Coordination & Control in Animals

• What does coordination in animals rely on?

  Coordination in animals relies not only on hormones but also on nervous impulses that travel through neurones.
• What are the two key differences between nervous + hormonal control?

  Nervous control is faster and more precise than hormonal control.
• What is the function of neurones?

  Neurones transmit electrical nerve impulses throughout the body.
• How is fine control + integration provided?

  Fine control and integration is provided through a system of synapses between the neurones that can control the pathways involved.
• What does the nervous system of a mammal consist of?

  The nervous system of a mammal consists of the CNS (brain and spinal cord) as well as a large number of peripheral nerves.
• What are nerves + how can they be categorised? (2)

  - Nerves are bundles of neurones grouped together within an outer protective layer.

  
  

  - They can contain only sensory or only motor, or they can be mixed and contain both sensory and motor neurones.
• What is the role of the CNS?

  The role of the CNS is to integrate the incoming information from the peripheral nerves and coordinate an effective response.
• There are 3 classes of neurone found in the nervous system and each has a slightly different structure reflecting their function. What are the common features with each of these cell types? (5)

  - Cell body or centron where most organelles e.g. nucleus, mitochondria and Nissl's granules (large groups of ribosomes) are found

  
  

  - Dendron(s) - these are fine threads of cytoplasmic threads that deliver impulses TOWARDS the cell body

  
  

  - Axons - these are fine cytoplasmic threads that carry impulses AWAY from the cell body

  
  

  - Dendrites are small and numerous extensions that can conduct impulses into a dendron or a cell body

  directly.

  
  

  - Synaptic bulb is where axons terminate
• Describe the structure of sensory neurones.

  Sensory neurones tend to have dendrons and axons that have a similar long length and therefore diagrams often show the cell body centrally placed.

  
• Describe the structure of motor neurones.

  Motor neurones have long axons but many short dendrons (often referred to as dendrites).

  
• Describe the structure of association neurones.

  Association / relay / connector neurones are shorter in overall length compared to the other types of neurones + have shorter dendrons and axons.

  
• What does myelinated mean + what type of animals have this? (2)


  - Most of the nerves in mammals are myelinated. This means they are wrapped in Schwann cells - creating an electrically insulating myelin sheath.

  
  

  - The Schwann cells are repeatedly wrapped around the axon, and the greatly extended cell surface membrane of the cell is rich in the lipid myelin.

  
• What are the Nodes of Ranvier?

  Between each Schwann cell a small patch of the neurone's membrane is exposed - the nodes of Ranvier.

  
• What is the significance of myelination?

  The significance of myelination is that it greatly speeds up the speed of nerve impulse transmission (see later notes) as well as protecting the neurone.

  
• What does the cell membrane result in, in all cells?

  In all cells, the presence of a cell membrane results in uneven distribution of charged ions across the membrane.
• What does potential difference mean?

  The difference in charge between 2 regions (e.g. inside and outside the cell) is referred to as the potential difference.
• What are polarised cells?

  Cells that have a potential difference across their membrane are said to be polarised.
• What is the resting potential of the neurone? (3)

  - At rest, neurones have an excess of positively charged ions (Na+) surrounding them (outside).

  
  

  - This results in a potential difference of -70mV existing as there are many more positively charged ions outside the cell + the

  cell contains many proteins which tend to be negatively charged.

  
  

  - This is the resting potential of the neurone.

  
• What can the resting potential be maintained at + what does this cause? (2)


  - This differential can be maintained as at rest, a neurone is largely impermeable to the flow of sodium ions.

  
  

  - For this reason there is an electrochemical gradient across the membrane (the positive ions want to move to the more negative region inside the cell, as well as there being a chemical diffusion gradient).
• What is the potential difference + how is it represented? (2)

  - The inside is less positive than the outside as there are fewer positive ions inside the cell.

  
  

  - This is the potential difference and is represented with the inside being negatively charged.

  
• What happens when a stimulus is applied to the neurone? (2)

  - When a stimulus is applied to the neurone the transmembrane proteins open and the positively charged (Na+) ions flood into the cell

  
  

  - The cell's potential difference rises (becoming less negative) i.e. the membrane becomes permeable to ions.
• When will the threshold potential be reached? (2)

  - If enough transmembrane proteins open and enough positively charged ions diffuse into the neurone down their electrochemical gradient, the threshold potential will be reached.

  
  

  - This is around -55mV in mammals.
• What happens as a product of the threshold potential? (3)

  - At this point, even more voltage gated ion channels open (increasing the rate of diffusion of positively charged Na+ ions into the axon) and the neurone becomes rapidly depolarised.

  
  

  - The P.D across the membrane reaches a peak of around +40mV. This is known as the action potential.

  
  

  - This sequence of events takes about 1millisecond.
• What is a "sub threshold stimulus"?

  If a stimulus does not result in enough ions channels being opened and the threshold potential is not reached then an action potential is not fired and the stimulus would be referred to as a "sub threshold stimulus".
• What is the "all or nothing" principle in terms of neurones?

  In this way neurones display the "all or nothing" principle - an action potential always peaks at the same value regardless of the intensity of the stimulus - more intense stimuli will result in a more frequent firing of action potentials.
• When does the recovery phase start + what is the refractory period? (3)

  - At the peak of the Action potential, the recovery phase starts and the positive ions both diffuse and are pumped out of the neurone.

