9.2 Flashcards
what is a neuron
cells specialized for the rapid transmission of impulses throughout an organism
what are receptor cells
specialized neurons that respond to changes in the environment
what are effector cells
specialized cells that bring about a response when stimulated by a neuron
what are sense organs
groups of receptors working together to detect changes in the environment
what is a sensory neuron
neurons that only carry information from the internal or external environment into the central processing areas
what is the CNS
central nervous system - specialised concentration of nerve cells where incoming information is processed and from where impulses are sent out through motor neurons
what are motor neurons
carry impulses to the effector organs
what is an axon
the long nerve fiber of a motor neuron which carries the nerve impulse
what is a dendron
the long nerve fiber of a sensory neuron which carries the nerve impulse
what is the peripheral nervous system
the parts of the nervous system that spread through the body that are not involved in the central nervous system
draw a motor neuron
draw a sensory neuron
draw a relay (connector) neuron
what are nerve impulses
minute electrical event that depend on the nature of the axon membrane and the maintenance if the sodium ion and potassium ion gradients across that membrane
what are dendrites
slender, finger-like processes that extend from the body cell of a neuron and connect with neighboring neurons
what is a schwann cell
specalised type of cell associated with myelinated neurons and the formation of myelin sheath
what is myelin sheath
fatty insulating layer around some neurons produced by Schwann cell
what are the nodes of ranvier
gaps between schwann cells that enable saltatory conduction
why is myelin sheath important
protects nerves from damage
speeds up transmission of the nerve impulses
what factors effect the speed at which electrical impulses travel across a neuron
diameter of nerve fiber - thicker fiber = impulse travels faster
absence or presence of myelin sheath
where are myelinated cells commonly found in vertebrates
neurons which transmit impulses to voluntary muscles (control movement)
where are unmyelinated cells commonly found
invertebrates
autonomic neurons in vertebrates (digestive system)
why are sensory nerve fibers hard to use for investigations
they are often connected to the brain or spinal cord making them difficult to access
why are motor axons good to use in investigations
they run directly to muscles in large motor nerves and the effect of stimulating them can be seen immediately
how is the axon membrane permeable to sodium and potassium ions
relatively impermeable to sodium ions
freely permeable to potassium ions
when is the axon at rest
when it is not conducting a nerve impulse and the extracellular concentration of ions is greater than the concentration in the axons cytoplasm
what is the sodium/potssium ion pump
contains enzyme Na+/K+ ATPase which uses ATP to move sodium ions out of the axon and potassium ions into the axon through active transport
what causes the membrane to become polarised
- NA+/K+ pump pumps sodium ions out of the axon lowering the concentration of sodium ions inside the axon
- due to the relative impermeability of the membrane to sodium ions they cannot diffuse back into the axon
- potassium ions are actively pumped into the axon but passively diffuse back out again along the concentration gradient through open potassium ion channels
- as a result the inside of the cell is left slightly negatively charged and so the membrane is polarised
what is the potential difference of the membrane at resting potential
-70mV
define polarised
the condition of a neuron when the movement of positively charged potassium ions out of the cell down the concentration gradient is opposed by the actively produced electrochemical gradient leaving the inside of the cell slightly negative to the outside
define resting potential
the potential difference across the membrane (roughly -70mV) when the neuron is not transmitting an impulse
what is a neurotransmitter
a chemical which transmits an impulse across a synapse
what are sodium gates
specific sodium ion channels in the nerve fiber membrane that open up, allowing sodium ions to diffuse rapidly down their concentration and electrochemical gradients
define depolarisation
the condition of the neuron when the potential difference across the membrane is briefly reversed during an action potential, with the cell becoming positive on the inside with respect to the outside for about 1 millisecond
what is action potential
when the potential difference across the membrane is briefly reversed to about +40mV on the inside with respect to the outside for about 1 millisecond
what is the threshold
the point when sufficient sodium ion channels open for the rush of sodium ions into the axon to be greater than the outflow of potassium ions resulting in an action potential
what is the refractory period
the time it takes for ionic movement to repolarise an area of the membrane and restore the resting potential after action potential
what is the absolute refractory period
the first millisecond after the action potential when is it impossible to re-stimulate the fiber as the sodium channels are completely blocked and the resting potential has not been restored
what is the relative refractory period
a period of several milliseconds after an action potential and the absolute refractory period when an axon may be re-stimulated but only by a much stronger stimulus than before
