CHAPTER 8
Which glial cell form myelin in the CNS Which glial cells form myelin in the PNS
CNS-Oligodendrocytes PNS-Schwann
Are there more neurons or glial cells in the nervous system.
Glial cells 10-50 to 1
For each of the 4 channels, identify the stimuli that causes them to open or close: Mechanically gated ion channel chemically gated ion channel voltage-gated ion channel
Na+----------voltage-gated ion channel K+------------voltage-gated ion channel Ca2+----------voltage-gated ion channel Cl- ------------
What membrane transporter maintains Na+ & K+ concentrations across the nerve cell membrane?
Na+-K+-ATPase pump
How do embryonic nerve cells establish synapses with the correct targets and what must happen in order for a new synapse to survive.
Send out growth cones through the extracellular compartments until they find their target cell. They "sniff out" the targets chemical sent. Neurotrophic factors secreted by neurons and glial cells must have a electrical or chemical activity or the synapse will disappear. "use it or lose it"
Enteric Nervous System (ENS)
the autonomic nerves in the walls of the GI tract. Controlled by the ANS
local current flow
the wave of depolarization that moves through the cell with a graded potential
What 2 factors influence membrane potential?
-The uneven distribution of ions across the cell membrane. ----Na+, Cl-, and Ca2+ are more concentrated in the extracellular fluid than in the cytosol. K+ is more concentrated in the cytosol. -Differing membrane permeability to those ions. ------resting membrane potential is much more permeable to K+ thats to Na+ or Ca2+. This makes K+ the major ion contributing to resting membrane potential.
Diagram saltatory conduction in axons. (Fig 8.16)
1- action potentials appears to jump from one node of Ranvier to the next. Only nodes have high voltage-gated Na+ channels 2-Demyelinating diseases reduce or block conduction when current leaks out of the previously insulated regions between the nodes. Current leaks reduces conduction and can lead to action potential failure.
Diagram the Ca2+- dependent exocytosis that releases neurotransmitters at the synapse. FIG8.19a
1- an action potential depolarizes the axon terminal 2-the depolarization opens voltage-gated Ca2+ channels and Ca2+ enters the cell. 3-Calcium entry triggers exocytosis of synaptic vesicle contents. 4- neurotransmitters diffuse across the synaptic cleft and bind with receptors on the postsynaptic cell. 5-neurotransmitter binding initiates a response in the postsynaptic cell.
List and describe the ways neurotransmitters activity can be terminated FIG 8.19B 8.20
1- can be returned to axon terminals for reuse or transported into glial cells. 2- Enzymes are inactivate neurotransmitters in the synaptic cleft. 3- Neurotransmitters can diffuse out of the synaptic cleft. Acetylcholine(ACh) is made from choline and acetyl CoA. in the Synaptic cleft ACh is rapidly broken down by the enzyme acetylcholinesterase. Choline is taken back into the axon terminal by cotransport with Na+ Then recycled choline is used to make more ACh.
Diagram the glutamate-based mechanism involved in long-term potentiation (LTP) FIG8.25
1- glutamat binds to AMPA and NMDA channels. 2- net Na+ entry through AMPA channels depolarizes the postsynaptic cell. 3- Depolarization ejects Mg2+ from NMDA receptor-channel and opens channel. 4- Ca2+ enter cytoplasm through NMDA channel. 5- Ca2+ activates second messenger pathways. 6- Paracrine from postsynaptic cell enhances glutamate release.
Diagram the changes in Na+ channel activation and inactivation gates as a depolarization stimulus is conducted along an axon (fig8.10)
1-at the resting membrane potential the activation gate closes the channel(-70) 2-Depolarization stimulus arrives at the channel, activation gate opens(-55) 3-With activation gate open, Na+ enters the cell. 4-Inactivation gate closes and Na+ entry stops(30+) 5-During repolarization caused by K+ leaving the cell, the 2 gates reset to their original positions.(-70)
What physical properties are affected by myelin so that the result is increased speed of conduction in an axon?
