Neurons, Synapses, and Signaling

Chapter 48 → Neurons, Synapses, and Signaling Student Guided Notes Overview: Lines of Communication • Neurons are nerve cells that ____________________________________________________________. • Communication by neurons is based on two distinct types of signals: long-distance _________________ signals and short-distance __________________________ signals. • If an organism does NOT have an integration center, it would not be able to interpret stimuli. Concept 48.1 Neuron structure and organization reflect function in information transfer Networks of neurons with intricate connections form nervous systems. • The neuron is the structural and functional unit of the nervous system. • Most of a neuron’s organelles, including its _____________________, are located in the _________________________. • Two types of extensions arise from the cell body: numerous dendrites and a single axon. o Dendrites (many) are _____________________________________________________________ _______________________________________________________________________________. o The _______________ (single) is a _______________________________________ that sends signals to neurons or effector cells. o Axons are covered in myelin sheaths (insulated layer) → these are necessary because regular cell membranes are made of lipids which are poor conductors of electrical currents, so the myelin works better o Each branched end of an axon transmits information to another cell at a junction called a _________________________. o At most synapses, information is passed from the transmitting neuron (the presynaptic cell) to the receiving/target cell (the postsynaptic cell) by means of chemical messengers called ___________________________________. • Glia are supporting cells that _______________________________, insulate the _________________ ___________________, and regulate the __________________________________________________. o Some glia form the blood brain barrier → restricts passage of most substances into the brain which controls the chemical environment of the CNS. Formed by TIGHT JUNCTIONS. 48-1 o __________________________________ (in CNS) and ___________________________ (in PNS) are glia that form _______________________ sheaths around the axons of neurons (LOOK ON PAGE 1070 FOR THIS BULLET) Information Processing Overview • The CNS (____________________________________________) includes the brain and spinal cord • The PNS (____________________________________________) includes neurons that bring information into and out of the CNS; communicates motor and sensory signals between the CNS and the rest of the body • Cerebrospinal Fluid → made in the brain by filtering the blood; fills the space in the brain and spinal cord • There are three stages in the processing of information by nervous systems: o Sensory input → sensory receptors take info from inside the body and the outside world and convey it to integration centers o Integration → carried out in the CNS – input is interpreted and sends a signal so body can respond appropriately o Motor output → signal is conducted from integration center to effector cells (ex. muscles or glands) that carry out the response Concept 48.2 Ion pumps and ion channels establish the resting potential of a neuron • Nerve signals are changes in voltage across the membrane due to movement of ions. • Because ions are unequally distributed between the interior of cells and the fluid that surrounds them, the inside of a cell is ________________________________________ relative to the outside. o _________________________________________________________________________________ ____________________________________________________ is called the membrane potential. • The membrane potential of a neuron that is NOT transmitting signals is called the _________________. The resting potential depends on ionic gradients that exist across the plasma membrane. • Concentration gradients of __________________ ions (____) and _______________ ions (______) across the plasma membrane of a neuron play critical roles in the formation of the resting potential. • In mammalian neurons, the concentration of K+ is highest _____________________ the cell, while the concentration of Na+ is highest ______________________. • These gradients are maintained by ___________________________________ in the plasma membrane. 48-2 o The pumps use the energy of ATP hydrolysis to ____________________________________ 3Na+ _________________________ and 2K+ ____________________________ (so the inside is MORE NEGATIVE and the outside is MORE POSITIVE.) o Gated ion channels are specialized proteins that span the membrane, and allow ions to diffuse back and forth across the membrane according to their respective gradients. o Chemically-gated ion channels → respond to a chemical stimulus (ex. neurotransmitter) o Voltage-gated ion channels → respond to a change in membrane potential o Allows only ONE kind of ion to pass through o Any resulting net movement of positive or negative charge generates a _____________________ ______________________ or ___________________________ across the membrane. o In other words: the action of the ion channels serves to “defeat” the action of the sodium potassium pump (because the ions move with their concentration gradients, not against) Concept 48.3 Action potentials are the signals conducted by axons SEE FIGURES 48.10 p. 1067 and 48.11 p. 1068 – KNOW THESE!!!!!! Gated ion channels are responsible for generating the signals of the nervous system. • Changes in membrane potential occur because neurons have gated ion channels, which open or