Nervous system I: Cells, Resting potential, Action potential
Central nervous system
- CNS is brain and spinal cord
- integration
Peripheral nervous system
- is all the rest of the nervous system
Cells of the nervous system
- Neurons (Fig 43.2)
o functional cell of the nervous system
o dendrites collect nerve impulses from other neurons (or can be quite specialized in sensory neurons)
o cell body contains nucleus and bulk of cytoplasm
o axon sends nerve impulses away from cell body
§ there is always only one axon
§ axon hillock – first part of axon, has key role in deciding whether to send a nerve impulse down the axon
§ synaptic terminal is specialized to communicate with another neuron or an effector cell at synapse
- Glia
o support cells of nervous system
o different types
§ Schwan cells
· Wrap membrane around axons of many neurons in peripheral nervous system, forming myelin sheath
§ oligodendrocytes
· make myelin sheath in CNS
§ astrocytes
· causes the cells of the endothelium in capillaries of the brain to form lots of tight junctions, thus creating the blood-brain barrier
o secretes factors that tell the endothelium to make more tight junctions
Types of neurons
- sensory (afferent)
o designed to detect stimuli (light, heat, pH, touch) and transfer info to CNS
o long axons
- motor (efferent)
o send signals away from CNS and produce some response
§ that can be movement, but also secretion or other effects
§ long axons
- interneurons
o connects neurons in the CNS to each other
o this allows lots of complex interactions (thinking)
o usually short axons
Reflex arcs (Fig. 43.4)
- shows how sensory, motor, interneurons work
- rapid response to simple stimuli
o sensory detects
o brings impulse to spinal cord
o synapses with a motor neuron and also (perhaps) an interneuron
o signal sent out the motor neuron to respond to stimulus
o interneuron then might send signal to brain
Electrical concepts in neuron function
- voltage is tendency for electrically charged particles to move between two points
o kind of like pressure is to water
o opposite charges attract, so positively charged things like to be next to negatively charged things
o like charges repel each other
- so separating ions causes a difference in electrical potential, or voltage
o if ions are separated across a membrane, called a membrane potential
§ measured in mV
The resting potential
- We can use a voltage meter to measure membrane potential, find it about -70mV (Fig. 43.5)
o First done in the ‘40s on giant squid axons
o Now use glass microelectrodes for much smaller neurons
o Due to differences in ion concentrations across membranes
- How is resting potential generated?
o sodium-potassium pump (Na+/K+ ATPase) protein pumps three Na+ OUT and two K+ IN
§ so three positive charges out for every two in
§ needs ATP to do that
§ excess of positive charge outside relative to inside
o ion channels in membrane – dominant are sodium and potassium ion channels
§ K+ channels open (“leak”) more than sodium ones
o Anionic proteins in the cell
§ Cannot cross membrane
- How do we know which way an ion will move across a membrane?
o For each ion there is a concentration gradient, but also a membrane potential or electrical gradient (an electrochemical gradient)
o So how do we combine these to know which way an ion goes?
o Nernst equation tells us
§ This allows you to find the equilibrium potential for an ion – that is, what potential would the membrane have to have in order to make that ion be at equilibrium and have no net movement across (Fig. 43.7)
§ E=60mVlog10 ([X extracellular]/[X intracellular])
· 60mV is a constant that depends on temp and the ion
§ Find that -70mV is close to equilibrium potential of K+ ions (-75mV)
§ BUT, far away from equilibrium potential of Na+ ions (+54mV)
· So under physiological conditions Na+ really wants into the cell because both electrical and concentration gradients favor that
The Action Potential (nerve impulse) (Fig. 43.10)
- Depolarization – when membrane potential becomes less negative
- Hyperpolarization – when membrane potential becomes more negative
- The action potential
o this is generated by voltage-gated channels
§ transmembrane ion channel proteins that open or close in response to changes in membrane potential
o if sodium channel opens, Na+ rushes in and depolarizes the cell
o at normal resting potential, most of these channels are closed
o cell has to depolarize to threshold potential for them to open, usually around -50 mV
o if a few enough Na+ channels open, threshold potential is reached
o then more sodium channels in that region of membrane open
§ Na+ rushes in and depolarizes membrane
o then channels close because of “inactivation gate”
o depolarizing to the threshold potential also causes voltage gated K+ channels to open, but there is a 1ms delay in these channels opening
§ K+ rushes out and repolarizes the membrane
o Then membrane slowly returns to normal
o there is a 1-2ms refractory period during which another action potential cannot be generated
§ this is when Na+ channels are closed and cannot be opened because inactivation gate is in the way
o action potential never lessens, it is all or nothing, either membrane is depolarized or it is not
o cannot reverse, because part of membrane it came from is in refractory period (Fig. 43.11)
Saltatory conduction (Fig. 43.12)
- most peripheral and some CNS neurons sheathed in myelin
- gaps in this are called nodes of Ranvier
- depolarization jumps from one to next
- called saltatory conduction
- increases speed of action potential down neuron
-