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
Why does the refractory period mean action potentials can only move in one direction?
Region behind is in refractory period
Na+ cannot move in so doesn’t move in both directions
Why does the refractory period mean only discrete impulses are made?
Time difference between the impulses due to the refractory period
Hence discrete impulses formed
Why does the refractory period mean it limits the number of action potentials?
Separated action potentials means limited number which can pass in a certain time
Limits strength of stimulus that can be detected
What is a synapse?
Point where one neurone communicates with another or an effector
What do synapses transmit?
Information not impulses
What is a neurotransmitter?
Chemicals are used to transmit information from one neurone to another
What is the synaptic cleft?
Small gap which separates the neurones
20-30 nm wide
What is the presynaptic neurone?
Neurone which releases the neurotransmitter
What is the presynaptic knob?
Swollen portion of the presynaptic neurone
Contains many mitochondria and ER
What is the function of the presynaptic knob?
Required for manufacture and storage of the neurotransmitter
What is the neurotransmitter stored in when in the presynaptic knob?
Synaptic vesicles
How is a synapse unidirectional?
Synapses only pass info in one direction
What is summation?
Processes used to ensure that sufficient neurotransmitter is released to cause a new action potential in the postsynaptic neurone
What are the two types of summation?
Spatial summation
Temporal summation
What is spatial summation?
Many presynaptic neurones release neurotransmitter simultaneously
Together release enough and can form a new action potential
What is temporal summation?
Single presynaptic neurone releases neurotransmitters many times in a very short period of time
If conc is larger than threshold it causes a new action potential
What is an inhibitory synapse?
Type of synapse that makes it less likely a new action potential will be created on the postsynaptic neurone
How does an inhibitory synapse work?
Releases neurotransmitter that binds to Cl- protein channel on postsynaptic neurone, opening them and causing Cl- to diffuse into it
Also causes opening of K+ channels which move out of neurone into synapse
Makes inside of postsynaptic membrane more -ve and outside more +ve, called hyperpolarisation
Requires more Na+
What is the membrane potential changed to in hyperpolarisation?
-65 mV to -80 mV
What do the structure of synapse allow?
Single impulse along one neurone can initiate many different neurones at a synapse, one neurone can make many responses
Number of impulses combined at a synapse so many can make one response
Where is the neurotransmitter produced?
Only in presynaptic neurone
How does a synapse work?
Acton potential causes Ca2+ gated channels to open in presynaptic neurone
Ca2+ diffuse into neurone and cause synaptic vesicles to fuse with presynaptic membrane
Releases neurotransmitter into cleft which diffuses and binds to specific receptor proteins on postsynaptic neurone
Leads to Na+ protein channels opening and Na+ diffuses into postsynaptic neurone causing an action potential to form
What is an excitatory synapse?
Synapses which produce new action potentials in synapses next to it
What is a cholinergic synapse?
One where the neurotransmitter used is acetylcholine
What is acetylcholine?
Neurotransmitter
Made of acetyl (ethanoic acid) and choline
Where are cholinergic synapses common?
Vertebrates
Found in CNS and at the neuromuscular junction
What is the abbreviation of acetylcholine?
ACh
How is transmission done at a cholinergic synapse?
Action potential opens Ca2+ in presynaptic neurone which enter by facilitated diffusion
Ca2+ cause synaptic vesicles to fuse with presynaptic membrane, releasing ACh
ACh diffuse across cleft and binds to receptor sites on Na+ protein channels in postsynaptic membrane
Na+ diffuse down conc gradient into postsynaptic neurone and creates a new action potential in it
How is acetylcholine recycled?
Acetylcholinesterase hydrolyses ACh to acetyl and choline
Diffuse into presynaptic neurone into synaptic vesicles
ATP from mitochondria used to recombine them
What does the breakdown of ACh prevent?
Prevents from continuously generating a new potential in the postsynaptic neurone
As Na+ channel proteins are allowed to close
Means discrete transfer can occur
Why are there many mitochondria in the presynaptic neurone?
Produce ATP for recombining ACh
What are the three types of muscle?
Cardiac
Smooth
Skeletal
What occurs when acetylcholinesterase is inhibited?
Causes constant muscle stimulation as it continually binds and leaves Na+ protein channels open
What is cardiac muscle?
Muscles only found in the heart
Involuntary/unconscious control
What is smooth muscle?
Walls which decrease in diameter when they contract
Found in walls of blood vessels and gut
Involuntary/unconscious
What is skeletal muscle?