1-axon diameter 2-myelination
Diagram the events of the rising phase of an action potential. Include relevant ions, ion channels, changes to membrane potential, electrochemical gradients, and direction of ion movement. (fig 8.9)
1-resting membrane potential at -70, both Na+ & K+ channels are closed. 2-action potential begins when a graded potential reaches the trigger zone & begins depolarization triggering Na+ channels to begin to open @ -55 3- Voltage gated channels to Na+ open, Na+ ions then flow into the cell, down the concentration gradient(attracted to the neg membrane potential inside the cell). RISING PHASE 4-Rapid Na+ entry Depolarizes the cell 5-The action potential peaks @ +30 when Na+ channels close and slower K+ channels open. FALLING PHASE 6-K+ moves from the cell to extracellular fluid. Sending the cell to resting potential @-70 7- K+ channels remain open and additional K+ leaves the cell through voltage gated channel & leaky channels, hyperpolarizing it @ -90. AFTER-HYPERPOLARIZATION 8-Voltages gated K+ channels close, less K+ leaks out of the cell. 9- Retention of K+ & Na+ makes the cell return to resting ion permeability and resting membrane potential @ -70.
Identify the 3 phases of an action potential (fig8.9)
1-rising phase 2-falling phase 3-after -hyperpolarization phase
Identify the components of a synapse.
A junction where information is transmitted from one neuron to the next. The target cell is called the postsynaptic cell, and the neuron that releases the information is the presynaptic cell. The region in-between these 2 cells in the synaptic cleft.
Distinguish between the absolute refectory period and the relative refractory period. Indicate the roles of various ion channels in each (fig. 8.12)
Absolute Refractory Period-Once an action potential begins, a second action potential can't be triggered for 1-2 msec. or until the first action potential has finished, no matter how large the stimulus. This allows the Na+ channel gates to reset their resting positions. Relative Refractory Period-A higher than normal grated potential is required to trigger an action potential.Na+ enters through reset Na+ channel, depolarization due to Na+ entry is off set by K+ loss through still-open K+ channels.
The CNS can release a variety of neruocrines, but the PNS secretes only 3 major ones: ________________ and __________________(neurotransmitters) and __________________(neurohormone) TBL 8.4
Acetylcholine Norepinephrine Epinephrine
Microglia
Act as phagocytes, eating damaged cells and bacteria, act as the brains immune system
How does axonal depolarization represent a positive feedback loop? What act of outside intervention breaks this positive feedback loop?(fig 8.11)
Additional positive charges that further depolarizes the inside of the cell creates a positive feedback loop which won't stop until Na+ inactive gates close. Inactivation gates in the Na+ channel are the outside intervention break that stops the pos. feedback loop.
Why is the Nerst equation ineffective for determining the resting membrane potential of neurons.
Because it would only result if the membrane was permeable to only one ion.
Why are graded potentials called "graded"? How are they initiated.
Because their magnitude varies directly with stimulus strength. Entry of ions through gated channels.
Why are action potentials called all or none phenomena?
Because they either occur as a maximal depolarization or don't occur at all.
Describe the structure of a Dendrite and its function.
Branchlike parts of a neuron that are specialized to receive information. Increase the surface area of a neuron.
Distinguish between Chemical Synapse and Electrical Synapse.
Chemical Synapse- a type of synapse at which a chemical (a neurotransmitter) is released from the axon of a neuron into the synaptic cleft, where it binds to receptors on the next structure (either another neuron or an organ) Electrical Synapse- A type of synapse in which the cells are connected by gap junctions, allowing ions (and therefore an action potential) to spread easily from cell to cell, usually in smooth and cardiac muscle. - compared to chemical synapse.
Describe the cell body (soma). what would the extensive cytoskeleton suggest about functions of neurons.
Contains the nucleus, located mainly in the CNS and direct cellular activity. The cytoskeleton extends outward into the axon and dendrites.
Distinguish between Divergence and Convergence
Divergence- When presynaptic neuron synapse on a larger number of postsynaptic neurons. Convergence-When several presynaptic neurons provide input to a smaller number of postsynaptic neurons.
The direction of ion movement depends on the _____________________ gradient of the ion.
