A&P Chapter 12
graded potential
- localized change in resting potential - Effect decreases with distance from stimulus
Temporal Summation
-Addition of multiple stimuli at a single synapse -If a second AP arrives before effects from 1st one have dissipated, the effects are magnified
sensory neurons (afferent neurons)
-Deliver info from periphery to CNS -Cell bodies located in sensory ganglia (PNS) -Mostly unipolar neurons
interneurons (all other neurons)
-Lie between sensory and motor neurons -Role in higher functions (learning, memory, planning, emotions, etc.)
Information Processing
-Occurs at axon hillock -Cell must "sum" all the EPSPs and IPSPs -If threshold is reached, a new AP is generated
Saltatory Propagation
-Occurs in myelinated axons -AP "jumps" from node to node -MUCH faster
microglia
-Smallest and fewest -Phagocytic cells -Immune cells for the CNS
Oligodendrocytes
-Wrap CNS neurons in myelin -One oligodendrocyte can produce several myelin sheaths
schwann cells
-Wrap PNS neurons in myelin -One Schwann cell produces just one myelin sheath
demyelination
-progressive destruction of myelin sheaths (CNS or PNS) - Impedes normal function of axon
four classifications of neurons by structure
1. Anaxonic- dendrites and axons appear similar 2. Bipolar- two distinct processes 3. Unipolar- dendrites and axons are continuous 4. Multipolar- two or more dendrites; single axon
Types of Neurotransmitters
Glutamate GABA Glycine Norepinephrine Dopamine Serotonin
Postsynaptic Potentials
If postsynaptic membrane is depolarized... -We call it a EPSP -Graded depolarization -Example: Influx of Na+ ions If postsynaptic membrane is hyperpolarized... -We call it a IPSP -Graded hyperpolarization -Example: Outflow of K+ ions
retrograde transport
Movement from axon terminal to cell body
Which two types of neuroglia insulate neuron cell bodies and axons in the PNS from their surroundings?
Neuroglia in the PNS are (1) satellite cells and (2) Schwann cells
Describe the conditions in a cell at rest (concentration gradients of molecules and distribution of charges)
There is more potassium inside the cell than outside the cell and more sodium outside the cell than inside the cell. Opposites attract so through diffusion sodium ions flow into the cell while potassium flows outside the cell to a low concentration.
Describe the organization of a synapse
There is the presynaptic membrane that is responsible for releasing the neurotransmitters from the synaptic vesicles. The postsynaptic membrane contains the receptors in order for the neurotransmitters to bind and the synaptic cleft is the narrow space.
Three types of neurons (axons):
Type A Fibers Type B Fibers Type C Fibers
axon collaterals
communication with multiple targets
sympathetic
fight or flight
gray matter
mainly cell bodies (Nissl bodies)
Synaptic cleft
narrow space (neurotransmitters can diffuse across)
Parasympathetic
rest or digest
neuroglia
supporting cells
what's the function of myelin?
to provide electrical insulation.
Describe the events that occur during nerve impulse transmission at a typical cholinergic synapse.
(1) The action potential arrives at the axon terminal, depolarizing it; (2) extracellular calcium enters the axon terminal, triggering the exocytosis of ACh; (3) ACh binds to the postsynaptic membrane and depolarizes the next neuron in the chain; (4) ACh is removed by AChE.
What three functional groups of neurons are found in the nervous system? What is the function of each type of neuron?
(1) sensory neurons: transmit impulses from the PNS to the CNS; (2) motor neurons: transmit impulses from the CNS to peripheral effectors; and (3) interneurons: analyze sensory inputs and coordinate motor outputs.
Describe the different types of neuroglia (4 types in the CNS; 2 types in the PNS) (CNS)
(CNS) 1. Ependymal cells- line central canal and ventricles 2. Astrocytes- largest and most numerous 3. Oligodendrocytes- wrap CNS neurons in myelin 4. Micoglia- smallest and fewest, phagocytic cells (PNS) 1. Satellite cells- surround cell bodies in peripheral ganglia 2. Schwann cells- wrap PNS neurons in myelin
Depolarization of a neuron plasma membrane will shift the membrane potential toward: (a) 0 mV, (b) −70 mV, (c) −90 mV, (d) all of these.
