Organisms response to stimuli (Unit 6) Flashcards
define stimuli
- changes in the environment (either external or internal) that produces a response in an organism
what are the three types of neurones and their function
- sensory neurones - posses receptors which are stimulated by a specific stimulus.
- motor neurones - transmit impulses to the appropriate effector (muscle or gland)
- relay neurones- act as a link between sensory and motor neurones ( also called intermediate or coordinator neurones)
name and describe the features of a myelinated motor neurone
- cell body - contains nucleus + most other organelles
- Axon - conducts nerve impulses which may myelinated or unmyelinated
- Myelin Sheath - made of up of Schwann cells and consists mainly of lipids providing electrical insulation
- Nodes of Ranvier - gaps in the myelin sheath ( only depolarised place on the axon)
- Synaptic Knobs - allow communication with other neurones or with effectors
what is a reflex
- a rapid automatic response to a stimulus
describe the reflex arc
- receptors detect the stimulus and transmit impulses along the sensory neurone to the brain/spinal cord
- sensory neurone synapses with relay neurone which passes impulses to the motor neurone
- impulses are then transmitted along the motor neurone to the effector
what are the advantages of reflexes
- they are innate
- automatic rapid response preventing damage to tissues
- increases the chance of escaping from predators
- important in detecting changes so homeostasis can be achieved
describe the nervous system
- rapid , short-lived and localised
define taxis
- a simple response in which the direction of the stimulus affects the direction of movement of the organism
define kineses
- a simple response in which the intensity of the stimulus affects the rate of turning or movement of the organism
define tropism
- the growth of part of a plant in response to a directional stimulus
describe how IAA affects growth in shoots
- IAA is negatively phototropic meaning it gathers on the shaded part of the plant
- in the shoots, high concentrations of IAA stimulates cell elongation and so there is greater elongation of the cells in the shaded part of the shoot therefore bending the shoot towards the light
describe how IAA affects growth in roots
- IAA gathers at the lower side of the roots
- in the roots, high concentrations of IAA inhibit cell elongation and so there is less elongation of the cells on the lower side of the root therefore bending the root downwards
define myogenic
- a muscle that can beat with it own rhythm (without an external impulse)
describe how a heartbeat is initiated and coordinated
- the SAN in the wall of the right atrium sends impulses through the walls of the atria causing them to contract
- the impulses reach the AVN between the atria and ventricles however do pass directly to the ventricles as they are blocked by a layer of non-conducting collagen tissue
- this causes a delay of 0.15 seconds before the AVN which allows the atria to fully empty
- AVN sends impulses from the AVN through the Purkyne tissue in the Bundle of His to all parts of the ventricles
- the ventricles are stimulated to contract together from the base upwards to push blood up and out into the arteries
how is heartrate controlled by the nervous system
- heart rate is sped up by the medulla sending more impulses along sympathetic neurones to the SAN (noradrenaline is released by sympathetic neurones which stimulates the SAN)
- heart rate is slowed down by the medulla sending more impulses along parasympathetic neurones to the SAN (acetylcholine is released by parasympathetic neurones which inhibits the SAN)
how is a high blood pressure detected and returned to normal
- increase in blood pressure is detected by baroreceptors in the wall of the aorta and carotid artery
- baroreceptors send more impulses to the cardiac centre in the medulla and therefore more impulses are sent from the cc along parasympathetic neurones to the SAN causing a decrease in heart rate
how is a change in blood pH detected and returned to normal
- decrease in the pH is detected by chemoreceptors in the aortic and carotid bodies
- chemoreceptors in the aortic and carotid bodies transmit more impulses to the cardiac centre in the medulla and therefore more impulses are sent from the cc to the SAN along the sympathetic neuron causing an increase in heart rate (to remove carbonic acid quicker)
why does exercise result in a lowering of the blood pH
- increased respiration = more CO2 produced which dissolves in the blood to form carbonic acid which lowers the pH
define resting potential
- the potential difference across a membrane of an axon when an impulse is not being transmitted
describe how the resting potential is maintained
- more potassium channel proteins are open than sodium channel proteins so there is a net loss of positively charged ions from the axon
- the sodium potassium pump actively transports 3 sodium out and 2 potassium in which maintains an electrochemical gradient
what would occur if neurones are treated with a respiratory inhibitor
- no respiration = no production of ATP = no release of energy = sodium/potassium pump cannot function therefore the concentration of the ions would eventually reach equilibrium, and the potential difference would be 0
describe what happens when a receptor is stimulated(nerve impulse)
- there is an increase in the permeability of the axon membrane to sodium ions and so they diffuse in down a conc gradient
- this causes more sodium channels to open and so more sodium ions diffuse in causing the negative resting potential to be cancelled out and eventually become positively charged (axon becomes depolarised)
- when the membrane is fully depolarised the sodium channels close and potassium channels open leading to more positively charged potassium ions diffusing out of the axon making inside less positive
- this starts the process of repolarisation and the sodium/potassium pump restores the resting potential by actively removing sodium ions which have entered and returning potassium ions back into the axon
how is an action potential transmitted along a neurone
- transmitted along a wave of depolarisation which occurs as an increase in permeability of the membrane to sodium ions in one area stimulates the same in the next area of the membrane and so on
what is the refractory period?
