Download Membrane Potential

Nervous System
Endocrine and nervous systems
cooperate to maintain homeostasis
Overview
• Sensory input
• Integration
• Motor output
CNS
• Brain and spinal cord
• Integration of sensory input
• Association stimuli with appropriate
motor output
PNS
• Network of nerves extending through
out body
• Carry sensory input to CNS
• Carry motor output from CNS
Unit of Structure -- neuron
• Cell body
• Dendrite – signals to cell body
• Short, branched (surface area)
Axons
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signals away from cell body
Long single process
Schwann cells  myelin sheath
Axon hillock – where impulses generated
Telodendria tipped with synaptic terminals
Release neurotransmitters
Types of neurons
• Sensory neurons
• Interneurons
• Motor neurons
Supporting Cells
• Reinforce, protect, insulate and
assist neurons
• Do not conduct impulses
• Much greater in number than
neurons
Glial cells – supporting cells of
the CNS
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Astrocytes
Encircle capillaries in the brain
Contribute to blood-brain barrier
Restricts passage of most substances
into CNS
• Oligodentrocytes-forms myelin sheaths
in CNS
Schwann Cells—supporting cells
of PNS
• Grow around axon forming concentric
layers
• Provides electrical insulation
• Membranes mostly lipidspoor conductors
• Increases speed of nerve impulse
propagation
• MS-deterioration of myelin sheaths
Electrical Membrane Potential
• Membrane potential range: -50 to –
100mV
• Indicates the charge of cytoplasm
side of membrane compared to
extracellular fluid
• -70mV is a resting neuron
Membrane Potential results from:
1. Differences in the ionic composition of the
intracellular and extracellular fluids
• Principal cation INSIDE cell is K+
• Principal cation OUTSIDE cell is Na+
• Principal anions inside cell: proteins,
amino acids, sulfate, phosphate
• Principal anions outside cell: Cl-
2. Selective permeability of the
plasma membrane
• Ions cannot readily diffuse through
hydrophobic core of phospholipid
bilayer
• Ion channels - carrier mediated
transport
• Large anions inside cell cannot cross
membrane
Ion Channel
• Allows specific ion to cross membrane
• Passive - open all the time
• Gated – require stimulu to change
conformation to open
• Selective for specific ion – Na+, K+, Cl• Membrane permeability is a function of
TYPE and NUMBER OF ION
CHANNELS
Maintaining Membrane Potential
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K+ diffuses out of cell
Down concentration gradient
Membrane has high permeability to K+
Transfers positive charges to outside
Cell’s interior becomes more negative
K+ move into cell down electrical gradient
Na+ move into cell down both gradients
What prevents weakening of
electrical gradient?
• Sodium-potassium pump
• Uses ATP
• Pumps Na+ out
gradients
of cell against both
• Pumps K+ into cell restoring
concentration gradient
Membrane potential changes
• All cells exhibit membrane potential
• Only neurons and muscles cells can
change membrane potentials in
response to stimuli
• These are excitable cells
• Resting potential – unexcited state, at
rest
Gated ion channels change the
membrane’s permeability
• Effect on the neuron depends on type
of gated ion channel the stimulus opens
• Stimuli that open K+ channels
• Hyperpolarize the neuron
• K+ effluxes from the cell
• Increases the electrical gradient
• more negative inside cell
Stimuli that open sodium
channels
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Depolarize the neuron
Na+ influxes into the cell
Reduce the electrical gradient
Inside of cell becomes more
positive
Graded potentials
• Voltage changes caused by stimulation
• Depends on strength of stimuls
• Threshold stimulus must be above this
critical intensity to stimulate the axon
• If depolarization reaches threshold
• Then cell triggers an action potential
Action Potential
• Rapid change in membrane potential