Electrochemical
Regarding to Fast & Slow mechanisms: If the synaptic potential is depolarizing, it is called _______________________. If the synaptic potential is hyperpolarizing, it is called ___________________.
Excitatory Postsynaptic Potential (EPSP)-making the cell more likely to fire a action potential Inhibitory Postsynaptic Potential (IPSP)-making the cell move away from threshold and less likely to fire an action potential.
Distinguish between graded potentials and action potentials
Graded potentials are changes in membrane potential that vary in size, as opposed to being all-or-none. Action potentials- are very brief, large depolarizations that travel long distance through a neuron without loosing strength.
For a neuron, a sudden increase in Na+ permeability would favor Na+ movement _________________(into or out of?) the cell. When Na+ moves in this case, it is moving _____________(down or up) its concentration gradient and __________________ (dn or against) its electrical gradient.
Into down down
Contrast ionotropic receptors and GPRC'S(G-protein coupled receptors)
Ionotropic- receptors that are coupled to ion channels and affect the neuron by causing those channels to open. Mediate rapid response, and are specific to a single ion Cl-. GPRC'S- Mediate slower response because signal transduction must go through 2nd messenger. GPRC's for neuromodulators are called metabotropic receptors; regulate the opening or closing of ion channels.
What happens when a suprathreshold graded potential reaches the trigger zone.
It initiates an action potential.
What causes after-hyperpolarization?
K+ is still leaving the cell through voltage-gated & leaky channels and the membrane hyper polarizes approaching K+ equilibrium potential.
How does the kiss-and-run pathway of neurotransmitter release compare to the classic exocytosis.
KISS-synaptic vescicles fuse to the presynaptic membrane at the fusion pore, then the neurotransmitter passes through, then the vescicle pulls back and returns to the pool of the vesicles in the cytoplasm. Exocytosis- membrane bound secretory vesicles are carried to the cell membrane and their contents are secreted into the extracellular environment.
Based on current info, describe the processes that seem to bring about long term depression (LTD)
LTD has two components : a change in the number of postsynaptic receptors and a change in isoforms of the receptor proteins. this causes down regulation or receptors in the endocrine system. Long term potentiation and Long term depression are related to neural processes for learning and memory and to changes in the brain.
Why do graded potentials lose strength as they move through the cytoplasm?
Loose strength the longer the time and distance.
What disease is the most common and best known demyelinating disease?
Multiple Sclerosis
Do action potentials lose strength as they are conducted through the axon? why not?
N0-passes kinetic energy, action potentials are replenished as it passes down the axon to maintain constant amplitude.
Which 3 ions are most influential in creating the membrane potential in resting cells. (Goldman-Hodgkin-Katz)
Na+, K+, Cl-
efferent neurons
Nerve cells that conduct impulses away from the central nervous system. Divided into 2 divisions Somatic Motor Division-which controls skeletal muscles Autonomic Division- which controls smooth and cardiac muscles, exocrine glands and some adipose tissue
what is the primary function of neuron?
Neurons receive, process, and transmit information.
Name 2 primary cells found in the nervous system.
Neurons-the basic signaling units of the nervous system Glial Cells- support cells (glia or neuroglia)
Differentiate between the following terms: neurotransmitters neuromodulators neurohormones
Neurotransmitters: acts at the synapse and elicits a rapid response, faster chemical messenger Neuromodulators: act at both synaptic and non synaptic sites, slower chemical messenger, usually a 2nd messenger, act on neurons to alter response to neurotransmitter. Often metabolic receptors Neurohormones: produced and released by neurons in the brain, rather than by the endocrine glands, and delivered to organs and tissues through the bloodstream
What happens when a subthreshold graded potential reaches the trigger zone.
No action potential- not enough strength to make an action potential.
Describe changes in neuron excitability that result from hypokalemia and hyperkalemia (fig 8.17)
Normal K+ levels, subthreshold graded potentials don't trigger action potentials and suprathreshold graded potentials do. An increase of blood K+ concentration(Hyperkalemia) shifts the resting membrane potential of a neuron closer to threshold and causes the cells to fire AP in response to a smaller graded potential. If blood K+ falls too low(hypokalemia) the resting membrane potential of the cells hyperpolarizes, moving it further from threshold.
