nervous coordination and muscles- chapter 15 Flashcards
what is the point of the nervous system
- Uses nerve cells to pass electrical impulses along their length.
- Stimulate their target cells by secreting neurotransmitters directly on to them.
- Results in rapid communication between specific parts of an organism.
- Responses are short-lived and restricted to the localised region of the body.
what is the point of the hormonal system
- Produces chemicals (hormones) that are transported in the blood plasma to their target cells.
- Target cells have specific receptors on their cell-surface membranes and the change in the concentration of hormones stimulates them.
- Results in slow, less specific form of communication between parts of an organism.
- Responses are long-lasting and widespread
what are neurones
Specialised cells adapted to rapidly carry electrochemical changes called nerves impulses from one part of the body to another
what is the mammalian motor neurone made up of
cell body, dendrons, axon, schwann cells, mylein sheath, nodes of ranvier
explain the cell body in a mammalian motor neurone
Contains a nucleus and large amounts of rough endoplasmic reticulum. This is associated with the production of protein and neurotransmitters.
explain the dendrons in a mammalian motor neurone
extensions of the cell body which subdivide into smaller branches fibres called dendrites, that carry nerve impulses towards the cell body
explain the axon in a mammalian motor neurone
A single long fibre that carries nerve impulses away from the cell body
explain the schwann cells in a mammalian motor neurone
surround the axon, protecting it and providing electrical insulation. 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
explain the mylein sheath in a mammalian motor neurone
which forms a covering to the axon and is made up of membranes of the Schwann cells. These membranes are rich in a lipid known as myelin. Neurones with a myelin sheath are called myelinated neurones
explain the nodes of ranvier in a mammalian motor neurone
constrictions between adjacent Schwann cells where there is no myelin sheath. The constrictions are 2-3um long and occur every 1-3mm in humans
explain the sensory neurones
transmit nerve impulses from a receptor to an intermediate or motor neurone. They 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
explain the motor neurones
transmit nerve impulses from an intermediate or relay neurone to an effector such as a gland or muscle. Motor neurones have a long axon and many short dendrites
explain the intermediate or relay neurones
transmit impulses between neurones, for example, from sensory to motor neurones. They have numerous short processes
what can a nerve impulse be defined as
- as a self-propagating wave of electrical activity that travels along the axon membrane.
- Temporary reversal of the electrical potential difference across the axon membrane
- This reversal is between two states, called the resting potential and the action potential.
how is the movement of ions, like sodium ions and potassium ions, across the axon membrane controlled
- The phospholipid bilayer of the axon plasma membrane prevents sodium and potassium ions diffusing across it.
- Proteins, known as channel proteins, span the phospholipid bilayer. These proteins have channels called ion channels which pass through them. These can open or close so that sodium or potassium ions can move through them by facilitated diffusion at any one time. Some channels remain open all the time, so the sodium and potassium ions move unhindered through them by facilitated diffusion.
- Some carrier proteins actively transport potassium ions into the axon and sodium ions out of the axon. Sodium- Potassium pump
what are 2 features of the axon
-the inside of an axon is negatively charged relative to the outside. This is known as resting potential and ranges from 50 – 90 mV but usually 65mV in humans.
- Axon is polarised.
explain the resting potential
- Sodium ions are actively transported out of the axon by the sodium-potassium pumps.
- Potassium ions are actively transported into the axon by the sodium-potassium pumps.
- The active transport of sodium ions is greater than that of potassium ions, so three sodium ions move out for every two potassium ions that move in.
- The outward movement of sodium ions is greater than the inward movement of potassium ions. As a result, there are more sodium ions in the tissue fluid surrounding the axon than in the cytoplasm, and more potassium ions in the cytoplasm than in the tissue fluid, thus creating an electrochemical gradient.
- The sodium ions begin to diffuse back naturally into the axon while the potassium ions begin 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, while most of the gates in the channels that allow the sodium ions to move through are closed.
what is the action potential
- When a stimulus of a sufficient size is detected by a receptor in the nervous system, its 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 inside the membrane become a positive charge of around +40mV. This is the active potential and this part of the axon becomes depolarised.
- This depolarisation occur because the channels in the axon membrane change shape, and open or close depending on the voltage across the membrane- voltage-gated channels.
explain the stages of the action potential
- At resting potential some potassium voltage-gated channels are open but the sodium voltage-gated channels are closed.
- The energy of the stimulus causes some sodium voltage-gated channels in the axon membrane to open and therefore sodium ions diffuse into the axon through these channels along their electrochemical gradient. Being positively charged, they trigger a reversal in the potential difference across the membrane.
- As the sodium ions diffuse into the axon, so more sodium channels open, causing an even greater influx of sodium ions by diffusion.
- Once the action potential of around +40mV has been established the voltage gates on the sodium ions channels close and the voltage gates on the potassium ion channels begin to open.
- With some potassium voltage-gated channels now open, the electrical gradient that was preventing further outward movement of potassium ions is now reversed, causing more potassium ion channels to open. This means that yet more potassium ions diffuse out, starting 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. The closable gates on the potassium ion channels now close and the activities of the sodium-potassium pumps once again cause sodium ions to be pumped out and potassium ions in. The resting potential of -65mV is re-established and the axon is said to be repolarised.
explain the steps of a passage of an action potential along an unmyelinated axon
Step 1: At resting potential the concentration of sodium ions out the axon membrane is high relative to the inside, whereas that of the potassium ions is high inside the membrane relative to the outside.
