- characteristic: they do not divide, (we are born with all the neurons we are going to have)
In the neuron cell body what is in the glandular cytoplasm?
- cytoplasmic inclusions
(glycogen, pigments [melanin], lipids)
- chromatophilic substance
(nissl bodies) (rough endoplasmic reticulum)
they are scattered throughout
many membranous packets
What are neurofibrils?
a network of fine threads that extend into the axon and support it
What are dendrites?
usually highly branched, ______,
provides receptive surfaces for other processes of neurons to communicate
What are dendritic spines?
they are tiny, thorn-like spines on the surfaces of dendrites, which are contact points for other neurons
What is axonal hillock?
it is slight elevation of the cell body
What is an axon?
it is slender and cylindrical in shape, with a uniform diameter and nearly smooth surface
arises from the axonal hillock
it's job is to conduct nerve impulses away from the the cell body
Where are ribosomes found?
they are found only in the cell body
The cytoplasm of the axon contains what?
mitochondria
microtubules
neurofibrils
The axon gives off many branches called what?
collaterals
Where is the axon terminal located and what does it do?
it is located near the end of the axon
they are the many fine extensions of the axon
each having a specialized ending called the synaptic knob
Where is the synaptic knob and what does it do?
it is found at the end of an axon terminal, where it is close to the receptive surface of another cell but is separated by a space (synaptic cleft)
What does the axonal transport do?
it conveys biochemicals produced in the neuron cell body
(such as: vesicles, mitochondria, ion, nutrients, and neurotransmitter)
move from the cell body to the end of the axon
Axons that have myelin sheaths are what?
myelinated axons
Axons that lack myelin sheaths are what?
unmyelinated axons
What is white matter?
are groups of myelinated axons, causing the appearance of white...
found in the PNS
What is gray matter?
is unmyelinated (nerve tissue) structures, cell bodies, dendrites...
in the CNS, many unmyelinated axons and neuron cell bodies
What are Schwann cells?
are neuroglia = nerve tissue, cells
found in the PNS that encase the large axons in lipid-rich sheets, tight coverings, forming as layers of cell membrane, winding around axons (myelin sheaths, neurilemma)
What are Nodes of Ranvier?
narrow gaps in the myelin sheeth, between Schwann cells
True or False: Neurons and Neuroglia descend form the same neural stem cells.
True
(they remain associated their entire existence)
What are 5 ways that neurons structurally differ?
size
shape
the lengths and sized of axons
the lengths and sizes of dendrites
the number of dendrites
What are the 3 major groups that neurons are structurally classified?
1.) Bipolar neurons
2.) Unipolar neurons
3.) Multipolar neurons
What is a bipolar neuron?
a neuron that has only 2 processes from the cell bodies
that could arises from either end
with end being an axon and the other a dendrite
but they are similar in structure
bipolar neurons are found mostly in specialized parts of the eyes, nose, and ears
What are unipolar neurons?
a neurons that has only 1 process extending from its cell body
has a short distance form the cell body
with the process dividing into two branches
yet really functioning as a single axon (peripheral process and central process)
and has ganglia
Describe peripheral process. (unipolar neuron)
associated with dendrites near a peripheral body part
Describe central process. (unipolar neuron)
enters the brain or spinal cord
What is ganglia?
the cell bodies of some unipolar neurons,
which aggregate in specialized masses of nerve tissue
located outside the CNS (of brain and spinal cord)
What are multipolar neurons?
neurons with many processes arising from their cell bodies
yet they only have 1 axon and all the rest are dendrites
* 99% of neurons are multi polar
these neurons are found mostly in the CNS
What are 3 ways that neurons are classified functionally?
- they carry information into the CNS = Sensory neurons
- they carry information completely within the CNS = Interneurons
- they carry information out of the CNS = Motor neurons
Describe sensory neurons.
carry impulse to the CNS form peripheral body parts
their distal ends, act as sensory receptors
most are unipolar, or bipolar
Describe interneurons.
lie within the CNS
forming links between other neurons (transporting impulses from one part of the brain or spinal cord to another)
most are multipolar
Describe motor neurons.
carry impulses away from the CNS
to effectors (of somatic and autonomic nervous system)
most are multipolar
Describe somatic nervous system.
control skeletal muscle contraction
under voluntary (conscious) control
Describe autonomic nervous system.
controls the cardiac and smooth muscle contraction and secretions of glands
under involuntary control
What are the 2 types of neuroglial cells in the PNS?