What the four types of glial cells in the CNS?
Oligodendrocytes-form myelin sheaths Astrocytes-source of neural stem cells,BBB, take up k+, maintain homeostasis in CNS, ATP production Microglia-Immune cells (scavengers)when activated remove all damaged cells and foreign invaders. Ependymal Cells- ependyma layer barriers between fluid compartments of CNS, source of neural stem cells.
Contrast injury and repair in CNS neurons and PSN.
PNS-Can regenerate and reestablish their synaptic connections, Schwann cells secrete neurotrophic factors that keep the cell body alive and stimulate regrowth. CNS-Glial cells tend to seal off and scar the damaged region, & damaged CNS cells secrete factors that inhibit axon regrowth.
Contrast the synthesis of polypeptide neurotransmitters with the synthesis of smaller neurotransmitters such as ACh, amines and purines. Where in the neuron does synthesis take place? Are there organelles involved? How do neurotransmitters get to the presynaptic axon terminal?
Polypeptide-must be made in the cell body because axon terminals don't have organelles needed for protein synthesis. ACh, Amines, Purines: are synthesized and packaged into vesicles in the axon terminal. There enzyme needed for their synthesis are made in the cell body and released into the cytosol. Neurotransmitters synthesis takes place in the nerve cell body and the axon terminal.
Contrast the effects of presynaptic modulation with the effects of postsynaptic modulation.
Presynaptic modulation- can be global or selective- Global input on the dendrites & cell body decrease neurotransmitter release. by all collaterals and all targets cells. Selective input, one collateral can be inhibitory while others remain unaffected. Provides precise means of control. Postsynaptic modulation- Change in the number of receptors Up- and down-regulation, change in the affinity of receptor for a ligand, effects on ion conductance
Contrast the mechanism between slow synaptic potentials and fast synaptic potentials.
SSP-Neurotransmitters that bind to G-protein coupled receptors that are linked to 2nd messenger slow the onset and have a longer lasting response. FSP-neurotransmitter binds to and opens a receptor channel on the postsynaptic cell, allowing ions to move between the postsynaptic cell and the ECF. Begins quickly but only last a short time.
what are the 2 types of glial cells in the PSN.
Schwann cells-have myelin Satellite Cells-don't have myelin
Neurons are separated into 3 groups: Sensory(afferent) Interneurons Efferent compare and contrast all three.
Sensory- carry information about temperature, pressure, light, and other stimuli to the CNS. Interneurons-lie entirely within the CNS, communicate with other neurons. Smaller with many dendrites. Efferent- both somatic(skeletal muscles) and autonomic(smooth&cardiac muscles, glands and adipose tissue); carry info away from the CNS back to the body.
Distinguish the difference between slow axonal transport and fast axonal transport. Identify the purpose and mechanisms for each.
Slow axonal transport-moves soluble proteins from cell body to the axon(.2-8mm/day)materials down axon and supplies new axoplasm needed for developing or regenerating neurons. Fast Axonal transport-moves material in anterograde or retrograde, occurs at a rate of 20 to 400 mm/day.
Distinguish between spatial summation and temporal summation. Describe how multiple signals must be integrated n to the postsynaptic cells response.
Spatial Summation-The summation of graded potentials from several sources in order to achieve threshold activation of an action potential. The summed input from these synapse determines the activity of the postsynaptic neuron. Temporal Summation- Two graded potentials that from the same presynaptic neuron and arrive at the trigger zone close enough together in time. Summation that occurs from graded potentials overlapping in time.
How does a cell change its ion permeability.
To open or close existing channels in the membrane. Or insert a new channel into the membrane or remove an existing channel. This is a slower method.
How are the channels classified? Name 4 types of selective ion channels in the neuron.
Usually named "classified" according to the primary ions they allow to pass through. 1-Na+ 2- K+, 3-Ca2+, 4-Cl-
Ohm's Law. What biological factors contribute to electrical resistance in neurons?
