Electrical Signals/Nervous System (Life 45-45.2.9)

Question 0

neurons (parts)

Answer 0

• neurons are excitable: they can generate and transmit electrical signals which are known as nerve impulses or action potentials
• long extension called axon enables neurons to conduct action potentials over long distances
• glia do not conduct action potentials but support neurons physically, immunologically, and metabolically
• a nerve is a bundle of axons that come from many different neurons
• axons wrapped in glia to electrically isolate them and increase their speed of conduction of action potentials

Question 1

Nervous systems are composed of two types of cells:

Answer 1

1. nerve cells (neurons)

2. glial cells (glia)

Question 2

neural networks

Answer 2

• neurons are organized into neural networks with three functional categories of neurons:
  1. afferent neurons-carry sensory information into the nervous system from specialized sensory neurons that transduce various sensory stimuli into action potentials
  2. efferent neurons-carry commands to physiological and behavioral effectors such as muscles and glands
  3. interneurons-integrate and store information and communicate between afferent and efferent neurons

Question 3

ganglia

Answer 3

• animals that need to integrate larger amounts of information have higher numbers of neurons organized into clusters called ganglia
• bilateral-ganglia come in pairs
• some ganglia may become enlarged or fused together at the anterior end, forming a larger, centralized integrative center or brain

Question 4

nervous system

Answer 4

• brain and spinal cord-the sites of most information processing, storage, and retrieval (central nervous system)
• neurons that extend or reside outside the brain and spinal cord, transfer information to the CNS (peripheral nervous system)
• information is passed from one neuron to another where they come into close proximity at structures called synapses
• the neurons sending the information is the presynaptic neuron and the neuron receiving the information is the post synaptic neuron
• synapses not constant but can be highly plastic

Question 5

Regions of a neuron

Answer 5

• cell body-contains the nucleus and most of the cell’s organelles
• dendrites-projections from the cell body which bring information from other neurons or sensory cells to the cell body
• axons-longer projection that carries action potentials away from the cell body
• axon terminal-comes close to the membrane of the target cell to form a synapse

Question 6

neurotransmitters

Answer 6

• most synapses in vertebrates are chemical synapses
• an action potential arriving at an axon terminal causes it to release chemical messenger molecules called neurotransmitters
• the neurotransmitters diffuse across the space and bind to receptors on the plasma membrane of the postsynaptic or target cell
• this binding alters the activity of the postsynaptic neuron

Question 7

glia

Answer 7

• do not generate or transmit electrical signals, but can release neurotransmitters
• in CNS, glia called oligodendrocytes wrap around the axons of neurons
• in PNS, glia called Schwann cells
• myelin is the covering produced by oligodendrocytes and Schwann cells allowing them to conduct action potentials more rapidly than axons that are not myelinated
• glia called astrocytes contribute to the blood-brain barrier, which protects the brain from toxic chemicals in the blood
• the blood-brain barrier usually prevents antibodies in the general circulation from entering the CNS. to provide the CNS with immune defenses, microglia, which originate during development from stem cells in the bone marrow, come to reside in the CNS and act as macrophages and mediators of inflammatory responses

Question 8

how do neurons generate and transmit electrical signals?

Answer 8

• more K+ inside and more Na+ outside cell so across the membrane there is an electrical charge difference
• leak currents int he cell membranes allow only certain ions (mostly K+) to leak passively across. K+ diffuses out the cell down a concentration gradient but when K+ leaks out, it leaves behind an unbalanced negative charge that pulls K+ back into the cell
• an equilibrium is reached when the tendency for K+ to diffuse out is countered by the electrical charge pulling K+ back in creating a membrane potential
• steady state membrane potential is the resting potential
• large shifts in membrane potential called action potentials or nerve impulses generated by sudden opening and closings of ion channels

Question 9

voltage

Answer 9

• electrical potential difference
• a force that causes electrically charged particles to move between two points
• in solutions and across cell membranes, electric current is carried by ions

Question 10

sodium-potassium pump

Answer 10

• expels Na+ ions from inside the cell, exchanging them for K+ ions from outside the cell
• antiporter, requires ATP to do work
• keeps the concentration of K+ inside the cell greater than that of the extracellular fluid and the concentration of Na+ inside the cell less than that of the extracellular fluid