explain depolarisation (the cause of an action potential)
- when a neuron is stimulated the axon membrane shows a sudden and dramatic increase in permeability of sodium ions
- specific sodium gates open allowing sodium ions to diffuse rapidly across their concentration and electrochemical gradient
- this results in the potential difference across the membrane across the membrane to be briefly reversed
- the inside of the cell becomes positive with respect to the outside
where does an action potential occur
any nerve fiber
explain repolarisation
- at the end of depolarisation sodium ion channels close and excess sodium is rapidly pumped out by the active sodium pump
- permeability of the membrane to potassium ions temporarily increases because voltage-dependent potassium channels open
- potassium ions diffuse out of the axon down their concentration and electrochemical gradient attracted by the negative charge outside the axon
- inside of the axon becomes negatively charged again and resting potential is restored
what is the oscilloscope trace
the rapid increase (spike) in potential difference on a graph
does action potential change depending on the stimulus after the threshold
no the size of the action potential is always the same after the threshold
how does the action potential cause the movement of an impulse across a nerve fiber of an unmyelinated fiber
depolarisation of the membrane in front of the action potential causes sodium ion channels to open, sodium channels behind the action potential can’t open due to the refractory period so the impulse is continually conducted in the required direction
what is saltatory conduction
the process by which action potentials are transmitted from one node of Ranvier to the next in a myelinated nerve
why are synapses needed
receptors must pass their information into the sensory nerves and sensory nerves must then pass their information to the CNS
information needs to be able to move freely around the CNS and be passed to motor neurons then to effector organs
define synapse
the junction between two neurons that nerve impulses cross via neurotransmitters
what is the synaptic knobs
bulges at the end of the presynaptic neurons where neurotransmitters are made
summarise the creation and movement of an impulse along a neuron fiber
- at resting potential there is a potential charge on the outside of the membrane and a negative charge on the inside due to high Na+ concentration outside and high K+ concentration inside
- when stimulated voltage gated Na+ channels open and sodium flows into the axon depolarising the membrane
- localised electric currents are generated in the membrane
- the potential difference of the adjacent membrane to the first action potential changes inititating a second action potential
- at the site of the first action potential voltage gated Na+ channels close and K+ channels open
- K+ leaves the membrane and it is repolarised
- a third action potential is initiated by the second causing the current to move along the axon
- at the site of the first action potential K+ ions diffuse back into the axon restoring resting potential
what is the presynaptic membrane
the membrane on the side of the synapse where the first impulse arrives from which neurotransmitters are release
what are the synaptic vesicles
membrane-bound sacs in the presynaptic knob which contain around 3000 molecules of neurotransmitter and move to fuse with the presynaptic membrane when an impulse arrives at the presynaptic knob
what is the synaptic cleft
the gap between the pre and post synaptic membranes in the synapse
what does the functioning of a synapse depend upon
movement of calcium ions
explain a synapse
- arrival of impulse at synaptic knob increases permeability of presynaptic membrane to calcium ions as calcium channels open
- calcium ions move into synaptic knob down their concentration gradient causing synaptic vesicles to move to the presynaptic membrane
- vesicles fuse with presynaptic membrane and release transmitter substance into synaptic cleft
- these molecules diffuse across the gap and become attached to specific protein receptors on the sodium channels of the post-synaptic membrane
- this opens the sodium ion channels in the membrane resulting in an influx of sodium ions into the nerve fiber
- which causes a change in potential difference across the membrane and an excitatory post-synaptic potential (EPSP) to be set up
- if sufficient EPSPs the positive charge in the post synaptic cell exceeds the threshold level and an action potential is set up which then travels along the post synaptic neuron
what happens if the neurotransmitters have an inhibiting effect
when the transmitter substance bonds to specific protein receptor sites different ion channels open allowing the inwards movement of negative ions which makes the inside more negative than resting potential and an inhibitory post-synaptic potential results (IPSP)
draw a synapse
what is the excitatory post-synaptic potential (EPSP)
the potential difference across the post synaptic membrane caused by an influx of sodium ions into the nerve fiber as the result of the arrival of a molecule of neurotransmitter on the receptors of the post-synaptic membrane that makes the inside more positive than resting potential increasing the chances of an action potential
what is the inhibitory post-synaptic potential (IPSP)
the potential difference across the post-synaptic membrane caused by an influx of negative ions as a result of the arrival of a molecule of neurotransmitter on the receptors of the post-synaptic membrane which makes the inside more negative than resting potential decreasing the change of a new action potential
