15: Nervous coordination and muscles Flashcards
What are the two main forms of coordination in animals?
Nervous system
Hormonal system
What are the features of the nervous system?
Communication by nerve impulses Transmission by neurones Very rapid transmission Travel to specific parts of the body Response is localised Response is rapid and short-lived Effect is temporary and reversible
How is communication done in the nervous system?
Nerve impulses
Transmission by neurones
What are the features of the hormonal system?
Communication by hormones Transmission by blood system Transmission is slow Hormones travelled all parts of body, only target cells respond Response is widespread Response is slow Response is often long-lasting Effect could be permanent and irreversible
What are hormones transported in?
Blood plasma
Why do hormones only affect target cells?
Specific receptor on membrane and the change in conc of hormones stimulate them
What is a neurone?
Nerve cells specialised to carrying nerve impulses from one part of the body to another
What is the composition of a neurone?
Cell body
Dendrons
Axon
Schwaan cells
What is a cell body in a neurone?
Cell which produces proteins and neurotransmitters
Contain a nucleus and a lot of RER
What is a dendron in a neurone?
Extensions of cell body which divide into dendrites
Carries nerve impulses to cell body
What is an axon in a neurone?
Single long fibre that carries nerve impulses away from the cell body
What is a Schwaan cell in neurones?
Surround the axon and provide electrical insulation
Membrane forms myelin sheath
Removes cell debris by phagocytosis
What is a myelin sheath?
Forms a covering to axon made of Schwaan cell membrane
Rich in myelin lipid
What are neurones with myelin called?
Myelinated neurone
What are nodes of Ranvier in a neurone?
Constrictions between Schwann cells where there is no myelin sheath
How close are nodes of Ranvier?
2-3 um long
Occur 1-3 mm in humans
What are the types of neurones?
Sensory neurones
Motor neurones
Intermediate or relay neurone
What is the function of a sensory neurone?
Transmit nerve impulses from a receptor to an intermediate or motor neurone
What is the structure of a sensory neurone?
One dendron that is often very long
One axon to transport away and towards from cell body
What is the function of a motor neurone?
Transmit nerve impulses from an intermediate or relay neurone to an effector
What is the structure of a motor neurone?
Long axon and many short dendrites
What is a intermediate/relay neurone?
Transmit impulses between neurones
What is the structure of an intermediate/relay neurone?
Numerous dendrons and dendrites
Small and thin axons
What is the definition of a nerve impulse?
Self-propagating wave of electrical activity that travels along the axon membrane
What are the two states of electrical activity on the axon membrane?
Resting potential
Action potential
How can Na+ and K+ cross the axon membrane?
Phospholipid bilayer prevents diffusion
Channel proteins allow them to move by facilitated diffusion
Sodium-potassium pump by active transport
How are Na+ and K+ transported at an axon using a pump?
Potassium ions are transported into the axon
Sodium ions are transported out of the axon
What are the types of protein channels found on an axon?
Leak - open all the time
Voltage-gated - open when depolarised
What is the resting potential value?
-50 to -90 mV but usually -65 mV in humans
Negatively charged relative to outside
When is the axon membrane polarised?
When it is at the resting potential
What occurs to form the resting potential?
Na+ actively transported out, K+ in
More Na+ (3) out and 2 K+ in, therefore causes relative negative charge on inside
K+ diffuses out of axon, Na+ not allowed to diffuse in as channels are closed
What is a voltage-gated channel?
Channels in axon membrane which open/close based on voltage across membrane
What is the change in membrane potential which forms an action potential?
-65 mV to +40 mV
What is the action potential?
Inside of axon membrane becomes + charge
Caused by a large enough stimulus is detected by a receptor
What is depolarisation?
When a part of the membrane becomes +vely charged
What causes an action potential?
Stimulus of a large enough size causes action potential if reversal of charge reaches threshold value
What are the stages in the formation of an action potential?
Resting potential Rising phase Overshoot phase Falling phase Undershoot phase Recovery
What occurs in the stages of an action potential?
