NERVOUS SYSTEM Flashcards
Stimulus
Change in an organism’s environment that can be detected by receptor cells
Receptor
Specialised cell that detects a stimulus and initiates a nerve impulse by creating a generator potential
Kinesis
Change in the speed of random movement in response to environmental stimulus (non-directional response)
Taxes
Directed movement toward or away from a stimulus (directional response)
Positive Taxes: Organism moves towards stimulus
Negative Taxis: Organism moves away from stimulus
Tropism
Response to stimulus by growing in a certain direction
Central Nervous System
The brain and the spinal cord
Peripheral Nervous System
Pairs of nerves that go to and from the brain and spinal cord (the CNS)
Sensory Neurones
Carries nerve impulses from receptors TO the CNS
Motor Neurones
Carries nerve impulses FROM CNS to the effectors, it is of two types:
Voluntary Nervous System: Caries nerve impulses to body muscles and is voluntary
Autonomic Nervous System: Carries nerve impulses to glands, smooth muscles and cardiac muscle and is involuntary
The Autonomic Nervous System is of two types
Sympathetic Nervous System: Stimulates effectors and therefore speeds up any activity
Parasympathetic Nervous System: Inhibits effectors and therefore slows down any activity
THEY ARE BOTH ANATGONISTIC
The Medulla Oblongata changes the heart rate in response to two receptors:
Chemoreceptors
Chemoreceptors in the Carotid and Aortic arteries which are sensitive to a change in CO2 level in blood
When CO2 levels high (e.g. during exercise):
There is a change in pH which is detected by chemoreceptor
Chemoreceptor sends impulses to Medulla Oblongata
Medulla Oblongata increases frequency of impulses down Sympathetic nerve to SAN
Heart beats faster
Increased blood flow removes CO2 faster
The Medulla Oblongata changes the heart rate in response to two receptors:
Pressure receptors
Pressure receptors in the Carotid Sinus which are sensitive to a change in blood pressure
If blood pressure is high, pressure receptors cause Medulla Oblongata to increase frequency of impulses down the Parasympathetic nerve to slow down the heart rate
Pacinian Corpuscle
it is a pressure receptor found on the skin and it detects SPECIFIC pressures and vibrations
Pressure on skin changes shape of pacinian corpuscle and Lamella layers are distorted
Distortion of lamella layers causes Stretch Mediated Sodium Channels to open which are on the sensory neurone
Sodium Channel allow positive Na+ ions to enter the negative sensory neurone causing depolarisation
This is known as a generator potential, if it reaches the threshold it triggers an action potential
The amount of sodium channels that open depends on the pressure applied by the stimulus
Therefore objects with small surface area cause a large stimulus as they cause a lot of pressure, like a pin
Rod Cells
they are photoreceptors that are found on the retina and they detect light, they then send impulses to the Optic Nerve which takes it to the brain
Many rods converge to one neurone (RETINAL CONVERGENCE) Low giving unclear image As brain cannot distinguish between the separate rods that generated the impulse Rhodopsin Found evenly all over retina Very sensitive to light due to RETINAL CONVERGENCE Black and white vision in poor light
Cone Cells
Only single cone per neurone
High giving sharp image
As brain can distinguish where the impulse came from as impulse was sent by single cone
Iodopsin
All over retina but more concentrated at fovea, therefore we move our heads to see stuff properly
Only functions in bright light, i.e not as sensitive to light
Seeing colour and detail in bright light
RETINAL CONVERGENCE:
Stimulation of several rods results in enough Neurotransmitters being released to reach the threshold value for an action potential in the bipolar neurone in low light intensities (also called spatial summation)
Why we have a high degree of visual sensitivity in low light levels
Several rod cells connected to each bipolar cell
Additive effect of small amount of light striking several rod cells
This creates a large enough depolarisation to generate an action potential
Why we have a high degree of visual acuity
Each cone cell connected to an individual neurone
Light strikes individual cone cell to generate a separate action potential
Very small area of retina stimulated, so very accurate vision
Reflex Arc/Spinal Reflex (this is a special pathway of neurones)
Rapid automatic response to a stimulus
Involves just 3 neurones
Does not involve the brain
One neurone is inside the Spinal Cord which is called the Intermediate/Relay Neurone, the other two neurones are the Sensory and Motor neurone
They are under genetic control and are effective from birth
The sequence is:
Stimulus → Receptor → Sensory neurone → Intermediate neurone → Motor neurone → Effector → Response
Importance of the Reflex Arc/Spinal Reflex
Brain is not overloaded with situations in which the response would be the same
Brain is free to carry out more complex responses
Protects from harmful stimuli as it is fast
Along the axon, there is
Active Transport: Na+/K+ pump; 3Na+ leaves axon and 2K+ enters. Therefore the p.d. of the axon is -70mV.
