nervous coordination and muscles year 2 Flashcards
nervous system classification
nervous system is split up into central N.S and peripheral N.S
- uses nerve cells to pass electrical impulses along their length. Stimulate target cells by secreting neurotransmitters directly onto them. This results in rapid communication between specific parts of an organism.
central nervous system
Contains the brain and the spinal cord.
peripheral nervous system
made up of autonomic nervous system and somatic nervous system.
somatic nervous system
The somatic nervous system communicates with sense organs and voluntary/ skeletal muscles
Contains sensory and motor neurones
autonomic nervous system
communicates with internal organs/ glands and smooth muscle.
Contains sympathetic and parasympathetic nervous system
sympathetic nervous system
Main nerve is sympathetic nerve
Main neurotransmitter is noradrenalin
Gives flight or fight response. Increases heart rate, pulmonary ventilation rate and causes pupils to dilate
parasympathetic nervous system
Main nerve is the vagus nerve
Main neurotransmitter is acetyl choline
Main effects is rest and digest
Has opposite effects of parasympathetic nervous system eg causes heart rate and pulmonary ventilation rate to decrease
what is resting potential?
Membrane potential difference is -70mV at rest.
- at rest high conc of Na+ outside the cell (in the tissue fluid) and high conc of K+ in the cell
how is movement of Na+ ions and K+ ions controlled
-phospholipid bilayer of the axon plasma membrane prevents sodium and potassium ions diffusing across it. This is because Na+ is hydrophilic so will repel the hydrophobic fatty acids in the membrane not allowing it to diffuse. This is despite the large concentration gradient of the Na+ and the large attraction between the positive sodium ions and the negative membrane potential difference
-Channel proteins span the phospholipid bilayer. These proteins have ion channels. However, some of these channels have gates which can be opened or closed, so that sodium and potassium ions can move through them by facilitated diffusion. However, most of the sodium gates remain closed so most sodium ions cannot move through the membrane.
- sodium potassium pump actively transports 3 sodium ions out of the neurone, allowing 2 potassium ions to enter. This requires ATP
how is resting potential difference established
-sodium ions are actively transported out of the axon by the sodium-potassium pump.
- potassium ions are actively transported into the axon by the sodium-potassium pump
- the active transport of sodium ions is greater than that of potassium ions
- although both are positive, there are more sodium ions in the tissue fluid surrounding the axon than in the cytoplasm, and their is more potassium ions in the cytoplasm than in the tissue fluid. This is because the outward movement of sodium is greater than the inward movement of potassium. This creates an electrochemical gradient
- the sodium ions will begin to diffuse by facilitated diffusion naturally into the axon while the potassium begins to diffuse back out of the axon. However, most of the gates in the channels that allow the potassium ions to move through are open, whilst most of the gates in the channels that allow the sodium ions to move through are closed
- the sodium ions want to move into the neurone as it is oppositely charged and they have a steep conc gradient.
- the potassium ions have a concentration gradient out of the neurone, however there is an equal force due to the charge difference of the potassium ions and potential difference keeping them in the cell. These forces are equal so they remain in the membrane.
hormonal system vs nervous system
- hormonal communicates using hormones nervous by nerve impulses
- hormonal transmits by the blood system the nervous transmits by neurones
- hormonal transmission is slow but nervous transmission is very rapid
- hormones travel to all parts of the body but only target cells respond, nerve impulses travel to specific parts of the body
- hormone response is widespread and slow, nervous response is localised and rapid
- hormone response long lasting, nervous short-lived
- hormone may have a permanent and irreversible effect but the nervous system has a temporary and reversable effect
neurone structure
- specialised cells adapted to rapidly carry electrochemical nerve impulses across the body.
Mammalian motor neurone is made of: - cell body, contains all the usual cell organelles, including a nucleus and large amounts of rough endoplasmic reticulum. Associated with production of proteins and neurotransmitters
- dendrons- extensions of the cell body which divide into smaller branched fibred called dendrites. Dendrons carry nerve impulses towards the cell body
- axon- single long fibre that carries nerve impulses away from the cell body
- Schwann cells- surround the axon protecting it and providing electrical insulation. Also carry out phagocytosis (the removal of debris) and play a part in nerve regeneration. They wrap themselves around the axon many times, so that layers of their membranes build up around it
- myelin sheath- covers the axon and is made up of the membranes of the Schwann cells. Membranes are rich in myelin. Neurones with a myelin sheath are called myelinated neurones
- nodes of Ranvier- constrictions between adjacent Schwann cells where there is no myelin sheath
types of neurones
- sensory- transmit nerve impulses from a receptor to an intermediate or motor neurone. Have one dendron that is often very long. It carries the impulse towards the cell body and one axon that carries it away from the cell body
- motor- transmit nerve impulses from an intermediate or relay neurone to an effector eg a gland or muscle. Have long axon and many short dendrites
- intermediate or relay neurones- transmit impulses between neurones eg sensory to motor. Have numerous short processes
action potential
- wave of depolarisation, followed by a wave of repolarisation, moving across a membrane
how does an action potential propagate?