1.) Schwann cells
2.) Satellite cells
What are Schwan(remember the difference, only one "n") cells in the PNS?
cells that produce myelin that are found in peripheral myelinated neurons
these cells speed up neruotransmission
What are satellite cells?
these cells support clusters of neuron cell bodies called ganglia
What are 4 types of neuroglial cells in the CNS?
1.) Astrocytes
2.) Oligodendrocytes
3.) Microglia
4.) Ependyma
Describe Astrocytes cells.
the most abundant type of neuroglial cells in the CNS
star-shaped cells* between neurons and blood vessels
provide:
structural support
formation of scar tissue
transportation of substances between blood vessels and neurons
communication between each other and neurons
mop up excess ions and neurotransmitters
induce synapse formation
Describe Oligodendrocytes cells.
shaped like astrocytes
but with fewer processes
occurring in rows along axons
in the CNS
provide:
forms myelin sheaths in the brain and spinal cord
produce nerve growth factors
Describe Microglia cells.
small cells
with few cellular processes
found throughout the CNS
provide:
structural support
phagocytosis (immune protection)
Describe Ependyma cells.
cuboidal and columnar cells
in the inner lining of the ventricles of the brain and the central canal of the spinal cord
provide:
form a porous layer through which substances diffuse between the interstitial fluid of the brain and spinal cord and the cerebrospinal fluid
True or False: Neuroglia comprise more than half of the volume of the brain.
True
True or False: Neuroglia outnumber neurons 6 to 1.
False
Neuroglia outnumber neurons 10 to 1
Name 2 disorders that abnormal neuroglia are associated with.
- most brain tumors
(neuroglia dividing too much)
- neurodegenerative disorders
(neuroglia that produce toxins)
True or False: Injury to the cell body usually kills the neuron, yet a damaged peripheral axon may regenerate.
True
Injury to the cell body usually kills the neuron. Why?
because mature neurons do not divide
the destroyed cell is not replaced unless neural stem cells become stimulated to proliferate
What is the average growth of a regenerating axon?
3-4 millimeter per day (slow)
Nerve growth factors, secreted by _______, may help direct the growing axon.
neuroglia
(still may end up in the wrong place) (full function often does not return)
True or False: Axon of neuron of CNS, seperated from the cell body (distor portion of axon) will degenerate more slowly than a seperated axon of PNS.
True
Axon of neuron of CNS, seperated from the cell body (distor portion of axon) will degenerate more slowly. Why?
lack neurilemma
oligodendrocytes (myelin-producing) do not proliferate following injury
Thus = proximal end of damaged axon that begins to grow, has not tube of sheath cells to guide it
True or False: If the peripheral nerve is severed, it is important that the two ends be connected as soon as possiable.
True
(so the regenerating sprouts of the axon can more easily reach the tubes formed by the basement membranes and connective tissue on the distal side of the gap)
What is neuroma and what does it do?
when the regenerating axons reach a gap of 3 millimeters or greater the axons may form a tangeled mass
this tangled mass composed of sensory axons, is painfully sensitive to pressure
Also, sometimes complicating a patient's recovery following limb amputation
Neurons do not divide, so what happens?
new neural tissue arises from neural stem cells --> which give rise to neural progenitor cells --> that can give rise to neurons or neuroglia
What happens at the synapse?
nerve impulses pass from neurons to neurons (or other cells)
What does the presynaptic neuron do?
it brings the impulse to the synapse
resulting = stimulation or inhibitation of a postsynaptic neuron (muscle or gland)
What does a synaptic cleft do?
this gap
seperates two cells, which are connected functionally but not physically
What is synaptic transmission?
the process by which the impulse in the presynaptic neuron signals the postsynaptic cell
Axons usually have several rounded ______ _____ at thier terminals, which dendrites lack.
synaptic knobs
What are the arry of memberanous sacs that synaptic knobs have called?
synaptic vessicles
What do synaptic vessicals contain?
neurotransmitter molecules
When a nerve impulse reaches a synaptic knob, what kind of channels open and what diffuses inward from the extracellular fluid?
voltage-sensitive calcium channels
calcium diffuses
What does an increased calcium concentration inside the cell initiate?
it inititates a series of events that fuses the synaptic vessicles with the cell membrane, where they release their neurotransmitter by exocytosis
Once the neurotransmitter binds to the receptors on a post-synaptic cell, the actions is either ______ or ______.
excitatory (turning a process on)
or
inhibitory (turning the process off)
What is released when the impulse reaches the synaptic knob?