(a) 0 mV
Receptors that bind acetylcholine at the postsynaptic membrane are: (a) chemically gated channels, (b) voltage-gated channels, (c) passive channels, (d) mechanically gated channels.
(a) chemically gated channels
Label the structures in the following diagram of a neuron
(a) dendrite (b) nucleolus (c) nucleus (d) axon hillock (e) initial segment (f) axolemma (g) axon (h) telodendria (i) axon terminals
What are the major components of (a) the central nervous system? (b) the peripheral nervous system? (c) the enteric nervous system?
(a) the CNS: brain and spinal cord (b) the PNS: all the nervous tissue outside the CNS and ENS divided between the efferent division (which consists of the somatic nervous system and the autonomic nervous system) and the afferent division (which consists of receptors and sensory neurons) (c) the ENS: network of neurons and nerve networks in the walls of the digestive tract.
Ligand Gated Channels
(aka chemically-gated channels) - open or close in response to binding of a specific molecule Example: ACh at the NMJ
The neural cells responsible for the analysis of sensory inputs and coordination of motor outputs are: (a) neuroglia, (b) interneurons, (c) sensory neurons, (d) motor neurons.
(b) interneurons
If the resting membrane potential of a neuron is −70 mV and the threshold is −55 mV, a membrane potential of −60 mV will: (a) produce an action potential, (b) make it easier to produce an action potential, (c) make it harder to produce an action potential, (d) hyperpolarize the membrane.
(b) make it easier to produce an action potential
What factor determines the direction that ions will move through an open membrane channel? (a) the membrane permeability to sodium ions, (b) the electrochemical gradient, (c) intracellular negatively charged proteins, (d) negatively charged chloride ions in the ECF.
(b) the electrochemical gradient
In the CNS, a neuron typically receives information from other neurons at its: (a) axon, (b) Nissl bodies, (c) dendrites, (d) nucleus.
(c) dendrites
Phagocytic cells in nervous tissue of the CNS are: (a) astrocytes, (b) ependymal cells, (c) oligodendrocytes, (d) microglia.
(d) microglia
Rate of Action Potential Generation
*Remember: APs are all-or-nothing events What happens when a neuron receives a strong stimulus? -Frequency of AP generation increases -Messages are often interpreted based on frequency of APs -The greater the depolarization, the higher the frequency of APs *Highest frequencies recorded in humans: 500 - 1,000 APs per second!
Differentiate between anterograde and retrograde axoplasmic transport
- Anterograde transport is the movement from the cell body to the axon terminal (moving forward). - Retrograde transport is the movement from the axon terminal to the cell body (moving backwards).
divisions of the nervous system and the function of each division
- Central nervous system (CNS), consists of the brain and spinal cord. Controls the body by processing and responding to sensory input from the peripheral nervous system. - peripheral nervous system (PNS), consists of all the nervous tissue not in the CNS. Communicates information from the body to the central nervous system (e.g. aches and pains) and to the body's organs, glands and muscles. The (PNS) is divided into two parts: Afferent Division- brings sensory info to the brain Efferent Division- carries commands from the brain
synaptic activity
- action at a synapse that produces graded potentials in postsynaptic cell
Electrical gradient (potassium ions)
- favor potassium entering the cell
Chemical gradient (sodium ions)
- favor sodium entering the cell
electrical gradient (sodium ions)
- favor sodium entering the cell
"Net Force" (potassium ions)
- for potassium to leave the cell If allowed to equalize, membrane potential would be -90 mV
"net force" (sodium ions)
- for sodium to enter the cell If allowed to equalize, membrane potential would be +66 mV
information processing
- integration of multiple stimuli within an individual cell
Voltage-Gated Channels
- open or close in response to changes in membrane potential - Have "activation" and "inactivation" gates - Can be open, closed, or "inactivated" - Most abundant on axons and axon hillock
Mechanically-Gated Channels
- open or close in response to distortions of the membrane
motor neurons (efferent neurons)
-Carry instructions from CNS to periphery -Somatic division: Cell bodies located in CNS -Autonomic division: Cell bodies in autonomic ganglia (PNS) -Mostly multipolar neurons
Differentiate between the neurotransmitters discussed (slide 43)
-Glutamate - primary excitatory NT in CNS Multiple types of receptors Plays a large role in learning and memory -GABA - primary inhibitory NT in CNS -Glycine - primary inhibitory NT in spinal cord -Norepinephrine - used in CNS and autonomic nervous system Synapses are called adrenergic synapses -Dopamine - used in CNS Role in modulating motor movements Role in reward circuits -Serotonin - used in CNS Role in attention and mood Many antidepressants
demyelination examples
-Heavy Metal Poisoning - chronic exposure to arsenic, lead, mercury -Diphtheria - bacterial toxin damages Schwann cells (can result in fatal paralysis) -Multiple Sclerosis - autoimmune attack on myelin (primarily CNS) - progressive -Guillain-Barré Syndrome - autoimmune attack on PNS (triggered by a virus; can fully recover)
ependymal cells
-Line central canal and ventricles -Secrete cerebral spinal fluid (CSF) -Circulate the CSF (cilia and microvilli)
Function of CSF
-Provides protective cushion -Transports gases, nutrients, wastes, etc.
