nerves arise from the cardiac centre
  in the brain

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Hormonal control of heartbeat

• Adrenaline speeds up the rate of the SAN,
  increasing heart rate.