  
  

  - This rapidly restores the resting potential and the cell membrane becomes largely impermeable again to Na+ ions.

  
  

  - This is known as the refractory period.
• What is repolarisation?


  Following depolarisation the neurone must actively re-establish the resting potential.
• What is the refractory period?


  The period of time taken for repolarisation is known as the refractory period and the neurone cannot be stimulated during this time, as the gated ion channels are closed and the resting potential has not been fully restored.
• How long does the electrochemical sequence of events associated with AP take?

  The entire electrochemical sequence of events associated with an AP takes about 4 milliseconds.
• Why is the refractory period significant? (2)

  - It limits the speed at which action potential fires and therefore allows the coordinator (the brain/spinal cord) to detect each action potential as a discrete event.

  
  

  - It also ensures that impulses can only travel in 1 direction. This is important as axons are actually physiologically capable of transmitting an impulse in either direction.
• Explain numbers 1-5 in the diagram which show the changes in the potential difference across the axon membrane as an action potential occurs.

  1. Resting potential with inside of axon -70mV relative to outside (another way of saying outside of the membrane is +70mV relative to inside).

  
  

  2. Membrane becomes permeable + positive ions diffuse into the axon → membrane is starting to become depolarised.

  
  

  3. At -55mV (inside relative to the outside) gated channels open + positive ions flood in at an even more rapid rate; rapid depolarisation of the membrane takes place + the inside becomes +40mV relative to the outside → the action potential.

  
  

  4. Positive ions diffuse out + are also pumped out of the axon.

  => This stage is called the refractory period as the membrane cannot be depolarised again until the resting potential is restored.

  => At the end of this stage there is a slight overshoot as the inside of the axon membrane becomes slightly more negative (hyperpolarisation) than in the normal resting potential.

  
  

  5. The resting potential is restored + the axon can conduct another nerve impulse if stimulated.

  
• What are the Threshold stimulus and the "all-or-nothing law"? (2)

  - The threshold stimulus refers to the level of stimulus a neurone requires before an action potential is produced, for example, a strong degree of depolarisation in the cell surface membrane of the neurone can occur without resulting in an action potential but at a critical point (the threshold potential) an action potential will result.

  
  

  - All-or-nothing law The all-or-nothing law refers to the principle that once the threshold stimulus is reached, the action potential results (i.e. an action potential either occurs or it doesn't). Different intensities of action potential do not occur → they are all the same.

  
• How fast do action potentials move?

  Action Potentials (or a wave of depolarisation) moves very rapidly along neurones (a bit like a Mexican wave).

  
• What is an impulse + what is it a result of? (2)

  - An impulse is the propagation of action potentials along the neurone.

  
  

  - This is as a result of the fact that the transmembrane proteins are "voltage gated ion channels".
• What is generated along the length of the membrane? (2)

  - There are localised circuits generated along the length of the membrane.

  
  

  - Areas where there are excess negative charges attract positive charges and vice versa.
• What happens as one part of the membrane becomes depolarised? (3)

  - As one part of the membrane becomes depolarised, it sets up local electrical currents with the areas immediately adjacent on either side.

  
  

  - +ve ions from the depolarised zone pass along the inside of the membrane toward the polarised zone immediately in front.

  
  

  - On the outside of the membrane +ve ions move back from the still polarised zone into the depolarised zone.
• What direction do the localised circuits flow + why?

  These localised circuits flow in both directions but due to the refractory period can only result in action potentials in areas of the membrane that have had the resting potential returned (so regions still in the refractory period cannot react).
• What happens in relation to the voltage gated ion channels + what does this result in? (3)

  - Voltage gated ion channels are able to sense these localised changes in charge and respond by opening their "gates" (really they just change shape in response to a change in localised charge).

  
  

  - As a result, positively charged ions flowing into the cell result in depolarisation (therefore making an action potential more likely).

  
  

  - This sequence of events continues along the length of the neurone - resulting in an impulse.

  
• What are three factors affecting the speed of conduction? (3)

  - In myelinated neurones the speed of impulse transmission is greatly increased as local current can only exist at the nodes of Ranvier.

  - The myelin sheath acts as an electrical insulator.

  - Therefore the localised currents leap (or jump) from one node to the next.

  - This is known as saltatory conduction, and produce speeds of impulses up to 100ms-1

  
  

  - Many species do not have myelin wrapping their neurones so rely on other physiological features to speed up transmission - the thicker the axon the faster the impulse.

  - This is because there is proportionally less leakage of ions in a neurone with a larger diameter.

  - If there is too much leakage of ions it is more difficult to maintain the potential gradients required to form RP's and AP's e.g. the larger diameter of neurones in squid species.

  
  

  - It should also be noted that temperature also affects the speed of transmission as the rate of diffusion is directly affected by temperature (cf. kinetic theory).

  
• What are synapses? (2)

  - A synapse is the junction between neurones.

  
  

  - The neurones are not in physical contact like the components of an electrical circuit and so looking at the structure and function of the synapse is important to allow understanding of how an electrical signal can jump across the "gap".