V(volts)=I(current flow) x R(resistance) - As resistance increases current flow decreases. Biological factors that contribute are: -resistance of cell membrane -internal resistance of the cytoplasm.
Identify the ion channel types required for conduction of an action potential.
Voltage gated K+ Channels Voltage gated Na+ channels some leaky channels.
Describe variation in ion channel behavior.
Voltage level, being open and the speed which they open. activation-channel opening to allow ion flow inactive-some channels that close spontaneously(certain time, from timing out)
Describe 2 examples of how ion movement can hyperpolarize a cell.
When the cell membrane suddenly become permeable to K+, positive charge is lost from inside the cell and the cell becomes more negative. Cell can also hyper polarize if a negatively charge ions (Cl-) entry the cell from the extracellular fluid.
Describe the cellular events that follow damage to a neuron.
When the neuron is damaged and the cell body dies, the entire neuron dies. When a axon is severed, the portion that is still connected to the cell body continues to live.The axon cytoplasm leaks out at the injury site until membrane is recruited to seal the opening. The segment with the cell body swells as organelles and filaments brought by axonal transport accumulate. Schwann cells near the injury site send chemical signals to the cell body alerting that an injury has occurred. The other portion will begin to disintegrate due to lack of protein sources. Then the myelin then begins to unravel and scavenger microglia or phagocytes ingest and clear away the debris. If severed axon belongs to a somatic motor neuron, death to the distal axon results in permanent paralysis of that skeletal muscle innervated by the neuron. If it is a sensory neuron the person may experience loss of sensation(numbness, tingling) in the region innervated by the neuron.
How is dendritic function different in the CNS than in the PNS?
Within the CNS- dendritic spines can unction as independent compartments, sending signals back and forth with neurons in the brain. They can change their size and shape in response to neighboring cells. Changes in the morphology associated with learning and memory, genetic disorders (Alzheimers)
What is myelin? Why is it important?
a mixture of proteins and phospholipids forming a whitish insulating sheath around many nerve fibers, increasing the speed at which impulses are conducted.
sympathetic branch
a part of the ANS that activates the body at times of stress. Fight or flight!
Astrocytes (CNS)
associated with synapse-take up and release chemicals, provide neurons with metabolic substrates for ATP production, maintain homeostasis in CNS by taking up K+, become part of the blood brain barrier
somatic motor division
carries signals to skeletal muscles Output produces muscular contraction as well as somatic reflexes—involuntary muscle contractions
KNOW PARTS OF NEURON FIG 8.2F
cell body, nucleus, axon hillock, axon, dendrites, nodes of ranvier
glial cells
cells in the nervous system that support, nourish, and protect neurons
Central Nervous System (CNS)
consists of the brain and spinal cord
autonomic nervous system (ANS)
divided into 2 divisions sympathetic-fight or flight parasympathetic-rest & digest the part of the peripheral nervous system that controls the glands and the muscles of the internal organs (such as the heart).
What are Nodes of Ranvier?
gaps in the myelin of axons
What are neural stem cells? Why could they be important in treating neural injuries and degenerative disease?
immature "uncommitted" cells in the nervous system that can mature into any type of neuron or glial cell needed by the brain. Stem cell transplant could potentially reverse the loss of function that comes with injury and degenerative neurology disease.
ependymal cells
line cavities of the brain and spinal cord, circulate cerebrospinal fluid
sensory neurons(afferent)
neurons that carry incoming information from the sensory receptors to the brain and spinal cord
Depolarization implies that a cell's membrane potential has become more ______________(neg or pos) as a result of ion movement.
positive.
What do satellite cells do?
surround neuron cell bodies within ganglia
What is synaptic plasticity? How can a neuron influence activity at the synapse.
the ability of a synapse to change over time through use or disuse. If reduced neuron activity at the synapse, the neurotransmitter release will decrease over time.
parasympathetic nervous system
the division of the ANS that calms the body, conserving its energy rest and digest
peripheral nervous system
the sensory(afferent) and Efferent motor neurons that connect the CNS to the rest of the body