The Nervous System C3 Flashcards
what two parts is the human nervous system composed of?
the central nervous system (CNS) and the peripheral nervous system (PNS)
what is the CNS composed of?
the brain and the spinal cord
what is the PNS composed of?
nerves and ganglia (collection of nerves) on the outside of the CNS
what do the responses of external or internal stimuli involve?
the reception of information and its transfer from a receptor to an effector via the nervous system or as hormones via the blood in the endocrine system
define stimulus
a detectable change in the external or internal environment of an organism
give examples of stimulus
light
pressure
temperature
define receptor
detects stimulus (one form of energy) and converts this to electrical energy to send the information as an electrical impulse to the central nervous system via neurones
give examples of receptors
photoreceptor
thermoreceptor
mechanoreceptor
osmoreceptor
chemoreceptor
define effector
receives electrical impulses from the CNS and brings about a response
give examples of body receptors
muscles
glands
what three functional types (neurones) are in mammals that generate and transmit nerve impulses?
sensory
motor
relay (connector)
what is a sensory neurone?
it brings impulses from the sense organs or receptors into the CNS
what is a motor neurone?
carries impulses from CNS to effector organs (which are muscles and glands)
what is a relay neurone?
receives impulses from certain neurones and passes them on to other appropriate neurones and it’s found within the spinal cord
where is the position of the cell body in a sensory neurone?
in the middle of the neurone
where is the position
of the cell body in the motor and relay neurone?
at the start of the neurone
what neurone out of the three is unmyelinated?
relay neurone
what is the function of the axon?
it is an extension of the cytoplasm that transmits impulses away from the cell body to the axon endings
what is the function of the dendrites?
they are many thin extensions of cytoplasm that receive impulses from other nerve cells and transmit impulses towards the cell body
what is the function of the cell body?
it contains the nucleus and groups of ribosomes in the cytoplasm (Nissl granules) that synthesise neurotransmitters
what is the synaptic end bulb
it is the swelling at the end of the axon in which neurotransmitters are stored
what do the axon endings do?
they secrete neurotransmitters by exocytosis into the synaptic cleft
what three components is only found in the peripheral nervous system?
schwann cells
myelin sheath
nodes of ranvier
what are schwann cells?
they surround peripheral neurones in vertebrates and grow around the axon to form a multi-layered myelin sheath
what is the myelin sheath?
they are formed as schwann cells grow around the axon in peripheral neurones and provide electrical insulation to speed up nerve impulse transmission
what are the nodes of Ranvier?
intervals in the myelin sheath between adjacent schwann cells
what is important to label if an exam question asked you to label a neurone and it was pointing to that structure?
the schwann cell nuclues
what is the spinal cord and what is it protected by? give 4 points
long, thin, tubular bundle of nervous tissue
support cells that extends from brain
most of peripheral nerves originate from spinal cord
protected by spinal column
what is the function of the spinal cord?
transmits neural signals between brain and rest of body
contains neural circuits that independently control many flexes
what is the spinal cord made up of?
a central area of grey matter which mainly contains nerve cell bodies and is surrounded by white matter which consists of nerve fibres surrounded by myelin sheath axons
the spinal cord is surrounded by membranes called meninges
what is meant by white matter and grey matter?
when the spinal cord is observed, the cell bodies in the grey matter absorb the dye better so it is a dark colour when looked at
the fatty tissue isn’t as water soluble so it repels the dye and leaves a white colour when observed
where are the sensory fibres and the motor fibres found in the spinal cord?
sensory fibres from the PNS enter the spinal cord on the dorsal (back) root and the cell bodies of the sensory fibres are found in the dorsal root ganglia
motor fibres leave via the ventral (front) roots
where is the central canal found in the spinal cord and what is its function?
it is a small circle in the middle of the spinal cord
it contains cerebrospinal fluid for protection
what are reflex arcs?
it is the simplest type of response and it is an inborn response to a stimulus and is rapid, automatic and beneficial
since the reaction is involuntary it does not involve the brain and such reactions are normally protective in function
show the pathway of a reflex arc
1- stimulus
2- receptor
3- sensory neurone
4- CNS
5- relay neurone
6- motor neurone
7- effector
8- response
give two examples of reflexes
blinking
retracting from pain
how are any reflex action impulses also sent to the brain and how may the brain react?
via ascending nerve fibres that synapse at grey matter of spinal cord
brain may store this info. / relate it to other sense data e.g. eyes
brain may override response using inhibitory nerve fibres
what does it mean by a neurone being an excitable cell?
it can change its resting potential (which is the potential difference across the membrane when no nervous impulse is being conducted)
how can the potential difference across membranes be measured?
in experiments involving inserting microelectrodes into axons and measuring the changes in electrical potential which can be read on a cathode ray oscilloscope
why are neurones from giant squids used?
