Ex. organelles to be degraded, signal molecules, viruses, bacterial toxins
Myelin sheath
Composed of myelin
Segmented sheath around most long or large diameter axons (myelinated fibers)
What is myelin
whitish protein-lipid substance
what are the functions of myelin
Protects and electrically insulates axon
Increases speed or nerve impulse transmission
Do nonmyelinated fibers conduct impulses more slowly
Yes
Myelination in the PNS
Formed by schwann cells (wrap around axon in jelly roll fashion, one cell forms one segment of myelin sheath)
Myelin sheath (concentric layers of schwann cell plasma membrane around axon)
Outer collar of perinuclear cytoplasm (peripheral bulge of schwann cell containing nucleus and most of cytoplasm)
Plasma membranes of myelinating have cells less protein (no channels or carriers, good electrical insulators, interlocking proteins bind adjavent myelin membranes)
Nodes of rangier (myelin sheath gaps between adjacent schwann cells, sites where axon collaterals can emerge)
Nonmyelinated fibers (thin fibers not wrapped in myeline, surrounded by schwann cells but no coiling, one cell may surround 15 different fibers)
Myelin sheaths in CNS
Formed by multiple flat processes of oligodendrocytes, not whole cells
Can wrap up to 60 axons at once
Nodes of ranvier present
No outer collar of perinuclear cytoplasm
Thinnest fibers are unmyelinated (covered by long extensions of adjacent neuroglia)
White matter & Gray matter
White matter
Regions of brain and spinal cord with dense collections of myelinated fibers, usually fiber tracts (tract=PNS)
Gray matter
mostly neuron cell bodies and nonmyelinated fibers
Structural classification of neurons
Grouped by number of processes; 3 types
Multipolar, Bipolar & Unipolar
Multipolar Neurons
3 or more processes
1 axon, others dendrites
most common; major neuron in CNS
Bipolar Neurons
2 processes
1 axon and 1 dendrite
rare e.g. retina and olfactory mucosa
Unipolar Neurons
1 short process
Divides T-like with both branches now considered axons (distal (peripheral) process is associated with sensory receptor and proximal (central) process enters CNS
What are the functional classifications of neurons
Grouped by direction in which nerve impulse travels relative to CNS; 3 types
Sensory (afferent), Motor (Efferent), Interneurons
Sensory Neurons
Transmit impulses from sensory receptors toward CNS
Almost all are unipolar
Cell bodies in ganglia in PNS
Motor Neurons
Carry impulses from CNS to effectors
Multipolar
Most cell bodies in CNS (except some autonomic neurons)
Interneurons (association neurons)
Lie between motor and sensory neurons
Shuttle signals through CNS pathways; most are entirely within CNS
99% of body's neurons
Most confined in CNS
How do neurons respond to adequate stimulus
by generating an action potential
what is an action potential
a nerve impulse
is the impulse always the same
yes; regardless of stimulus
if opposite charges are separated the system has what kind of energy
potential energy
voltage
a measure of potential energy generated by separated charge
current
flow of electrical charge (ions) between two points
resistance
is a hindrance to charge flow; an be an insulator or conductor
insulator
substance with high electrical resistance
conductor
substance with low electrical resistance
Ohms Law
current (I)=voltage (V)/resistance (R)
*current is directly proportional to voltage
*no net current flow between points with same potential
*current inversely related to resistance
what is the role of membrane ion channels
large proteins serve as selective membrane ion channels; there are two types; leakage (nongated) or gated
leakage channels
nongated channels that are always open
gated channels
part of protein changes shape to open/close channel
what are the three types of gated channels
chemically gated
voltage-gated
mechanically gated
chemically gated (ligand-gated) channels
open with binding of a specific neurotransmitter
voltage-gated channels
open and close in response to changes in membrane potential
mechanically gated channels
open and close in response to physical deformation of receptors, as in sensory receptors
explain what happens when gated channels are open
-ions diffuse quickly across membrane along electrochemical gradients
-ion flow creates an electrical current and voltage changes across membrane
resting membrane potential
-potential difference across membrane of resting cell (-70)
-membrane is polarized
-generated by differences in ionic makeup of ICF and ECF/differential permeability of the plasma membrane
ECF
extracellular fluid
ICF
intracellular fluid
which has a higher concentration of Na+ (ICF or ECF)
ECF
which has a higher concentration of K+ (ICF or ECF)
ICF
What chemical compound plays the most important role in membrane potential
K+
Explain the difference in plasma membrane permeability with K+ Na+ and Cl-
-slightly permeable to Na+ through leakage channels
-25 times more permeable to K+ than sodium (more leakage channels)
