AP Bio Test Unit #4 Nervous System

neuron communication at synapse

When an action potential reaches the axon terminal, it depolarizes the membrane and opens voltage-gated Na+ channels. Na+ ions enter the cell, further depolarizing the presynaptic membrane. This depolarization causes voltage-gated Ca2+ channels to open. Calcium ions entering the cell initiate a signaling cascade that causes a release of small, membrane-bound vesicles, called synaptic vesicles, containing neurotransmitter molecules . These synaptic vesicles fuse with the presynaptic membrane. Fusion of a vesicle with the presynaptic membrane causes neurotransmitters to be released into the synaptic cleft: the extracellular space between the presynaptic and postsynaptic membranes . The neurotransmitter diffuses across the synaptic cleft, binding to receptor proteins on the postsynaptic membrane. The binding of a specific neurotransmitter causes particular ion channels, in this case ligand-gated channels, on the postsynaptic membrane to open. Neurotransmitters can either have excitatory or inhibitory effects on the postsynaptic membrane.

stages of an action potential

An action potential occurs when an electrical signal disrupts the original balance of Na+ and K+ within a cell membrane, briefly depolarizing the concentrations of each. An electrical impulse travels along the axon via depolarized voltage-gated ion channels in the membrane, and can either "jump" along a myelinated area or travel continuously along an unmyelinated area. action potential is a rapid change in polarity that moves along the nerve fiber from neuron to neuron. In order for a neuron to move from resting potential to action potential—a short-term electrical change that allows an electrical signal to be passed from one neuron to another—the neuron must be stimulated by pressure, electricity, chemicals, or another form of stimuli. The level of stimulation that a neuron must receive to reach action potential is known as the threshold of excitation, and until it reaches that threshold, nothing will happen. A stimulus starts the depolarization of the membrane. Depolarization, also referred to as the "upswing," is caused when positively charged sodium ions rush into a nerve cell. As these positive ions rush in, the membrane of the stimulated cell reverses its polarity so that the outside of the membrane is negative relative to the inside. Repolarization. Once the electric gradient has reached the threshold of excitement, the "downswing" of repolarization begins. The channels that let the positive sodium ion channels through close up, while channels that allow positive potassium ions open, resulting in the release of positively charged potassium ions from the neuron. This expulsion acts to restore the localized negative membrane potential of the cell, bringing it back to its normal voltage. The refractory phase takes place over a short period of time after the depolarization stage. Shortly after the sodium gates open, they close and go into an inactive conformation. The sodium gates cannot be opened again until the membrane is repolarized to its normal resting potential. The sodium-potassium pump returns sodium ions to the outside and potassium ions to the inside. During the refractory phase this particular area of the nerve cell membrane cannot be depolarized. Therefore, the neuron cannot reach action potential during this "rest period."

oligodendrocytes

a glial cell similar to an astrocyte but with fewer protuberances, concerned with the production of myelin in the CENTRAL NERVOUS SYSTEM

motor neurons

a nerve cell forming part of a pathway along which impulses pass from the brain or spinal cord to a muscle or gland.

2 differences between a typical stimulus pathway and a reflex

a reflex arc is involuntary. That means there is no conscious control so it does not need to be processed by the brain. Another difference is there are fewer neurons. This means the synapses are involved and a shorter distance to travel, which makes it much quicker than the typical stimulus-response.

axon hillock

a specialized part of the cell body (or soma) of a neuron that connects to the axon. The axon hillock is the last site in the soma where membrane potentials propagated from synaptic inputs are summated before being transmitted to the axon.

cell body

also called the soma, is the spherical part of the neuron that contains the nucleus. when information is received from another neurons, the dendrites pass the signal to the cell body. The cell body then may send the information to the axon, depending on the strength of the signal.

ligand gated ion channels

are a group of transmembrane ion channel proteins which open to allow ions such as Na+, K+, Ca2+, or Cl− to pass through the membrane in response to the binding of a chemical messenger (i.e. a ligand), such as a neurotransmitter.

nodes of ranvier

are periodic gaps in the insulating myelin sheaths of myelinated axons where the axonal membrane is exposed to the extracellular space. as an action potential is sent down a myelinated neuron, it is depolarized on each node of ranvier and skips the myelin, producing a faster response.

synaptic vesicles

contain neurotransmitter. bind to membrane to release when calcium rushes into synaptic bulb

interneurons

create neural circuits, enabling communication between sensory or motor neurons and the central nervous system (CNS).

typical stimulus response way (through the neurons)

sensory neurons receive signal that sends this to CNS, to an interneuron that sends signals to the brain. then the CNS sends a signal to an effector, or motor neuron, that creates a response to the stimulus. ex) changes in temperature

dendrite

short branched extension of a nerve cell, along which impulses received from other cells at synapses are transmitted to the cell body. (receives signals)

threshold

that some event (a stimulus) causes the resting potential to move toward 0 mV. When the depolarization reaches about -55 mV a neuron will fire an action potential. This is the threshold.

