Chapter 15: Coordination Flashcards
What forms part of the central nervous system?
Brain and spinal cord.
What forms part of the peripheral nervous system?
Cranial and spinal nerves
In what form is information transferred?
Nerve impulses/ electrical impulses
What are nerve cells also called?
Neurones
What are the three types of neurone?
Sensory neurone, intermediate neurones, motor neurones
What does the motor neurone do?
It transmits impulses from the brain or spinal cord to a muscle or gland.
Where is the nucleus of a neurone?
In its cell body
Where is the cell body of a motor neurone?
In the spinal cord or the brain
Give a brief overview of the structure of an unmyelinated neurone
Thin cytoplasmic processes extend from the cell body known as dendrites which give a large surface area. The axon is much longer and conducts impulses over long distances. There are small regions of RER which synthesise proteins present in the cytoplasm. The branches of the axon as well as the cytoplasm contain mitochondria.
Where is the cell body of a sensory neurone found?
It may be found near the source of stimuli or in a swelling of a spinal nerve known as a ganglion.
Where are relay neurones found?
They are found entirely within the central nervous system.
Describe the myelin sheath of neurones.
Myelin is made when Schwann cells wrap themselves around the axon all along its length, enclosing the axon in many layers of its cell surface membrane. This enclosing sheath, called the myelin sheath, is made largely of lipid, together with some proteins.
What does the myelin sheath affect?
It affects the speed of conduction of the nerve impulse.
What are nodes of Ranvier?
The small, uncovered areas of axon between Schwann cells are called nodes of Ranvier.
How often do nodes of Ranvier occur and how big are they?
They occur about every 1-3mm in human neurones and nodes are around 2-3um long.
What is a reflex arc?
It is a pathway along which impulses are transmitted from a receptor to an effector without involving the ‘conscious’ regions of the brain.
Can some reflex arcs not contain an intermediate neurone?
Yes
What is a reflex action?
It is a response to a stimulus before there is any voluntary response involving the ‘conscious’ regions of the brain.
In what part of the neurone is there the travel of electrical impulses?
Cell surface membrane of the neurone.
In a resting axon, is the inside or the outside of the axon more negative?
The inside of the axon has a slightly negative electrical potential compared with the outside.
What is potential difference?
It is the difference between the voltage.
What is resting potential? ref. pd
The potential difference is often between -60mV and -70mV. In other words, the electrical potential of the inside of the axon is between 60 and 70mV lower than the outside. This difference is the resting potential.
What is the resting potential maintained by?
The sodium-potassium pumps in the cell surface membrane of the neurones.
Explain what happens at the resting potential.
The Na-K pumps constantly move 3Na ions out for every 2K ions that are brought in using the energy from the hydrolysis of ATP to move both of these ions against their concentration gradients. The membrane has protein channels for K and Na open all the time, however there are far more of these for the K than for Na. Therefore, K diffuses back out again much faster than sodium diffuses back in. In addition, there are many large, negatively charged molecules inside the cell that attract the K ions reducing the chance that they will diffuse out. The result of these effects is an overall excess of negative ions inside the membrane compared with the outside. The membrane is relatively impermeable to sodium.
What are two things that influence the inward movement of sodium ions during an action potential? ref. gradient
- There is a steep concentration gradient.
- The inside of the membrane is negatively charged which attracts positively charged ions.
A double gradient like this is known as an electrochemical gradient.
What is an action potential and what is it caused by?
An action potential is a potential difference across the membrane, caused by changes in the permeability of the cell surface membrane to Na and K ions.
What is a voltage gated channel?
As well as the channels open all the time, there are other channels in the membrane that open and close depending on the electric potential(or voltage) across the membrane, and therefore known as voltage gated channels.
When are the voltage gated channels closed?
At the resting potential
What happens in an action potential? *full
First, the electric current is used to stimulate the axon causes the opening of the voltage gated channels in the membrane which allow Na ions to pass through. As there is a much greater concentration of Na ions outside the axon than inside, Na ions enter through the open channels. This changes the potential difference across the membrane, making the inside less negative. This depolarisation triggers some more channels to open so that more sodium ions enter, creating more depolarisation. If the potential difference reaches about -50mV, then many more channels open and the inside reaches a potential of +30mV compared with the outside. This is an example of positive feedback because a small depolarisation leads to a greater and greater depolarisation.