close ____________________________________________________________. o Opening K+ channels increases ______________________________________________________ ______________________, increasing the net diffusion of K+ _____________________ the neuron. o This INCREASE in the membrane potential, is called _____________________________________, and makes the inside of the cell _______________________________________. o SO → Open K+ channel; K+ flows out and causes the inside of the cell to become more negative o Opening Na+ channels increases the membrane’s permeability to Na+, increasing the net diffusion of Na+ into the neuron o This __________________________ in the membrane potential is called ___________________________, and makes the inside of the cell MORE POSITIVE. o SO → Open a Na+ channel; Na+ flows in and causes the inside of the cell to become more positive 48-3 • These changes in membrane potential are called graded potentials because the _________________ ________________________________________—either hyperpolarization or depolarization—varies with the __________________________________________ (larger stimulus will open more channels). Changes in membrane voltage accompany an action potential. • If a depolarization shifts the membrane potential sufficiently, the result is _______________________ ______________________________________________________________ called an action potential. o Unlike graded potentials, actions potentials are “all or nothing” depolarizations. o (NOTE: hyperpolarizations do not cause action potentials!) o Action potentials can therefore spread along axons, making them well suited for _______________ ________________________________________________________________________________. • Action potentials arise because some __________________________________________________ are voltage-gated ion channels, opening or closing ____________________________________________ ______________________________________________________________. • Action potentials occur whenever a ____________________________ increases the _______________ ______________________ to a particular value, called the ___________________________. The threshold is usually 15-20 mV more positive than the resting potential. The generation of action potentials. THE BASICS: • Action potentials are “all or none”, NOT “graded” events • The neuron is at RESTING STATE • DEPOLARIZATION causes an action potential to start • When the THRESHOLD is reached, it causes an ACTION POTENTIAL DEPOLARIZATION causes an ACTION POTENTIAL. In RESTING neurons, the membrane potential is more NEGATIVE than the threshold potential. (the nerve impulse) • After the neuron “fires”, REPOLARIZATION occurs • After a short UNDERSHOOT, the RESTING POTENTIAL is restored and the neuron may generate another impulse 48-4 THE MECHANISM OF ACTION POTENTIALS: (Read along with Figure 48.11 p. 1068 referenced earlier) 1) _______________________________ → the gated Na+ and K+ gates are _________________ (so only the sodium potassium pump and ungated channels are moving ions to maintain resting potential) 2) ___________________________ → triggered by an action potential which signals the opening of ____ gates, and sodium flows INTO the cell, making the inside more POSITIVE. A THRESHOLD is then reached and the ___________________________________ can be triggered. 3) Rising phase of the action potential → depolarization continues with sodium gates open and __________________________ gates closed until ACTION POTENTIAL occurs 4) Repolarization (falling phase of the action potential)→ _______ gates start to close and ________ gates open, allowing potassium to LEAVE the cell, making the inside more _______________________ 5) _____________________________ → sodium channels close but some potassium gates are still open causing a lag in the time to restore _____________________________________. a. During this brief refractory period, the neuron is unable to depolarize. ***It is the number of action potentials per second, not their amplitude, that codes for a stimulus intensity in the nervous system*** Axon diameter and myelination affect conduction speed. • One factor that affects the speed at which action potentials are conducted along an axon is the diameter of the axon: The larger the axon’s diameter, the faster the conduction. • Saltatory conduction → action potentials that jump from __________________________________ Concept 48.4 Neurons communicate with other cells at synapses SEE FIGURE 48.16 p. 1072 • In MOST cases, action potentials are not transmitted from neurons to other cells. o However, information is transmitted at the synapse. • Synapses → unique cell junctions that control communication between a neuron and another cell; two types: o _____________________________ synapses → contain gap junctions that allow electrical current to flow directly from cell to cell 48-5 o Chemical synapses → involve the ________________________________________________ _____________________________________________________________________________; most neurons are this type o The presynaptic neuron synthesizes the neurotransmitter and packages it in membrane-bound _____________________________________, which are stored in the neuron’s synaptic terminals. THE MECHANISM OF CHEMICAL TRANSMISSION ACROSS A SYNAPSE: (read along with Figure 48.16 p. 1072 referenced earlier) 1) An action potential DEPOLARIZES the PRESYNAPTIC membrane 2) The depolarization opens voltage gated channels, allowing ___________ to enter the cell (neuron). 