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 is a resting potential established?
The phospholipid bilayer prevents Na+ and K+ ions passing through by simple diffusion
Channel proteins form a sodium-potassium pump - they actively transport sodium ions out of the axon and potassium ions in
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 the myelin sheath impact the speed of the impulse?
Saltatory conduction (impulse jumps between nodes of Ranvier)
How does temperature impact the speed of an impulse?
Rate of diffusion of ions
Enzymes
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 closed
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 actin?
A long, helical strand of protein
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
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 is the phosphate supply regenerated?
When muscle relaxes
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
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?
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
Wha 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
How is information passed across a synapse?
Neurotransmitters diffuse across
What is the gap between neurones called?
The synaptic cleft
What is the neurone before the synapse called?
The presynaptic neurone
What is the neurone after the synapse called?
The postsynaptic neurone
Where are neurotransmitters released from?
The presynaptic knob
How is the presynaptic knob adapted for its function?
Possesses many mitochondria and lots of endoplasmic reticulum for the production of neurotransmitters
What are neurotransmitters stored in?
Vesicles
Why are synapses unidirectional?
Information can only pass from the presynaptic neurone to the post
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
What is temporal summation?
A single presynaptic neurone releases neurotransmitter many times over a short period exceed the threshold value of 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 are the two functions of synapses?
Allows a single impulse to initiate new impulses in numerous neurones
Allows a number of impulses to be combined at a synapse
What are excitatory synapses?
Synapses that produce a new action potential when neurotransmitters bind with receptors in the postsynaptic neurone
How does an impulse pass along an axon?
The action potential acts as a travelling wave of depolarisation
How does an action potential pass along a myelinated axon?
Action potentials jump between nodes of Ranvier - saltatory conduction
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 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
Charge of resting potential
65mv