Muscle attached to the bone either directly or by tendons
Acts under voluntary control, conscious
What is the prefix used for muscle cells?
Sarco-
What is a muscle made of?
Millions of tiny muscle fibres called myofibrils
Why are myofibrils grouped together?
Lined up parallel as it maximises its collective strength
Why could the muscle not be made of individual cells joined up end to end?
Junction between cells would be a point of weakness
What is a muscle fibre?
Separate cells which have fused together to form longer
Share nuceli, sarcoplasm etcl.
What is the name of the cytoplasm in muscle fibres?
Sarcoplasm
What is the name of the endoplasmic reticulum in muscle fibres?
Sarcoplasmic reticulum
What is found in the sarcoplasm in high conc?
Mitochondria
Sarcoplasmic reticulum
What is body movement caused by?
Contraction of skeletal muscle
What is a muscle split into?
Muscle Bundle of muscle fibres Muscle fibres Myofibrils Sarcomere
What is the sarcomere?
Smallest unit of skeletal muscle that can contract
What is a myofibril made of?
Protein filaments:
Actin
Myosin
What is actin?
Protein filament
Thinner and consists of two strands twisted around each other
What is myosin?
Protein filament
Thicker and consists of long rod-shaped tails with bulbous heads that project to the side
What is myoglobin?
Protein in muscle which carries oxygen
What are the bands present in a sarcomere?
I bands
A bands
H-zone
Z-line
What is the I band?
Lighter bands
Only actin present, area between myosin and z-line
What is the z-lines?
Lines which separate each sarcomere
Middle of I-band
What is the A band?
Myosin (thick) and actin (thin) overlap
Produces dark region
What is the H-zone?
Region where only myosin is present
Darker than I band but lighter than A-band
What is tropomyosin?
Protein which forms a fibrous strand around the actin filament
What is troponin?
Protein bound to tropomyosin which is wrapped around the actin
What are the two types of muscle fibre?
Slow-twitch fibres
Fast-twitch fibres
What are the features of slow-twitch fibres?
Contract slowly and less powerfully
Contractions over a long period
Adapted to endurance work in muscles where constantly used
Adapted for aerobic respiration
What are the features of fast-twitch fibres?
Contract rapidly and more powerfully
Contracts over a short period
Adapted to intense exercise in muscles needed for short-bursts of activity
Adapted for anaerobic respiration
Name and explain an example where slow-twitch fibres are used?
Calf muscle
Constantly contracts for keeping the body upright
Why are slow-twitch muscles adapted for aerobic respiration?
Prevent build-up of lactic acid
Lactic acid would cause them to function less effectively, stops long-duration contraction
How are slow-twitch muscle fibres adapted for aerobic respiration?
Large store of myoglobin which stores O2
Rich supply of blood vessels to deliver O2 and glucose
Many mitochondria to produce ATP
How are fast-twitch fibres adapted to their role?
Thicker and more numerous myosin filaments
High conc of glycogen
High conc of enzymes for anaerobic respiration providing ATP rapidly
Large store of phosphocreatine to produce ATP
What is the neuromuscular junction?
Point where a motor neurone meets a skeletal muscle fibre
How many neuromusclar junctions are found along a muscle and why?
Many junctions
Ensures fibres all contract simultaneously therefore rapid and powerful
What would occur if there was just 1 neuromuscular junction per muscle?
Wave of contraction not all contract at same time
Slow and weaker contraction
Why is rapid and coordinated contraction required?
Needed for survival
What is a motor unit?
All muscles fibres which are supplied by a single motor neuron
Acts as a single functional unit
What do motor units allow for?
Varying amounts of force applied
If a large force needed then more motor units are stimulated
What is the structure of the neuromuscular junction?
Presynaptic neurone is cholinergic
Postsynaptic is muscle fibre
What is the membrane of the muscle fibre called?
Sarcolemma
What occurs when a nerve impulse is received at a neuromuscular junction?
Ca2+ voltage-gated channels open in presynaptic neurone
Synaptic vesicles fuse with presynaptic membrane
Releases ACh which then diffuses to sarcolemma
Binds to receptor and opens Na+ channels, which diffuse into the muscle, depolarising the membrane
This then leads to contraction
What prevents the muscle from being over-stimulated by ACh?
Acetylcholineterase enzymes found at neuromuscular junctions
What are the similarities of the neuromuscular junction and a synapse?
