Nervous Coordination And Muscles Flashcards
Cons To Specialised Cells?
Cons:
- as species have evolved, certain cells have lost the ability to perform certain functions, therefore, they rely on other cells to do these other functions.
- all the cells must be able to function and be coordinated to perform efficiently.
In nervous coordination and muscles, we look at how these coordinations work.
Forms Of Coordination In Animals?
There are two main ways in which animals coordinate:
- the hormonal system,
- the nervous system.
The Nervous System?
The nervous system uses electrical impulses along nerve cells.
These electrical impulses stimulate target cells by secreting chemicals, known as neurotransmitters directly onto the target cells.
This results in rapid communication between parts of the organism.
The responses in the nervous system are short-lived and occur in a localised region of the body.
An example of nervous system of coordination:
- Reflex action,
- Withdrawal of hand from an unpleasant stimulus,
- For obvious reasons, this response is rapid and short-lived.
The Hormonal System?
The hormonal system produces chemicals (hormones) that are transported in blood plasma to their target cells.
The target cells have specific receptors on their cell-surface membranes and the change in the concentration of hormones stimulates them.
This results in a less-specific and slower communication between the parts of organism.
The responses are often long-lived and widespread.
An example of hormonal system:
- control of blood glucose concentration, which produces a slower response but is longer lasting and more widespread.
Simplified Hormonal System List?
- Communication is by chemicals called hormones,
- Transmission is by blood system,
- Transmission is slow,
- Hormones travel all over the body but only target receptors can respond to their specific hormones,
- Response is widespread,
- Response is slow,
- Response is often long-lasting,
- Effect might be irreversible and permanent.
Simplified Nervous System List?
- Communication is by nerve impulses,
- Transmission is by neurones,
- Transmission is very rapid,
- Nerve impulses travel to specific places in the body,
- Response is rapid,
- Response is often short-lived,
- Response is localised,
- Effect is usually temporary and reversible.
Neurones?
Neurones (nerve cells) are adapted to rapidly carrying electrochemical changes called nerve impulses from one part of the body to the other.
Mammalian Motor Neurone Properties?
The mammalian neurone contains a:
- cell body,
- dendron,
- axon,
- Schwann cells,
- Myelin sheath,
- Nodes of Ranvier.
Cell Body?
Cell body: includes all of the usual cellular organelles, including a nucleus and large amounts of rough endoplasmic reticulum. This is due to the production of proteins and neurotransmitters.
Dendrons?
Dendrons: extensions of the cell body which subdivide into smaller branched fibres, called dendrites, that carry nerve impulses toward the cell body.
Axon?
Axon: single long fibre that carries nerve impulses away from the cell body.
Schwann Cells?
Schwann Cells: surround the axon and protect it.
They provide electrical insulation.
They also carry out phagocytosis (the 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.
Phagocytosis?
The removal of cell debris.
Myelin Sheath?
Myelin Sheath: forms a covering to the axon and is made up of membranes of Schwann cells.
These membranes are rich of liquid known as myelin.
Neurones with a myelin sheath are called myelinated neurones.
Nodes Of Ranvier?
Nodes Of Ranvier: constrictions between adjacent Schwann cells where there is no myelin sheath.
The constructions are 2-3um long and occur every 1-3mm in humans.
Sodium ion channels are conc at nodes.
This means depolarisation only happens at node (when sodium ions can get through membrane).
The neurones cytoplasm conducts enough electrical chant he to depolarise the next node, so the impulse jumps. This is called Saltatory conduction - fast.
Sensory Neurones?
Transmit nerve impulses from a receptor to an intermediate or motor neurone.
They have one dendron that is often very long.
The dendron carries the impulse toward the cell body and then an axon carries the impulse away from the cell body.
Cell body in middle, in dorsal root ganglion.
Impulse travels from the nerve endings (dendron) at the skin to the axon.
Myelinated.
Impulse travels from the dendrites and cell body to the axon.
Motor Neurones?
Transmit nerve impulses from an intermediate or relay neurone to an effector.
Effector examples: gland or muscle.
Motor neurones have a long axon and then lots of short dendrites.
Usually attached to some sort of muscle tissue.
Cell body at same side as dendrites.
Myelinated.
Intermediate Neurones?
Also known as relay neurones.
Transmit impulses between neurones, for example, from sensory to motor neurones.
They have numerous short processes.
Cell body in middle.
Not myelinated.
Impulses travel from the dendrites inwards to the cell body in middle of neurone. They all travel inward (almost like a inward spiral).
What Is A Nerve Impulse?
Self-propagating wave of electrical activity that travels along the axon membrane.
It is a temporary reversal of the electrical potential difference across the axon membrane.
This reversal is between two states, the resting potential and the action potential.
