Nervous coordination and muscles - A-Level Biology notes PDF

Preview

Summary

A-Level Biology notes...

Description

Chapter 15: Nervous coordination and muscles 15.1 Neurons and nervous coordination Principles of coordination: Hormonal system Nervous system Communication by hormone Communication by nerve impulses Transmission by the blood system Transmission by neurons Transmission is slow Transmission is fast Hormones travel to all parts of the Nerve impulses travel to specific body but only target cells respond parts of the body Response is widespread Response is localised Response is slow Response is fast Response is often long-lasting Response is short-lived Effect is permeable and irreversible Effect is temporary and reversible 1. Cell body: production of proteins and neurotransmitters 2. Dendrons: extensions of the cell body that branch out into smaller dendrites carrying nerve impulses towards the cell body. 3. Axon: a single long fibre that carries nerve impulses away from the cell body 4. Schwann cells: provide the axon with protection and insulation: They can also undergo phagocytosis and nerve regeneration. These cells wrap themselves around the axon many times, building layers around the axon. 5. Myelin sheath: made up of Schwann cells membranes which are in myelin (a lipid) 6. Nodes of Ranvier: space between adjacent Schwann cells where there’s no myelin sheath.

15.2 The nerve impulse: a self-propagating wave of electrical activity that travels along the axon membranes Resting potential: the inside of an axon is negatively charged relative to the outside. It ranges from -92 -50 mV. The axon is polarized at this stage.Movement of sodium ions (Na+) and potassium ions (K+) across the axon membrane is done in a number of different ways: 1. Phospholipid by layer of the axon plasma membrane prevents Na+ and K+ plus diffusion 2. Channel proteins have gates that can be opened or closed to move Na+ and K+ through by facilitated diffusion at one time 3. Some carrier proteins actively transport K+ into the axon and Na+ out, which is the sodium potassium pump How is the resting potential established? 1. Na+ are actively transported out of the axon by the pump 2. K+ are actively transported into the axon by the pump 3. There’s a greater active transport of Na+, so 3Na+ move out for 2K+ that move inside. 4. An electrochemical gradient is created because there are more Na+ in the tissue fluid surrounding the axon than in the cytoplasm where there’s a higher concentration of K+ 5. Na+ begin to diffuse back into the axon while K+ will diffuse out 6. Most of the potassium voltage gated channels are open while all sodium voltage gated channels are closed

Chapter 15: Nervous coordination and muscles Action potential: the stimulus needs to exceed the threshold to cause a temporary reversal of the charges from -65 mV to +45 mV, which depolarizes the axon membrane 1. Some potassium voltage gated channels are open while the sodium voltage gated channels are closed 2. Energy of the stimulus causes sodium voltage gated channels to open which allows Na+ to diffuse into the axon along the electrochemical gradient. been positively charged it reverses the potential difference 3. More sodium channels open leading to a greater influx of Na+ by diffusion 4. Once the action potential reaches +40 mV the sodium voltage gated channels close and the potassium voltage gated channels open 5. More K+ enters the axon which repolarizes the axon 6. This causes an overshoot of the gradient and the potassium voltage gated channels close allowing the sodium potassium pump to resume. When resting potential of -65 mV is reestablished the axon is said to be re-polarized 15.3 Passage of an action potential As one region of the axon produces an action potential and becomes depolarized, it acts as a stimulus for the depolarization of the next region of the axon. Action potentials are a traveling wave of depolarization Passage of the action potential along an unmyelinated axon:

1. Resting potential: High concentration of Na on the outside and high concentration of K on the inside; the axon is polarized

2. Stimulus causes the influx of Na+ hence reversal of charge. The membrane is depolarized creating an action potential

3. Localized electrical currents established by the influx of Na+ cause the opening of Na+ voltage gated channels resulting in depolarization. The new region that follows will open K+ channels and close Na+ channels. K+ ions leave the axon by diffusion allowing the depolarization wave to move

4. The region that follows depolarization, the action potential returns to its original state, meaning it has been re-polarized

5. Repolarization: Na+ are actively transported out returning the axon into it resting potential Passage of an action potential along a myelinated axon: Fatty sheath of myelin acts as an electrical insulator, preventing action potentials from forming. Action potentials occur at the nodes of Ranvier:

Chapter 15: Nervous coordination and muscles localize circuits arise between adjacent nodes of Ranvier and the action potential jumps from one node to the next saltatory conduction. This allows the action potential to pass faster than in an unmyelinated neuron. This is because in the unmyelinated neuron, events of depolarization have to happen along the entire axon which takes more time