In the CNS...
-Restoration of normal function highly unlikely -Damage will involve more cells - Astrocytes will produce scar tissue - Astrocytes release chemicals that block axon regrowth
neuroglia
-Roughly 1:10 ratio with neurons -In adults, only actively dividing cells in nervous system (tumors) -Different types in CNS vs. PNS
Chemical gradient (potassium ions)
-favor potassium leaving the cell
Describe the events that occur at a synapse (using a cholinergic synapse as an example)
1) AP arrives; depolarizes synaptic terminal 2) Voltage-gated calcium channels are activated Calcium rushes into cell Triggers exocytosis of vesicles containing neurotransmitter 3) Neurotransmitter binds to receptors on postsynaptic membrane In the case of ACh... Depolarization of postsynaptic membrane If threshold is met, an AP is generated in the postsynaptic cell 4) Neurotransmitter is removed from synaptic cleft ACh is removed by AChE
Astrocytes functions
1) Maintain blood-brain barrier (BBB) 2) Structural framework 3) Repair of damage 4) Help guide neuron development 5) Control of interstitial environment
What is the function of each type of neuroglia?
1. Ependymal cells- secrete cerebral spinal fluid and circulate it. 2. Astrocytes- maintain blood-brain barrier (BBB), structural framework, repair of damage, help guide neuron development, control of interstitial environment. 3. Oligodendrocytes- produce several myelin sheets 4. Micoglia- immune cells for the CNS (PNS) 5. Satellite cells-regulate environment around the neuron 6. Schwann cells- produce one myelin sheath
Explain some causes of demyelination and why this is a bad thing
1. Heavy Metal Poisoning - chronic exposure to arsenic, lead, mercury. - This is a bad thing because the myelin sheath covers and insulates the axons, aiding the conduction of electrical signals between nerves. The process of demyelination disrupts this electrical nerve conduction, which leads to symptoms of neurodegeneration.
Briefly contrast the response to neuronal injury in the PNS and CNS
1. In the PNS... Schwann cells can help provide a path for the axon to regrow 2. In the CNS... Restoration of normal function highly unlikely. Damage will involve more cells, Astrocytes will produce scar tissue Astrocytes release chemicals that block axon regrowth
three classifications of neurons by function
1. Sensory neurons- afferent neurons, deliver info from periphery to CNS 2. Motor neurons- efferent neurons, carry instructions from CNS to periphery 3. Interneurons- all other neurons, role in higher functions (learning, memory, planning, emotions, etc.)
Events at a Synapse Step 2:
2) Voltage-gated calcium channels are activated -Calcium rushes into cell -Triggers exocytosis of vesicles containing neurotransmitter
Events at a Synapse Step 3:
3) Neurotransmitter binds to receptors on postsynaptic membrane In the case of ACh... -Depolarization of postsynaptic membrane -If threshold is met, an AP is generated in the postsynaptic cell
Events at a Synapse Step 4:
4) Neurotransmitter is removed from synaptic cleft -ACh is removed by AChE
Describe the structure of a neuron
A neuron is constructed of a cell body, dendrites, and an axon.
Spatial Summation
Addition of simultaneous stimulations from multiple places
peripheral nervous system (PNS)
All nervous tissue not in the CNS
What is the difference between an absolute refractory period and a relative refractory period?
