✅15 - Nervous Coordination And Muscles Flashcards
How does the nervous system control actions?
It uses nerve cells to pass electrical impulses along their length and stimulate target cells by secreting neurotransmitters.
What is the main benefit of control via the nervous system?
The response is very quick, reflex action
What is the main potential drawback of control via the nervous system?
The response is short lived and restricted to one part of the body.
How does the hormonal system have control over the body?
It produces hormones which are transported in the blood plasma to their target cells, which have specific receptors on the cell surface membrane, sensitive to hormone concentration.
What are the main parts of a nerve cell?
A cell body
Dendrons
An axon
Schwann cells//myelin sheath
What does the cell body contain?
It contains all the usual cell organelles, including a nucleus and large amounts of rough endoplasmic reticulum, associated with the productions of proteins and neurotransmitters.
What are the dendrons?
Extensions of the cell body which subdivide into smaller branched fibres called dendrites that carry nerve impulses towards the cell body.
What is the axon?
A single long fibre that carries nerve impulses away from the cell body
What do the Schwann cells do?
The surround the axon, protecting it and providing electrical insulation. They also carry out phagocytosis and play a part in nerve regeneration. They wrap around the axon many times so the layers build up.
What is the structure and function of the myelin sheath?
Covers the axon and is made up of the membranes of the Schwann cells. Membranes are rich in the lipid myelin.
What is the structure and function of the nodes of Ranvier?
Constrictions between adjacent Schwann cells where there is no myelin sheath. 2-3 micro metres long and occurs every 1-3mm in humans.
Describe the structure and function of Sensory neurones:
Transmit nerve impulses from a receptor to an intermediate or motor neurone. One dendron that is often very long, carries nerve impulse towards cell body and one axon carries away from ell body
What is the structure and function of motor neurones?
Transmit nerve impulses from an intermediate or ready neurone to an effector, such as a gland or muscle. Motor neurones have a long axon and many short dendrites.
What is the structure and function of intermediate neurones?
Transmit impulses between neurones. For example from sensory to motor neurones. Have numerous short processes.
What can a nerve impulse be described as?
A sell propagating wave of electrical activity that travels along the axon membrane
What are the two states of the axon?
Resting potential and action potential
How is the movement of ions across the axon membrane controlled?
Phospholipid bilayer prevents Na+ and K+ diffusing across it
Gated ion channels only allow ions through at certain times or under certain conditions, some all the time
Some carrier proteins actively transport ions in and out of the axon, sodium-potassium pump
What does the control of ion movement result in?
The inside of the axon being negatively charged relative to the outside - RESTIG POTENTIAL, usually around 65mV
How is the potential difference between the axon and outside established?
Na+ actively transported OUT of axon by pump
K+ actively transported IN to axon by pump
Active transport of Na+ greater, so 3 Na+ move out for every 2 K+ in
More Na+ in tissue fluid outside, creates electrochemical gradient
Sodium ions begin to diffuse back in naturally, Potassium diffuse out
Most K+ gates are open while most Na+ gates are closed
How is an action potential created?
When a stimulus of a sufficient size is detected by a receptor, energy causes a temporary reversal of charge either side of this part of the axon membrane, from -65mV to 40mV
When an action potential is caused, the axon membrane is…
…depolarised
How does depolarisation occur?
Channels in the axon membrane change shape and hence open or close depending on the voltage across the membrane (voltage gated channels) at a perticular point on the axon membrane
Describe the process of creating an action potential:
Energy from stimulus causes Na+ channels to open, Na+ diffuse in and reverse potential difference
As Na+ diffuse in, more channels open, greater influx
Once action potential of 40mV established, Na+ voltage gates close and K+ open
K+ voltage gated channels open and reverse electrochemical gradient, more K+ in and repolarisation started
Outward diffusion of K+ causes temporary overshoot with inside of axon being more negative and K+ channels close
What are action potentials caused by?
Diffusion
What are resting potentials maintained by?
Active transport
Does the size of the action potential change from one end of the axon to the other?
No
How is an action potential passed along the axon?
As one region of the axon produces an action potential and becomes depolarised, it acts as a stimulus for the depolarisation of the next region of the axon
What is saltatory conduction?
Localised currents arise between adjacent nodes of Ranvier and the action potentials ‘jump’ between nodes
Does an action potential move faster along a myelinated or unmyelinted axon?
Myelinated - because the events of depolarisation don’t have to take place all the way down the neurone, saltatory conduction occurs instead
What are the factors affecting the speed at which action potentials travel?
The myelin sheath
Diameter of the axon
Temperature
How does the myelin sheath affect the speed at which an action potential travels?
The myelin sheath increases speed at which the action potential travels because it allows saltatory conduction
How does the diameter of the axon affect the speed at which an action potential travels?
The greater the diameter, the faster the speed of conductance due to less leakage from a large axon (meaning membrane potentials are easier to maintain)
How does temperature affect the speed at which an action potential travels?
Temperature affects the rate of diffusion of ions and therefore the higher the temperature, the faster the nerve impulse.
Respiration is controlled by enzymes, functioning more rapidly at high temperatures, so the higher the temperature, the more ATP and the more active transport.
What is the all or nothing principal?
There is a certain level of stimulus, called the threshold value, which triggers an action potential. Any stimulus below threshold will not create AP, any stimulus above will.
How is the size of an impulse perceived by an organism?
- By the number of impulses passing in a given time. Larger stimuli generate more impulses
- By having different neurones with different threshold values, as the brain interprets the number and type of neurones the pass impulses as a result of a given stimulus and thereby determines its size.