neurotransmitters
The net effect on the post-synaptic cell depends on the combined effect of the _________ and ___________ inputs from as few as _____ to ___________ or more presynaptic neurons.
excitatory
inhibitory
1
100,00
A critical part of the synaptic transmission is what?
the polararization of the membrane
A cell membrane is usually electrically charged or polarized, what does that mean?
it means that the inside of the membrane is negatively charged, and thus the outside is positively charged
What is polarization due to?
it is due to unequal distribution of positive and negative ions on either side of the membrane
[Distribution of Ions]
What are Potassium (K+) ions?
they are the major intracellular positive ions (cations)
[Distribution of Ions]
What are Sodium (Na+) ions?
they are the major extracellular positive ions (cations)
[Distribution of Ions]
The distribution of ions is largely created by what?
the Sodium Potassium Pump (Na+/K+ pump)
[Distribution of Ions]
What does the Sodium Potassium Pump do?
it actively transports sodium ions out of the cell and potassium ions into the cell
[Distribution of Ions]
Channels formed by membrane proteins can be selective, what does that mean?
means a particular channel may allow only one type of ion to pass through and exlude all other ions of different size and charge
[Distribution of Ions]
Membrane permeability to ions is also due in part to ___________ in the cell membrane.
channels
[Distribution of Ions]
Both _______ and ______ electrical factors can affect the opening and closing these gated channels.
chemical
electrical
[Distribution of Ions]
True or False: Some channels are always open, wheras others may be either open or closed (somewhat like a gate).
True
[Distribution of Ions]
The ability of ions to diffuse across cell membranes depends on the presence of what?
the presence of channels
[Resting Potential]
What is it called when a nerve cell is not being stimulated to send a nerve impulse?
a resting nerve cell
[Resting Potential]
Under resting conditions what determines the membrane permeability to sodium and potassium ions?
the nongated (always open) channels do
[Resting Potential]
Sodium and potassium follow what laws?
the laws of diffusion
[Resting Potential]
The resting cell membrane is only slightly permable to sodium and potassium ions, yet which one is the membrane more permeable to?
the membrane is more permeable to the potassium ions than the sodium ions
[Resting Potential]
What are negatively charged ions called?
anions
[Resting Potential]
True or False: A resting nerve cell's cytoplasm has many negatively charged ions (phospate, sulfate, proteins) that are synthesized inside the cell but can easily diffuse through the cell membrane.
False
these ions cannot diffuse through the cell membrane
[Resting Potential]
What is potential difference?
it represents stored electrical energy that can be used to work at some future time
[Resting Potential]
What is membrane potential?
the potential difference across the cell membrane
[Resting Potential]
True or False: With the resting membrane potential established, a few sodium ions and potassium ions continue to diffuse across the cell membrane.
True
[Resting Potential]
The negative membrane potential helps and hinders what?
helps sodium ions to enter the cell despite sodium's low permeability
hinders potassium ions from leaving the cell despiste potassium's higher permeability
[Resting Potential]
Describe RMP.
RMP = -70 mV (inside to outside of cell)
it is a polarized membrane
inside of cell is negative, realative to outside of cell (due to distribution of ions inside vs. outside)
Na+/K+ pump restores
[Local Potential Changes]
What does neurons are excitable mean?
it means they can respond to changes in their surroundings (inside and outside the body)
[Local Potential Changes]
Environmental changes affect the membrane potential by opening a what?
a gated ion channel
[Local Potential Changes]
What is hyperpolarized?
as a result (nerons are excited, causes change, etc) the membrane potential becomes more negative than the resting potential
[Local Potential Changes]
What is depolarized?
the membrane becomes less negative (more positive) than the resting potential
[Local Potential Changes]
Local potential changes are graded, what does that mean?
means the degree of change in the restin potential is directly proportional to the intensity of the stimulation
[Local Potential Changes]
What is threshold potential?
approximately -55 millivolts in a neuron
when neurons are sufficiently depolarized, the membrane potential reaches a level that is such called
[Local Potential Changes]
What is action potential?
this is the results of the threshold potential being reached
(the basis for the nerve impulse)
[Local Potential Changes]
What is another way threshold potential may be reached and action potential results?
if the presynaptic neurons release more neurotransmitter
or
if other neurons that synapse with the same cell join in the effort to depolarize
[Local Potential Changes]
What are the 3 ion channels?
