Ch. 12 Part 2: Membrane Potentials, Synapse

Slowing of Na+ inflow and acceleration of K+ outflow cause the membrane potential to change from

+30 mV to -70 mV

the resting membrane potential ranges from

-40 to -90 mV. A typical value is -70mV. The minus sign indicates that the inside of the cell is negative relative to the outside

The inflow of Na+ changes the membrane potential from

-55 mV to 30 mV

how does a chemical synapse transmit a signal

1 A nerve impulse arrives at a synaptic end bulb (or at a varicosity) of a presynaptic axon. 2 The depolarizing phase of the nerve impulse opens voltage-gated Ca2+ channels, which are present in the membrane of synaptic end bulbs. Because calcium ions are more concentrated in the extracellular fluid, Ca2+ flows inward through the opened channels. 3 An increase in the concentration of Ca2+ inside the presynaptic neuron serves as a signal that triggers exocytosis of the synaptic vesicles. As vesicle membranes merge with the plasma membrane, neurotransmitter molecules within the vesicles are released into the synaptic cleft. Each synaptic vesicle contains several thousand molecules of neurotransmitter. 4 The neurotransmitter molecules diffuse across the synaptic cleft and bind to neurotransmitter receptors in the postsynaptic neuron's plasma membrane. The receptor shown in Figure 12.23 is part of a ligand-gated channel (see Figure 12.11b); you will soon learn that this type of neurotransmitter receptor is called an ionotropic receptor. Not all neurotransmitters bind to ionotropic receptors; some bind to metabotropic receptors (described shortly). 5 Binding of neurotransmitter molecules to their receptors on ligand-gated channels opens the channels and allows particular ions to flow across the membrane. 6 As ions flow through the opened channels, the voltage across the membrane changes. This change in membrane voltage is a postsynaptic potential. Depending on which ions the channels admit, the postsynaptic potential may be a depolarization (excitation) or a hyperpolarization (inhibition). For example, opening of Na+ channels allows inflow of Na+, which causes depolarization. However, opening of Cl− or K+ channels causes hyperpolarization. Opening Cl− channels permits Cl− to move into the cell, while opening the K+ channels allows K+ to move out—in either event, the inside of the cell becomes more negative. 7 When a depolarizing postsynaptic potential reaches threshold, it triggers an action potential in the axon of the postsynaptic neuron.

postsynaptic neuron responses

1. EPSP. If the total excitatory effects are greater than the total inhibitory effects but less than the threshold level of stimulation, the result is an EPSP that does not reach threshold. Following an EPSP, subsequent stimuli can more easily generate a nerve impulse through summation because the neuron is partially depolarized. 2. Nerve impulse(s). If the total excitatory effects are greater than the total inhibitory effects and threshold is reached, one or more nerve impulses (action potentials) will be triggered. Impulses continue to be generated as long as the EPSP is at or above the threshold level. 3. IPSP. If the total inhibitory effects are greater than the excitatory effects, the membrane hyperpolarizes (IPSP). The result is inhibition of the postsynaptic neuron and an inability to generate a nerve impulse.

two main advantages of electrical synapses

1. Faster communication. Because action potentials conduct directly through gap junctions, electrical synapses are faster than chemical synapses. At an electrical synapse, the action potential passes directly from the presynaptic cell to the postsynaptic cell. The events that occur at a chemical synapse take some time and delay communication slightly. 2. Synchronization. Electrical synapses can synchronize (coordinate) the activity of a group of neurons or muscle fibers. In other words, a large number of neurons or muscle fibers can produce action potentials in unison if they are connected by gap junctions. The value of synchronized action potentials in the heart or in visceral smooth muscle is coordinated contraction of these fibers to produce a heartbeat or move food through the gastrointestinal tract.