myelin sheath

the lipid-rich sheath surrounding axons in both the central and peripheral nervous systems. The myelin sheath is an electrical insulator and allows faster and more energetically efficient conduction of impulses. The sheath is formed by the cell membranes of glial cells (Schwann cells in the peripheral and oligodendroglia in the central nervous system). loss or damage leads to severe loss of neural function, as in multiple sclerosis.

postsynaptic cell

the postsynaptic membrane is the membrane that receives a signal (binds neurotransmitter) from the presynaptic cell and responds via depolarisation or hyperpolarisation. The postsynaptic membrane is separated from the presynaptic membrane by the synaptic cleft.

depolarization

As the sodium rushes back into the cell the positive sodium ions raise the charge inside of the cell from negative to positive. Once the interior of the cell becomes positively charged, depolarization of the cell is complete.

schwann cells

IN THE PERIPHERAL NERVOUS SYSTEM. insulate (myelinate) individual nerve fibers (axons), which is necessary for sending appropriate electrical signals throughout the nervous system.

synapse

Information from one neuron flows to another neuron across a synapse. The synapse contains a small gap separating neurons. The synapse consists of: a presynaptic ending that contains neurotransmitters, mitochondria and other cell organelles a postsynaptic ending that contains receptor sites for neurotransmitters a synaptic cleft or space between the presynaptic and postsynaptic endings.

inhibitory postsynaptic potentials (IPSP's)

Release of neurotransmitter at inhibitory synapses causes inhibitory postsynaptic potentials (IPSPs), a hyperpolarization of the presynaptic membrane. For example, when the neurotransmitter GABA (gamma-aminobutyric acid) is released from a presynaptic neuron, it binds to and opens Cl- channels. Cl- ions enter the cell and hyperpolarize the membrane, reducing the probability that the neuron will fire an action potential.

membrane potential of action potentials

-70 mV at rest -55 mV during action potential

synaptic terminal

A bulb at the end of an axon in which neurotransmitter molecules are stored and released.

excitatory postsynaptic potentials (EPSP's)

This depolarization, called an excitatory postsynaptic potential (EPSP), increases the probability that the postsynaptic neuron will fire an action potential

hyperpolarization

is a change in a cell's membrane potential that makes it more negative. It is the opposite of a depolarization. It inhibits action potentials by increasing the stimulus required to move the membrane potential to the action potential threshold. Hyperpolarization prevents the neuron from receiving another stimulus during this time, or at least raises the threshold for any new stimulus. Part of the importance of hyperpolarization is in preventing any stimulus already sent up an axon from triggering another action potential in the opposite direction. In other words, hyperpolarization assures that the signal is proceeding in one direction.

saltatory conduction

is the propagation of action potentials along MYELINATED axons from one node of Ranvier to the next node, increasing the conduction velocity of action potentials.

axon

long, slender projection of a nerve cell, or neuron, that typically conducts electrical impulses away from the neuron's cell body. Myelinated axons are known as nerve fibers. The function of the axon is to transmit information to different neurons, muscles and glands.

resting potential and how it is established

membrane is at rest, the resting potential is negative inside cell due to the accumulation of more sodium ions outside the cell than potassium ions inside the cell. Sodium-potassium pumps move two potassium ions inside the cell as three sodium ions are pumped out to maintain the negatively-charged membrane inside the cell; this helps maintain the resting potential. (−70 mV) inside cell (-55 mV creates depolarization)

reflex arc

most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This characteristic allows reflex actions to occur relatively quickly by activating spinal motor neurons without the delay of routing signals through the brain, although the brain will receive sensory input while the reflex is carried out. Analysis of the signal takes place after action has been taken. ex) needle prick It is the pathway followed by sensory nerve in carrying the sensation from receptor organ to spinal cord and then the pathway followed by motor nerve in carrying the order from spinal cord to effector organ during a reflex action.

sensory neurons

nerve cells within the nervous system responsible for converting external stimuli from the organism's environment into internal electrical impulses. For example, some sensory neurons respond to tactile stimuli and can activate motor neurons in order to achieve muscle contraction.

Ca2+ channels

once the action potential reaches the axon terminal to the synaptic terminal, it causes the bulb to depolarize and Ca2+ channels open. As calcium rushes in, the synaptic vesicles containing neurotransmitter fuse to the membrane, releasing the neurotransmitter into the synaptic cleft.

presynaptic cell

presynaptic membrane is the cell membrane of an axon terminal that faces the receiving cell. The postsynaptic membrane is separated from the presynaptic membrane by the synaptic cleft.

repolarization

refers to the change in membrane potential that returns it to a negative value just after the depolarization phase of an action potential