Chapter 15 - Nervous Coordination and Muscles Flashcards
Characteristics of the nervous system
Nerve cells transmit electrical impulses along their length
Impulse stimulates the secretion of neurotransmitters onto target cells
Short-lived, affect a small area
Characteristics of the hormonal system
Hormones transported in blood plasma to target cells
Slow, widespread, long-lasting effect
What is the cell body?
Contains organelles, produces neurotransmitters
What are dendrons?
Extensions of cell body
Divide into dendrites
Carry impulse TO cell body
What is an axon?
A long fibre that carries impulses FROM the cell body
Function of Schwann cells
Electrical insulation
Function of myelin sheath
Covers the axon
What are the nodes of Ranvier?
No myelin sheath
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 cell 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.
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 voltage gated channels close. During this time, it is impossible for further AP.
What are the purposes of the refractory period?
- Ensures that APs are propagated in one direction only
- It produces discrete impulses
- It limits the number of action potentials
How does a refractory period ensure that action potentials only propagate in one direction?
APs can only pass from an active region to a resting region, as APs cannot propagate to a region that is refractory, so can only move in the direction away from the region in the refractory period.
How does a refractory period ensure that discrete impulses are produced?
Due to the refractory period, a new action potential cannot be formed immediately behind the first one, ensuring they are separated
How does a refractory period ensure that the number of actin potentials are limited?
As action potentials are separated from one another, this limits the number of APs that can pass along an axon in a given time, limiting the strength of the stimulus.
What is the synaptic cleft?
The gap separating two neurones
What is the presynaptic neurone?
The neurone releasing the neurotransmitter
What is the synaptic knob?
The portion at the end of the presynaptic neurone that is swollen.
What are synaptic vesicles?
Pockets storing neurotransmitters
Why are there many mitochondria and smooth ER in the synaptic knob?
They are required for producing neurotransmitters
What is unidirectionality?
Synapses can only pass information in one direction
What is spatial summation?
A number of presynaptic neurones together release enough neurotransmitter to exceed the threshold value of the postsynaptic neurone, triggering an AP
What is temporal summation?
A single presynaptic neurone releases neurotransmitter many times over a very short period. If the concentration exceed threshold value, an AP is triggered
What are inhibitory synapses?
Synapses that make it less likely that a new AP will be created on the postsynaptic neurone
How do inhibitory synapses operate?
- Presynaptic neurone releases neurotransmitter that binds to chloride ion protein channels, causing them to open
- Cl- move into postsynaptic neurone by facilitate diffusion
- Binding of neurotransmitter causes opening of K+ protein channels, K+ move out of neurone into synapse
- Combined effect of Cl- moving in and K+ moving out makes inside more negative and outside more positive
- Membrane potential increases to up to -80mV with -65mV resting potential
- Hyperpolaristion - makes it less likely that a new AP will be created because larger influx of Na+ needed to produce one
What are the functions of synapses?
- Allow a single impulse along one neurone to initiate new impulses in a number of different neurones at a synapse, allowing a single stimulus to create a number of simultaneous responses
- Allow a number of impulses to be combined at a synapse, allowing nerve impulses from receptors reacting to different stimuli to contribute to a single response
How are neurotransmitters recycled?
Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid which diffuse back across the synaptic cleft into the presynaptic neurone
How is recycling neurotransmitters beneficial?
It prevents it from continuously generating new action potentials in the postsynaptic neurone, so leads to the discrete transfer of information across synapses.
How is the membrane polarised?
Na+ transported out
K+ transported in
3 Na+ out for every 2 K+ in
An electrochemical gradient is established - there are more Na+ ions in the tissue fluid around the axon and more K+ ions in the cytoplasm
Na+ diffuse back in and K+ diffuse out
The Na+ gates are closed but the K+ gates are open
How is the membrane depolarised and repolarised?
Some K+ gates are open but the Na+ gates are closed
The stimulus causes some Na+ gates to open so Na+ ions can diffuse into the axon
As more diffuse into the axon, more voltage gates open
At a limit, the gates close and the K+ gates open
This means K+ can diffuse out of the axon and causes more K+ gates to open - the membrane is repolarised
This action causes a temporary overshoot where the inside of the axon is more negative than the outside so the K+ gates close and Na+ is pumped in
Which three factors affect the speed of an impulse?
Temperature, diameter of the axon, myelin sheath
How does temperature impact the speed of an impulse?