-Very permeable to Cl-
what happens to chloride when it goes into the cell
it becomes hyperpolarized
More potassium diffuses out than sodium diffuses in (true or false)
True; cell more negative inside and establishes resting membrane potential
is the sodium-potassium pump always going
Yes; it stabilizes resting membrane potential, maintains concentration gradients for Na+ and K+
-for every 3 Na+ pumped out of cell 2 K+ pumped in
When does membrane potential change
-when concentrations of ions across membrane change
-membrane permeability to ions change
what are the two types of signals changes in the membrane potential produce
graded potentials and action potentials
what is graded potential
incoming signals operating over short distances
action potentials
long-distance signals of axons
what are changes in the membrane potential used as
signals to receive, integrate, and send information
Depolarization
-decrease in membrane potential (toward zero and above)
-inside of membrane becomes less negative than resting membrane potential
-increases probability of producing a nerve impulse
is the inside of a cell positive of negative and why
it is negative because of protein
what is the membrane potential threshold
-55; -55 and above is called action potential & -54 and below is graded potential
do you have to have graded potential to have action potential
YES
hyperpolarization is the opposite of what in terms of resting membrane potential
depolarization
hyperpolarization
-an increase in membrane potential (away from zero)
-inside of cell more negative than resting membrane potential
-reduces probability of producing a nerve impulse
graded potentials
-short-lived, localized change sin membrane potential (magnitude varies with stimulus strength; stronger stimulus=father current flow)
-either depolarization of hyperpolarization
-triggered by stimulus that opens gated ion channels
-current flows but dissipates quickly and decays (graded potentials are signals only over short distances)
change of voltage of 100 mV --> how do you get this number
-70 resting membrane potential --> +30 =100
what is the principle way neurons send signals
action potentials
what is the principal means of long-distance neural communication
action potentials
what kind of potential only occur in muscle cells and axons of neurons
action potentials
do action potentials decay over distance
No they don't; graded potentials do though
each Na+ channel has two voltage-senstive gates, what are they?
activation gates and inactivation gates
*they shut out unnecessary stimuli
activation gates
closed at rest; open with depolarization allowing Na+ to enter the cell
inactivation gates
open at rest; block channel once it is open to prevent more Na+ from entering cell
what are the properties of K+ gated channels
-each K+ channel has one voltage-senstive gate
-closed at rest
-opens slowly with depolarization
generation of an action potential: resting state
-all gated Na+ K+ channels are closed
-Only leakage channels for Na+ and K+ are open; which maintains the resting membrane potential
generation of an action potential: depolarizing phase
depolarizing local currents opens voltage-gated Na+ channels
Na+ influx causes more depolarization which then opens more channels making ICF less negative
at threshold, positive feedback causes opening of ALL Na+ channels; creates a spike of action potential
generation of an action potential: repolarizing phase
Na+ channel slow inactivation gates close
membrane permeability to Na+ declines (AP spike stops rising)
slow voltage-gated K+ channels open and K+ exits the cell so internal negativity is restored
generation of an action potential: hyperpolarization
some k+ channels remain open, allowing excessive K+ efflux (membrane becomes more negative than -70)
this causes hyperpolarization of the membrane
Na+ channels begin to reset
role of sodium-potassium pump
repolarization resets electrical conditions no ionic conditions
after depolarization Na+/K+ pumps restore ionic conditions
threshold
-55
not all depolarization events produce APs
for the axon to "fire" signals, depolarization must reach threshold
@ threshold:
membrane has been depolarized by 15-20 mV
Na+ permeability increases
Na+ influx exceeds K+ efflux
the positive feedback cycle begins
all or none phenomenon
an AP either happens completely or not at all
propagation of an action potential
propagation allow AP to serve as a signaling device
Na+ influx causes local currents, which causes depolarization of adjacent cells in the direction AWAY from the AP origin
Na+ channels closer to AP origin are inactivated so no new AP is generated there
once initiated, AP is self-propagating
propagation of an action potential in nonmyelinated axons
each successive segment of membrane depolarizes and then repolarizes
coding for stimulus intensity
all APs are alike, regardless of stimulus intensity
CNS differentiates between weak and strong stimuli by frequency of impulses sent per second.