How Is Movement Of Ions Across A Membrane Controlled?
The movement of ions (e.g. Na2+ and K+) across the axon membrane is controlled in many ways:
- The phospholipid bilayer of the axon plasma membrane prevents sodium and potassium ions diffusing across it.
- Proteins, such as channel proteins, span these phospholipid bilayers. These proteins have channels, called ion channels, which pass through them. These channels can open or close so that sodium or potassium ions can move through them via facilitated diffusion at any one time. Some of the channels are open all the time.
- Some carrier proteins actively transport potassium ions into the axon and sodium ions out of the axon. This mechanism can be called a sodium-potassium pump.
Resting Potential?
As a result of the control of ions that can cross a membrane, the inside of an axon is negatively charged relative to the outside of the axon.
This is known as resting potential. This negative charge is usually around (minus) 50-90 millivolts (mV) but is usually around (minus) 65mV in humans.
In this condition, the axon is said to be polarised.
Steps Of Resting Potential?
- Sodium ions are actively transported out of the axon by sodium-potassium pumps.
- Potassium ions are actively transported into the axon via sodium-potassium pumps.
- The active transport of sodium ions is greater than that of potassium ions. Three sodium ions move out for every two that move in.
- Although both sodium and potassium ions are positive, the outward movement of sodium ions is greater than the inward movement of potassium. Therefore, there are more sodium ions in the tissue fluid outside the axon than in the cytoplasm. And more potassium ions in the cytoplasm than in the tissue fluid outside the axon. This creates an electrochemical gradient.
- The sodium ions begin to diffuse back naturally into the axon while the potassium begins to diffuse back out of the axon.
- Most of the gates in the channels that allow the potassium ions to move through are open, whilst most of the gates in the changes that allow sodium ions to move through are closed.
Action Potential?
When a stimulus of a sufficient size is detected by a receptor in the nervous system, it’s energy causes a temporary reversal of the charges either side of this part of the axon membrane.
If the stimulus is great enough, the negative charge of -65mV in the axon becomes +40mV.
This is known as action potential and the part of the axon membrane is said to be depolarised.
Why Does Depolarisation Occur?
This depolarisation occurs because the channels in the axon membrane change shape, and hence, open or close.
This depends on the voltage across the membrane.
They are therefore called voltage-gates channels.
The sequence of events in the steps of action potential all occur on a specific point on the axon and not the whole axon.
Steps Of Action Potential?
At resting potential, some of the potassium voltage-gated channels are open but sodium voltage-gated channels are closed.
The energy of the stimulus causes some sodium voltage-gated channels to open. Therefore, sodium ions diffuse into the axon through these channels along their electrochemical gradient.
Because sodium ions are positively charged, they trigger a reversal in the potential difference across the membrane.
As more sodium ions diffuse across the membrane, more sodium ion channels open. This causes an even greater influx of sodium ions into the axon.
Once the action potential of around +40mV has been established, the voltage gates on the sodium ion channels close and voyage gates on the potassium ions channels begin to open.
With potassium voltage-gates channels now open, the electrical gradient that was previously preventing further outward movement of potassium ions is now reversed. This means more potassium ions now flow out of the axon. This causes repolarisation of the axon.
The outward diffusion of these potassium ions causes a temporary overshoot of the electrical gradient, with the inside of the axon being more negative than usual (hyperpolarisation).
The closable gates on the potassium ions channels now close and the activities of the sodium-potassium pumps once again cause sodium ions to be pumped out and potassium ions to be pumped in.
The resting potential is then reastablisbed and the axon is reploarised.
Passive And Active Processes?
Action potential is maintained via diffusion whilst resting potential is maintained via active transport.
This means action potential is a passive process.
Resting potential is an active process.
The Size Of Action Potential?
Once an action potential has been created, it moves quickly along the axon.
The size of action potential remains the same from one side of the axon to the next.
As one region of the axon produces an action potential and becomes depolarised, it acts as a stimulus for the next region of the axon to become depolarised.
Action potential is therefore a travelling wave of depolarisation.
How Is An Impulse Propagated In An Unmyelinated Axon?
- At resting potential, no nerve impulse is being travelled and so no impulse is travelling yet.
- A stimulus causes a sudden influx of sodium ions and hence a reversal of charge on the axon membrane. This is the action potential and the membrane is depolarised.
- The localised electrical currents established by the influx of sodium ions cause the opening of sodium voltage-gates channels a little further along the axon. The influx of sodium ions causes depolarisation.
- Behind this region of depolarisation, the sodium voltage-gated channels close and the potassium ones open. Potassium ions leave the axon along their electrochemical gradient. This explains how the depolarisation current moves along the axon.
- The action potential (depolarisation) is propagated in the same way further along the axon. The outward movement of potassium ions eventually causes repolarisation.