Rate of diffusion of ions
Enzymes
How does the myelin sheath impact the speed of the impulse?
Saltatory conduction (impulse jumps between nodes of Ranvier)
How does the diameter of the axon impact the speed of an impulse?
Big diameter = less leakage = faster
What is the all-or-nothing principle?
Above the threshold value, an action potential is triggered
Below the threshold value, no action potential is triggered
What is the refractory period?
When an action potential is generated, there is a time delay when Na+ ions can’t enter the axon because the gates are close
What are the three purposes served by the refractory period?
Limits the number of action potentials
Produced discrete impulses
Action potentials only go in one direction
How does an action potential pass alone the neurone?
Saltatory conduction
‘Jumps’ between unmyelinated nodes of Ranvier
How is the axon depolarised?
Sudden influx of Na+ ions
In what direction will muscles work?
They can only pull, not push
Why do skeletal muscles only work in antagonistic pairs?
The pairs work in opposite directions - when one is relaxed, the other contracts
Where will myofibrils be darker in colour?
Where the actin and myosin filaments overlap
What changes happen to the sarcomere when the muscle contracts?
The I-band becomes narrower
The sarcomere shortens
The H-zone becomes narrower
What is myosin?
Made of the tail (fibrous proteins) and the head (bulbous structures)
What is tropomyosin?
Wound around actin to block binding sites
What is the sliding filament theory of muscle contraction?
The layers of actin and myosin slide past each other to contract the muscle
What is actin?
A long, helical strand of protein
How are muscles stimulated to contract?
An action potential reaches neuromuscular junctions
Ca2+ protein channels open and Ca2+ diffuses into synaptic knob
Ca2+ causes vesicles to release acetylcholine into the cleft
Acetylcholine diffuses across the cleft and binds with receptors on neighbouring muscles, depolarising them
How do muscles relax?
Hydrolysis of ATP provides the energy to actively transport Ca2+ into the endoplasmic reticulum
This makes tropomyosin block the binding sites again
Myosin can’t attach so muscles relax
What is energy needed for during muscle contraction?
Movement of myosin heads
Active transport of Ca2+
How is energy provided for muscle contraction?
ATP -> ADP
What does phosphocreatine do?
Acts as a source of phosphate
Phosphate + ADP -> ATP
Demand for oxygen outweighs supply - another way to form ATP must be used
How do muscles contract?
Action potential travels through T-tubules into endoplasmic reticulum
Endoplasmic reticulum has actively transported Ca2+ from muscle so has low Ca2+ concentration
Protein channels open, Ca2+ diffuses in down concentration gradient
Causes tropomyosin to move from binding sites
Myosin head and associated ADP bind to actin
Myosin heads change shape, releasing ADP and pulling actin along
ATP attaches to myosin head
Myosin head detaches
Ca2+ activates ATPase (ATP - ADP)
Energy released use to move myosin head to original position
Myosin head reattaches to actin
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Why will two actin filaments move in opposite directions?
The myosin heads are joined tail to tail so the movement of one set of heads is in the opposite direction to the other
Why does the sarcomere shorten when a muscle contracts?
Because the actin is moving in opposite directions, they are pulled towards each other
What are the three types of muscle?
Cardiac, smooth and skeletal
Where is cardiac muscle found?
In the heart
Where is smooth muscle found?
The walls of blood vessels and gut
Where is skeletal muscle found?
Attached to bone, acts under voluntary control
What are the ‘monomers’ of muscles called?n
Myofibrils
Why is it advantageous that muscle cells merge together?
There are no points of weakness
What is sarcoplasm?
Cytoplasm found in myofibrils
Characteristics of actin
Thinner, twisted strands
Characteristics of myosin
Thicker, bulbous heads project to side
Why do myofibrils appear striped?
Alternating light and dark coloured bands
What are the light bands called?
I bands (isotropic bands)
What are the dark bands called?
A bands (anisotropic bands)
Why do I bands appear lighter and A bands appear darker?
The thick and thin filaments overlap in the I band
What is at the centre of each A band?
A lighter coloured zone called the H-zone
What is at the centre of each I band?
The Z-line
What is the sarcomere?
The distance between adjacent z-lines
What are slow twitch fibres?
Endurance
What are the two types of muscle tissue?
