3.5 - the nervous sytem Flashcards
state the components of a nervous response
- receptors
- effectors
- nervous system or hormones transfer information from receptors to effectors
name the 2 main divisions of the nervous system
- central nervous system (CNS)
- peripheral nervous system (PNS)
outline the gross structure of the mammalian nervous system
peripheral:
- voluntary
- automatic:
— sympathetic
— parasympathetic
central:
- spinal cord
- brain
what’s the CNS
brain + spinal cord
what’s the PNS
pair of nerves originating from CNS + carry nerve impulses into + out of CNS
what’s the dorsal root
- one of 2 roots that emerges from spinal cord
- travels to dorsal root ganglion
- sensory neurones enter spinal cord via dorsal root
what’s the ventral root
- one of 2 roots emerging from spinal cord
- motor neurones leave spinal cord via ventral root
what’s a reflex
rapid, automatic response to sensory stimulus by body
serves as protective mechanism
outline a simple reflex arc
stimulus → receptor → sensory neurone → relay neurone (in CNS) → motor neurone → effector → response
how does a reaction differ from a reflex
- reaction is voluntary + coordinated by brain
- reflex is non-voluntary + doesn’t involve the brain
what’s a nerve net
- simplest form of nervous system found in Cnidarians
- consists of interconnected nerve cells w/ short extensions allowing response to limited number of stimuli
state the 3 types of functional neurones
- sensory neurone
- relay neurone
- motor neurone
state the function of a sensory neurone
carries nerve impulses from receptors to CNS via dorsal root
state the function of a motor neurone
carries nerve impulses from CNS to effectors via 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 spinal cord
- links sensory neurone to motor neurone
what’s te function of dendrites
- short, branched extensions of cell body
- receive nerve impulses from other neurones
describe the cell body
region of neurone that contains organelles, notably nucleus + RER
what’s the function of the axon
long fibre that conducts nerve impulses away from cell body
what are axon terminals
branched endings of axon that approach muscle fibre
what’s a synaptic end bulb
end of axon that’s bulbous shaped + contains synaptic vesicles filled w/ 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 theres no myelin sheath
define action potential
temporary change in electrical potential across membrane of axon in response to transmission of a nerve impulse
what’s resting potential
potential difference (voltage) across 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 cells and 2K+ into cell
- organic phosphates + large protein anions remain in cytoplasm
- establishes electrochemical gradient: cell contents more negative than extracellular environment
name the stages of action potential
- depolarisation
- repolarisation
- hyperpolarisation
- return to resting potential
what happens in depolarisation
- stimulus causes change in voltage across axon membrane, opening voltage-gated Na+ channels
- Na+ diffuse into axon
- potential difference across membrane becomes more positive, membrane depolarises
what happens in repolarisation
- membrane potential reaches +40mV
- voltage-gated Na+ channels close and voltage=gated K+ channels open
- facilitated diffusion of K+ ions out of cell down electrochemical gradient
- potential difference across membrane becomes more negative, membrane repolarises
what happens in hyperpolarisation
- ‘overshoot’ when K+ ions diffuse out
- potential difference becomes more negative than resting potential
- membrane hyperpolarises, preventing another impulse occurring
what’s the refractory period
time period after action potential during which further action potentials are prevented
why is the refractory period important
ensures action potentials can only be propagated in one direction
describe the ‘all or nothing’ law
principle states that all stimuli above a certain threshold value will generate the same size of action potential, regardless of strength of stimulus
state the factors affecting the speed of conduction of a nervous impulse
- temperature
- 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 can’t occur where myelin sheath acts as electrical insulator)
impulse doesn’t travel along whole axon length
how does temperature affect speed of conduction of nervous impulses
higher temp = faster speed of conduction
how does axon diameter affect speed of conduction of nervous impulses
larger diameter = faster speed of conduction
what’s a synapse
junction between 2 nerve cells or nerve cell + effector
what’s the function of synapses
- electrical impulse can’t cross junction
- neurotransmitters send impulses between neurones or from neurones to effectors
- summation of sub-threshold impulses
- new impulses can be initiated in several different neurones for multiple simultaneous repsonses
describe the structure of a synapse
- presynaptic neurone ends in synaptic knob
- synaptic knob contains high conc of mitochondria, ER, + vesicles of neurotransmitter
- synaptic cleft, 20-30nm gap
- postsynaptic neurone has complementary receptors to neurotransmitter (ligand-gated Na+ channels)
what’s the synaptic cleft
small gap between neurones across which a nerve impulse is transmitted via neurotransmitters
describe synaptic transmission in presynaptic neurone
- wave of depolarisation travels down presynaptic neurone, causing voltage-gated Ca2+ channels to open
- Ca2+ cause vesicles of acetylcholine to move towards + fuse w/ presynaptic membrane
- exocytosis of neurotransmitter (e.g: acetylcholine) into synaptic cleft
how do neurotransmitters cross the synaptic cleft
via simple diffusion
describe synaptic transmission in 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 generated
how is 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 postsynaptic neurone
- products diffuse back across cleft
what’s the effect of organophosphates on transmission of impulses
- act as cholinesterase inhibitors, preventing hydrolysis of acetylcholine in postsynaptic neurone
- results in continuous stimulation of neurone
where are organophosphates commonly found
component of insecticides
how do psychoactive drugs (e.g: amphetamine) affect transmission of impulses
- excitatory drugs
- stimulate release of neurotransmitters such as noradrenaline