they are very large
when, in potential difference experiments, is there no difference in electrical potential?
when the two microelectrodes are both outside the neurone
what is known as potential difference?
when two electrodes are placed on either side of the axon and the difference between the outside and inside of the axon is measured
how do neurones transmit electrical impulses along the cell membrane?
by changing the potential difference across the axon membrane of the neurone
what is resting potential?
when no impulse is being sent and the inside of the cell has a negative electrical charge compared to the outside
how is resting potential maintained?
via the movement of sodium and potassium ions
why do we say the membrane is polarised? (carrying on from resting potential)
the large protein anions (negatively charged protein) and the organic phosphates ( eg ATP^-4) remain in the cytoplasm inside the axon thus producing a negative potential difference across the membrane at around -70mV relative to the exterior of the axon
describe the mechanism of the sodium and potassium ions in resting potential during nerve transmission
3 Na+ ions are pumped by active transport out of the axon for every two K+ ions that are pumped in by the sodium-potassium pump
voltage-gated K+ ion channels, some of which are open, allow K+ to diffuse back out of the axon
as the voltage-gated Na+ ion channels are closed, the membrane is much more permeable to K+ than Na+
the neurone has a resting potential of -70mV
some K+ channels allow the K+ ions to leave the axon through leakage
what is depolarisation?
when the nerve impulse is initiated, the action potential is created
how is the action potential generated?
the energy of a stimulus causes some Na+ ion-gated channels to open and the voltage-gated potassium ion channels close
if a threshold voltage is reached, the permeability of the membrane to Na+ increases and they rush into the axon, depolarising the membrane
the negative charge of -70mV inside the axon rapidly becomes a positive charge of +40mV
the membrane is said to be depolarised
summarise how repolarisation occurs
the voltage-gated Na+ channels close and the voltage-gated K+ channels open
this causes K+ to rapidly diffuse out of the axon, reducing the potential difference across the membrane
an overshoot causes the membrane to become hyper-polarised
describe the refractory period
during this time the concentrations of K+ and Na+ are restored to that of the resting potential
during this time - about 1ms - this portion of the axon cannot transmit another action potential, ensuring that transmission is in one direction only
what is the refractory period also known as?
hyperpolarisation
describe the initiation or no initiation of an action potential using the threshold
if the intensity of a stimulus is below the threshold (approx. -55mV) then no action potential is initiated
if the intensity of the stimulus exceeds the threshold then an action potential is generated (+40mV)
this ensures that low level stimuli are filtered out
what does the all-or-nothing law state?
an action potential is either initiated or is not and it is always the same size
explain the all-or-nothing law
the size of the impulse is independent of the stimulus
the speed of conduction of the stimulus is not altered by the intensity of the stimulus
a large stimulus will simply produce a greater frequency of impulses
describe once again the nerve impulse transmission which we have learnt for a non-myelinated neurone
Na+ rapidly diffuse down their concentration gradients into the axon during depolarisation to the next part of the axon due to local current effects
this begins to depolarise the adjacent section of the membrane causing voltage-gated Na+ channels to open in these regions to allow more Na to diffuse in, generating an action potential and depolarising this part of the membrane
this spreads a wave of depolarisation along the axon
at the site of the initial action potential generated, Na+ channels are inactivated and can’t reopen again until the resting potential has been re-established
in what species is the myelin sheath present in?
vertebrates
describe the nerve impulse transmission in a myelinated neurone
the axon is surrounded by Schwann cells that form a myelin sheath to act as an electrical insulator
the nerve impulse appears to jump from one node of Ranvier to the next, as myelin is a lipid-type substance that is impermeable to ions
therefore depolarisation and action potential can only take place at the nodes where there is an abundance of ion channels and Na/K+ pumps
the local currents caused by the diffusion of Na+ are elongated and the speed of transmission is increased. this is known as the saltatory propagation
what are three factors affecting the speed of conduction of an impulse?
temperature
size/diameter of an axon
myelination
explain temperature as a factor of speed conduction
ions move faster at higher temperatures as they have more kinetic energy
warm-blooded animals transmit nervous impulses more quickly and have faster responses
explain the size/diameter of an axon as a factor of speed conduction
the larger the diameter, the greater its volume in relation to the area of the membrane
more Na+ can flow along the axon so impulses travel faster
explain the myelination as a factor of speed conduction
this electrical insulation of the axon speeds up the rate of transmission as the action potential jumps from node to node in a process called saltatory propagation
so while nerve impulses in unmyelinated neurones have a maximum speed of around 1m/s, in myelinated neurones that travel at 100m/s
what is cnidaria?