15.4 Speed of the nerve impulse A nerve impulse is the transmission of an action potential along the axon. 3 Factors affecting the speed at which an action potential travels: All-or-nothing principle: if the threshold is not reached, 1. The myelin sheath acts as an electrical insulator so the no action potential is generated. action potential jumps from node to node (saltatory conduction) Organisms can perceive size of a stimulus in 2 ways: 2. Diameter of the axon: the greater the diameter of the axon 1. Number of impulses passing in a given time. The the faster the speed of conductance. This is due to less larger the stimulus, the more impulses are leakage of ions from a large axon. generated. 3. Temperature: higher the temperature the faster the rate of 2. Different neurons have different threshold values, diffusion hence speed of nerve impulse. Temperature also this way the brain can determine the size. affects the speed and strength of muscle contractions. The refractory period: An inward movement of Na+ is prevented because the sodium-voltage-gated channels are closed. During this time an action potential can’t be generated. The refractory period serves 3 purposes: 1. Ensured that action potentials can only travel one direction 2. Produces discrete impulses: a new action potential can be formed behind the previous one. This ensures that action potentials are separated from each other. 3. It limits the number of action potentials, which limits the strength of stimulus that can be detected. 15.5 Structure and function of synapses Neurotransmitters are stored in vesicles and made only in the presynaptic neuron. Excitatory synapses are those that generate an action potential

Features of neurotransmitters: 1. Unidirectional 2. Summation: a rapid build-up of neurotransmitter in the synapse by spatial summation (where several presynaptic neurons fire at the same time) or by temporal summation (where a single presynaptic neuron fires many times over a short period of time)

Inhibition: Synapses that make it less likely for an action potential to be generated follows the following sequence: 1. The presynaptic neuron releases the type of neurotransmitter that binds to chloride ions protein channels on the postsynaptic neuron 2. The neurotransmitter causes the chloride ions protein channels to open

Chapter 15: Nervous coordination and muscles 3. Chloride ions move into the postsynaptic neuron by facilitated diffusion 4. Binding of the neurotransmitter causes nearby potassium ion channels to open 5. Potassium ions move out of the postsynaptic neuron into the synapse 6. This makes the inside of the postsynaptic membrane more negative and the outside more faster 7. The membrane potential increases to -80mV 8. This is hyperpolarisation which makes it less likely for a new action potential to be generated Synapses act like junctions, allowing: 1. A single impulse along one neuron to initiate other impulses which allows a single stimulus to contribute to a number of simultaneous responses 2. A number of impulses to be combined at a synapse, which allows nerve impulses from receptors reacting to different stimuli to contribute to a single response 15.6 Transmission across a synapse A cholinergic synapse (common in vertebrates) is one in which the neurotransmitter present is acetylcholine. Process of transmission: 1. Arrival of an action potential at the end of the presynaptic neuron causes calcium (Ca) ion channels to open and Ca2+ to enter the synaptic knob by facilitated diffusion 2. The influx of Ca2+ into the presynaptic neuron causes synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the cleft 3. Acetylcholine diffuses across and binds to receptor sites on Na+ channels in the membrane of the postsynaptic neuron. This causes the Na+ channels to open, allowing Na+ to diffuse in 4. Influx of Na+ generates an action potential in the postsynaptic neuron 5. Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid (acetyl), which diffuses back into the cleft (recycling). This also prevents continuous generation of action potentials and leads to discrete transfer amount of information across synapses. 6. Acetyl and choline combine by ATP released from the mitochondria and acetylcholine is then stored in vesicles for future use. The Na+ channels close. 15.7 Structure of skeletal muscles Slow twitch muscles fibers: - Contract more slowly - Provide less powerful contractions - Adapted to endure work (marathons) - Common in calf muscles - Adapted for aerobic respiration Adaptations: - Large store of myoglobin (bright red molecules that store oxygen) - Rich supply of blood vessels to deliver oxygen and glucose for aerobic respiration - Numerous mitochondria to produce ATP

Fast twist muscles fibers: - Contract more rapidly - Produce powerful contractions for a short period of time - Adapted to intense exercise (weight-lifting) - More common and biceps of the upper arm Adaptations: - Thicker and more numerous myosin filaments - A high concentration of glycogen - High concentration of enzymes involved in anaerobic respiration which rapidly produces ATP - Store of phosphocreatine which rapidly generate ATP from ADP in anaerobic conditions

Chapter 15: Nervous coordination and muscles -

-

-

-

Muscles (effectors) respond to nervous stimulation by contracting and thus bringing about movement The cardiac muscle (the heart) and the smooth muscles (walls of blood vessels and the gut) are under unconscious control Skeletal muscles are attached to the bone and act through voluntary conscious control Myofibrils are stronger collectively Muscle fibres share a sarcoplasm (cytoplasm) which is found around the circumference of the fibre. There’s a high concentration of mitochondria and endoplasmic reticulum within the sarcoplasm.