Both have neurotransmitters which move by diffusion
Receptors which upon binding cause the influx of Na+
Na+/K+ pump used to re-polarise axon
Enzymes used to break down neurotransmitter
What are the differences between the neuromuscular junction (NJ) and a cholinergic synapse?
NJ is only excitatory, cholinergic can also be inhibitory
NJ only links neurones to muscles, cholinergic can link to other neurones or other effector organs
Action potential ends at NJ, new action potential can be produced in cholinergic synapse (if another synapse)
ACh binds to receptor on sarcolemma at NJ but on post-synaptic neurone in cholinergic
What is a transverse tubule?
Structures in muscle fibres which carries depolarisation from membrane to inside of muscle fibres
Ensures all contract at same time
Made of cell-surface membrane
What is the mechanism by which muscle contracts?
Sliding filament mechanism
What does antagonistic muscles mean?
Muscles movements which oppose each other
What is an antagonistic pair of muscles?
Muscles which cause movement of a limb
One contracts whilst other relaxes to move in one direction, vice versa in other direction
What is the flexor?
Muscle which contracts meaning the limb bends
What is the extensor?
Muscle which contracts meaning the limb extends
How does Ca2+ affect protein filaments?
Binds to troponin, changes its conformational shape
No longer binds to tropomyosin
Tropomyosin moves and exposes binding sites on actin
How is contraction done by the sliding filament mechanism?
T-Tubule transfers action potential deep into fibre
Causes Ca2+ channels in sarcoplasmic reticulum to open and diffuse into sarcoplasm down conc gradient
Ca2+ moves causes tropomyosin to move, exposing actin binding site
Myosin head with ADP binds to form cross-bridge with actin
ADP is released and ATP binds
Ca2+ activates ATPase, energy from ATP hydrolysis gives energy for movement of myosin head puling actin along and to stop them being bound
Head returns to original position and binds further along actin with ADP to repeat process whilst [Ca2+] is high
Myosin molecules joined tail to tail pull in opposing directions, meaning actin pulls towards eachother and shortens muscle
What occurs in muscle relaxation in the sliding filament mechanism?
Occurs when nervous stimulation stops
Ca2+ actively transported into sarcoplasmic reticulum using energy from ATP hydrolysis
Tropomyosin blocks actin filament
Myosin head unable to bind to actin filaments, contraction ceases
What is energy required for in muscle contraction?
Movement of myosin heads
Reabsoprtion of Ca2+ into sarcoplasmic reticulum by active transport
Does muscle contraction require a large amount of energy?
Considerable energy
Supplied by hydrolysis of ATP
How is ATP regenerated in the muscle?
Mostly by aerobic respiration of pyruvate (large amount in muscle)
Anaerobically respires or uses phosphocreatine to donate phosphate groups to ADP
What is phosphocreatine?
Molecule which acts as a phosphorus store in muscle
Donates Pi to ADP when not enough provided by respiration
How is phosphocreatine made?
Using phosphate from ATP when the muscle is relaxed
What are the changes to the sarcomere when muscle contracts?
I bands become narrower
Z lines move closer together
H zone becomes narrower
A band remains same width
Why does the A band remain the same width in contraction?
Determined by length of myosin
Therefore myosin itself is not getting shorter
What disproves the theory that contraction is due to the filaments themselves shortening?
A band remains same width
Therefore myosin length remains constant as it is determined by it
What is the function of tropomyosin in myofibril contraction?
Moves out of the way when Ca2+ binds
Allows myosin to bind actin
What is the function of myosin in myofibril contraction?
Head of myosin binds to actin and pulls actin past
Myosin detaches from actin and moves further along actin
This uses ATP
Why is there a high conc of glycogen in fast muscle fibres?
Glycogen broken down to glucose for glycolysis as anaerobic
Many needed as glycolysis yields very few (2 ATP) per glucose
Why are many capillaries a benefit in slow muscle fibres?
Gives high [O2]
Allows higher rate of aerobic respiration
Why is there variation in the time taken for phosphocreatine to reform?
Genetic differences
Fitness
Fast or slow muscle fibres
If myosin cannot bind to one another why can muscle contraction not occur?
Cant form thick myosin filament
Can’t pull the actin filament and myosin itself moves
Actin doesn’t move and can’t shorten sarcomere so no contraction
Where are mitochondria located in slow-twitch fibres and why?
Near the edges
Short diffusion pathway for oxygen which is used in the ETC