The outside is positive compared with the inside.
The axon membrane is polarised.
Step 2: 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.
Step 3: The localised electrical currents established by the influx of sodium ions cause the opening of sodium voltage-gated channels a little further along the axon.
The resulting influx of sodium ions in this region causes depolarisation.
Behind this new region of depolarisation, the sodium voltage- gated channels close and the potassium ones open.
Potassium ions begin to leave the axon along their electrochemical gradient.
Step 4: The action potential (depolarisation) is propagated in the same way further along the axon.
The outward movement of the potassium ions has continued to the extent that the axon membrane behind the action potential has returned to its original charged state (positive outside, negative inside).
It has been repolarised
Step 5: Repolarisation of the axon allows sodium ions to be actively transported out, one again, returning the axon to its resting potential in readiness for a new stimulus if it comes.
what are action potentials prevented along the myelin sheath
as it acts as an electrical insulator
where does the action potentials take place
- At intervals of 1-3mm there are breaks called nodes of Ranvier
- The actional potentials in effect jump from node to node in a process known as saltatory conduction.
- This means action potentials travel faster than on an unmyelinated one of the same diameter.
what are factors affecting the speed at which ac action potential travels and explain them
- The myelin sheath- Acts as an electrical insulator, preventing an action potential forming in the part of the axon covered in myelin. The action potential jumps from one node of Ranvier to another. This increases the speed of conductance from 30cm-1 in an unmyelinated neurone to 90ms-1 in a similar myelinated one.
- The diameter of the axon- the greater the diameter of an axon, the faster the speed of conductance. This is due to less leakage of ions from the large axon (leakage makes membrane potentials harder to maintain)
- Temperature- this affects the rate of diffusion of ions and therefore the higher the temperature the faster the nerve impulse. The energy for active transport comes from respiration. Respiration, like the sodium-potassium pump is controlled by enzymes.
what is the all-or-nothing principle
- 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 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 the action potentials.
- The size is detected in two ways:
1. By a number of impulses passing in a given time. The larger the stimulus, the more impulses that are generated in a given time.
2. 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.
what three purposes does the refractory period serve
- It ensures that action potentials are propagated in one direction only- they can only pass from an active region to a resting region.
- It produces discrete impulses- 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- they are separated from one another this limits the number of action potentials that can pass along an axon in a given time, so limits the strength of stimulus that can be detected.
explain the structures of a synpase
- Synapses transmit information, but not impulses, from one neurone to another by neurotransmitters.
- Neurones are separated by a small gap, called the synaptic cleft, which is 20-30nm wide.
- 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- to make the neurotransmitter.
- 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 is receive it.
what does it mean that a synpase is unidirectional
synapses can only pass information in one direction
what does summation mean for synpases and what are the two types
- low frequency action potentials often lead to the release of insufficient concentrations of neurotransmitter to trigger a new action potential in the postsynaptic neurone. They can in a process called summation- rapid build-up of neurotransmitter in the synapse by:
1. Spatial Summation- number of different presynaptic neurones together release enough neurotransmitter to exceed the threshold value of the postsynaptic neurone. Together they therefore trigger a new action potential.
2. Temporal Summation- a single presynaptic neurone releases neurotransmitter many times over a very short period. If the concentration of neurotransmitter exceeds the threshold value of the postsynaptic neurone, then a new action potential is triggered.
describe inhibitory synpases
- Synapses that makes it less likely that a new action potential will be created.
- Presynaptic neurone releases a type of neurotransmitter that binds to chloride ion protein channels on the postsynaptic neurone.
- Neurotransmitter causes the chloride ion protein channels to open.
- Chloride ions (Cl-) move into the postsynaptic neurone by facilitated diffusion.
- Binding of the neurotransmitter causes the opening of nearby potassium (K+) protein channels.
- Potassium ions move out of the postsynaptic neurone into the synapse.
- 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.
- The membrane potential increases to as much as -80mV compared with the usual -65mV at resting potential.
- This is called hyperpolarisation and makes it less likely that a new action potential will be created because a larger influx of sodium ions is needed to produce one.
why is it good that synapses act as junctions (transmitting information from one neurone to another)
- 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.
what is a chemical made of (synpases)
made only in the presynaptic neurone and not in the postsynaptic neurone
where is a neurotransmitter stored
- in synaptic vesicles.
- When an action potential reaches the synaptic knob the membranes of these vesicles fuse with the pre-synaptic membrane to release the neurotransmitter.
- When released, the neurotransmitter diffuses across the synaptic cleft to bind to specific receptor proteins which are found only on the postsynaptic neurone.
- The neurotransmitter binds with the receptor proteins and this leads to a new action potential in the postsynaptic neurone. Synapses that produce new action potentials in this way are called excitatory synapses
what is acetylcholine made up of and where is it found
acetyl and choline
common in vertebrates, which they occur in the central nervous system and at neuromuscular junctions