An absolute refractory period: another stimulus will have no effect -Voltage-gated sodium channels are "inactivated" -Prevents AP from moving backwards A relative refractory period: another stimulus could trigger another AP, but it would have to be stronger than normal -Cell is partially hyperpolarized and farther from threshold
What is the difference between anterograde flow and retrograde flow?
Anterograde flow is the movement of materials from the cell body to the axon terminals. Retrograde flow is the movement of materials toward the cell body
Why is axoplasmic transport necessary?
Axonal transport is defined as the process by which proteins and other substances synthesized in the neurosome are transported to the nerve endings through cytoskeleton. Axonal transport is essential for neuronal function, and many neurodevelopmental and neurodegenerative diseases result from mutations in the axonal transport machinery.
How Neurotransmitters Work... Indirect Effect
Binding of NT causes change in metabolism of postsynaptic cell (metabotropic effects) -NT is a "first messenger" -Activates other molecules ("second messengers") -Usually involves G proteins -Many effects in the cell -Can eventually open ion channels and depolarize or hyperpolarize the membrane
How Neurotransmitters Work... Direct Effect
Binding of NT causes ion channels to open or close (ionotropic effects) Depolarize or Hyperpolarize Postsynaptic Membrane
How can membrane potential change?
By opening/closing different types of channels
In a graded potential pt2.
Change in local potential depends on strength of the stimulus -As stimulus increases, more channels open -More channels = Larger change in local potential
Describe the electrochemical gradient of potassium ions (know potassium's equilibrium potential)
Chemical gradient - favor potassium leaving the cell Electrical gradient - favor potassium entering the cell "Net Force" - for potassium to leave the cell If allowed to equalize, membrane potential would be -90 mV
Describe the electrochemical gradient of sodium ions (know sodium's equilibrium potential)
Chemical gradient - favor sodium entering the cell Electrical gradient - favor sodium entering the cell "Net Force" - for sodium to enter the cell If allowed to equalize, membrane potential would be +66 mV
What is the functional difference among chemically gated (ligand-gated), voltage-gated, and mechanically gated ion channels?
Chemically gated (ligand-gated) ion channels open or close when they bind specific extracellular chemicals (ligands). Voltage-gated ion channels open or close in response to changes in the membrane potential. Mechanically gated ion channels open or close in response to physical distortion of the membrane surface.
Speed of AP Propagation
Depends on size and myelination.... -As diameter increases, speed increases
Action Potential Propagation
Depolarization in one area depolarizes neighboring areas -APs start at the initial segment (by axon hillock) -AP "moves" down the length of the axon -One-way movement because of sodium channel inactivation
How is an action potential propagated down an axon?
Depolarization in one area depolarizes neighboring areas next ... APs start at the initial segment (by axon hillock) then ... AP "moves" down the length of the axon (One-way movement because of sodium channel inactivation)
Differentiate between depolarization and hyperpolarization
Depolarization is a shift towards a more positive membrane potential. Hyperpolarization is a shift towards a more negative membrane potential.
Synaptic Communication
Electrical vs. Chemical Synapses Chemical Synapse - most abundant -use of neurotransmitters to transfer message from one cell to the next *KEY POINT: The effect of a NT depends on the properties of the receptor -Some NTs can be excitatory or inhibitory
Events at a Synapse Step 1:
Example: Cholinergic Synapse Uses acetylcholine as a neurotransmitter 1) AP arrives; depolarizes synaptic terminal
Compare and contrast graded potentials and action potentials
Graded potentials (local potentials) occur in the receptive (dendrites & cell bodies) due to opening of chemically gated channels that allow small ion amounts to cross the membrane and the altered charge may result in a change in polarization. Degree of change depends on how many molecules make it through and decreases intensity over distance-usually only lasting a short time. Action potentials (AP) are the result of voltage gated channels opening (not chemically gated ones like GP's) and require a threshold value to be reached. Voltage below this is not sufficient to create an AP but once the threshold value is reached a temporary reversal of polarity across the plasma membrane occurs. AP's are self propagated or transmitted and maintain intensity along the synaptic knob because of the successive opening of other voltage gated channels. AP's obey the "all or none" law but not all APs have the same intensity under the same conditions.
