Coordination Flashcards
Describe the neurone cell body.
Contains a nucleus and large amounts of rough endoplasmic reticulum. This is associated with the production of proteins and neurotransmitters.
Describe a neurones dendrons.
Small extensions of the cell body which subdivide into smaller branched fibres, called dendrites, that carry nerve impulses towards the cell body.
Describe the neurone axon.
A single long fibre that carries nerve impulses away from the cell body.
Describe the Schwann cells.
Surround the axon, protecting it and providing electrical insulation. They also carry out phagocytosis (removal of cell debris) and play a part in nerve regeneration. Schwann cells wrap themselves around the axon many times, so that layers of their membranes build up around it.
Describe the myelin sheath.
Forms a covering to the axon and is made up of the membranes of the Schwann cells. These membranes are rich in a lipid known as myelin. Neurones with a myelin sheath are called myelinated neurones.
Describe the nodes of Ranvier.
Gaps between adjacent Schwann cells where there is no myelin sheath. The gaps are 2-3 um long and occur every 2-3 mm in humans.
Describe sensory neurones.
Transmit nerve impulses from a receptor to an intermediate or motor neurone. They have on dendron that carries the impulse towards the cell body and one axon that carries it away from the cell body.
Describe motor neurone.
Transmit nerve impulses from an intermediate or sensory neurone to an effector, such as a gland or muscle. They have a long axon and many short dendrites.
Describe intermediate neurone.
Transmit nerve impulses between neurones, for example, from sensory to motor neurones. They have numerous short processes.
Define a nerve impulse.
A self propagating wave of electrical disturbance that travels along the surface of the axon membrane. Temporary reversal of the electrical potential difference across the axon membrane.
How is the movement of ions, such as sodium (Na+) ions and potassium (K+) ions, across the axon membrane controlled?
- The phospholipid bilayer of the axon plasma membrane prevents sodium and potassium ions diffusing across it.
- Molecules of proteins, known as intrinsic proteins, span phospholipid bilayer. These proteins contain channels, called ion channels, which pass through them. Some of the channels have gates, which can be opened or closed in order to allow Na+ or K+ ions to move through them or stop them from passing through. Some channels stay open all the time, allowing Na+ and K= ions to diffuse through them.
- Some intrinsic proteins actively transport K+ ions into the axon and sodium ions out of the axon. Called sodium-potassium pump.
What is the resting potential (charge)
Ranges from 50 to 90 mV but usually 65 mV. Axon is polarised
What events cause the establishment of the potential difference during the resting potential.
- Na+ ions actively transported OUT of the axon by Na-K pumps.
- K+ ions actively transported into axon by Na-K pumps.
- The active transport of Na+ ions is greater than that of K+ ions, so 3 Na+ ions move out for every 2 K+ in.
- Na+ and K+ ions are positive, however the outward movement of Na+ ions is greater than the inward movement of K+ ions. As a result, there are more Na+ ions in tissue fluid surrounding the axon than in the cytoplasm, and more K+ ions in the cytoplasm than in the tissue fluid, thus creating a chemical gradient.
- The Na+ ions begin to diffuse back naturally into the axon while the K+ ions begin to diffuse back out.
- However, most of the K+ gates in the channels are open, while most of Na+ gates are closed.
- As a result the axon membrane is 100 times more permeable to K+ ions, which therefore diffuse back out of the axon faster than Na+ ions diffuse back in. This further increases potential difference (difference in charge) between -ve inside and +ve outside of axon.
- There is also an electrical gradient. As more K+ ions diffuse out of the axon, so the outside of the axon becomes more and more +ve. Further outward movement of K+ ions then becomes difficult because, being +ve they are attracted to the overall -ve state inside the axon which compels them to move in and repelled by +ve outside when prevents them moving out.
- An equilibrium is established in which the chemical and electrical gradients are balanced and there is no net movement of ions.
What is the action potential? (simple)
When a stimulus is received by a receptor or nerve ending, its energy causes a temporary reversal of charges on the axon membrane. As a result the -ve charge of -65 mV becomes +40mV. Membrane is said to be depolarised.
Why does depolarisation occur during the action potential.
The channels in the axon membrane change shape, and hence open or close, depending on the voltage across the membrane. Therefore called voltage gated channels.
Describe the action potential.
- At resting potential some K voltage gated channels are open (permanently) but some Na voltage gated channels are closed.
- The energy of the stimulus cause some Na voltage gates channels to open and therefore sodium ions diffuse into the axon through these channels along their electrochemical gradient. Being +vely charged they trigger a reversal of potential difference across the membrane.
- As the Na+ ions diffuse into the axon, so more Na channels open, causing an even greater influx of Na+ ions by diffusion.
- Once the action potential of around +40mV has been established, the voltage gates on Na+ ion channels close (preventing further influx) and voltage gates on K+ ions begin to open.
- The electrical gradient that was preventing further outward movement of K+ ions is now reversed, causing more K+ ion channels to open. This means that yet more K+ ions diffuse out, causing repolarisation of the axon.
- The outward diffusion of these K+ ions causes a temporary overshoot of the electrical gradient, with the inside of the axon being more -ve (relative to outside) than usual (HYPERPOLARISATION). The gates on the K+ ion channels now close and the activities of the sodium-potassium pumps once again cause Na+ ions to be pumped out and K+ ions in. The resting potential of -65mV is re-established. Axon is repolarised.
Does the size of the action potential change?
No, remains the same from one end of an axon to other.
How (simply) does an action potential pass along a membrane?
Nothing physically moves but rather the reversal of electrical charge is reproduced at different points along axon.
