Speed of nerve impulse Flashcards
(13 cards)
What happens after an action potential has been set up?
- once an action potential has been set up, it moves rapidly from one end of the axon to the other without any decrease in size
- in other words, the action potential at the end of the axon is the same size as when it starts
- this transmission od an action potential along the axon of a neurone is the nerve impulse
What are the factors affecting the speed at which an action potential travels?
- they myelin sheath: -the myelin sheath acts as an electrical insulator, preventing an action potential forming in the part of the axon covered in myelin
- it does however jumps from one node of Ranvier to another (saltatory conduction)
- this increases the speed of conductance from 30ms-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 a 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
- enzymes function more rapidly at higher temperatures up to a point
- above a certain temperature, enzymes and the plasma membrane proteins are dentures and impulses fail to be conducted at all
- temperature is clearly an important factor in response times in cold-blooded (ectothermic) animals, whose body temperature varies in accordance with the environment
- temperature also effects the speed amend strength of muscle contractions
What is the all-or-nothing principle?
- nerve impulses are describes 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, ands therefore no impulse is generated
- any stimulus of whatever strength, that is below the threshold value will fail to generate an action potential - this is the nothing part
- 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
How does the organism perceive the size of a stimulus?
- by the number of I’m, pulses 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
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 sodium ions is prevented because the sodium voltage-gated channels are closed
- during this time it is impossible for the further action potential to be generated which is known as the refractory period
Which purposes does the refractory period serve?
- 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 refectory period 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, which means that they can only move in forward direction
- this prevents action potentials from spreading out in both directions, which they would otherwise do
How does the refectory period 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
How does the refectory period limit 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 and thus limits the strength of stimulus that can be detected
What are the properties of the human motor axon to leg muscle?
- it has myelin
- it is 20um in diameter
- its transmission speed is 120ms-1
What are the properties of the human sensory axon from skin pressure receptor?
- it has myelin
- it is 10um in diameter
- its transmission speed is 50ms-1
What are the properties of the squid giant axon?
- it has no myelin
- it is 500um in diameter
- its transmission sped is 25ms-1
What are the properties of the human motor axon to interval organ?
- it has no myelin
- it is 1um in diameter
- its transmission speed is 2ms-1