- At resting potential some POTASSIUM voltage-gated channels are open (mostly those that are permanently open) but the sodium voltage-gated channels are closed (as these channels are voltage gated they are able to detect changes in potential difference)
- Local currents will be induced in the membrane when a voltage/ stimuli is applied. This occurs as a current moves from positive to negative charge. The stimulus/ receptor cell will cause the initial depolarisation
- Na+ channels in the axon membrane will then open due to localised electrical currents, causes initially by the stimuli. This allows sodium ions to move into the axon down the electrochemical gradient by facilitated diffusion
- This reduces the potential difference across the axon membrane as the inside of the axon becomes less negative, as positive ions are moving in. This depolarisation triggers more channels to open, allowing more sodium ions to enter and causing more depolarisation and increasing the influx of sodium ions into the neurone.
- When threshold potential of -50mV is reached, many more channels will open until axon reaches potential of around +40mV. The sodium ions will diffuse in along the axon, depolarising the membrane in the next section of the axon. This triggers the production of another action potential in this section of the axon membrane and the process continues. This allows action potentials to begin at one end of an axon and then pass along the entire length of the axon membrane
- this is an example of positive feedback as a small initial depolarisation leads to greater and greater levels of depolarisation
- MAKE SURE TO LEARN THE GRAPH
- when action potential is traveling, the charge/ potential difference on the neurone is flipping
repolarisation
- when the action potential of around +40mV has been established, the voltage gates on the sodium ion channels close and the voltage gates on the potassium ions begins to open
- This allows potassium ions to diffuse out of the axon, down their concentration gradient, causing an efflux of potassium ions out of the neurone, as they repel the positive potential difference. This is because the electrical gradient that was preventing the outward movement of potassium has now been reversed, causing the potassium ion channels to open.
- The outward diffusion of these potassium ions cause a temporary overshoot of the electrical gradient, with the inside of the axon being more negative than usual. This is called hyperpolarisation. The hyperpolarisation inhibits the synapse for a short period of time, as it means more sodium ions have to enter for threshold to be reached
- The potassium ion voltage-gated channel proteins then close and the sodium-potassium pump once again causes sodium ions to be pumped out and potassium ions in. The resting potential of -70 mV is re-established and the axon is said to be repolarised
This occurs along the length of the whole neurone, so as the action potential is still traveling, repolarisation occurs behind it
Effect of distance along the neurone on action potentials amplitude and duration
Amplitude and duration of action potentials dont change as you move along the neurone. This is because the concentration gradient of sodium ions, resting membrane potential and number of Na+ channels all remain constant
passage of an action potential along a myelinated axon
In a myelinated axon, the fatty sheath of myelin around the axon acts as an electrical insulator, preventing action potentials from forming. At intervals there are breaks in myelin insulation called nodes of Ranvier. Myeline sheath occurs from the membranes of the Schwann cells that cover the neurone.
Action potentials can occur at the nodes of Ranvier, but Na+ and K+ cannot be exchanged across Schwann cells, but can across nodes of Ranvier. The localised circuits therefore arise between adjacent nodes of Ranvier and the action potentials jump from node to node in a process known as saltatory conduction.
As a result, an action potential passes along a myelinated neurone faster than along the axon of an unmyelinated neurone of the same diameter. This is because in an unmyelinated neurone, the events of depolarisation have to take place all the way along an axon and this takes more time.
Myelinated neurones, therefore have less influx of Na+ and less K+ efflux. This ,means there is less use of the sodium potassium pump, so less ATP is used for active transport and more can be conserved for other uses
factors affecting conduction velocity (the speed at which an action potential travels)
- Saltatory conduction. If myelinated, saltatory conduction will occur, allowing the action potential to jump between nodes of Ranvier, and therefore will have a higher conduction velocity as the action potential doesnt need to travel across the whole neurone
- Axon diameter. The greater the diameter of an axon, the greater the conduction velocity. This is due to less leakage of ions from a large axon, so easier to maintain membrane potentials
- Temperature. Affects the rate of diffusion of ions and therefore the higher the temperature the higher the conduction velocity. If temperature is low the local currents will be set up more slowly so sodium ions will diffuse into the cell more slowly. Also, at higher temperatures, before denaturing, enzymes in respiration occur more rapidly, so more ATP produced, to allow for more movement of Na+/K+ pump, increasing speed the action potential travels at
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 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.
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 a stimulus cannot be detected by the size of action potentials. If in an experiment voltage is increased, there will be no effect on amplitude or potential difference, as there will be either no action potential or a full action potential
- In an experiment, the higher the stimulus intensity, the higher the frequency of action potential traveling along a neurone. This shows that the action potential will still stay the same
- If in an experiment the stimulating and recording electrode are moved closer together, then small bumps will appear on the oscilloscope. Some voltage causes a small rise in potential difference, but not enough to cause an action potential. Some sodium channels open, but not enough to start positive feedback. When the potential difference threshold is reached, an action potential is also reached
- can act as a filter to prevent minor stimuli from setting up nervous impulses and overloading the brain
how can the size of a stimulus be measured?
