Animal Physiology 2 Flashcards
(39 cards)
Explain neurons? What is the origin of nerve impulses?
Neurons are specialized cells that function to transmit electrical impulses within the nervous system.
The nervous system converts sensory information into nerve impulses (electrical signals) in order to rapidly detect/respond to stimuli.
What are the three basic components that most neurons share, and what are their functions? (Suck A Dick)
Soma: a cell body containing the nucleus and organelles – where essential metabolic processes occur to maintain cell survival
Axon: elongated nerve fibre that transmits electrical signals to terminal regions for communication with other neurons/effectors
Dendrites: short-branched nerve fibres that convert chemical information from other neurons / receptor cells into electrical signals
Explain the myelin sheath.
- An insulating layer on some nerve fibres
- Consists of many layers of phospholipid bilayer
- Deposited by Schwann cells that grow around the nerve fibre
- Improves conduction speed of electrical impulses along the axon
What is the node of Ranvier?
A gap in the myeline between adjacent Schwann cells.
How does saltatory conduction work?
In myelinated nerve fibres, the nerve signal is forced to jump from one node of Ranvier to the next, creating a faster speed of impulse transmission.
Define the resting potential of neurons.
The potential difference/voltage across the membrane of a non-transmitting neuron.
It’s created due to an imbalance of positive/negative charges across the membrane.
Explain how resting potential in neurons works.
- Sodium-potassium pumps transfer sodium (Na+) and potassium (K+) ions across the membrane
- Na+ ions are pumped out, K+ ions are pumped in.
What creates the concentration gradient and imbalance with regards to resting potential? What is the typical resting membrane potential of a neuron?
- 3 Na+ pumped out for every 2 K+ pumped in, unequal; creates concentration gradients for both ions.
- Membrane is more permeable to K+, so it leaks back across the membrane faster; Na+ concentration gradient is steeper, creating charge imbalance.
- Organic anions inside the nerve fibre are negatively charged, increasing charge imbalance.
- Resting membrane potential is approx -70 mV.
What is an action potential?
- Action potential: a rapid change in membrane polarization, consisting of depolarization (negative to positive) and repolarization (positive back to negative)
How does depolarization work in neurons?
- Due to the opening of sodium channels, allowing Na+ to diffuse into neuron down concentration gradient
- Entry of ions reverses charge imbalance; inside is positive relative to outside
- membrane potential of +30 mV
How does repolarization work in neurons?
- Occurs rapidly after depolarization
- Sodium channels close, potassium channels open
- K+ diffuses out of neuron down concentration gradient, restoring relative negativity of cell
- Channels remain open until membrane falls to -70mV potential
- Once concentration gradients are re-established, neuron can transmit another nerve impulse
What is a nerve impulse and how do they work?
Nerve impulse: action potential beginning at one neuron end, propagated along the axon the the other end
- Propagation occurs because depolarizing ion movements trigger depolarization in neighbouring parts
- Impulses can only be initiated at one terminal, must be passed on to other neurons / different cells at another terminal
- Refractive period after depolarization prevents backwards propagation
What are local currents, and how do they work?
Local currents: movements that reduce the concentration gradient in the polarized part of the neuron
Inside the axon
- Higher Na+ concentration in depolarized part of axon, ions diffuse along axon to polarized part
Outside the axon
- Higher Na+ concentration in polarized part, ions diffuse along axon to depolarized part
- Membrane potential rises from -70mV to -50mv;
- Threshold potential of voltage gated sodium channels is reached, allowing depolarization (then repolarization)
How can membrane potentials in neurons be measured/analyzed?
- By placing electrodes on each side of the membrane, potentials can be displayed using an oscilloscope.
- Time on x-axis, membrane potential on y-axis
- Rising and falling signifies depolarization and repolarization
- May show potential rising before threshold potential is reached
Explain synapses, where they exist, and how they’re used.