what is acetylcholine (ACh)
a neurotransmitter found in the synapses of motor neurons, the parasympathetic nervous system and cholinergic synapses in the brain synthesised in the synaptic knob using ATP
what are cholinergic nerves
nerves using acetylcholine as their transmitter
what is acetylcholinesterase
an enzyme found embedded in the post-synaptic membrane to cholinergic nerves that break down acetylcholine in the synapse after it has triggered a post-synaptic potential
what is noradrenaline
a neurotransmitter found in the synapses of the sympathetic nervous system and adrenergic synapses of the brain
what are adrenergic nerves
use noradrenaline as the neurotransmitter in their synapses
how can drugs increase response
- increase the amount of neurotransmitter synthesised
- increase the release of neurotransmitter from the vesicles at the presynaptic membrane
- binds to post-synaptic receptors and activates them or increases the effect of the normal neurotransmitter
- prevents the degradation of neurotransmitter by enzymes or prevents reuptake into presynaptic knob
how can drugs decrease response
- blocks synthesis of neurotransmitter
- causes neurotransmitter to leak from vesicles and be destroyed by enzymes
- prevents release of neurotransmitter from vesicles
- blocks the receptor and prevents neurotransmitter binding
how does nicotine work
mimics effect of acetylcholine and binds to specific acetylcholine receptors in post-synaptic membrane (nicotine receptors)
triggers an action potential in the post synaptic neuron but then the receptor remains unresponsive to more stimulation for some time
what effects does nicotine have in general
raises heart rate & blood pressure
triggers release of dopamine
what are the effects of nicotine in small and large doses
small dose - stimulating
large dose - blocks acetylcholine receptors and can be deadly
what is lidocaine used as
local anaesthetic
how does lidocaine work
lidocaine molecules block voltage gates sodium channels so raises the depolarisation threshold and prevents the production of an action potential in sensory nerves so prevents you from feeling pain
how is cobra venom fatal
it binds to acetylcholine receptors in the post-synaptic membranes and neuromuscular junctions and prevents the transmission of impulses across the synapse so muscles are not stimulated to contract and gradually the person affected becomes paralyzed and when it reaches muscles involved in breathing it causes death
when can cobra venom be beneficial
in very low concentrations it can be used to relax the muscles in the trachea and bronchi in severe asthma attacks
what is a primary receptor
simple sensory receptors where the dendrite receives a stimulus, chemical events occur that result in an action potential in the nerve fiber of the neuron
what are secondary receptors
consist of one or more completely specialised cells that are sensitive to a particular type of stimulus which synapse with a normal sensory neuron which carries impulses to the CNS
how do sensory receptors work (in depth)
receptor cells have a resting potential that depends on maintaining the charge of the cell interior negative in relation to the outside using membrane sodium pumps
when a receptor receives a stimulus sodium ions move rapidly across the cell membrane along the concentration and electrochemical gradient which sets up a generator potential
a large stimulus = a large generator potential
a small stimulus = a small generator potential
if the generator potential produced is large enough to reach the threshold of the sensory neuron an action potential will occur
summarise the stimulation of a sensory neuron
stimulus -> local change in permeability -> generator potential -> action potential
what is convergence
when several receptor cells synapse with a single sensory neuron so that if the generator potential from an individual receptor cell is insufficient to trigger an action potential several generator potentials may add together to trigger an action potential
why is convergence useful
because it increases sensitivity of a sensory system to low-level stimulus
how does the eye make us aware of varying degrees of light and shade not just light and dark
a weak stimulus results in a low frequency of action potential whereas a strong stimulus results in a rapid stream of action potentials therefor a graded response is still possible giving information about the strength of the stimulus in the eye
what is the range of wavelength of light that human eyes are sensitive to
400 - 700nm
what do the ciliary muscles do
change the shape of the lens
what is the cornea
clear layer involved in focusing light and protecting the eye
what is the pupil
circular hole in the iris which light enters the eye
what is the iris
a circular sheet of muscle that controls the amount of light entering the eye
what is the lens needed for
focusing light onto the retina
what is the choroid
pigmented cells that prevent internal reflection of light
what is the retina
layer of light sensitive cells (rods & cones)
what is the fovea
area of the retina containing only cones, region of highest visual acuity
what is the optic nerve
bundle of nerve fibers carrying impulses from the retina to the brain
what is the blind spot
the point where the optic nerve leaves the eye so there are no photoreceptor cells so no vision
what are secondary exteroceptors (e.g. rods & cones)
they signal changes in the external environment