Fast-twitch and slow-twitch fibres
Adaptations of slow-twitch fibres:
Lots of myoglobin
Blood vessels for oxygen
Mitochondria for ATP
What are fast-twitch fibres?
Intense, short exercise
Adaptations of fast-twitch fibres:
Lots of myosin
Lots of glycogen
Lots of enzymes for anaerobic respiration
Phosphocreatine
What is a neuromuscular junction?
The point that a motor neurone meets a muscle fibre
What happens when a nerve impulse is received at a neuromuscular junction?
Synaptic vesicles fuse with presynaptic membrane
Acetylcholine released and diffuses to postsynaptic membrane
Increased permeability to Na+ - enters rapidly
Membrane depolarised
How is a neuromuscular junction ‘reset’?
Acetylcholine broken down by acetylcholinerase
Choline and ethnic acid diffuse back to neurone
Mitochondria provide energy to reform acetylcholine
Similarities between neuromuscular junction and synapse:
Both have neurotransmitters moved by diffusion
Receptors that cause influx of Na+
Sodium-potassium pump
Differences between neuromuscular junction and synapse:
Neuromuscular junction links muscles to neurones, synapses link neurones to neurones
The action potential ends at a neuromuscular junction but can continue after a synapse
Neuromuscular junction only involves motor neurones
What is a cholinergic response?
Neurotransmitter is acetylcholine
What are the two components making up acetylcholine?
Ethnic acid and choline
How does an impulse cross a synapse?
Action potential reaches presynaptic neurone
Ca2+ channels open, Ca2+ enters presynaptic knob by facilitated diffusion
This causes vesicles to fuse with membrane, releasing acetylcholine into cleft
Acetylcholine diffuses across cleft to Na+ channels, increasing their permeability to Na+
Influx of Na+ by diffusion
This generates new action potential
Acetylcholinesterase hydrolyses acetylcholine into ethnic acid + choline which diffuse back to presynaptic neurone
ATP used to reform acetylcholine which is stored in vesicles
Why is information transferred discretely across a synapse?
Acetylcholinesterase hydrolyses acetylcholine into ethanoic acid + choline
This prevents it from continuously generating new action potentials
What are synapses?
Points where neurones communicate either with other neurones or effectors
Charge of resting potential:
65mv
How does an action potential pass along an unmyelinated axon?
Resting potential: higher concentration of positive ions on outside compared with inside of membrane - membrane is polarised
Stimulus causes sudden influx of Na+ which reverses the charge on the axon membrane causing it to depolarise
This is the action potential
The influx of Na+ causes Na+ channels to open further along the axon causing depolarisation
Beyond this region, Na+ channels close and K+ voltage gates open
K+ begins to diffuse out of membrane
The depolarisation occurs along the axon
The outward movement of K+ has meant that the original area of depolarisation has repolarised
Repolarisation means Na+ is actively transported out, returning the axon to its resting potential
Why is the moment of an action potential along a myelinated axon faster than an unmyelinated one?
In an unmyelinated axon, depolarisation has to occur across the whole length of the axon, which is time consuming
How does an action potential pass along a myelinated axon?
Action potentials jump between nodes of Ranvier - saltatory conduction
What are excitatory synapses?
Synapses that produce a new action potential when neurotransmitters bind with receptors in the postsynaptic neurone
How do inhibitory synapses work?
Presynaptic neurone releases a neurotransmitter that binds to Cl- channels on postsynaptic neurone
This causes channels to open
Cl- moves in by facilitated diffusion
K+ channels open
K+ moves out of postsynaptic neurone into synapse
Inside of postsynaptic membrane is more negative and outside is more positive
This is hyper polarisation - a larger influx of Na+ is needed to create a new action potential so it is less likely
What is temporal summation?
A single presynaptic neurone releases neurotransmitter many times over a short period exceed the threshold value of the postsynaptic neurone
What are the two types of summation?
Spatial and temporal
What is spatial summation?
Many presynaptic neurones together release enough neurotransmitter to reach over the threshold value of the postsynaptic neurone and trigger an action potential
Why are synapses unidirectional?
Information can only pass from the presynaptic neurone to the post
What are neurotransmitters stored in?
Vesicles
How is the presynaptic knob adapted for its function?
Possesses many mitochondria and lots of endoplasmic reticulum for the production of neurotransmitters
What is the neurone after the synapse called?
The postsynaptic neurone
What is the neurone before the synapse called?
The presynaptic neurone