Myofibrils are made up of: 1. Actin: 2 thin strands twisted around one another 2. Myosin: 2 thicker, rod-shaped strands with bulbous heads Neuromuscular junctions: the point Junction Synapse where a motor neuron meets a skeletal Excitatory Excitatory and inhibitory muscle fibre Only links neurons to muscles Links neurons to neurons or to other - There are numerous junctions for effector organs rapid movement Only motor neurons All neurons - The spread of junctions across the Action potential ends here A new action potential can begin whole of the muscle ensures rapid Acetylcholine binds to receptors Acetylcholine binds to receptors on and powerful contractions on membrane of muscle fibre membrane of postsynaptic neuron - All muscle fibres supplied by a single motor neuron act together as a single functional motor unit. Weak force stimulates only a few motor units, whereas strong force requires numerous units to be activated. 15.8 Contraction of skeletal muscles- Skeletal muscles are attached to the bone Muscles can only pull, therefore to move the limb in the opposite direction requires a second muscle that works opposite to the first one. These two muscles are antagonists. Skeletal muscles must occur in antagonistic pairs. Sliding filaments mechanism involves the actin and myosin filaments sliding past one another, evidence of this: - When a muscle contracts the sarcomere changes: o I band becomes narrower o H zone becomes narrower o Z lines move closer together - A band remains the same with which is determined by the length of myosin filaments; this discounts the theory that muscle contraction is due to the filaments themselves shortening Main proteins involved: 1. Myosin: the tail is a fibrous protein arranged into a filament that’s made up of several molecules; the head is a globular protein formed into 2 bulbous structures at one end 2. Actin: a globular protein whose molecules are arranged into long chains that are twisted around one another to form a helical strand

Chapter 15: Nervous coordination and muscles 3. Tropomyosin: forms long, thin threads that are wound around actin filaments Muscle stimulation: 1. An action potential reaches many neuromuscular junctions simultaneously causing Ca2+ channels to open and allow Ca2+ to diffuse into the synaptic knob 2. The Ca2+ causes synaptic vesicles to fuse with the presynaptic membranes and release their acetylcholine into the cleft 3. Acetylcholine diffuses across the cleft and binds to receptors on the muscle cell-surface membrane causing it to depolarise Muscle contraction: (continuous process) 1. The action potential travels deep into the fibre through T-tubules, which are extensions of the cell-surface membrane, and branch throughout the sarcoplasm 2. Tubules are in contact with the sarcoplasmic reticulum, which has actively transported Ca2+ from the sarcoplasm leading to a very low concentration of Ca2+ 3. The action potential opens the Ca2+ channels on the sarcoendoplasmic reticulum and Ca2+ diffuse into the sarcoplasm down a concentration gradient 4. Ca2+cause the tropomyosin molecules that were blocking the binding sites on the actin filament to pull away 5. ADP molecules attached to the myosin heads means they can bind to the actin filament and form a cross-bridge 6. Once attached to the actin filament the myosin heads change their angle and pull the actin filaments along and release a molecule of ADP 7. An ATP molecule attaches to each myosin head causing it to become detached from the actin filament 8. Ca2+ activate ATPase which hydrolyses the ATP to ADP to provide energy for the myosin head to return to its original state 9. The myosin head re-attaches itself further along the actin filament and the cycle is repeated as long as the concentration of Ca2+ in the myofibril remains high 10. As the myosin molecules are joined tail to tail in 2 oppositely facing sets, the movement of one set is in the opposite direction to the other set, meaning that actin filaments also move in opposite directions 11. Movement of actin filaments in opposite directions pulls them towards each other, shortening the distance between the two adjacent Z lines Muscle relaxation: 1. When nervous stimulation ceases, Ca2+ or actively transported back into the endoplasmic reticulum using energy from the hydrolysis of ATP 2. This reabsorption of the Ca2+ allows tropomyosin to block the actin filament again 3. Myosin heads are now unable to bind to actin filament and contractions cease = muscle relaxes 4. In this state, force from antagonistic muscles can pull actin filaments out from between myosin Energy supply during muscle contraction: Energy is supplied by the hydrolysis of ATP to ADP and inorganic phosphate. Energy released is needed for: o Movement of myosin heads o Reabsorption of Ca2+ into the endoplasmic reticulum by active transport -

In an active muscle the demand for ATP and oxygen is greater than the rate at which the blood can supply oxygen. ATP needs to be generated quicker aerobically, which is achieved by more glycolysis and phosphocreatine Phosphocreatine generates ATP; it’s stored in muscles and acts as a reserve supply of phosphate. The store is replenished when muscle is relaxed...