what are rods
photoreceptors found in the retina which contain the visual pigment rhodopsin and respond to low light intensities, gives black and white vision and are very sensitive to movement
what are cones
photoreceptors found in the fovea of the retina which contain the visual pigment iodopsin which respond to bright light, give clarity of vision and colour
what are the benefits and drawbacks of rod cells not being tightly packed together
-/ they don’t give a very clear picture
+/ they are extremely sensitive to low light levels and movement because several of them synapse to the same neuron
what is the outer segment of a rod or cone cell
light-sensitive region that contains flattened membranous vesicles filled with photosynthetic pigments
what is the constriction section of a rod or cone cell
narrow region between the outer and inner segments
what is found in the inner segment of rod and cones cells
mitochondria and ribosomes
what forms rhodopsin
opsin and retinal
what vitamin effects site and why
vitamin A because it is needed for the formation of retinal which then forms rhodopsin
which geometric isomer of retinal prefers the dark
cis-retinal
what is bleaching
when a photon of light hots a molecule of rhodopsin it converts the cis-retinal into trans-retinal which changes its shape and puts strain on the bonding between opsin and retinal and the rhodopsin breaks into opsin and retinal
how is a generator potential geneorated in a rod cell
- rod cells are usuallt very permeable to Na+ ions
- when rhodopsin is bleached it closes sodium ion channels but the sodium pump continues to work so the interior becomes more negative
- this is hyperpolarisation and causes generator potential
how is generator potential passed on to action potential
if the generator potential is above the threshold neurotransmitter substances are released into the synapse with the bipolar cell and an action potential is set up in the bipolar cell that passes across the synapse and causes an action potential in the sensory neuron
why do rod cells need a lot of mitochondria
once rhodopsin has been bleached the rod cannot be stimulated again so it must be resynthesised using ATP
why do you become almost blind when you walk from a sunny garden into a house
the bright light has completely bleached rhodopsin you need to see in dimmer light and as rhodopsin reforms your vision returns as your eyes become dark-adapted again
why are cone cells less sensitive to low light intensities
it requires more energy to break down iodopsin than rhodopsin
what pigment so cone cells have and how does it allow us to see in colour
iodopsin
there are three types which are sensitive to one of the primary colours of light
what is the role of the brain
information can be processed, instructions can be issued to give fully coordinated responses to situations
what is the role of the spinal cord
carries nerve fibers into and out of the brain
coordinates unconscious reflex actions
what is the forebrain in embryos
old factory lobes which forms the cerebral hemispheres
what is the midbrain
contains optic lobes
what is the hindbrain
forms cerebellum and the medulla
what is the cerebrum
the area of the brain responsible for conscious thought, personality and control of movement
what are the cerebral hemispheres
two parts of the cerebrum joined by the corpus callosum
what is grey matter
consists of the cell bodies of neurons in the CNS
what is white matter
consists of the nerve fibers of neurons in the CNS
what is the corpus callosum
band of axons (white matter) that join the left and right cerebral hemispheres of the brain
what is the hypothalamus
the area of the brain that coordinates the autonomic (unconscious) nervous system, thermoregulation, osmoregulation
what is the cerebellum
the area of the brain that coordinates smooth movements (balance and posture)
what is the medulla oblongata
the most primitive part of the brain that controls reflex centers controlling functions such as breathing and heart rate
what is a reflex response
rapid responses that take place with no conscious thought involved
where do impulses enter the spinal cord from
dorsal roots
where do impulses leave the spinal cord
ventral roots
what is the function of a reflex arc
brings about an appropriate response to a particular stimulus as rapidly as possible
what is a relay neuron
relays impulses from sensory neurons to motor neuron
what is the voluntary nervous system
involves motor neurons that are under voluntary or conscious control involving the cerebrum
what is the autonomic nervous system
the involuntary nervous system involved in controlling bodily functions not involving conscious area of the brain
give similarities and differences in the structure of the sympathetic and parasympathetic nervous system
similarities/ myelinated preganlionic fibers leave the CNS and synapse in ganglion with unmyelinated post-ganglionic fibers
sympathetic/ ganglia very close to CNS
short preganglionic fibers & long post ganglionic fibers
parasympathetic/ ganglia close to CNS
long preganglionic fibers & short post ganglionic fibers
what is the neurotransmitter produced in the synapse of the sympathetic nervous system
noradrenaline
what is the neurotransmitter produced in the synapse of the parasympathetic nervous system
acetylcholine
what is the function of the sympathetic nervous system
produces a rapid response
fight or flight
what is the function of the parasympathetic nervous system
slower inhibitory effect
maintains normal functioning of the body & restores calm after a stressful situation