consequences of flow of current across the membrane only at the nodes of Ranvier

1. The action potential appears to "leap" from node to node as each nodal area depolarizes to threshold, thus the name "saltatory." Because an action potential leaps across long segments of the myelinated axolemma as current flows from one node to the next, it travels much faster than it would in an unmyelinated axon of the same diameter. 2. Opening a smaller number of channels only at the nodes, rather than many channels in each adjacent segment of membrane, represents a more energy-efficient mode of conduction. Because only small regions of the membrane depolarize and repolarize, minimal inflow of Na+ and outflow of K+ occurs each time an action potential passes by. Thus, less ATP is used by sodium-potassium pumps to maintain the low intracellular concentration of Na+ and the low extracellular concentration of K+.

resting membrane potential arises from

1. Unequal distribution of ions in the ECF and cytosol. 2. Inability of most anions to leave the cell. 3. Electrogenic nature of the Na+-K+ ATPases.

neurotransmitter is removed by

1. diffusion 2. enzymatic degredation 3. uptake by cells

polarized membrane potential varies from

5 to -100 mV

muscle action potential

A stimulating impulse that propagates along the sarcolemma and transverse tubules; in skeletal muscle, it is generated by acetylcholine, which increases the permeability of the sarcolemma to cations, especially sodium ions (Na+).

positive feedback system with Na+

As more channels open, Na+ inflow increases, the membrane depolarizes further, and more Na+ channels open

two enzymes that break down catecholamines

COMT and MAO

inhibitory nts that are amino acids

GABA and glycine

most synapses between neurons are

axodendritic

B fiber axons

axons with diameters of 2-3 . Like A fibers, B fibers are myelinated and exhibit saltatory conduction at speeds up to 15 m/sec (34 mi/hr). B fibers have a somewhat longer absolute refractory period than A fibers. B fibers conduct sensory nerve impulses from the viscera to the brain and spinal cord. They also constitute all of the axons of the autonomic motor neurons that extend from the brain and spinal cord to the ANS relay stations called autonomic ganglia.

Certain amino acids are modified and decarboxylated (carboxyl group removed) to produce

biogenic amines

postsynaptic neuron

carries a nerve impulse away from a synapse

Norepinephrine, dopamine, and epinephrine are classified chemically as

catecholamines; all have an amino group (—NH2) and a catechol ring composed of six carbons and two adjacent hydroxyl (—OH) groups. Catecholamines are synthesized from the amino acid tyrosine

graded potentials occur mainly in

dendrites and cell body

substance P

eleased by neurons that transmit pain-related input from peripheral pain receptors into the central nervous system, enhancing the perception of pain

absolute refractory period

even a very strong stimulus cannot initiate a second action potential. This period coincides with the period of Na+ channel activation and inactivation

EPSP

excitatory postsynaptic potential; A depolarizing postsynaptic potential

The electrical signals produced by neurons and muscle fibers rely on

four types of ion channels: leak channels, ligand-gated channels, mechanically-gated channels, and voltage-gated channels

How can your sensory systems detect stimuli of differing intensities if all nerve impulses are the same size

frequency of action potentials

one-way information transfer

from a presynaptic neuron to a postsynaptic neuron or an effector, such as a muscle fiber or a gland cell.

Most excitatory neurons in the CNS and perhaps half of the synapses in the brain communicate via

glutamate

excitatory nts that amino acids

glutamate and aspartate

which potentials are used for short-distances communication only

graded potential

postsynaptic potential

graded potential occurs in the dendrites or cell body of a neuron in response to a neurotransmitter

gate that is open in the resting state of a voltage-gated Na+ channel

inactivation gate

small molecule neurotransmitters

include acetylcholine, amino acids, biogenic amines, ATP and other purines, nitric oxide, and carbon monoxide

IPSP

inhibitory postsynaptic potential; hyperpolarizing postsynaptic potential; During hyperpolarization, generation of an action potential is more difficult than usual because the membrane potential becomes inside more negative and thus even farther from threshold than in its resting state

continuous conduction

involves step-by-step depolarization and repolarization of each adjacent segment of the plasma membrane. In continuous conduction, ions flow through their voltage-gated channels in each adjacent segment of the membrane. Note that the action potential propagates only a relatively short distance in a few milliseconds. Continuous conduction occurs in unmyelinated axons and in muscle fibers.

