15.2 Nervous communication
Structure
Central nervous system
Brain
Spinal cord
Peripheral nervous system
Cranial and spinal nerves
Neurones
Sensory neurones
Transmit impulses from receptors to CNS
Relay neurones
Transmit impulses from sensory neurones to motor neurones
Motor neurones
Transmit impulses from CNS to effectors
Found only in CNS
Many branched dendrites provides large surface area for axon terminals of other neurones
Terminal branches have mitochondria and vesicles containing neurotransmitters
Cell body lies along axon
Cell body lies within CNS
Has synaptic endings instead of dendrites
Cytoplasm contains many mitochondria and extensive RER for protein synthesis
Myelin
Many axons are myelinated
Schwann cells wrap themselves around the axon, forming the myelin sheath
Myelin sheath is largely lipid-based
Gaps between Schwann cells are called Nodes of Ranvier
Resting potential
The difference in electrical potential between the inside and the outside of the axon
-70mV
Maintained by sodium-potassium pumps (3 sodium out, 2 potassium in)
Membrane is impermeable to ions, and voltage-gated channel proteins are closed
Action potential
Electric current used to stimulate the axon opens voltage-gated Na+ channels
Na+ enter, making the inside of the axon less negative, causing more voltage-gated channels to open
Positive feedback loop of depolarisation
Inside potential reaches +30mV
Action potential is only initiated if the threshold potential of -60 to -50mV is reached
All or nothing event
Na+ channels close, K+ channels open
Membrane is hyperpolarised, voltage-gated K+ channels close, K+ ions diffuse back into the axon to restore resting potential of -70mV
Travel along an axon
Action potential on an axon cell surface membrane triggers action potentials on either side of that point
Current flows in a local circuit due to differences in charge between site of action potential and adjacent areas
Action potentials only occur ahead as the area behind is still in the refractory period and the ion channels cannot reopen
Action potentials are discrete events
Minimum time between action potentials and therefore maximum frequency of impulses is limited by refractory period
Impulses are unidirectional
Carrying of information
Constant size
Constant speed of travel
Strong stimulus = high frequency of impulses, and more neurones carrying impulses
Type of stimulus is deduced by brain from position of the sensory neurone carrying the stimulus
Speed of conduction of impulses
Myelin speeds up rate of action potential travel
Thick axons transmit impulses faster, as they have a greater surface area over which diffusion of ions can occur
Saltatory conduction is faster than normal transmission
Myelin sheath is an insulator, and is impermeable to sodium and potassium ions
Action potentials can only occur at nodes of Ranvier
Receptors
Can be specialised cells that activate sensory neurones
Can simply be the end of a sensory neurone itself
Salt receptors in the taste buds are directly affected by sodium ions
Receptor potential - change in resting potential across membrane of receptor cell caused by a stimulus
Sodium ions flow in through channel proteins
Membrane is depolarised and causes receptor potential
Voltage-gated calcium ion channel proteins open
Calcium ions enter the cytoplasm and lead to exocytosis of vesicles containing neurotransmitters
Synapses
Synaptic cleft - 20nm
Cholinergic synapse - uses acetylcholine as neurotransmitter
Arrival of action potential stimulates opening of Na+ and Ca2+ voltage-gated channel proteins
Ca2+ ions move in, causing vesicles containing ACh to move into presynaptic membrane and fuse with it, releasing ACh into synaptic cleft
Receptor proteins on cell surface membrane of postsynaptic neurone are complementary to ACh and allow it to bind
Receptor proteins undergo complementary shape change and allow Na+ ions to enter the postsynaptic neurone
Postsynaptic membrane is depolarised and new impulse occurs
Acetylcholinesterase hydrolyses ACh to acetate and choline
Prevents permanent depolarisation of postsynaptic neurone
Allows ACh to be recycled - products are brought back to presynaptic neurone
ACh diffuses across the synaptic cleft
Roles of synapses
Ensures one-way transmission, as receptors are only on one side
Allows interconnection of nerve pathways - many neurones to one, or one to many