more impulses per second=stronger stimulus
absolute refractory period
when voltage-gated Na+ channels open neuron cannot respond to another stimulus
time from opening of Na+ channels until resetting of the channels
ensures that each AP is an all-or-none event
enforces one-way transmission of nerve impulses
relative refractory period
-follows absolute refractory period
-Most Na+ channels have returned to their resting state
-some K+ channels still open
-repolarization is occuring
-threshold for AP generation is elevated
-inside of the membrane is more negative than resting state
-only exceptionally strong stimulus could stimulate an AP
conduction velocity
varies widely
rate of AP propagation depends on the axon diameter and degree of myelination
axon diameter: larger diameter fibers have less resistance to current flow so a fast impulse conduction
degree of myelination: continuous conduction in unmyelinated axons is slower than saltatory conduction in myelinated axons
saltatory conduction
faster than continuous conduction
happens in myelinated axons
30x faster
myelin sheaths insulate and prevent leakage of charge
continuous conduction
slower than saltatory conduction
happens in unmyelinated axons
how are nerve fibers classified
diameter
degree of myelination
speed of conduction
what are the three nerve fiber classifications
group A, group B, group C
group A fibers
large diameter
myelinated somatic sensory and motor fibers of skin, skeletal muscles, joints
transmit at 150 m/s
group B fibers
intermediate diameter
lightly myelinated
transmit at 15 m/s
group C fibers
smallest diameter
unmyelinated ANS fibers
transmit at 1 m/s
what are the cell bodies and processes for neurons called in the CNS
body: nuclei
process: tract
what are the cell bodies and processes for neurons in the PNS called
body: ganglia
process: nerve
how are neurons functionally connected
through synapses
they mediate information transfer from neuron to neuron or neuron to effector cell
what are the two main synapse classifications
axodendritic: between axon terminals of one neuron and dendrites of others
axosamatic: between axon terminals of one neuron and soma of others
presynaptic neuron
neuron conducting impulses towards the synapse
sender of the information
most cells function as both post & pre synaptic neurons
postsynaptic neuron
neuron transmitting electrical signals away from the synapse
receives the information
most cells function as both post & pre synaptic neurons
electrical synapses
less common than chemical synapses
neurons electrically coupled
communication very rapid
may be unidirectional or bidirectional
synchronize activity
more abundant in nervous tissue
nerve impulse remains electrical
chemical synapse
specialized for release and reception of chemical neurotransmitters
composed of two parts: axon terminal of presynaptic neuron and neurotransmitter receptor region of postsynaptic neuron's membrane
two parts separated by synaptic cleft
electrical impulse changed to chemical across synapse, then back to electrical
axon terminal of presynaptic neuron
contains synaptic vesicles filled with neurotransmitters
neurotransmitter receptor region on postsynaptic neuron's membrane
usually on dendrite or cell body
the synaptic cleft
fluid-filled space
30-50 nm wide
prevents nerve impulses form directly passing from one neuron to the next
transition across the synaptic cleft
chemical event (not electrical)
depends on release, diffusion, and receptor binding of neurotransmitters
ensures unidirectional communication between neurons
information transfer across chemical synapses
AP arrives at axon terminal of presynatic neuron
causes voltage gated Ca2+ channels to open (Ca2+ floods into cell)
synaptotagmin protein binds Ca2+ and promotes fusion of synaptic vesicles with axon membrane
exocytosis of neurotransmitter into synaptic cleft occurs (the higher the impulse frequency, the more neurotransmitters released)
neurotransmitter diffuses across synapse
binds to receptors of postysnaptic neuron (chemically-gated ion channels)
ion channels are opened
causes an excitatory or inhibitory event (graded potential)
neurotransmitter effects terminated
whats the name of the protein that binds Ca2+ and fuses the synaptic vesicles with the axon membrane
