3.8 The Nervous system 5/6 Flashcards
state the components of a nervous response
receptors
effectors
nervous system or hormones transfer info from receptors to effectors
name 2 main divisions of the nervous system
CNS Central nervous system
PNS Peripheral nervous system
what is the CNS?
brain and spinal cord
What is PNS?
pairs of nerves that originate from the CNS and carry nerve impulses into and out of CNS
what is the dorsal root?
one of two roots that emerges from the spinal cord
travels to the dorsal root ganglion
sensory neurones enter spinal cord via the dorsal root
what is the ventral root?
one of two roots that emerges from spinal chord
motor neurones leave spinal cord via the ventral root
what is a reflex??
a rapid automatic response to a sensory stimulus by the body, serves as a protective mechanism
outline a simple reflex arc
stimulus -> receptor -> sensory neurone -> relay neurone (in CNS) ->motor neurone -> response
how does a reaction differ from a reflex?
reaction is voluntary & coordinated by the brain
reflex is non-voluntary & does not involve the brain
what is a nerve net?
simplest form of nervous system found in Cnidarians
consists of interconnected nerve cells with short extensions allowing a response to a limited number of stimuli
3 types of functional neurones
sensory neurone
relay neurone
motor neurone
state the function of a sensory neurone
carries nerve impulses from receptors to the CNS via the dorsal root
what is the function of the motor neurone?
carries nerve impulses from the CNS to the effectors via the ventral root
describe the structure of a motor neurone??
short dendrites carry impulses from CNS to cell body
cell body found at one end of neurone
long axon carries impulses from cell body to effectors
state the function of a relay neurone??
located in the spinal cord
links sensory neurone to motor neurone
what is the function of dendrites??
short branched extensions of the cell body
receive nerve impulses from other neurones
describe the cell body
the region of neurone that contains organelles notably the nucleus and rough endoplasmic reticulum
what is the function of the axon?
a long fibre tht conducts nerve impulses away from the cell body
what are axon terminals??
branched endings of an axon that approach the muscle fibre
what is a synaptic end bulb??
the end of an axon that is bulbous shaped contains synaptic vesicles filled with neurotransmitters
describe the additional features of a myelinated neurone??
schwann cells: wrap around axon involved in electrical insulation, phagocytosis, nerve regeneration
myelin sheath: made from myelin-rich membranes of schwann cells
nodes of ranvier: small gaps between neighbouring schwann cells where there is no myelin sheath
action potential??
temporary change in electrical potential across the membrane of an axon in response to the transmission of a nerve impulse
resting potential??
potential difference (voltage) across a neurone membrane when not stimulated (-70mV)
how is resting potential established?
- membrane more permeable to K+ than Na+
- Sodium-potassium pump actively transports 3Na+ out of cell and 2K+ into cell
- organic phosphates & large protein anions remain in cytoplasm
- establishes electrochemical gradient: cell contents more -ve than extracellular environment
name the stages of an action potential:
- depolarisation
- repolarisation
- hyperpolarisation
- return to resting potential
what happens during depolarisation?
- stimulus causes a change in the voltage across axon membrane, opening voltage-gated Na+ channels
- Na+ diffuse into axon
- potential diff across membrane becomes more +ve, membrane depolarises
what happens during repolarisation??
- membrane potential reaches +40mV
- voltage-gated Na+ channels close and voltage gated K+ channels open
- falicitated diff of K+ ions out of cell down their electrochemical gradient
4.potential difference across membrane becomes more -ve, membrane repolarises
what happens during hyperpolarisation?
- ‘overshoot’ when K+ ions diffuse out
- potential difference becomes move -ve than resting potential
- membrane hyperpolarises, preventing another impulse occurring
refractory period??
time period after an action potential during which further action potentials are prevented
why is the refractory period imp??
ensures that action potentials can only be propagated in one direction
describe the shape of an oscilloscope trace showing the passage of an action potential
graph y axis:voltage(mV) x axis:time resting potential -70mV stimulus shoots up depolarisation (dotted line at -55mv threshold potential) peak action potential down repolarisation below -70mV refractory period then back up to resting state
describe the ‘all or nothing’ law
principle that states that all stimuli above a certain threshold value will generate the same size of action potential regardless of the strength of the stimulus
state the factors affecting the speed of conduction of a nervous impulse
temp
axon diameter
myelin sheath
explain why myelinated axons conduct impulses faster than unmyelinated axons
saltatory propagation: impulse ‘jumps’ from one node of Ranvier to another (depolarisation cannot occur where myelin sheath acts as electrical insulator) so impulse doesnt travel along whole axon length
how does temp affect speed of conduction of nervous impulses?
higher the temp the faster the speed of conduction
how does axon diameter affect speed of conduction of nervous impulses?
larger diameter faster the speed of conduction
synapse??
junction bet 2 nerve cells or a nerve cell & an effector
what is the function of synapses?
- electrical impulse cant cross junction
-neurotransmitters send impulses bet neurones or from neurones to effectors - summation of sub-threshold impluses
-new impulses can be initiated in several diff neurones for multiple simultaneous responses
parts of a sypanse
presynaptic knob
postsynaptic knob
vesicles containing neurotransmitters
receptors
synaptic cleft
describe structure of a synapse:
-presynaptic neurone ends in synaptic knob
-synaptic knob contains a high conc of mitochondria, endoplasmic reticulum & vesicles of neurotransmitter
- synaptic cleft 20-30nm gap
-postsynaptic neurone has complementary receptors to neurotransmitter (ligand-gated Na+ channels)
what is a synaptic cleft??
a small gap bet neurones across which a nerve impulse is transmitted via neurotransmitters
describe synaptic transmission in the presynaptic neurone:
- wave of depolarisation travels down presynaptic neurone causes voltage-gated Ca2+ channels to open
2.Ca2+ cause vesicles of acetylcholine to move towards & fuse w presynaptic membrane
3.exocytosis of neurotransmitter(e.g. acetylcholine) into synaptic cleft
how do neurotransmitters cross the synaptic cleft?
via simple diffusion
describe synaptic transmission in the postsynaptic neurone:
- acetylcholine diffuses across synaptic cleft binds to specific receptors on postsynaptic membrane
- ligand-gated Na+ channels open
- if influx of Na+ ions raises membrane to threshold potential action potential is generated
how is the merging of impulses prevented during synaptic transmission?
-active transport of Ca2+ out of synaptic knob
-role of cholinesterase
-reabsorption of neurotransmitters
describe the role of cholinesterase in synaptic transmission:
-hydrolyses acetylcholine in the postsynaptic neurone
-products diffuse back across the cleft
what is the effect of organophosphates on the transmission of impulses?
- act as cholinesterase inhibitors, preventing the hydrolysis of acetylcholine in the postsynaptic neurone
-results in continuous stimulation of the neurone
where are organophosphates commonly found?
component of insecticides
how do psychoactive drugs (e.g. amphetamine) affect the transmission of impulses??
- excitatory drugs
-stimulate the release of neurotransmitters such as noradrenaline
