Week 4: Neurophysiology, Neurotransmitters, and the Nervous System Flashcards
Neurons role
- Excitable
-Receive sensory information from the outside world
-storing and transmitting that information
-controlling the function of muscles and glands
Glia role
-Play a structural support role
-Also are protective and supply neurons with energy
-Active communicators with neurons via chemical transmission
-More than just support cells
Who first discovered that the nervous system communicated via electrical impulses?
-Italian scientist Luigi Galvani
Who first discovered that the nervous system communicated via electrical impulses?
Italian
scientist Luigi Galvani
Gulvani’s experiments on “animal electricity”. What? + What technology did it inspire?
-Frog legs that were dissected from rest of body. If had copper wire attached to nerves in spinal cord + legs –> sent electrical impulse —> muscles in legs contracted without input of the brain
-Further experiment = frog legs on table when there was lighting/ electrical energy in the air the legs would move
-Galvani’s experiments on “animal electricity” paved the way for the technique of electrophysiology, from which the electrical properties of neurons has been uncovered
-It is now known that neurons communicate by means of all-or-nothing electrical signals called action potentials
Important parts of neuron
-Dendrites (input zone —> connected other neuron or receiving direct sensory input)
-Soma (cell body)
-Axon hillock
-Axon (with Schwann cells/ myelin sheath for insulation —> spaces = nodes of Ranvier)
-Terminals
Generation of action potentials
-Resting potential (-70mv)= due to balance of ions in the extra + intra cellular space
-As the distribution of ions changes within the neuron (i.e. as sodium enters and potassium leaves), the electrical charge changes
-If the neuron is depolarized to around -55 mV (the threshold), the delicate balance that maintains the resting potential breaks down
—> action potential
Hyperpolarization versus depolarization
-Hyperpolarization – when the charge becomes even more negative (even harder to get to threshold)
-Depolarization – when the charge becomes less negative
Action potential course
Conduction of action potentials along the cell
-An action potential is generated at the axon hillock and travels down the cell
-Successive portions of the cell are depolarized and generate their own action potential at the Nodes of Ranvier
Is the strength of the action potentials decreased as it travels down the length of the axon?
Action potentials are non-decremental
—-> They reach the axon terminal with the same strength as which they were initiated near the axon hillock
(occurs because new AP at each node of Ranvier)
The all-or-none law
-Action potentials are always the same
-They do not vary in strength with the strength of the depolarizing stimulus
-Information about stimuli is transmitted by changes in the rate of action potential firing
Rate law
-Information about stimuli is transmitted by changes in the rate of action potential firing (as APs are not graded)
-More rapid APs = stronger stimulus
-Slower rate of firing + more uneven distribution = weak stimulus
Stimulation of the dendrites and cell body
-The dendrites and cell body contain many other proteins and enzymes that modulate cell excitability
—-> Allows for modulation by a number of factors
-Unlike the all-or-none action potential (at the axon hillock), disturbance of the resting potential of dendrites and cell bodies has variable effects i.e. EPSP, IPSP
Post-synaptic potentials (PSPs)
Excitation – opens voltage gated Na+ channels so Na+ comes in
-Depolarization
-Excitatory post-synaptic potential (EPSP)
-Brings the cell closer to firing an action potential
Inhibition – opens voltage gated K+ channels so K+ leaves
-Hyperpolarization
-Inhibitory post-synaptic potential (IPSP)
-More difficult to fire action potentials
Summing of excitation and inhibition
-PSPs (many thousands at a time) are integrated by neurons to determine the rate at which action potentials are generated
Two types of integration:
-Temporal summation
-Spatial summation
Temporal summation
-Signals come in a small time window
—> e.g. 2 EPSPs in rapid succession sum to make depolarisation to threshold more likely
Spatial summation
-Signals from two different inputs arrive at time will summate
How do neurons communicate with one another?
-Communication takes place at the synapse between adjacent neurons
-Voltage-gated calcium channels open in response to AP arriving at terminal of the pre-synaptic action potential
-Calcium causes vesicle contents to be released via exocytosis
-Neurotransmitters in vesicles cross the clef and bind to postsynaptic receptors
-Can have number of effects e.g. open channels to allow sodium in = EPSP.
Types of receptors
-Ionotropic = Neurotransmitter binds to receptor causes opening of ion channel and ions can come into =and out of the cell.
-Metabotropic =
Neurotransmitter binds to receptor but no channel. Instead the attached G-protein (sometimes called second messenger) breaks off and results in a signaling cascade.
Signaling cascade
-Can happen as a result of g-protein coupled metabotropic receptors
= Chain reaction that causes biochemical changes in the cell
-More long lasting/ permanent changes (e.g. changes in gene expression) than what it typically seen as a result of ionotropic receptors
Presynaptic effects of neurotransmitters
Autoreceptors =
-Contain a binding site for neurotransmitters
-Regulate the amount of neurotransmitter released from the presynaptic neuron (can act as a brake)
i.e. if enough neurotransmitter is being released that it is ‘spilling out’ and reaction the auto receptors at ‘the sides’ of the presynapse then this is too much.
Heteroreceptors =
-Like autoreceptors, but respond to chemicals released by the post-synaptic cell
-Regulate the amount of neurotransmitter released from the presynaptic neuron
—–> in other words, a number of ways that release from the presynaptic cell is regulated
Terminating the signal
-Reuptake = taking the neurotransmitter back into the presynaptic neuron for repacking and retransmission. Most common method (as don’t want to waste what has already been made). Accomplished by special proteins called transporters.
-Deactivation = synapse may contain an enzyme which breaks down the neurotransmitter. Parts may be taken back into the presynaptic cell for remanufacturing
Two divisions of the nervous system
Central nervous system – CNS
The brain and spinal cord
Cell bodies called nuclei (grey matter), axons called tracts (white matter)
Peripheral nervous system – PNS
Everything outside the brain and spinal cord
Cell bodies called ganglia, axons called nerves
Somatic nervous system
-Made up of all the sensory nerves from the conscious senses
-Allows us to interact with our environment
-Sensory system as well as motor nerves
-Uses acetylcholine as its primary neurotransmitter
Autonomic nervous system
-Sensory systems we are not usually aware of – blood pressure, functioning of organs, levels of hormones
-Parasympathetic (rest + digest) and sympathetic systems (fight or flight)
Parasympathetic
-In charge most of the time
-Rest and digest system
Keeps the internal functioning of the body running smoothly
Sympathetic
-Connected to the same organs as parasympathetic division
-In times of danger, takes over to help body prepare for sudden expenditures of energy – Fight or flight response —> overactive in those with anxiety
Difference in neurotransmitter use (sympathetic versus parasympathetic)
-Parasympathetic uses acetylcholine
-Sympathetic uses adrenaline