Synapses: junctions between neurons; between neurons and receptor/effector cells
Sense organs
- Synapses between sensory receptor cells and neurons
Brain / Spinal cord
- Immense number of synapses between neurons
Muscles + glands
- Synapses between neurons and muscle fibres / secretory cells
Chemicals called neurotransmitters send signals across synapses; system used at all synapses where presynaptic and postsynaptic cells are separated by the synaptic cleft (fluid-filled gap) that electrical impulses cannot cross
How does synaptic transmission work?
- Propagated nerve impulse reaches pre-synaptic membrane
- Depolarization causes Ca2+ to diffuse through membrane channels into neuron
- Calcium influx causes vesicles containing neurotransmitter to fuse with presynaptic membrane
- Neurotransmitter released into synaptic cleft by exocytosis
- Neurotransmitter diffuses across synaptic cleft, binds to postsynaptic membrane receptors
- Binding causes adjacent sodium channels to open
- Sodium ions diffuse into postsynaptic neuron, which reaches threshold potential
- Action potential triggered and propagated
Explain what acetylcholine is and how it works.
- Acetylcholine is a neurotransmitter in many synapses
- Produced in presynaptic neuron by combining choline (from diet) with an acetyl group (from aerobic respiration)
- Loaded into vesicles and released into synaptic cleft during transmission
- Binds to receptors on postsynaptic membranes
- Remains bound for one action potential initiation – acetylcholinesterase in synaptic cleft breaks it down
- Choline reabsorbed into presynaptic neuron, converted back into an active neurotransmitter with new acetyl group
What are neonicotinoids, and how do they work? Explain their advantages and disadvantages.
- Synthetic compounds – bind to the acetylcholine receptor in cholinergic synapses in CNS of insects
- Irreversible binding blocks receptors and prevents transmission – acetylcholinesterase does not break down neonicotinoids
- Causes paralysis and death in insects
Advantages – less toxic!
- More CNS synapses in insects are cholinergic than humans/mammals
- Neonicotinoids bind less strongly to mammal receptors
Disadvantages
- May impact honeybees and other beneficial insects
What happens at a synapse if the amount of neurotransmitter secreted is not sufficient?
- Threshold potential will not be reached
- Postsynaptic membrane does not depolarize
- Newly entered sodium ions are pumped out by sodium potassium pumps
- Postsynaptic membrane returns to resting potential
How do nerve impulses display positive feedback?
Opening of some sodium channels and inward diffusion of sodium ions increases membrane potential, causing more sodium channels to open.
How do bones and exoskeletons support muscles?
- They provide an anchorage
- They act as levers.
Explain levers and different classes of levers.
Levers change the size/direction of forces with three parts – the effort force, the fulcrum (pivot point), the resultant force. The relative positions of the three determine the lever’s class.
First class lever (e.g. spine when nodding head)
- Effort force
- Fulcrum
- Resultant force
Second class lever (rising onto balls of feet)
- Fulcrum
- Resultant force
- Effort force
Third class lever (grasshopper leg)
- Fulcrum
- Effort force
- Resultant force
How do pairs of skeletal muscles work?
- Skeletal muscles work in antagonistic pairs to faciliate movement.
- When one muscle contracts, the other relaxes
- Opposite movements are produced at a joint
Example: in the elbow, the triceps extend the forearm and the biceps flex the forearm
Explain the mechanism of antagonistic muscles in a grasshopper.
The hindlimb of a grasshopper is specialized for jumping – jointed appendage with three main parts.
- Below the joint is the tibia; at the tibia’s base is another joint, the tarsus
- Above the joint is the femur, with extensor and flexor muscles
- (Flexion) When the grasshopper prepares to jump, the flexor muscles contract and the extensor muscles relax. This brings the tibia and tarsus into a Z-like position, and brings the femur and tibia closer together.
- (Extension) Extensor muscles contract, extending the tibia and producing a powerful propelling force.
