The kidneys transport...
-a number of molecules from the blood filtrate which becomes urine, back into the blood.
-Glucose, for example in normally completely reabsorbed so that urine is normally free of glucose.
*glycosuria
-glucose found in the urine due to the Transport Maximum being exceeded.
-This may result from the consumption of too much sugar or from inadequate action of the hormone insulin in the disease diabetes mellitus.
*hyperglycemia.
If the glucose concentration of the blood and filtrate is too high
*hypoglycemia.
When the plasma glucose concentration is abnormally low
The rate of the facilitated diffusion of glucose into tissue cells depends directly on
the plasma glucose concentration.
Oral rehydration therapy is effective for diarrhea because:
-The absorption of water by osmosis across the intestine is proportional to the absorption of Na+.
-The intestinal epithelium cotransports Na+ and glucose.
-The glucose in the mixture promotes the cotransport of Na+ and the Na+ transport promotes the osmo
Ion channel
-are integral proteins that span the membrane and, when open, permit the passage of certain ions.
Ion channels are selective;
they permit the passage of some ions, but not others. Selectivity is based on the size of the channel and the distribution of charges that line it.
Ion channels may be open or closed..
When the channel is closed, ions cannot flow through.
The conductance of a channel depends on
-the probability that the channel is open.
-The higher the probability that a channel is open, the higher the conductance, or permeability.
-Opening and closing of channels are controlled by gates
Voltage-gated channels
-are opened or closed by changes in membrane potential
-The activation gate of the Na+ channel in nerve is opened by
-depolarization;
-when open, the nerve membrane is permeable to Na+
The inactivation gate of the Na+ channel in nerve is closed by
-****repolarization;
-when closed, the nerve membrane is impermeable to Na+ (e.g., during the repolarization phase of the nerve action potential).
Ligand-gated channels
-open or closed by hormones, second messengers, or neurotransmitters.
Diffusion Potential
-the potential difference generated across a membrane because of a concentration difference of an ion.
-A diffusion potential can be generated only if the membrane is permeable to the ion.
-the size of the diffusion potential depends on the size of the co
The equilibrium potential
-the diffusion potential that exactly balances (opposes) the tendency for diffusion caused by a concentration differences.
At electrochemical equilibrium,
-the chemical and electrical driving forces that act on an ion are equal and opposite, and more net diffusion of the ion occurs.
1. Example of a Na+ diffusion potential (NaCl)
2. Example of a Cl- diffusion potential
Using the Nernst equation to calculate equilibrium potentials
-used to calculate the equilibrium potential at a given concentration difference of a permeable ion across a cell membrane.
-It tells us what potential would exactly balance the tendency for diffusion down the concentration gradient;
--in other words, at
Approximate values for EP in nerve and muscle:
ENa+ +65mV
ECa2+ +120mV
EK+ -85mV
ECl- -85mV
Resting membrane potential
-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 70mV
Depolarization
makes the membrane potential less negative (the cell interior becomes less negative).
Hyperpolarization
makes the membrane potential more negative (the cell interior becomes more negative).
Inward current
-the flow of positive charge into the cells.
-depolarizes the membrane potential.
Outward current
-the flow of positive charge out of the cell.
-hyperpolarizes the membrane potential
Action potential
-a property of excitable cells (nerve, muscle)
-consists of a rapid depolarization, or upstroke, followed by repolarization of the membrane potential.
-Action potentials have stereotypical size and shape, are propagating, and are all-or-none.
Threshold
-the membrane potential at which the action potential is inevitable.
-Inward current depolarizes the membrane.
--If the inward current depolarizes the membrane to threshold, it produces an action potential.
--If the inward current is not sufficient to dep
Upstroke of the action potential
1. Inward current depolarizes the membrane potential to threshold.
2. Depolarization causes rapid opening of the activation gates of the Na+ channel, and the Na+ conductance of the membrane promptly increases.
3. The Na+ conductance becomes higher than th
Repolarization of the action potential
1. Depolarization also closes the inactivation gates of the Na+ channel. Closure of the inactivation gates results in closure of the Na+ channels, and the Na+ conductance returns toward zero.
2. Depolarization slowly opens K+ channels and increases K+ con
Absolute refractory period
-is the period during which another action potential cannot be elicited, no matter how large the stimulus.
Relative refractory period
-begins at the end of the absolute refractory period and continues until the membrane potential returns to the resting level.
-An action potential can be elicited during this period only if a larger than usual inward current is provided.