Differentiate between grey matter and white matter
Gray matter is composed mainly of cell bodies whereas white matter is composed of mainly myelinated axons.
What is the resting membrane potential in neurons?
In a resting neuron, there are concentration gradients across the membrane for Na+ and K+. Ions move down their concentration gradients via channels, leading to a separation of charge that creates the resting potential.
What is meant by saltatory propagation? How does it differ from continuous propagation?
In saltatory propagation, which occurs in myelinated axons, only the nodes along the axon can respond to a depolarizing stimulus. In continuous propagation, which occurs in unmyelinated axons, an action potential appears to move across the membrane surface in a series of tiny steps.
What is meant by the "all-or-none" principle of action potentials?
It means that an AP is either generated or not
Astrocytes
Largest and most numerous
Differentiate between chemically gated ion channels, voltage-gated ion channels, and mechanically gated ion channels
Ligand gated channels open or close in response to binding to a specific molecule, Voltage- Gated channels open or close in response to changes in the membrane potential. Mechanically- Gated Channels open or close in response to distortions of the membrane.
Why can't most neurons in the CNS be replaced when they are lost to injury or disease?
Neurons lack centrioles and therefore cannot divide and replace themselves
gaps between myelin sheaths are called....
Nodes of Ranvier
In the PNS...
Schwann cells can help provide a path for the axon to regrow
What are the sequence of steps that occur during an action potential?
Step 1: A graded depolarization brings an area of excitable membrane to threshold (-60mv) Step 2: Voltage-gated sodium channels open and sodium ions move into the cell. The transmembrane potential rises to +30mv Step 3: Sodium channels close, voltage-gated potassium channels open, and potassium ions move to of the cell. Repolarization begins Step 4: Potassium channels close, and both sodium and potassium channels return to their normal states.
satellite cells
Surround cell bodies in peripheral ganglia Similar function as an astrocyte Regulate environment around the neuron
What is the difference between temporal summation and spatial summation?
Temporal summation is the addition of stimuli that arrive at a single synapse in rapid succession. Spatial summation occurs when simultaneous stimuli at multiple synapses have a cumulative effect on the membrane potential.
State the all-or-none principle of action potentials.
The all-or-none principle of action potentials states that any depolarization event sufficient to reach threshold will generate an action potential of the same strength, regardless of the amount of stimulation above threshold.
Describe the steps involved in the generation of an action potential
The membrane depolarizes to threshold. Next, voltage-gated sodium ion channels are activated, and the membrane rapidly depolarizes. These sodium channels are then inactivated, and voltage-gated potassium ion channels are activated. Finally, normal permeability returns. The voltage-gated sodium channels become activated once the repolarization is complete; the voltage-gated potassium channels begin closing as the membrane potential reaches the normal resting membrane potential.
Describe what determines the resting membrane potential
The resting potential is determined by concentration gradients of ions across the membrane and by membrane permeability to each type of ion.
What is the difference between type A, type B, and type C fibers?
Type A Fibers are largest diameter; myelinated APs move at a rate of 120 m/s (268 mph) Type B Fibers are smaller and myelinated 18 m/s (40 mph) Type C Fibers are small and unmyelinated 1 m/s (2 mph)
What are the structural and functional differences among type A, B, and C fibers?
Type A fibers are myelinated and carry action potentials very quickly (120 m/sec). Type B fibers are also myelinated, but carry action potentials more slowly due to their smaller diameter. Type C fibers are extremely slow due to their small diameter and lack of myelination.
In a graded potential pt3.
When a chemically-gated potassium channel opens... Potassium rushes out which causes... Hyperpolarization - a shift towards a more negative membrane potential
In a graded potential
When a chemically-gated sodium channel opens... Sodium rushes in which causes.. depolarization - a shift towards a more positive membrane potential
What does a neuromodulator do?
a neuromodulator adjusts the sensitivities of another neuron to specific neurotransmitters.