synaptotagmin
what are the three ways neurotransmitter effects are terminated
reuptake: by astrocytes or axon terminal
degradation: by enzymes
diffusion: away from synaptic cleft
synaptic delay
time needed for neurotransmitter to be released, diffuses across the synapse, and bind to receptors
is rate-limiting step of neural transmission
postsynaptic potentials
neurotransmitter receptors cause graded potentials that vary in strength with the amount of neurotransmitter released and time neurotransmitter stays in area
moves membrane potential away from threshold for generating an AP
terms for output
output, efferent, motor
terms for input
input, afferent, sensory
EPSPs process
neurotransmitter binding opens chemically gated channels and allows simultaenous flow of Na+ and K+ in opposite directions
Na+ influx greater than K+ efflux --> net polarization called EPSP (not AP)
EPSP help trigger AP if EPSP is of threshold strength (can spread to axon hillock, trigger opening of voltage-gated channels and cause AP to be generated)
IPSPs process
reduces postsynaptic neuron's ability to produce an action potential (by making membrane more permeable to K+ or Cl-; if K+ channel is open, it moves out of cell, if Cl- channel is open, it moves into cell)
neurotransmitter hyperpolarizes cell (inner surface of membrane becomes more negative, AP less likely to be generated)
syanptic integration: summation
a single EPSP cannot induce an AP
EPSPs can summate to influence postsynaptic neuron
IPSPs can also summate
most neurons receive both excitatory and inhibitor inputs from thousands of other neurons
Action potential only happens if EPSPs predominate and bring to threshold
what are the two types of summation
temporal summation: one or more presynaptic neurons transmit impulses in rapid-fire order
spatial summation: postsynaptic neuron stimulated simultaneously by large number of terminals at same time
integration: synaptic potentiation
repeated use of synapse increases ability of presynaptic cell to excite postsynaptic neuron: Ca2+ concentration increases in presynaptic terminala nd postsynaptic neuron
brief high-frequency stimualtion partially depolarizes postsynaptic neuron: chemically gated channels allow Ca2+ entry, Ca2+ activates kinase enzymes that promote more effective responses to subsequent stimuli
integration: presynaptic inhibition
excitatory neurotransmitter release by one neuron inhibited by another neuron via axoaxonic synapse
less neurotransmitter released
smaller EPSPs formed
neurotransmitters
language of nervous system
50 or more have been identified
most neurons make 2 or more neurotransmitters
usually released at different stimulation frequencies
classified by chemical structure and function
acetylcholine (ACh)
first identified; best understood
released at neuromuscular junctions by some ANS neurons, by some CNS neurons
synthesized from acetic and choline by enzyme choline acetyltransferase
degraded by enzyme acetylcholinesterase (AChE)
what are the two categories of biogenic amines
catecholamines: dopamine, norepinephrine (NE) and epinephrine; synthesized from amino acid tyrosine
indolamines: serotonin and histamine; serotonin synthesized from amino acid tryptophan, histamine synthesized from amino acid histidine
biogenic amines (catecholamines and indolamines) traits
broadly distributed in the brain
plays roles in emotional behavior and biological clock
some ANS motor neurons (esp. NE)
imbalances associated with mental illness
GABA
gamma aminobutyric acid (amino acid)
primarily inhibitory 99.9% of time
amino acid of neurotransmitters
peptides of neurotransmitters
substance P: mediator of pain signals
endorphins: beta endorphin, dynorphin and enkaphalins; act as natural opiates and reduce pain perception (e.g. tattoos)
bind with G protein-coupled receptors in the brain
lipid soluable
synthesized on demand
NO involved in learning and formation of new memories; brain damage in stroke patients, smooth muscle relaxation in intestine
H2S acts directly on ion channels to alter function
classification of neurotransmitters based on function
diverse functions
effects: excitatory versus inhibitory
actions: direct versus indirect