Accommodation
-occurs when the cell membrane is held at a depolarized level such that the threshold potential is passed without firing an action potential.
-is demonstrated in HYPERKALEMIA,
*HYPERKALEMIA
--Demonstrates ACCOMMODATION due to the skeletal muscle membranes being depolarized by the high serum K+ concentration.
-Although the membrane potential is closer to threshold, action potentials do not occur because inactivation gates on Na+ channels are
Conduction velocity is increased by:
a. Increased fiber size.
b. Myelination.
Increased fiber size.
-Increasing the diameter of a nerve fiber results in decreased internal resistance; thus, conduction velocity down the nerve is faster.
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.
General characteristics of chemical synapses
1. An action potential in the presynaptic cell causes depolarization of the presynaptic terminal.
2. As a result of the depolarization, Ca2+enters the presynaptic terminal, causing release of neurotransmitter into the synaptic cleft.
3. Neurotransmitter d
Neuromuscular junction
-is the synapse between axons of motoRneurons and skeletal muscle.
-The neurotransmitter released from the presynaptic terminal is Ach, and the postsynaptic membrane contains a nicotinic receptor
How Synapses happens in the Neuromuscular Junction
1. Synthesis and storage of Ach in the presynaptic terminal -Choline acetyltransferase catalyzes the formation of Ach from acetyl coenzyme A (CaA) and choline in the presynaptic terminal.
-Ach is stored in synaptic vesicles with ATP and proteoglycan for l
*myasthenia gravis
-is caused by the presence of antibodies to the Ach receptors.
--Antibodies block Ach Receptors
-is characterized by skeletal muscle weakness and fatigability resulting from a reduced number of Ach receptors on the muscle end plate.
-The size of the EPP i
Types of Synaptic transmission:
-One-to-one synapses
-Many-to-one synapses
One-to-one synapses
-an action potential in the presynaptic element (the motor nerve) produces an action potential in the postsynaptic element (the muscle).
Many-to-one synapses
-an action potential in a single presynaptic cell is sufficient to produce an action potential in the postsynaptic cell.
-Instead, many cells synapse on the postsynaptic cell to depolarize it to threshold.
-The presynaptic input may be excitatory or inhib
Input to synapses
-Excitatory postsynaptic potential (EPSP)
-Inhibitory postsynaptic potentials (IPSP
Excitatory postsynaptic potential (EPSP)
-are inputs that depolarize the postsynaptic cell, bringing it closer to threshold and closer to firing an action potential.
-are caused by opening of channels that are permeable to Na+ and K+, similar to Ach channels.
Example ENT: include Ach, norepineph
Inhibitory postsynaptic potentials (IPSP)
-are inputs that hyperpolarize the postsynaptic cell, moving it away from threshold and farther from firing and action potential.
-are caused by opening Cl- channels.
Example: INT are gama-aminobutyric acid (GABA) and glycine.
Neurotransmitters
a. Ach
b. Norepinephrine, epinephrine, and dopamine
c. Seretonin
d. Histamine
e. Glutamate
f. GABA
g.Glycine.
Norepinephrine
- is the primary transmitter released from postganglionic sympathetic neurons.
-is synthesized in the nerve terminal and released into the synapse to bind with alpha or beta receptors on the postsynaptic membrane.
-is removed from the synapse by reuptake
Epinephrine
-is synthesized from norepinephrine by the action of phenylethanolamine-N-methyltransferase.
-is secreted, along with norepinephrine, from the adrenal medulla.
Dopamine
-is prominent in midbrain neurons.
-is released from the hypothalamus and inhibits prolactin secretion.
-is metabolized by MAO and COMT.
A. D1 receptors activate adenylate cyclase via a Gs protein.
B. D2 receptors inhibit adenylate cyclase via a Gi protei
Serotonin
-is present in high concentrations in the brain stem.
-is formed tryptophan.
-is converted to melatonin in the pineal gland.
Histamine
-is formed from histidine.
-is present in the neurons of the hypothalamus.
Glutamate
-is the most prevalent excitatory neurotransmitter in the brain.
-has a kainate receptor, which is an ion channel for Na+ and K+.
GABA
-is an inhibitory neurotransmitter.
-is synthesized from glutamate by glutamate decarboxylase.
-has two types of receptors:
1. The GABAa receptor increases Cl- conductance and is the site of action of benzodiazepines and barbiturates.
2. The GABAb recepto
Glycine
-is an inhibitory neurotransmitter found primarily in the spinal cord and brain stem.
-increases Cl- conductance.