Membranes and Action Potential Flashcards
Membrane Lipids—Phospholipids
Major
• Glycerophospholipids/
phosphoglycerides
– Phosphatidylcholines (PC),
phosphatidylserines (PS),
phosphatidylethanolamines (PE)
– Polar head group = choline,
serine, ethanolamine
• Minor: include
phosphatidylinositols (PI)
• PC (lecithin), mainly in outer
leaflet
• PE, PI mainly in inner leaflet
• PS exclusively in inner leaflet
Membrane Lipids—Sphingolipids
Major
• Located mainly in outer
leaflet
• Derived from ceramide
Membrane Lipids—Glycolipids
Glycolipids
– Most made from ceramide:
glycosphingolipids
– Eg. gangliosides, ABO blood
group Ags
– Minor but essential; ~2% of
membrane lipids
– Exclusive to outer leaflet
Membrane Lipids—Cholesterol
Major
– Amphipathic
– Interdigitates between
phospholipids in inner and
outer leaflets
– Helps maintain structural
integrity of plasma
membrane
Membrane Proteins
Integral, peripheral (membrane associated)
• All TM proteins integral
Transport Across Membranes
• Selectively permeable barrier
• Main modes of transport:
– osmosis, simple diffusion, facilitated transport,
active transport, exocytosis, endocytosis,
transcytosis
• Size exclusion limit
Simple Diffusion
• Small, lipid
-soluble
molecules can diffuse
through membrane
according to their
concentration gradient
• Movement in both
directions but net flow in
one direction until
concentration on both sides
of membrane equal – Eg. NO diffuses through lipid
bilayer
Osmosis
Aquaporins—channels
used for transport of water
– Integral membrane proteins
Facilitated Diffusion
Some molecules too large, charged or hydrophilic→ cannot simply diffuse through lipid bilayer • Transporters (transmembrane proteins) change conformation to rapidly move a molecule across the membrane, down its electrochemical gradient (Passive transport)
Active Transport
ENERGY used to transport molecules across membrane
• Primary active transport
• Secondary active transport; co-transport
Active Transport—
Primary Active Transport
Eg. Na/K ATPase, expressed in most cells
– Pumps Na+
ions out of cell and K+
ions into cell, with
hydrolysis of ATP, ie. against each ion’s electrochemical
gradient
• NOTE: [Na+
] in the ECF than in the ICF; [K+
] higher in the ICF
Active Transport—
Secondary Active Transport
Transport of one molecule
coupled to movement of
another molecule
• Symport
• Antiport
Active Transport—
Secondary Active Transport
Eg. Na/Glucose, SGLT1;
symport
– Both glucose and Na+ bind
symporter and are transported
into cell
– Energy from Na+ moving down
its electrochemical gradient is
maintained by Na/K ATPase
– Na+ and glucose in intestinal
lumen are transported across
apical membrane of
enterocytes into these cells,
against glucose concentration
gradient
Ion Channels
Transmembrane proteins that form aqueous
conduits, selective for types of charged species that
can flow through them (facilitated diffusion)
• Move ions at relatively high rate
– Examples: conduction of nerve impulses, muscle
contraction
• (Not permanently open—many are gated, ie. they
open and close in response to specific stimuli)
• Gap junctions
– Ions, other solutes
Ion Channels—Gating
• Permeability of membranes must be
regulated
• Three major classes of gated channels:
–Voltage-, ligand-, mechanically-gated
Ion Channels—Voltage-gated Channels
Open in response to change in electrical potential across cell membrane
CFTR
- Anion channel
- Chloride, bicarbonate
Ion Channels—Mechanosensitive
Open in response to mechanical forces
Exocytosis
Cell releases molecules into
extracellular environment
through fusion of transport
vesicles with plasma membrane
• Material to be exocytosed may
be synthesized and:
– Released immediately from cell,
– Stored in secretory vesicles near
the membrane until needed. Eg.
