Nervous system Flashcards
What is the the ‘all or nothing’ nature of a nerve impulse?
All action potentials are the same size and have a minimum threshold value for action potential to occur
How do we get a Resting Potential (-70mv, inside of the axon is negative in relation to the outside)
Sodium potassium pump actively transports sodium out of axon and potassium into the axon
Membrane is more permeable to potassium than sodium, sodium gates are closed
Potassium diffuses out of the axon
Negatively charged proteins are found inside the axon
What is depolarisation and how is it achieved?
Change in the membrane potential. Inside of neurone becomes positive with respect to outside
Voltage Gated sodium channels open
Inflow of sodium ions down electrochemical gradient causes depolarisation
How is an action potential propagated?
Depolarisation of axon membrane causes local currents to be set up
These currents cause a change in the permeability of the adjoining region as voltage gated Na+channels open (in adjoining region)
Sodium ions enter adjoining region causing depolarisation
How is repolarisation achieved?
Sodium channels voltage gates close (absolute refractory: no new action potential can be produced)
Potassium voltage gates open
Potassium ions leave down electrical/chemical gradient causing Repolarisation;
hyperpolarisation (more negative inside the axon than at rest) relative refractory: larger stimulus is required to produce a new action potential
Sodium-potassium pump restores resting potential
What is myelination and how does it affect an action potential?
Myelin insulates axon depolarisation so ions can only pass through (plasma membrane
of axon) at gaps in myelin sheath (nodes of Ranvier) impulse jumps from node to node (saltatory conduction). In non myelinated neurones depolarisation occurs along the entire length of membrane. Thus energy demand is lower in myelinated neurones as active transport of ions, for maintaining and restoring resting potential occurs only at the nodes, so less ATP required, less respiration needed
How do we get a high degree of visual acuity?
Cone cells (responsible for acuity);
Each cone cell connected to an individual neurone;
idea of light striking each individual cone cell to generate a separate
action potential / impulse;
very small area of retina stimulated, so very accurate vision;
How do we get high sensitivity in low light conditions?
Rod cells (responsible for sensitivity);
Several rods connected to each bipolar cell;
Additive effect of small amount of light striking several rod cells;
creating a large enough depolarisation to generate an action potential in the ganglion cell
What is the purpose of a reflex?
Reflexes (purpose)
- Automatic (adjustments to changes in environment)/ involuntary;
- Reducing/avoiding damage to tissues / prevents injury/named injury
- Role in homeostasis/example;
- Posture/balance;
- Finding/obtaining food/mate/suitable conditions;
- Escape from predators;
What is a receptor?
A specialised cell which detects a stimulus
Which receptor detects pressure?
A pacinian corpuscle
Explain the advantages of a reflex being rapid and automatic?
Rapid:
Only involves 3 neurones: receptor, relay and effector neurone;
myelination: saltatory conduction;
Few synapses;
chemical / synaptic transmission is slow OR electrical / nervous transmission is fast;
Automatic
does not necessarily involve passage to brain / only spinal cord;
same pathway used each time;
higher brain centres not involved / no thinking;
How a resting potential is maintained in a neurone?
Membrane less permeable to sodium ions gated channels are closed / fewer channels;
Sodium ions actively transported out;
By sodium ion carrier / intrinsic proteins;
Higher concentration of sodium ions outside the neurone;
Inside negative compared to outside / 3 sodium ions out for two potassium ions in
Negatively charged proteins / large anions inside axon;
When pressure is applied to a Pacinian corpuscle, an impulse is produced in its sensory neurone. Explain how.
(Pressure) deforms and opens (sodium) channels Entry of sodium ions; Causes depolarisation (generator potential) Ions diffuse downstream and when threshold of nearby voltage gated channels is reached they open and sodium diffuses in causing depolarisation
Describe the events which produce an action potential.