The splitting of ATP controls this active transport.
Facilitated Diffusion: There are Sodium and Potassium ion channels (INTRINSIC PROTEINS) in the membrane. The Na+ ions come into axon through their channel and K+ ions come out of axon through their channel. There are more K+ channels than Na+ channels. At resting, both channels are closed, but they still ‘leak’.
Therefore, the resting potential is -70mV
The Myelin Sheath
provides protection and electrical insulation
Action Potential
Depolarisation:
Neurone stimulated causing a few chemical-gated Na+ channels to open allowing Na+ ions to diffuse into axon
If stimulus large enough to make enough Na+ move in that the potential changes from -70 to -50 (threshold value), then all the other Na+ channels open that are VOLTAGE gated
More Na+ enter (positive feedback – change creating more change)
Repolarisation:
When potential reaches +40mV, then K+ channels open causing K+ to rush out making potential negative again
Hyperpolarisation:
The K+ channels remain open a little longer causing a little extra K+ to leave, making the potential -80mV
At -80mV, the K+ channels close and the Na+/K+ pump restores the potential to -70mV using ATP
All action potentials are the same size
Speed of transmission
Axon Diameter: the larger, the quicker the impulse
Less resistance to flow of ions in/out of axon, therefore depolarisation reaches other parts of neurone cell membrane quicker. Also this allows less leakage of ions.
Temperature: the higher, the quicker the impulse
Ions diffuse faster in facilitated diffusion, also in Na+/K+ pump is controlled by active transport which needs ATP from respiration, respiration depends on enzymes which rely on temperature
Saltatory Conduction; action potential goes from one Node of Ranvier to another due to insulating myelin. Therefore action potentials can only occur at Nodes of Ranvier
Therefore is something has a very low temperature, to compensate, it may have a large axon diameter.
In exam, if asked to explain Depolarisation
Sodium ion channel proteins open allowing Sodium ions to enter
Changes membrane potential and reaches threshold
More channel open this is called positive feedback
In exam, if asked to explain Repolarisation
Potassium channels open
Potassium leaves axon
Sodium channels close
Purpose of Refractory Period
Separates nerve impulses as another nerve impulse cannot happen during the refractory period
Limits number of impulses per second
Process of transmission across a neuromuscular junction
Nerve impulse depolarises the presynaptic membrane
Calcium channels open allowing Ca2+ ions to enter presynaptic membrane
Synaptic vesicle move towards presynaptic membrane
Release of transmitter across cleft using exocytosis
Transmitter attaches to receptor sites on post synaptic membrane
Sodium channels on post synaptic membrane open causing an influx of sodium ions resulting in a depolarisation
Neurotransmitter broken down by enzyme Acetylcholinesterase in the cleft
The products are reabsorbed by pre-synaptic knob where they are re-synthesis using energy from ATP in mitochondria
Temporal Summation
where two or more impulses arrive in quick succession down the same neurone, due to the post synaptic membrane having a high threshold
Spatial Summation
where two or more impulses arrive at the same time down different neurone, due to the post synaptic membrane having a high threshold
How transmission of information in the nervous system may be modified by summation
Summation is the addition of a number of impulses converging on a single post synaptic neurone
This allows integration of stimuli from a variety of sources, this is called SPATIAL SUMMATION
This allows weak background stimuli to be filtered out before reaching the brain, this is called TEMPORAL SUMMATION
How a neurotransmitter contributes to a synapse being unidirectional