Neurotransmitters, hormones
– …Or, synthesized as precursors.
When needed, converted to active
proteins before or after exocytosis
Endocytosis
Cell takes up
macromolecules, fluid,
solutes, membrane
components
• Phagocytosis, pinocytosis
• Material enclosed by
plasma membrane which
eventually pinches off to
form endocytic vesicle
(EV)
Endocytosis
EV may fuse with
receiving
compartment→ early
endosome (EE)
– sorting→ recycling,
degradation, other
• Endosome maturation:
EE→→ LE
• Fusion of LE with
lysosomes→
endolysosomes;
degradation
Phagocytosis
Endocytosis of large particle followed by fusion with specialized vesicle →phagosome – Bacteria, viruses, cells that
have died by apoptosis• Phagosome fuses with lysosome and ingestedmaterial is degraded
• Some types ofphagocytosis enacted by specialized cells, such
as macrophages and neutrophils
Receptor-mediated Endocytosis
Some endocytosis events (incl. phagocytosis) require binding of
extracellular macromolecule to a membrane-bound receptor
• Example: Cholesterol (LDL) taken up from bloodstream via RME
Transcytosis
• Vesicle-mediated transcellular transport
A 55-year-old woman presents to her physician with complaints
of substernal chest pain while at rest. Upon physical
examination, the physician notes that her heartbeat is less
rhythmic than it should be. A defect in diffusion of ions between adjacent affected cardiac myocytes is identified. Expression of the gene coding for which of the following is most likely affected?
A) Actin
B) Aquaporin
C) Gap junction
D) Na/K ATPase
E) Tubulin
The structure of neurons includes the cell body, or soma; the dendrites; the axon; and the presynaptic terminals
Glial cells, which greatly outnumber neurons, include astrocytes, oligodendrocytes, and microglial cells; their function, broadly, is to provide support for the neurons.
The nervous system is composed of two divisions: the central nervous system (CNS), which includes the brain and the spinal cord, and the peripheral nervous system (PNS), which includes sensory receptors, sensory nerves, and ganglia outside the CNS. The CNS and PNS communicate extensively with each other.
Structure of the Neuron
Cell Body: The cell body, or soma, surrounds the nucleus of the neuron and contains the endoplasmic reticulum and Golgi apparatus. It is responsible for the neuron’s synthesis and processing of proteins.
Dendrites: Dendrites are tapering processes that arise from the cell body. They receive information and thus contain receptors for neurotransmitters that are released from adjacent neurons.
Axon: The axon is a projection arising from a specialized region of the cell body called the axon hillock, which adjoins the spike initiation zone (or initial segment) where action potentials are generated to send information.
Whereas dendrites are numerous and short, each neuron has a single axon, which can be quite long (up to 1 meter in length). The cytoplasm of the axon contains dense, parallel arrays of microtubules and microfilament that rapidly move materials between the cell body and the axon terminus. Axons carry action potentials between the neuron cell body and the targets of that neuron, either other neurons or muscle. Axons may be insulated with myelin ,which increases conduction velocity; breaks in the myelin sheath occur at the nodes of Ranvier.
Presynaptic Terminals: The axon terminates on its target cells (e.g., other neurons) in multiple endings, called presynaptic terminals. When the action potential transmitted down the axon reaches the presynaptic terminal, neurotransmitter is released into the synapse. The transmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane (e.g., of dendrites of other neurons). In this way, information is transmitted rapidly from neuron to neuron (or, in the case of the neuromuscular junction, from neuron to skeletal muscle)
Based on the number of processes that emanate from their cell body, neurons can be classified as unipolar, bipolar, pseudounipolar, and Multipolar
True
A) Unipolar neurons have one process, with different segments serving as receptive surfaces and releasing terminals.
True
.B) Bipolar neurons have two specialized processes: a dendrite that carries information to the cell and an axon that transmits information from the cell.