- Repolarisation of the axon allows sodium ions to be actively transported out. This returns the axon to resting potential. The axon is ready for a new stimulus and action potential to occur.
How Does An Impulse Travel Along A Myelinated Axon?
The fatty sheath of myelin acts as an electrical insulator.
This prevents the action potentials from forming.
At intervals of 1-3mm, there are breaks in this myelin called the Nodes of Ranvier.
Action potentials can occur at these points.
The localised circuits therefore arise between these adjacent nodes of Ranvier and jump between the nodes.
What Is Saltatory Conduction?
The process of the impulse jumping from node to node in a myelinated neurone.
Saltatory conduction is faster than a normal conduction (I.e in an unmyelinated neurone of the same diameter).
This conduction is faster because the process and events of depolarisation must occur all the way along an axon that is unmyelinated and this takes more time than the process of depolarisation occuring just at nodes.
What Does A Localised Current Look Like On A Myleinated Axon?
Look at page 356.
Factors Effecting Speed At Which An Action Potential Travels?
Once an action potential begins, it travels at quickly at the same size.
However, there are factors that could effect the speed at which the impulse (action potential) travels:
- Myelin sheath,
- The diameter of the axon,
- Temperature.
What Actually Is A Nerve Impulse?
The transmission of an action potential along the axon of the neurone is the nerve impulse.
Speed That Action Potential Can Travel?
As little as 0.5ms-1 to as much as 120ms-1.
Factors affect the speed at which a nerve impulse travels.
Myelin Sheath As A Factor Effecting Nerve Impulse Speed?
The myelin sheath acts as an electrical insulator.
It prevents an action potential forming in a part of the axon covered by myelin sheath.
Myelin sheath makes the nerve impulse jump from one node of Ranvier to the next node of Ranvier (places with no myelin sheath). This is called Saltatory conduction.
This increases the speed of conductance from 30ms-1 (in an unmyelinated neurone) to 90ms-1 (in a myelinated neurone of the same diameter).
Diameter Of The Axon As A Factor Effecting Nerve Impulse Speed?
The greater the diameter of the axon, the faster the speed of conductance.
This is due to less leakage of ions from a large axon.
Leakage makes membrane potentials harder to maintain.
Temperature As A Factor Effecting Nerve Impulse Speed?
Temperature effects the rate at which ions diffuse.
Therefore, the higher the temperature, the faster the impulse travels along an axon.
This is because the energy for active transport comes from respiration. Respiration is controlled by enzymes and enzymes work more at a higher temperature (to a certain point).
So, the higher the temperature, the more respiration occurs and therefore, more energy is available for active transport.
Active transport is used in the conduction of an impulse along an axon FINISH THIS, DONT UNDRRTSNAD WHAT ACTIVE TRANSPORT IS NEEDED FOR
Temperature As A Factor Effecting Nerve Impulse Speed Especially In Animals?
If temperature is too high, plasma membranes and proteins (enzymes) are denatured.
This means a nerve impulse cannot travel along an axon at all.
Temperature is important for response time in cold-blooded animals (ectothermic) whose body temperatures vary in accordance with the environment.
Temperature also effects the speed and strength of muscle contractions.
All Or Nothing Principle?
Nerve impulses are described as all-or-nothing responses.
There is a certain level of stimulus called the threshold value, which triggers an action potential.
Below the thread hold value, no action potential can be generated. Therefore, no no impulse is generated.
Any stimulus that is below the threshold value will fail to generate an action potential.
Any stimulus above the threshold value will succeed in generating an action potential and so a nerve impulse will travel.
All action potentials are more or less the same size, and so the strength of the stimulus can not be detected by the size of the action potential.
The all or nothing principle acts as a filter, preventing minor stimuli from setting up nerve impulses and thus, preventing the brain from becoming overloaded with information.
How Can An Organism Perceive The Size Of A Stimulus?
This is achieved in two ways:
- By the number of impulses passing in a given time. The larger the stimulus, the more impulses 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.
The Refractory Period?
One an action potential has been created in a region of the axon, there is a period afterwards where inward movement sodium ions is prevented.
This is because the sodium voltage-gated channels are closed.
During this time, it is impossible for a further action potential to be generated.
This is known as the refractory period.
The refractory period limits the strength of stimulus that can be detected.
The Refractory Period Serves 3 Purposes?
- It ensures that action potentials are propagated in one direction only.
- It produces discrete impulses.
- It limits the number of action potentials.
How Does The Refractory Period Ensure Action Potentials Are Propagated In Only One Direction?
Action potentials can only pass from an active region to a resting region.
This is because action potentials can not be propagated in a region that is refractory, which means that they can only move in a forward direction.
This prevents action potentials from spreading out in both directions (which they would otherwise do).