Question 11

potassium channels

Answer 11

• most common open or leak channels
• resting neurons more permeable to K+ than any other ion
• K+ diffuses down its electrochemical gradient out of the cell leaving behind unbalanced negative charges, generating an electric potential across the membrane that tends to pull K+ back into the cell
• the membrane potential at which the net diffusion of K+ out of the cell ceases is the potassium equilibrium potential
• the value of the potassium equilibrium potential can be calculated from the concentrations of K+ on the two sides of the membrane using the Nernst equation

Question 12

types of gated ion channels

Answer 12

• voltage-gated channels open or close in response to a change in voltage across the plasma membrane
• chemically gated channels open or close depending on the presence or absence of a specific molecule that binds to the channel protein
• mechanically gated channels open or close in response to mechanical force applied to the plasma membrane

Question 13

changing membrane potential

Answer 13

• openings and closings of gated channels alter the resting potential
• when Na+ channels open, Na+ diffuses into the neuron down its electrochemical gradient, so the inside of the cell becomes less negative
• when the inside of a neuron becomes less negative (or more positive) in comparison to its resting condition, its plasma membrane is depolarized
• when gated K+ channels open, the membrane potential becomes more negative and the plasma membrane is hyperpolarized

Question 14

graded membrane potential

Answer 14

• a change from the resting potential
• changes can be due to chemical or mechanical influences on ion channels
• graded potentials are a means of integrating inputs to a cell because the membrane can respond to those inputs with proportional amounts of depolarization or hyper-polarization
• can transmit signals over very short distances, plays an important role at the neuromuscular junction
• axons too long to transmit information as a continuous flow of electric current so axons code information as discrete action potentials that travel along their membranes. graded potentials play an important role in the generation of action potential

Question 15

action potential

Answer 15

• sudden, large changes in membrane potential
  1. gated channels closed, open leak K+ channels create the resting potential (-65mV)
  2. activation gates of some Na+ channels open, depolarizing the cell to threshold
  3. additional voltage gated Na+ channels open causing a rapid spike of depolarization (an action potential)
  4. Na+ channel inactivation gates close, gated K+ channels open, repolarizes and hyperpolarizes the cell
  5. all gated channels close, the cell returns to its resting potential, Na+ inactivation gates reopen
• voltage gated Na+ channels have a refractory period where they cannot open again because it has both an activation and inactivation gates
• under resting conditions, activation gate is closed and inactivation gate is open
• depolarization to threshold level causes both gates to change state, but the activation gate responds faster so the channel is open for a brief time
• this allows for the dip in the membrane potential following an action potential called after-hyperpolarization or undershoot

Question 16

conduction of action potentials

Answer 16

• can travel over long distances with no loss of signal
• the magnitude of the action potential does not change between sites of axon because an action potential is an all-or-none, self-regenerating event
  1. all-or-none: bc of the interaction between the VG Na+ channels and the membrane potential. membrane slightly depolarized, VG Na+ channels open, causing continuing depolarization. positive feedback mechanism ensures that action potentials always rise to their maximum value
  2. self-regenerating: bc it spreads by local current flow to adjacent regions of the plasma membrane bringing neighboring areas to threshold. action potential at one location stimulates adjacent regions of axons to generate action potentials down the length of the axon
• action potentials only in one direction
• travel faster in large diameter axons than small diameter axons because the resistance to current flow decreases as an axon’s diameter gets bigger
• travel faster in myelinated than in nonmyelinated axons because they can jump from one node to another without traversing the intervening space

Question 17

myelination

Answer 17

• when glia wrap around axons, they leave regularly spaced gaps called nodes of Ranvier where the axon is not covered
• the leakage of ions across the regions of the plasma membrane tat are wrapped in myelin is reduced, so electric current can spread farther along the inside of a myelinated axon than it can along a nonmyelinated axon
• VG ion channels are clustered at the nodes of Ranvier
• axon can fire action potentials only at nodes and those action potentials cannot be propagated through the adjacent patch of membrane covered with myelin
• action potentials jump from node to node along the axon
• the speed of conduction is increased in myelinated axons because electric current flows much faster through the cytoplasm than ion channels can open and close forming rapid impulse propagation called saltatory conduction