1 Stimulus to threshold / critical firing level; 2 Sodium channels/gates open; 3 Sodium ions enter; 4 Down electrical/chemical gradient; 5 Positive feedback; 6 Depolarisation; 7 Inside becomes positive / membrane potential reverses; 8 Potassium channels/gates open; 9 Potassium ions leave; 10 Down electrical/chemical gradient 11 Repolarisation; 12 Sodium channels/gates close; 13 Undershoot / hyperpolarisation; 14 Sodium-potassium pump restores resting potential;
Describe how an action potential passes along the neurone - .propagation.
(Depolarisation of axon membrane causes) local currents to be set up;
Change permeability (of adjoining region) to Na+/open Na+ gates (in adjoining region);
sodium ions enter adjoining region;
adjoining region depolarises;
This process repeated along axon / self propagation;
Correct reference to/description of saltatory conduction;
Describe the events which allow transmission to take place across the synapse (6)
Increased permeability of (presynaptic) membrane to calcium ions;
Ca 2+enter;
vesicles fuse with membrane;
exocytosis of / release of acetylcholine / neurotransmitter
diffuses across synaptic cleft;
binds to receptors on postsynaptic membrane / side
increased Na + permeability / opens sodium channels / depolarises membrane
acetylcholine broken down by acetylcholinesterase
How may transmission of information may be modified by summation?
Summation = addition of a number of impulses converging on a single post synaptic neurone;
allows integration of stimuli from a variety of sources (spatial
summation);
allows weak background stimuli to be filtered out before reaching the brain (temporal summation)
What is the function of the mitochondria in the synaptic bulb?
Active transport of ions/ ionic pump; (reject active transport of Ach)
Synthesis of acetylcholine / neurotransmitter/ reform vacuole;
Reabsorption of acetylcholine, or acetyl + choline (from cleft);
Movement of vesicles (to membrane);
Synthesis of relevant enzyme, e.g. acetylcholinesterase.
Why does transmission in myleinated neurones uses less energy?
Myelinated leads to saltatory conduction
Active transport of ions is used less only at nodes of Ranvier;
Less respiration needed less ATP needed;
For repolarisation of ion balance;
Why are impulses along a non-myelinated axon slower than along a myelinated axon?
Non-myelinated – next section of membrane depolarised / whole membrane;
myelinated – depolarisation / ion movement only at nodes;
impulse jumps from node to node /saltatory conduction
What do baroreceptors do?
Baroreceptors monitor the pressure of the blood flowing into and out from the heart
What do chemoreceptors monitor?
Chemoreceptors monitor the pH of the blood flowing in the heart and the brain
If there is a rise in pressure, what effect does this have on the heart?
Rise in pressure stimulates heart to slow
Pressure receptors (baroreceptors) in aorta and carotid sinus send impulses to cardio-inhibitory centre in the medulla
Impulses are sent along the vagus nerves of the parasympathetic nervous system to SAN;
The release of ACh decreases impulses from SAN and this decreases impulses to AVN decreased stimulation of AVN
How does exercise cause the heart rate to increase?
Respiration increases
CO2 produced acidity of blood increases
Detected by chemoreceptors in aortic and carotid bodies and medulla
Impulses to cardio accelerator centre
More impulses along sympathetic nerves
Noradrenaline released at SAN increase heart rate
Higher pressure of blood in venous return (muscle contraction stronger)
Detected by pressure receptors in aorta and carotid artery
Impulses sent to the cardio acceleratory centre
More impulses via sympathetic nerves affecting the SAN
What is the mechanism of a heart beat?
Cardiac muscle is myogenic
SAN sends out a wave of depolarisation across the atria
Atrial systole occurs
AVN relays the impulse to the ventricles
(Impulse is delayed to allow time for the ventricles to fill)
Impulse passes to the ventricles travelling down the Bundle of His and up the ventricular
walls along the purkyne fibres causing ventricular systole
What is a rod?
A sensory detector in the retina that detects light
What is a cone?
A sensory receptor that detects colour
What is acetylcholinesterase?
An enzyme that catalyses the breakdown of Acetylcholine
What is an action potential?
A change in potential difference across the membrane of a neurone or muscle cell