3) Calcium causes __________________________________ to fuse with the pre-synaptic membrane 4) Neurotransmitters are released into the _____________________________ 5) Neurotransmitters bind to the ________________________________ membrane 6) Ion channels open, allowing Na+ and K+ ions to enter the postsynaptic cell ***A series of events converts the electrical signal of the action potential arriving at the synaptic terminal into a chemical signal that travels across the synapse, where it is converted back into an electrical signal in the postsynaptic cell. (electrical signal → chemical signal → electrical signal) Structure of a Chemical Synapse • Neurotransmitters (intracellular messengers) are held in the tip of the presynaptic axon. • Action potential releases these neurotransmitter molecules into the synapse (the action potential depolarizes the membrane) • The postsynaptic membrane has special receptors for neurotransmitters o Neurotransmitter binds → opens ion channels (chemically gated!) – postsynaptic membrane is either hyperpolarized or depolarized (depending on receptor) o Neurotransmitter is removed quickly (enzyme degradation) → therefore the effect is brief and precise o NOTE: nerve impulses can only transmit ONE way Neural integration occurs at the cellular level. • At some synapses, the _______________________________________ ion channel is permeable to both Na+ and K+. (Na+ IN and K+ OUT) 48-6 o When the channel opens, the depolarization brings the membrane potential toward threshold, and it is called an ____________________________________________________________ (_________). o This DEPOLARIZES the plasma membrane (inside of the cell becomes more positive) • At other synapses, the ligand-gated ion channel is selectively permeable to only K+ or Cl-. (K+ OUT and Cl- IN) o When the channel opens, the hyperpolarization moves the membrane potential away from threshold, and produces an __________________________________________________________ (_______). o This HYPERPOLARIZES the membrane (the inside of the cell becomes more negative) ** BOTH EPSP AND IPSP ARE GRADED POTENTIALS • Summation → Several synaptic terminals working simultaneously on the same postsynaptic cell and have an ADDITIVE effect Temporal summation → Two ________________ produced in rapid succession at the _________ ____________________ Spatial summation → Two __________________ produced nearly simultaneously by _________ ____________________ on the same postsynaptic neuron The same neurotransmitter can produce different effects on different types of cells. o Same neurotransmitter can produce different effects on different types of cells (depends on the receptors). SEE FIGURE 48.2 p. 1075 for major neurotransmitters • Acetylcholine (most common) is vital for nervous system functions that include __________________ ________________________, ____________________________________, and __________________. o It can be either excitatory or inhibitory • Amino acid neurotransmitters are active in the vertebrate CNS and PNS. o Glutamate is the most common __________________________________________. • The amino acid gamma-aminobutyric acid (GABA) is the neurotransmitter at most _______________ ______________________________________________________________________. o Binding of GABA to receptors in postsynaptic cells increases membrane permeability to ______, resulting in an ______________ (no action potential). • Biogenic amines are synthesized from _________________________________. 48-7 o _________________________________, made from ___________________, is an excitatory neurotransmitter in the ______________________________ nervous system. o Outside the nervous system, norepinephrine functions as a _____________________, as does the related biogenic amine _____________________________. • The biogenic amines ________________________, made from tyrosine, and ____________________, made from ___________________________, are released at many sites in the brain and affect _________________, ______-__________, ______________________, and ____________________. o Depression can be treated with drugs that increase the brain concentrations of biogenic amines: Prozac enhances the effect of serotonin by inhibiting its reuptake after release. • Neuropeptides, _____________________________________________________, serve as neurotransmitters that operate via metabotropic receptors. o The neuropeptide substance P is an excitatory neurotransmitter that mediates ________ perception. • Neuropeptide endorphins function as _______________________________________, decreasing __________________________________________. o Endorphins are produced in the brain during times of _____________________________________. • Some neurons in vertebrates release dissolved gases like nitric oxide (NO) that act as local regulators. • Unlike other neurotransmitters, NO is not stored in ________________________________________ but is instead __________________________________________. • NO diffuses into ________________________________________________, produces a change, and is _________________________________________ within a few seconds. o In its target cells, NO works like many hormones, stimulating an enzyme _____________________ ________________________________________________________________________________. • Vertebrates produce small amounts of CO as a neurotransmitter. • __________________________________________ (gas) is generated in certain populations of neurons in the brain and PNS. o In the brain, CO regulates the _______________________________________________________. o In the PNS, it acts as an __________________________________________________ that hyperpolarizes intestinal smooth muscle cells. 48-8