1.) chemically gated
2.) voltage gated
3.) mechanically gated
[Action Potentials]
In a multipolar neuron, the first part of the axon (intial segment), is often referred to as ____________ and why?
as the trigger zone
because it contains many voltage-gated sodium channels
[Action Potentials]
True or False: Voltage-gated sodium channels remain closed at the resting membrane potential.
True
[Action Potentials]
What happens to voltage-gated sodium channels when threshold is reached?
they open for an instant
briefly increasing sodium permeability
[Action Potentials]
At the peak of the action potential, the membrane potential may reach ________.
+30 mV
[Action Potentials]
What is happening when voltage-gated sodium channels are quickly closing?
the slower voltage gated potassium channels open and briefly increase postassium permeability
[Action Potentials]
What does the active transport mechanism in the membrane do?
it works to maintain the original concentrations of sodium and potassium ions
[Action Potentials]
Which one is capable of action potential and which ones are not? Axons Cell body Dendrites
Axons are capable
Cell body and dendrites are not
[Action Potentials]
When an action potential is at the trigger zone what does it cause?
it causes an eletrical current to flow further down the axon
[Action Potentials]
What is the propagation of actions potentials along an axon?
nerve impulse
[All-or-None Response]
True or False: Nerve impulse conduction is an all-or=none response.
True
(if a neuron responds at all, it responds completely)
What is the refractory period?
it is a short time following an a nerve impulse, where a threshold stimulus will not trigger another impulse on axon
What are the two periods of the refractory period?
- absolute refractory period
- relative refractory period
What does the refractory period limit in a neuron?
it limits how many action potentials may be generated in a neuron in a given period
What is the absolute refractory period?
time when threshold stimulus doesn not start another action potential
What is the relative refractory period?
time when stronger threshold stimulus can start another action potential
[Impulse Conduction]
An unmyelinated axon conducts an impulse over its ________ surface.
entire
[Impulse Conduction]
Concerning myelinated axons, myelin serves as an ___________ __________ and ___________ almost all flow of ions through the membrane that it encloses.
electrical insulator
prevents
[Impulse Conduction]
Why doesn't a myelin sheath not prevent the conduction of a nerve impulse?
because the sheaths are not continuous
(nodes of ranvier between schwann cells, or oligodendrocytes interrupt the sheath)
(at the nodes, axon membrane has channels for sodium and potassium ion that open during threshold depolarization)
[Impulse Conduction]
When a myelinated axon is stimulated, where does the action potential occur?
at the trigger zone
[Impulse Conduction]
In a nerve impulse traveling along a myelinated axon, action potentials occur only where?
action potentials occur only at the nodes
[Impulse Conduction]
What is saltatory conduction?
it is when the action potentials appear to jump from node to node
[Impulse Conduction]
Is conduction on myelinated axons faster than on unmyelinated axons?
yes, many times faster
[Impulse Conduction]
Does the diameter of the axon affect the speed of the nerve impulse conduction?
Yes,
the greater the diameter, the faster the impulse conduction
[Synaptic Transmission]
Released neurotransmitters molecules diffuse across the _____ _____ and react with specific molecules called ______ in the _______ neuron membrane.
synaptic cleft
receptors
postsynaptic
[Synaptic Transmission]
What channels respond to neurotransmitter molecules?
chemically gated channels
[Synaptic Transmission]
What are the local potential called that are created when there is changes in chemically-gated channels?
synaptic potentials
(enable one neuron to affect another neuron)
[Synaptic Potentials]
What can synaptic potentials do to the receiving cell membrane?
synaptic potentials can depolarize or hyperpolarize the receiving cell membrane
[Synaptic Potentials]
What is EPSP?
excitatory postsynaptic potential
- neurotransmitter binds to a postsynaptic receptor
- opens sodium ions channels, that diffuse inward
- depolarizing the membrane
- possiably, triggering an action potential, more likely
- (lasts about 15 milliseconds)
[Synaptic Potentials]
What is IPSP?
inhibitory postsynaptic potential
- different neurotransmitter binds other receptors
- increase in membrane permeability to potassium ions, that diffuse outward
- hyperpolarizing the membrane
- action potential is less likely
[Synaptic Potentials]
True or False: The integrated sum of EPSPs and IPSPs determines whether an action potential results.
True
[Synaptic Potentials]
What is the process called when EPSPs and IPSPs are added together?
it is called the summation
[Synaptic Potentials]
The integrated sum of EPSPs and IPSPs determines whether an action potential results. How?
More EPSP's lead to greater probibility of an action potential