As one region of the axon produces an action potential and becomes depolarised, it acts as a stimulus for the depolarisation of the next region of the axon. In this manner, action potentials are regenerated along each small region of the axon membrane. In the meantime, the previous region of the membrane returns to resting potential, that is, it undergoes repolarisation.
Describe the process of the passage of an action potential along an unmyelinated axon.
- At resting potential the concentration of Na+ ions outside the axon membrane is high relative to the inside, whereas that of the K+ ions is high inside the membrane relative to outsied. The overall conc. of +ve ions is, however, greater on the outside, making this +ve compared to inside. Axon membrane is POLARISED.
- A stimulus causes a sudden influx of Na+ ions and hence a reversal of charge on the axon membrane. This is the action potential and the membrane is DEPOLARISED.
- The localised electrical circuits established by the influx of Na+ ions cause the opening of Na voltage-gated channels a little further along the axon. The resulting influx of Na+ ions in this region causes DEPOLARISATION. Behind the new region of depolarisation, the Na voltage-gated channels close and K ones open, K ions begin to leave the axon along their electrochemical gradient.
- The action potential is propagated in the same way further along the axon. The outward movement of the K+ ions has continued to the extent that the axon membrane behind the action potential has returned to its original state (+ve outside, -ve inside). REPOLARISED.
- Repolarisation of the axon allows Na+ ions to be actively transported out, once again returning the axon to its resting potential in readiness for new stimulus if it comes.
Describe the passage of an action potential across a myelinated axon.
In myelinated axons, the fatty sheath of myelin around the axon act as an electrical insulator, preventing action potentials from forming. At intervals of 1-3 mm there are breaks in this myelin insulation, called nodes of Ranvier. Action potentials occur at these points. the localised circuits therefore arise between adjacent nodes of Ranvier and the action potentials in effect “jump” from nose to node in a process known as saltatory conduction. As a result, an action potential passes along a myelinated neurone faster than along an unmyelinated one.
What factors affect the speed at which action potentials travel.
- The myelin sheath. Acts as an electrical insulator, Action potential jumps from one node of Ranvier to another.. Increases speed of conductance from 30ms-1 to 90ms-1.
- Diameter of axon. Greater the diameter the faster the speed of conductance. Due to less leakage of ions from a large axon (leakage makes membrane potentials harder to maintain).
- Temperature. Affects the rate of diffusions of ions and therefore the higher the temp. the faster the nerve impulse. the energy for active transport comes from respiration which is controlled by enzymes (as is Na-K pump). Enzymes function faster at higher temps. to a point. Above a certain temp. enzymes and plasma membrane protein denature.
What is the refractory period.
Once an action potential has been created in any region of an axon, there is a period afterwards when inward movement of Na+ ions is prevented because the Na voltage gated channels are closed. During this time it is impossible for a further action potential to be generated.
What is the purpose of the refractory period (3).
- Ensures that an action potential is propagated in one direction only. An action potential can only pass from an active region to a resting region. This is because an action potential cannot be propagated in a region that is refractory, which means it can only move in forwards direction. Prevents action potential spreading in both directions.
- Produces discrete impulses. Due to the refractory period, a new action potential cannot be formed immediately behind the first one. This ensures that action potentials are separated from one another.
- It limits the number of action potentials. As action potentials are separated from one another this limits the number of action potentials that can pass along an axon in a given time.
Define threshold value.
The minimum intensity that a stimulus must reach in order to trigger an action potential in a neurone.
Why are nerve impulses described as all or nothing responses?
There is a certain level of stimulus, called the threshold value, which triggers an action potential. Below the threshold value, no action potential, and therefore no impulse, is generated. Any stimulus, of whatever strength, that is below the threshold value will fail to generate an action potential- nothing part. Any stimulus above threshold value will succeed in generating an action potential. It does not matter how much above threshold a stimulus is, it will only generate one action potential- all part.
How can an organism perceive the size of an impulse?`
- By the number of impulses passing in a given time. The larger the stimulus, the more impulses that are generated in a given time.
- By having different neurones with different threshold values. The brain interprets the number and type of neurones that pass impulses as a result of a given stimulus and thereby determines its size.
Why do cells need to be coordinated?
As species have evolved, their cells have become adapted to perform specialist functions. This means cells have lost the ability to perform other functions. This makes cells dependent upon others to carry out the functions they have lost.
These different functional systems must be coordinated if they are to perform efficiently in a coordinated fashion.
What are the two main systems of coordination in mammals?
- The nervous system
- The hormonal system
How does the nervous system pass impulses?
Use nerve cells to pass electrical impulses along their length.
How does the nervous system stimulate target cells?
They stimulate their target cells by secreting chemicals known as neurotransmitters directly on to them. (quick)
What are the responses of the nervous system like?
Rapid communication between specific parts of an organism. The responses produced are often short lived and restricted to a localised region of the body.
Effect is temporary and reversible.
What is an example of nervous coordination?
A reflex action, such as the withdrawal of the hand from an unpleasant stimulus.
How does the hormonal system stimulate target cells?
Produces chemicals that are transported in the blood plasma to their target cells, which they then stimulate.
What are the responses of the hormonal system like?
Transmission and response slower, less specific form of communication between parts of an organism. The responses are often long lasting and widespread. Travel to all parts of the body but only target organs respond.
Effect may be permanent and irreversible.
What is an example of hormonal communication?
The control of blood glucose.
What are chemical mediators?
These are chemicals that are released from certain mammalian cells and have an effect on cells in their immediate vicinity.