- 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. Brain interprets the number and type of neurones that pass impulses as a result of a given stimulus and thereby determines its size
the refectory period
Once an action potential has been created in any region of an axon, there is a period afterwards where inward movement of sodium ions is prevented 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
- each sodium channel needs a refractory period where they close and remain closed
- the sodium channels open at different points along the wave of depolarisation. This means that there refractory periods will end at different points, leading to a small wave of depolarisation that doesn’t result in an action potential
- when all of the sodium channels refractory periods end an action potential occurs
purposes of the refractory period
- To ensure that action potentials are propagated in one direction only. Action potentials can only pass from an active region to a resting region. This is because action potentials cannot be propagated in a region that is refractory, as the sodium channels will remain closed, which means that can only move in a forward direction
- To produce 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 they are separated from one another, this limits the number of action potentials that can pass along an axon in a given time, and thus limits the strength of stimulus that can be detected
structure of a synapse
Synapses transmit information, but not impulses, from one neurone to another by means of chemicals called neurotransmitters.
Neurones are separated by a small gap called the synaptic cleft.
The neurone that releases the neurotransmitter is called the presynaptic neurone. The axon of this neurone ends in a swollen portion known as the synaptic knob. This possesses many mitochondria and large amounts of endoplasmic reticulum. These are required in the manufacture of the neurotransmitter which takes place in the axon.
The neurotransmitter is stored in the synaptic vesicles
Once the neurotransmitter is released from the vesicles it diffuses across to the postsynaptic neurone, which possesses specific receptor proteins on its membrane to receive it
functions of synapses
- transmit information from one neurone to another. In doing so they act as junctions, allowing:
- a single impulse along one neurone to initiate new impulses in a number of different neurones at a synapse. This allows a single stimulus to create a number of simultaneous responses
- a number of impulses to be combined at a synapse. This allows nerve impulses from receptors reacting to different stimuli to contribute to a single response
- prevent action potentials going the other way around a neural circuit, as vesicles only found in pre-synaptic cleft and receptors only found on post-synaptic cleft
- they can also break/reform to modify themselves and allow new connections eg in the brain
cholinergic synapse
- once in which the neurotransmitter is a chemical called acetylcholine. Acetylcholine is made up of: acetyl and choline. Cholinergic synapses are common in vertebrates, where they occur in the central nervous system and at neuromuscular junctions
1. The arrival of an action potential at the end of the presynaptic neurone causes calcium ion protein channels to open and calcium ions to enter the synaptic knob by facilitated diffusion
2. The influx of calcium ions into the presynaptic neurone causes synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft
3. Acetylcholine molecules diffuse across the narrow synaptic cleft very quicky because the diffusion pathway is short. Acetylcholine then binds to receptor sites on sodium ion protein channels in the membrane of the postsynaptic neurone. This causes the sodium ion protein channels to open, allowing sodium ions to diffuse in rapidly along a concentration gradient
4. The influx of sodium ions generates a new action potential in the postsynaptic neurone
5. Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid (acetyl) , which diffuses back across the synaptic cleft into the presynaptic neurone, recycling. In addition to recycling the choline and ethanoic acid , the rapid breakdown of acetylcholine also prevents the generation of continuous new action potentials in the postsynaptic neurone, and so leads to discrete transfer of information across synapses
6. ATP released by mitochondria is used to recombine choline and ethanoic acid into acetylcholine. This is stored in synaptic vesicles for future use. Sodium ion protein channels close in the absence of acetylcholine in the receptor sites
inhibition
Some synapses make it less likely that a new action potential will be created on the postsynaptic neurone. These are known as inhibitory synapses
1. The presynaptic neurone releases a type of neurotransmitter that binds to chloride ion protein channels on the postsynaptic neurone
2. The neurotransmitter causes the chloride ion protein channels to open
3. Chloride ions move into the postsynaptic neurone by facilitated diffusion
4. The binding of the neurotransmitter causes the opening of nearby potassium protein channels
5. Potassium ions move out of the postsynaptic neurone into the synapse
6. The combined effect of negatively charged chloride ions moving in and positively charged potassium ions moving out is to make the inside of the postsynaptic membrane more negative and the outside more positive
7. The membrane potential difference increases to as much as -80mV
8. This is called hyperpolarisation and it makes it less likely that a new action potential will be created because a larger influx of sodium ions are needed for threshold to be reached
summation
Low frequency action potentials often lead to the release of insufficient concentrations of neurotransmitter to trigger a new action potential in the postsynaptic neuron. They can however, do so in a process called summation. This entails a rapid build-up of neurotransmitter in the synapse by:
1. Spatial summation. A number of different presynaptic neurones together release enough neurotransmitter to exceed the threshold value of the postsynaptic neurone. Together they trigger a new action potential. Different small, sub-threshold depolarisations have added together
2. Temporal summation. Sub-threshold depolarisations along a single neurone are added together to form an action potential, when threshold is reached. A single presynaptic neurone releases neurotransmitter many times over a very short period