Resting
Rising - energy from stimulus opens Na+ voltage-gated channels, Na+ diffuses in and only depolarises if large enough stimulus
Overshoot - Na+ channels open, causes even greater +ve charge
Falling - after +40mV Na+ voltage-gated channels close and K+ voltage-gated channels open, K+ leaves axon and repolarises membrane
Undershoot - K+ voltage and leak channels open to remove K+ and all others close, hyperpolarisation as more -ve than resting
Recovery - K+ voltage close and Na+/K+ pump removes Na+ and K+ in causes resting potential to be reached
Where does an action potential form?
Particular point on the axon membrane not the whole membrane
How is the resting and action potential maintained?
Action potential - passive
Resting potential - active
What are the features of an action potential?
Moves rapidly along axon
Size of action potential remains constant
Part of axon which is depolarised, acts as a stimulus for depolarisation of next region of axon
Action potential is travelling wave of depolarisation
What are the stages of the passage of an action potential along an unmyelinated neurone?
Stimulus causes Na+ into axon causing depolarisation
Localised electrical current causes Na+ voltage-gated channels open further along axon, causing depolarisation further along
Behind depolarisation Na+ voltage close and K+ open, K+ move down electrochemical gradient
Repolarised membrane follows area of depolarised
How does a myelin sheath work?
Acts an electrical insulator
Prevents action potentials from forming
What is saltatory conduction?
Process whereby localised currents form between adjacent nodes and action potentials jump from node to node to Ranvier
Where can an action potential occur along a myelinated axon?
Only at the nodes of Ranvier
Does an action potential travel faster in an unmyelinated and myelinated axon?
Myelinated axon
Why do myelinated axons conduct impulses faster than unmyelinated?
Depolarisation only at nodes of myelinated
Saltatory conduction - jumps from node to node
Myelinated - impulse does not travel along whole length
Why can destruction of the myelin sheath cause problems with muscle control?
Action potential moves slower
Causes delays in muscle contractions
How does the size of the action potential change along the neurone?
Remains the same size throughout
Why does reploarisation occur behind the depolarisation in the axon?
Outward movement of K+ ions meaning it returns to relative -ve charge
Caused as K+ voltage open and Na+ close
What actually travels between adjacent nodes of Ranvier in myelinated neurons?
Current move from one to another
Causes voltage-gated channels to open/close
Define nerve impulse
Transmission of an action potential along the axon
What are the main factors which affect the speed at which an action potential travels?
Myelin sheath
Diameter of the axon
Temperature
What is the range for how fast an action potential travels?
0.5 ms-1 to 120 ms-1
How does the diameter of the axon affect the speed of action potentials?
Greater the diameter the faster the conductance speed
Why does the diameter of the axon affect the speed of action potentials?
Faster conductance
Due to less leakage of ions from a large axon so membrane potential is easier to maintain
Why do larger diameters cause less leakage?
Ions collide with the axon less and hence less leak
How does the temperature the speed of action potentials?
The higher the temp the faster the nerve impulse
Over a certain temp it slows/stops it
Why does the temperature affect the speed of action potentials?
Increasing temp increases rate of diffusion of ions and respiration enzymes act faster so energy more available for active transport
Over certain temp denatures enzymes for respiration and channel proteins, impulses stop
Which type of animal’s speed of conduction will be most affected by changes of temp?
Ectothermic (cold-blooded) animals
Temperature varies massively and can also affect muscle contractions
What does the all-or-nothing response mean?
Any impulse above the threshold value causes an action potential of the same size
Below causes no response
What occurs if the stimulus is below the threshold value?
Does never generate an action potential part
How can an organism perceive the size of a stimulus?
Number of impulses in a given time
Different neurones with different threshold values
How does the number of impulses change based on the size of the stimulus?
Larger the stimulus the more impulses generated in a given time
What is the refractory period?
Period after the depolarisation where sodium voltage-gated channels are closed so cannot move into membrane
Hence membrane cannot have another action potential generated across it
What are the main purposes of the refractory period?
Ensures action potentials only propagate in one direction
Produces discrete impulses
Limits number of action potentials