C) Some sensory neurons are in a subclass of bipolar cells called pseudounipolar cells. As the cell develops, a single process splits into two, both of which function as axons—one going to skin or muscle and another to the spinal cord.
True
D) Multipolar cells have one axon and many dendrites. Examples include motor neurons, hippocampal pyramidal cells with dendrites in the apex and base, and cerebellar Purkinje cells with an extensive dendritic tree in a single plane.
Gilial Cells
How do ions move in and out of the cell
RMP is expressed as the measured potential difference across the cell membrane in millivolts (mV). is, by convention, expressed as the intracellular potential relative to the
extracellular potential. Thus, a resting membrane potential of −70 mV means 70 mV, cell negative
- The resting membrane potential is established by diffusion potentials that result from concentration differences of permeant ions.
- Each permeable ion attempts to drive the membrane potential toward its equilibrium potential. Ions with the highest permeabilities, or conductances, will make the greatest contributions to the resting membrane potential, and those with the lowest permeabilities will make little or no contribution.
- For example, the resting membrane potential of nerve is −70 mV, which is close to the calculated K+ equilibrium potential of , but far from the calculated Na+ equilibrium potential. At rest, the nerve membrane is far more permeable to K+ than to Na+.
- The Na+–K+ pump contributes only indirectly to the resting membrane potential by maintaining, across the cell membrane, the Na+ and K+ concentration gradients that then produce diffusion potentials. The direct electrogenic contribution of the pump (3 Na+ pumped out of the cell for every 2 K+ pumped into the cell) is small.
Graded Potential
Local, non-propagated, in response to a depolarizing or hyperpolarizing stimulus of lesser strength•
Catelectrotonic potential: Electrically stimulated cathodal end of the stimulator evokes a depolarizing response•
Anelectrotonic potential: The hyperpolarizing potential produced due to stimulation at anodal end
the figure shows the voltage-gated sodium channel in three separate states. This channel has two gates—one near the outside of the channel called the activation gate, and another near the inside called the inactivation gate.
The upper left of the figure depicts the state of these two gates in the normal resting membrane when the membrane potential is −70 millivolts. In this state, the activation gate is closed, which prevents any entry of sodium ions to the interior of the fiber through these sodium channels.
A) Na+ channels exert positive feedback. B) K+ channels exert negative feedback. PNa, PK is permeability to Na+ and K+, respectively.
Sodium and Potassium Gating Summary
Graded vs. Action Potential
Conduction velocity :The speed at which action potentials are conducted along a nerve or muscle fibre
Conduction velocity is increased by:
a. ↑ fiber size. Increasing the diameter of a nerve fiber results in decreased internal resistance; thus, conduction velocity down the nerve is faster.
b. Myelination. Myelin acts as an insulator around nerve axons and increases conduction velocity. Myelinated nerves exhibit saltatory conduction because action potentials can be generated only at the nodes of Ranvier, where there are gaps in the myelin sheath
MYELIN being an insulator increasing membrane resistance hence less current is lost across the cell membrane .More current flows down the axon ,conduction velocity increases
Myelin is an effective insulator, and current flow through it is negligible. Instead, depolarization in myelinated axons travels from one node of Ranvier to the next, with the current sink at the active node serving to electrotonically depolarize the node ahead of the action potential to the firing level .This “jumping” of depolarization from node to
node is called saltatory conduction. It is a rapid process that allows myelinated axons to conduct up to 50 times faster than the fastest unmyelinated fibers.
Propagation in a demyelinating disorder
•Multiple sclerosis- demyelinating disorder of CNS••Thus membrane resistance decreases (current leaks out) thus faster decay of currents, insufficient to produce an AP.••Hence all the symptoms•
Facilitated Diffusion
Substances that move across the plasma membrane via facilitated diffusion are typically too bulky or charged to pass through the lipid bilayer and instead pass through the membrane via two classes of membrane proteins: carriers and channels. These proteins act like a gate to control the entry and exit of substance that cannot otherwise pass through the lipid bilayer