- the period in after an action potential when a neurone cannot generate another action potential
what does the refractory period allow
- allows for discrete (separate) impulses
- ensures that the impulse travels in one direction along an axon
what is the ‘All or Nothing’ principle
- a stimulus must be above a certain threshold level for an impulse to be generated
- stronger stimuli result in greater frequency of impulses than a weaker stimuli however amplitude of impulse always remains the same
what factors affect the speed of transmission along neurones
- temperature
- axon diameter
- myelination
how does temp affect speed of transmission
- increase in temp increases speed of transmission due to the ions having a greater kinetic energy and therefore diffusing faster
- above a certain temp, channel proteins and enzymes (affecting respiration) would be denatured
how does axon diameter affect speed of transmission
- greater the diameter the faster the transmission due to the reduced resistance
how does myelination affect speed of transmission
- as myelination of axon increases , transmission speed increases as there are less gaps where depolarisation occur (node of Ranvier)
- depolarisation occurs along the whole membrane in non-myelinated neurones = slower speed of transmission
what type of neurones contain acetylcholine (neurotransmitter)
- cholinergic neurones
describe the mechanism of synaptic transmission
- arrival of action potential at synaptic knob causes depolarisation of presynaptic membrane which stimulates calcium channels to open and calcium ions to diffuse into the synaptic knob
- calcium ions cause synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitter (acetylcholine) which diffuses across the synaptic cleft
-acetylcholine attaches to specific protein receptor sites on the postsynaptic membrane stimulating the entry of sodium ions - as more neurotransmitter attach more sodium ions enter leading to depolarisation of postsynaptic neurone and transmission of an impulse
- acetylcholine is broken down in the postsynaptic membrane by the enzyme acetyl cholinesterase forming acetyl and choline which are both taken up to the synaptic knob by active transport and acetylcholine is re-synthesised
- active transport and re-synthesis require ATP yielded from mitochondria
What causes synapses to be unidirectional?
- only the presynaptic neurone has vesicles to release the neurotransmitter into the synapse
- only the post-synaptic neurone has receptors for the neurotransmitter
What is the difference between spatial and temporal summation
Spatial summation - two or more impulses arrive in different regions of the same neurone at the same time
Temporal summation - two or more impulses arrive in rapid succession at the same place
How can synapses have an inhibitory effect on the postsynaptic neurone
- by stimulating the influx of negative (chloride) ions or by stimulating the removal of positive potassium ions
- both cause a more negative charge inside axon therefore reaching the threshold level during depolarisation is more difficult to achieve
How can different drugs affect the speed of transmission of nerve impulses
- same structure and effect = bind to receptor, faster transmission
- same structure and different effect = bind to receptor and slows transmission
- prevents release of transmitter = transmission is stopped
- inhibits enzymes that break down transmitter = faster transmission (more transmitters)
what does summation mean
the additive effect of several impulses causing depolarisation of the postsynaptic membrane
what type of receptors detect changes in mechanical pressure
- pacinian corpuscles
describe how the pacinian corpuscles detects and transmits an impulse
- when pressure is applied the receptor changes shape and the membrane becomes stretched
- the stretching deforms the stretch mediated sodium ion channels and sodium ions diffuse in causing depolarisation
- if the generator potential>threshold level , an action potential is produced and nerve impulses are transmitted along the neurone
what are the two types of light sensitive photoreceptor cells in the eye + their uses
rods - high visual sensitivity
cones- high visual acuity and enable colour vision
2 reasons why rods have higher visual sensitivity
1 - rhodopsin, the photopigment in rods is more sensitive to light than iodopsin (photopigment in cones)
2 - several rods synapse to a single bipolar cells providing an additive effect (spatial summation) enabling the threshold level to be reached
why do cones have lower visual sensitivity
- they have a 1:1 relationship with the bipolar neurone so no spatial summation occurs, making it harder to reach the threshold value
why do cones have high visual acuity
- cones have a 1:1 relationship with bipolar neurones therefore several stimulated cones = several impulses
- several impulses means that there is no blurring/combination of information = higher visual acuity
why is acuity greatest at the fovea
- it contains only cones which have a high visual acuity
why do rods have lower visual acuity
- rods share bipolar neurone therefore even if several rods are stimulated, only one neurone will be stimulated and send only one set of impulses
how does colour vision work?