decremental conduction

mode of travel by which graded potentials die out as they spread along the membrane

state of K+ channels

most voltage-gated K+ channels do not exhibit an inactivated state. Instead, they alternate between closed (resting) and open (activated) states

which neurons secrete hormones

neurosecretory cells

NOS

nitric oxide synthase; catalyzes formation of NO from the amino acid arginine

plays roles in arousal (awakening from deep sleep), dreaming, and regulating mood

norepinephrine

neuropeptides

numerous and widespread in both the CNS and PNS. Neuropeptides bind to metabotropic receptors and have excitatory or inhibitory actions, depending on the type of metabotropic receptor at the synapse

ligand-gated channel

opens and closes in response to the binding of a ligand (chemical) stimulus. A wide variety of chemical ligands—including neurotransmitters, hormones, and particular ions—can open or close ligand-gated channels. The neurotransmitter acetylcholine, for example, opens cation channels that allow Na+ and Ca2+ to diffuse inward and K+ to diffuse outward. Ligand-gated channels are located in the dendrites of some sensory neurons, such as pain receptors, and in dendrites and cell bodies of interneurons and motor neurons.

voltage-gated channel

opens in response to a change in membrane potential (voltage). Voltage-gated channels participate in the generation and conduction of action potentials in the axons of all types of neurons.

mechanically-gated channel

opens or closes in response to mechanical stimulation in the form of vibration (such as sound waves), touch, pressure, or tissue stretching. The force distorts the channel from its resting position, opening the gate. Examples of mechanically-gated channels are those found in auditory receptors in the ears, in receptors that monitor stretching of internal organs, and in touch receptors and pressure receptors in the skin.

cell that receives the signal

postsynaptic cell

leak channels

randomly alternate between open and closed positions. Typically, plasma membranes have many more potassium ion (K+) leak channels than sodium ion (Na+) leak channels, and the potassium ion leak channels are leakier than the sodium ion leak channels. Thus, the membrane's permeability to K+ is much higher than its permeability to Na+. Leak channels are found in nearly all cells, including the dendrites, cell bodies, and axons of all types of neurons.

ACh

released by many PNS neurons and by some CNS neurons. ACh is an excitatory neurotransmitter at some synapses, such as the neuromuscular junction, where the binding of ACh to ionotropic receptors opens cation channels. It is also an inhibitory neurotransmitter at other synapses, where it binds to metabotropic receptors coupled to G proteins that open K+ channels

effector cell

responds to the impulse at the synapse.

localized flow

spreads to adjacent regions along the plasma membrane in either direction from the stimulus source for a short distance and then gradually dies out as the charges are lost across the membrane through leak channels

spatial summation

summation of postsynaptic potentials in response to stimuli that occur at different locations in the membrane of a postsynaptic cell at the same time

temporal summation

summation of postsynaptic potentials in response to stimuli that occur at the same location in the membrane of the postsynaptic cell but at different times

chemical synapses are separated by

synaptic cleft; a space of 20-50 nm* that is filled with interstitial fluid

gate open in the activated state of a voltage-gated Na+ channel

the activation and inactivation gates in the channel are open and Na+ inflow begins

receptor & generator potentials

the graded potentials that occur in sensory receptors and sensory neurons

A fiber axons

the largest diameter axons (5-20 ) and are myelinated. A fibers have a brief absolute refractory period and conduct nerve impulses (action potentials)

repolarizing phase

the membrane potential is restored to the resting state of -70 mV

after-hyperpolarizing phase

the membrane potential temporarily becomes more negative than the resting level; the voltage-gated K+ channels remain open and the membrane potential becomes even more negative

depolarizing phase

the negative membrane potential becomes less negative, reaches zero, and then becomes positive.

current carried by Na+ and K+ flows across the membrane mainly at

the nodes

relative refractory period

the period of time during which a second action potential can be initiated, but only by a larger than normal stimulus. It coincides with the period when the voltage-gated K+ channels are still open after inactivated Na+ channels have returned to their resting state

summation

the process by which graded potentials add together. If two depolarizing graded potentials summate, the net result is a larger depolarizing graded potential

The slower opening of voltage-gated K+ channels and the closing of previously open voltage-gated Na+ channels produce

the repolarizing phase of the action potential.