a phylum containing aquatic animals such as hydra, jellyfish and sea anemone
describe a nerve net
it is the simplest type of nervous system
the sense receptors only respond to a limited number of stimuli and there is only a small number of effectors
the system consists of simple nerve cells with short extensions joined to each other and branching in a number of different directions
once again, give a summary of nerve impulse initiation and transmission in 10 points
1- Na+/K+ pumps are working: 3Na+ are actively transported out and 2K+ are transported in and this process requires ATP as transport is against the conc. gradient. causes the membrane to be polarised and the resting potential is -70mV
2- pressure receptor cells in your skin detect an external stimulus and if the threshold is reached, the voltage-gated Na= channels in the area open
3- Na+ diffuses rapidly into the cell resulting in depolarisation
4- an action potential is generated and the membrane potential is now +40mV
5- voltage-gated K+ channels open, Na+ channels close and K+ rapidly diffuse out of the cell which repolarises the membrane
6- the amount of K+ ions that diffuse out is a slight overshoot so the membrane potential falls to approximately -75mV aka hyperpolarisation
7- the resting potential is restored during the refractory period which ensures that the nerve impulse continues in one direction
8- once an impulse is made, a local current is set up between the area where there is an action potential and the resting area next to it
9- Na+ diffuses down their conc. gradient and depolarise the adjacent area to propagate the action potential along the axon
10- in a myelinated neurone the impulse is conducted more rapidly, as the impulse ‘jumps’ from one node of Ranvier to the next as myelin is an electrical insulator
what is a synapse?
a junction between two neurones
what are the two types of synapses?
electrical and chemical
what is an electrical synapse?
a tiny gap approx. 3nm that is small enough for an electrical impulse to be transmitted directly across - an electrical impulse
what is a chemical synapse?
involves a gap around 20nm
branches of axons lie close to dendrites and the impulse is transmitted across the synaptic cleft chemically as a neurotransmitter before being converted back into an electrical impulse
name a few neurotransmitters
acetylcholine
serotonin
glutamate
histamine
dopamine
give the labels of a cholinergic synapse
axon ending
dendrite
neurotransmitters
neurotransmitter transporter
synaptic vesicle
presynaptic cell membrane
Ca2+ channel
synaptic cleft
post synaptic cell membrane
receptor
sodium ion channels
action potential reaches the next neurone
describe the mechanism involved in synaptic transmission
an action potential arrives at the axon terminal which causes voltage-gated Ca2+ ion channels to open. the calcium ions diffuse rapidly into the presynaptic knob
the influx of Ca2+ causes vesicles containing acetylcholine to fuse with the presynaptic membrane
the acetylcholine is released into the synaptic cleft by exocytosis and diffuses over the synaptic cleft and binds to receptors on the post synaptic membrane
this causes the sodium ion channels in the post synaptic membrane to open and Na+ diffuses in, depolarising the post synaptic membrane. if the depolarisation reaches the threshold value an action potential is generated in the post synaptic neurone
acetylcholine in the receptors is broken down by cholinesterase into ethanoic acid and choline which diffuse back into the axon terminal through the presynaptic membrane
ATP is then required to re-synthesise and package the neurotransmitters into vesicles
Ca2+ ions use a different protein to be actively transported back out of the synaptic knob to reset its electrochemical gradient
what are organophosphates?
organophosphates insecticides acetylcholinesterase inhibitors, which cause repeated firing of the post-synaptic neurone. this results in repeated, uncontrolled contractions of the muscle
what do psychoactive drugs do?
act on the CNS and can alter brain function resulting in temporary changes in perception, mood, consciousness and behaviour
what do excitatory drugs do?
stimulate the nervous system - increase the number of action potentials
what are the possible methods of action with excitatory drugs?
they mimic the action of natural NT - have similar shape and bind to post-synaptic neurone, agonist
prevent breakdown of NT by inhibiting Ach esterase
block reuptake of natural NT
give two examples of excitatory drugs
caffeine
cocaine
what do inhibitor drugs do?
inhibit the nervous system - decrease the number of action potentials
what are the possible methods of action with inhibitor drugs?
blocking receptors in post-synaptic membrane preventing NT binding to them - antagonist
blocking calcium ion uptake
inhibit exocytosis of NT
give two examples of inhibitor drugs
cannabis
alcohol
how do neurotransmitters work once released from a neuron?
they are released into the synaptic cleft where they cross the gap and land on receptors of the dendrites of the next neuron.
the neurotransmitters are hydrolysed and their constituents are returned to the original cell through transporters in a process called re-uptake.
as long as the neurotransmitter remains in the cleft, the receptors will continue to be stimulated
explain how SSRI’s (selective serotonin re-uptake-inhibitor) can treat mood disorders
it prevents the re-uptake of serotonin, so that it continues to stimulate the post-synaptic neurone and this can prolong feelings of happiness and satisfaction
what is synaptic pruning?
the process by which some synapses (the ones that are frequently used) become more stable, and the synapses that are barely used wither away