- each cone contains only one pigment type which each has its own absorption peak at a particular wavelength of light
- when a cone absorbs more light - greater frequency of impulses
- the colour perceived is determined by the relative frequency of impulse from each cone type
what is another term for skeletal muscle
- striped/striated muscle
describe the structure of muscle fibres
- cylindrical in shape and enclosed by a sarcolemma
- possess many nuclei (multinucleate)
- possess many protein strands/myofibrils
- arranged parallel to each other
describe the fine/ultrastructure of muscle
- z-line connects actin filament
- sarcomere is length between each z-line
- A-band (dark band) is the length of myosin filaments
- h-zone is centre of A-band and only contains myosin
- I-band (light band) is outer region of A-band and only contains actin
- m-line connects myosin filaments in A-band
describe how the muscle structure changes during contraction?
- H-zone narrows
- I-band narrows
- outer region of A-band becomes wider
- A-band remains same size
- length of each sarcomere decreases (z-lines closer)
describe the mechanism of muscle contraction
- tropomyosin covers the binding site on the actin filaments
- calcium ions are released from the sarcoplasmic reticulum when the muscle fibre is stimulated
- calcium ions binds to tropomyosin causing it to change shape and move from binding site
- this allows myosin heads to bind to the actin filaments forming actinomyosin crossbridges
- calcium ions stimulates ATP hydrolase which hydrolyses ATP for the release of energy to enable the actin filament to be pulled and to detach the myosin head breaking the actinomyosin bridges
what happens to the calcium ions after muscle contraction
- they are actively transported back into the sarcoplasmic reticulum via active transport which requires energy
describe the transmission of nerve impulse across a neuromuscular junction
- arrival of action potential at synaptic knob causes depolarisation of presynaptic membrane which stimulates calcium channels to open and calcium ions to diffuse into the synaptic knob
- calcium ions cause synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitter (acetylcholine) which diffuses across the synaptic cleft
-acetylcholine attaches to specific protein receptor sites on the sarcolemma stimulating the entry of sodium ions causing depolarisation - this results in an action potential along the sarcolemma and into the muscle fibre stimulating the release of calcium ions which initiates muscle contraction
- acetylcholine is broken down in the postsynaptic membrane by the enzyme acetyl cholinesterase forming acetyl and choline which are both taken up to the synaptic knob by active transport and acetylcholine is re-synthesised
- active transport and re-synthesis require ATP yielded from mitochondria
describe the differences between slow and fast muscle fibres
- slow contract slowly, fast contracts quickly
- slow has many mitochondria, fast have few mitochondria
- slow is red due to presence of myoglobin (stores oxygen), fast is whitish due to minimal amount of myoglobin
- slow has low conc of phosphocreatine, fast has high conc of phosphocreatine
- slow has low conc of glycogen as lots of energy released per glucose, fast has high conc of glycogen as less energy released per glucose
what is ATP required for in muscles
to provide energy for:
- the movement of myosin head to pull the actin filament
- the detachment of myosin head breaking the actinomyosin bridges
- the reabsorption of calcium ions into the endoplasmic reticulum via active transport
what are the three sources of ATP
- ATP-Phosphocreatine system
- Anaerobic Respiration
- Aerobic Respiration
describe the ATP-Phosphocreatine system
- ADP reacts with phosphate donated by phosphocreatine to form ATP and creatine
- produces ATP very quickly but phosphocreatine is used up very quickly
- phosphocreatine is reformed with a using phosphate from ATP when the muscle is relaxed
what type of muscle fibre is the ATP-Phosphocreatine system particularly important
- fast muscle fibres in order to help produce ATP quickly in anaerobic conditions