After about 1ms, all the sodium voltage gated channels close, so the sodium ions stop diffusing into the axon. At the same time, the potassium ion channels open causing potassium ions to diffuse out of the axon, down their conc gradient. The outward movement of potassium ions removes positive charge from inside the axon to the outside, thus returning the potential difference back to normal(-70mV). This is called repolarisation.
In fact, the membrane briefly becomes even more negative than the normal resting potential. The K ion channels then close and the sodium ions channels become responsive to depolarisation. The Na-K pump continues to pump sodium ions out and K ions in and this helps to maintain the distribution of sodium ions and potassium ions across the membrane.
What determines whether an action potential is generated?
Action potentials are only generated if the potential difference reaches a value between -60mV and -50mV. This value is called the threshold potential. If it less than this, then an action potential does not occur.
What is the function of a neurone?
To transmit electrical impulses along itself.
Where does an action potential begin on an axon?
Anywhere along the axon’s cell surface membrane.
Why can’t action potentials be generated behind with reference to refractory period?
This is because the region behind it will still be recovering from the action potential it has just had and the sodium ion voltage gated channels are shut and cannot be stimulated to open. This period of recovery is the refractory period when the axon is unresponsive.
What are the differences between a strong and weak stimulus?
A strong stimulus produces a rapid succession of action potentials, therefore having a high frequency of action potentials. A weak stimulus results in fewer action potentials per second.
Moreover, a strong stimulus is more likely to stimulate more neurones than a weak stimulus.
What provides the brain with information about the strength of the stimulus being detected?
The brain can interpret the frequency of action potentials arriving along the axon of a sensory neurone and the number of neurones carrying action potentials, to get information about the strength of the stimulus being detected.
How is the nature of the stimulus deduced by the brain?
It is deduced from the position of the sensory neurone bringing the information.
How does myelin speed up the rate at which action potentials travel? Where do action potentials occur? \What is saltatory conduction?
It does this by insulating the axon membrane. Na and K ions cannot flow out of the membrane and so it is not possible for depolarisation or action potentials to occur in myelinated parts of the axon. Action potentials can only occur at nodes of Ranvier, where all the channel proteins and pump proteins are concentrated. The local circuits exist from one node to the next. Thus, action potentials jump from one node to the next, a distance of 1-3mm. This is called saltatory conduction. This can increase the speed of transmission.
How does diameter affect the speed of transmission?
Thick axons transmit impulses faster than thin ones, as their resistance is much less.
What is a receptor cell?
A receptor cell converts energy from one form into energy in an electrical impulse, responding to a stimulus by initiating an action potential.
Where are chemoreceptors found?
In the taste buds.
What is every chemoreceptor covered with?
It is covered with different receptor proteins that each detect different chemicals.
What are the five tastes?
Sweet, sour, salt, bitter and umami
What happens when the chemoreceptors detect salt?
When the chemoreceptors in the taste buds of papillae detect salt, the sodium ions diffuse through highly selective channel proteins in the cell surface membrane of the microvilli and this leads the the depolarisation of the membrane. The increase in positive charge inside the cell is the receptor potential. If there is sufficient stimulation by Na ions in the mouth then the receptor potential becomes large enough to stimulate the opening of voltage gated calcium ion channels. Calcium ions enter the cytoplasm and lead to the exocytosis of vesicles containing neurotransmitter from the basal membrane. The neurotransmitter stimulates an action potential in the sensory neurone that transmits impulses to the taste centre in the cerebral cortex of the brain.
What is the all or nothing law with action potentials?
When receptors are stimulated they are depolarised. If the receptor potential is below a certain potential, the stimulus only causes local depolarisation of the receptor cell and the sensory neurone is not stimulated to send impulses and vice versa. Neurones either transmit impulses or they do not.
What is a synaptic cleft?
Where two neurones meet, there is a gap called the synaptic cleft.
What is the average measurement of a synaptic cleft?
20nm
What is a synapse?
The parts of the two neurones near the cleft and the cleft itself make up a synapse.
What is the outline of the sequence of synaptic transmission?
■ An action potential occurs at the cell surface
membrane of the presynaptic neurone.
■ The action potential causes the release of molecules of transmitter substance into the cleft.
■ The molecules of transmitter substance diffuse across the cleft and bind temporarily to receptors on the postsynaptic neurone.
■ The postsynaptic neurone responds to all the impulses arriving at any one time by depolarising; if the overall depolarisation is above its threshold, then it will send impulses.
What are some examples of neurotransmitters in the nervous system?
Noradrenaline and acetylcholine(ACh)