All-or-None Principle
an AP is either generated or not The size of an Action Potential does not depend on strength of stimulus
Absolute Refractory Period
another stimulus will have no effect -Voltage-gated sodium channels are "inactivated" -Prevents AP from moving backwards
Relative Refractory Period
another stimulus could trigger another AP, but it would have to be stronger than normal -Cell is partially hyperpolarized and farther from threshold
peripheral ganglia
are areas that are found out in the peripheral
neurons
basic functional units
central nervous system (CNS)
brain and spinal cord
Afferent division of PNS
brings sensory info to the brain
Neuromodulator
can affect activity at synapses -Alter rate of NT release by presynaptic cell -Change response of postsynaptic cell to NT Example: opioids Help to relieve pain Inhibit the release of "substance P" at synapses
Efferent division of PNS
carries commands from the brain
Excitatory Neurotransmitters
cause depolarization and generation of APs
Inhibitory Neurotransmitters
cause hyperpolarization and suppression of APs
cell body
contains organelles
postsynaptic membrane
contains receptors for neurotransmitters
What is the difference between continuous propagation and saltatory propagation
continuous propagation occurs in unmyelinated axons and saltatory propagation occurs in myelinated axons
somatic nervous system
control of skeletal muscles
autonomic nervous system
control of smooth muscle, cardiac muscle, glandular secretions
anaxonic
dendrites and axons appear similar
unipolar
dendrites and axons are continuous (2 axons, one side has dendrites and other side has synaptic terminals.)
resting potential
difference in charges across membrane at rest
action potential
electrical impulse propagated along membrane surface
Types of Neuroglia in the CNS:
ependymal cells astrocytes oligodendrocytes microglia
What is the difference between excitatory and inhibitory neurotransmitters?
excitatory neurotransmitters cause depolarization and generation of APs and inhibitory neurotransmitters cause hyperpolarization and suppression of APs
Type A Fibers
largest diameter; myelinated -APs move at a rate of 120 m/s (268 mph)
Explain what a "graded potential" is
local changes in membrane potential
white matter
mainly myelinated axons
myelin
membranous sheath that serves as electrical insulation - is white
Anterograde transport
movement from cell body to axon terminal
Continuous Propagation
occurs in unmyelinated axons
Synaptic Fatigue
occurs when resynthesis and transport of NT is insufficient to keep up with demand -Activity at that synapse weakens until NT stores are replenished
Glutamate
primary excitatory NT in CNS -Multiple types of receptors -Plays a large role in learning and memory
GABA
primary inhibitory NT in CNS
Glycine
primary inhibitory NT in spinal cord
dendrites
receive signals -surface area increased with "spines"
presynaptic membrane
release of neurotransmitters from synaptic vesicles
Types of Neuroglia in the PNS
schwann cells satellite cells
axon
send signals
axon (synaptic) terminals
sites of communication
Type C Fibers
small and unmyelinated -1 m/s (2 mph)
Type B Fibers
smaller and myelinated -18 m/s (40 mph)
What is the definition of synaptic delay and synaptic fatigue
synaptic delay: is the time between the arrival of an AP and the effect on the postsynaptic membrane and..... synaptic fatigue: occurs when resynthesis and transport of NT is insufficient to keep up with demand
Threshold
the membrane potential at which an action potential is generated -Usually between -60 and -55 mV -In order for a graded potential to become an Action Potential, the membrane must be depolarized by 10-15 mV
Synaptic Delay
the time between the arrival of an AP and the effect on the postsynaptic membrane -Time for calcium influx and NT release -Typically 0.2-0.5 msec
bipolar
two distinct processes
multipolar
two or more dendrites; single axon
Serotonin
used in CNS -Role in attention and mood -Many antidepressants are "Selective Serotonin Reuptake Inhibitors" (SSRIs)
Dopamine
used in CNS -Role in modulating motor movements -Role in reward circuits
Norepinephrine
used in CNS and autonomic nervous system -Synapses are called adrenergic synapses
What is the difference between presynaptic facilitation and presynaptic inhibition
with presynaptic facilitation activity at an axoaxonic synapse increases the amount of neurotransmitter released when an action potential arrives at the axon terminal and the neurotransmitter serotonin is involved in presynaptic facilitation. with presynaptic inhibition the release of GABA inhibits the opening of voltage-gated calcium ion channels in the axon terminal and This inhibition reduces the amount of neurotransmitter released when an action potential arrives there, and thus reduces the effects of synaptic activity on the postsynaptic membrane.