C fiber axons

the smallest diameter axons (0.5-1.5 ) and all are unmyelinated. Nerve impulse propagation along a C fiber ranges from 0.5 to 2 m/sec (1-4 mi/hr). C fibers exhibit the longest absolute refractory periods.

saltatory conduction

the special mode of action potential propagation that occurs along myelinated axons, occurs because of the uneven distribution of voltage-gated channels

The after-hyperpolarizing phase occurs when

the voltage-gated K+ channels remain open after the repolarizing phase ends

synaptic delay

time for presynaptic neuron to convert electrical signal and postsynaptic neuron to receive the chemical signal and convert to electrical; is the reason that chemical synapses relay signals more slowly than electrical synapses

graded electrical signals

vary in amplitude (size), depending on the strength of the stimulus

enkephalins

Scientists discovered that certain brain neurons have plasma membrane receptors for opiate drugs such as morphine and heroin. The quest to find the naturally occurring substances that use these receptors brought to light the first neuropeptides: two molecules, each a chain of five amino acids

inactivated state of Na+ channel

Shortly after the activation gates of the voltage-gated Na+ channels open, the inactivation gates close

refractory period

The period of time after an action potential begins during which an excitable cell cannot generate another action potential in response to a normal threshold stimulus

nitrix oxide

an important excitatory neurotransmitter secreted in the brain, spinal cord, adrenal glands, and nerves to the penis and has widespread effects throughout the body

first channels that open during action potential

Na+

depolarizing graded potential

When the response makes the membrane less polarized (inside less negative)

hyperpolarizing membrane potential

When the response makes the membrane more polarized (inside more negative)

presynaptic neuron

a nerve cell that carries a nerve impulse toward a synapse. It is the cell that sends a signal

graded potential occurs when

a stimulus causes mechanically-gated or ligand-gated channels to open or close in an excitable cell's plasma membrane

threshold stimulus

a stimulus that is just strong enough to depolarize the membrane to threshold

suprathreshold stimulus

a stimulus that is strong enough to depolarize the membrane above threshold. Each of the action potentials caused by a suprathreshold stimulus has the same amplitude (size) as an action potential caused by a threshold stimulus

lower motor neuron

a type of motor neuron that directly supplies skeletal muscle fiber

upper motor neuron

a type of motor neuron that synapses with a lower motor neuron farther down in the CNS in order to contract a skeletal muscle. The graded potential subsequently causes a nerve action potential to occur in the axon of the upper motor neuron, followed by neurotransmitter release.

ionotropic receptor

a type of neurotransmitter receptor that contains a neurotransmitter binding site and an ion channel. In other words, the neurotransmitter binding site and the ion channel are components of the same protein. An ionotropic receptor is a type of ligand-gated channel

metabotropic receptor

a type of neurotransmitter receptor that contains a neurotransmitter binding site but lacks an ion channel as part of its structure. However, a metabotropic receptor is coupled to a separate ion channel by a type of membrane protein called a G protein

subthreshold stimulus

a weak depolarization that cannot bring the membrane potential to threshold

threshold value

about -55mV

connexons

act like tunnels to connect the cytosol of the two cells directly

electrical synapse

action potentials (impulses) conduct directly between the plasma membranes of adjacent neurons through structures called gap junctions

speed of propagation of an action potential is affected by

amount of myelination, axon diameter, and temperature

the presynaptic neuron converts

an electrical signal (nerve impulse) into a chemical signal (released neurotransmitter)