Neurotransmitters released from presynaptic neurone
Diffusion of neurotransmitter from high concentration to lower concentration
Only the synaptic neurone contains receptors which the neurotransmitter can bind to
How inability to break down Acetylcholinesterase (enzyme) will lead to death
Acetylcholine (neurotransmitter) will not be broken down and will stay bound to receptor
Na+ ions continues to enter and continues to cause depolarisation
Muscles stay contracted
Skeletal Muscle
Muscle is made up of large bundles of long CELLS called muscle fibres, the membrane if muscle fibre CELLS is called sarcolemma, bits of sarcolemma fold inwards into a sarcoplasm, these bits are called T-tubules and they help to spread electrical signals, there are sarcoplasmic reticulums that run through the sarcolemma which release Ca2+ ions needed for contraction. The sarcolemma also has the myofibril, where the main muscle contractions take place.
Inhibitory Ion Channel Synapses
Neurotransmitter is Cl- ions which causes hyperpolarisation making action potential unlikely
Slow-twitch fibres: FOUND IN HEART
Contract more slowly and are provide less powerful contractions over a long period, therefore for endurance
Suited to their role by being adapted for aerobic respiration by:
Large store of myoglobin
Supply of glycogen to provide source of metabolic energy
Supply of blood vessels to deliver good amount of Oxygen and Glucose to maintain aerobic respiration
Numerous mitochondria to provide AT
The mitochondria in the slow-twitch fibre will be distributed to the edges of the fibre because
Allows rapid diffusion of Oxygen
Oxygen is used in the mitochondria
Site for Krebs Cycle and Electron Transport Chain
Fast-twitch fibres: FOUND IN FINGER MUSCLES, ARM MUSCLES
Contract more rapidly and provide more powerful contractions over a short period, therefore for intense exercise
They are adapted to their role for anaerobic respiration by:
Thicker and more numerous myosin filaments
High concentration of enzymes used in aerobic respiration
High store of Glycogen which can be broken down into glucose, there is a high store because anaerobic respiration is not very efficient
The Myosin
has a tail (fibrous protein)
has a head (globular protein) which contains ADP
Actin
A globular protein which has ACTIVE SITES that are covered by a protein called TROPOMYOSIN
When muscle contracts BANDS
I-Bands become narrower
Z-Lines move closer together
H-Zones become narrower
A-Bands remain the SAME (This fact proves that the myosin does not shorten and therefore is a Sliding Filament Mechanism)
Overall, the whole sarcomere gets shorter
Sliding Filament Theory
Calcium ions bind to troponin
This removes the TROPOMYOSIN in the actin, therefore exposing actin binding sites
ATP allows myosin to bind to actin
As myosin attaches, it changes its angle causing the actin to slide along and also causing the release of ADP
ATP combines to myosin allowing detachment of myosin from the actin binding site
ADP combines with myosin therefore causing normal angle to be assumed
Phosphocreatine allows regeneration of ATP without respiration by releasing Pi which combines with ADP to form ATP
Energy (ATP) is used for
Attachment between actin and myosin
2) Pulling of actin
3) Detachment of myosin heads
4) To move myosin head back to original position
5) Absorption of Ca2+ into endoplasmic reticulum by active transport
Phosphocreatine
Sometimes a muscle may be so active that Oxygen is rapidly used up and therefore ATP has to be generated using a chemical called Phosphocreatine. Phosphocreatine is stored in muscle and breaks down into ATP when Oxygen is being rapidly used up. When the muscles are relaxed, the phosphocreatine is made, and when short burst exercise takes place, e.g. 100m sprint, then phosphocreatine is converted to loads of ATP