neurotransmitters classification: effects
excitatory vs inhibitory
neurotransmitter effects can be excitatory (depolarizing) and/or inhibitory (hyper polarizing)
effect determined by receptor to which it binds e.g. GABA usually inhibitory vs glutamate usually excitatory
neurotransmitters classification: actions
direct vs indirect
direct: neutrotransmitter binds to and opens ion channels, promotes rapid responses by altering membrane potential (ACh and amino acids)
indirect: neurotransmitter acts through intracellular second messengers usually G protein pathways, broader long-lasting effects similar to hormones (epinephrine and norepinephrine)
what are the two types of neurotransmitter receptors
channel-linked: mediate fast synaptic transmission
g protein-linked: oversee slow synaptic responses
channel-linked receptors
ligand-gated ion channels
action is immediate and brief
excitatory receptors are channels for small cations (Na+ influx contributes most to depolarization)
inhibitory receptors allows Cl- influx that causes hyper polarization
g protein-linked receptors
responses are indirect, complex, slow and prolonged
transmembrane protein complexes
cause widespread metabolic changes
examples ACh receptors
neurotransmitters binds to g protein-linked receptor, activates g protein, and activated g protein controls production of second messengers (Ca2+)
second messenger proteins functions
open or close ion channels
activate kinase enzymes
phosphorylate channel proteins
activate genes and induce protein synthesis
basic concepts of neural integration
neurons function in groups
groups contribute to broader neural functions
there are billions of neurons in CNS, so there must be integration so the individual parts fuse to make a smoothly operating whole
neuronal pools
functional groups of neurons:
integrate incoming information received from receptors or other neuronal pools
forward processed information to other destinations
simple neuronal pool:
single presynaptic fiber branches and synapses with several neurons in pool
discharge zone:neurons most closely associated with incoming fiber
facilitated zone:neurons farther away form incoming fiber
circuits (definition and 4 types)
patterns of synaptic connections in neuronal pools
four types:
diverging: away from
converging: together
reverberating: again and again and again
parallel after discharge: parallel
patterns of neural processing: serial processing
input travels along one pathway to a specific destination
system works in all-or- none manner to produce specific anticipated response
e.g. spinal reflexes -5 steps (receptor, sensory neuron, CNS integration center, motor neuron, effector)
patterns of neural processing: parallel processing
input travels along several pathways
different parts of circuitry deal simultaneously with the information
important for higher-level mental functioning
e.g. certain scent may remind one of an odor and associated experiences
neuronal cell death
about 2/3 of neurons die before birth
if they do not make a connection, they die
also due to apoptosis
comparing AP to GP
GP:
location of event: cell body and dendrites
distance traveled: short (cell body to axon hillock)
size: various sizes; decays with distance
stimulus for opening ion channels: chemical or sensory (temp, light)
positive feedback: absent
repolarization: voltage independent, occurs when stimulus is no longer present
summation: stimulus can summate to increase amplitude of graded potential (temporal or spatial)
function: EPSP or IPSP
initial effect: opens chemically gated channels that allow NA+ K+ fluxes or opens chemically gated K+ or Cl- channels
peak membrane potential: depolarizes and hyperpolarizes
AP:
location of event: axon hillock and axon
distance traveled: long, entire length of axon
size: always the same, all or none
stimulus for opening ion channels: voltage (depolarization triggered by GP reaching threshold)
positive feedback: present
repolarization: voltage regulated; occurs when Na+ channels inactivate and K+ channels open
summation: does not occur, all or none phenomenon
function: long distnce signaling; constitutes the nerve impulse
initial effect of stimulus: opens voltage-gated channels; first Na+ channels then K+ channels