Exam 2 Worksheet Answers Flashcards
what are the two primary functional divisions of the nervous system
CNS and PNS
how do the CNS and PNS differ
The two systems differ both anatomically and functionally. Looking at anatomy first, the CNS is composed of the brain and the spinal cord while the PNS is composed of the cranial nerves and the spinal nerves. The PNS can be further subdivided into the sensory (afferent) and motor (efferent) divisions. Both divisions are still composed of cranial and spinal nerves, but the nerves in each division are carrying different information (sensory or motor). Looking at function, the CNS is responsible for integration of information and contains regulatory control centers while the PNS is responsible for communication between the CNS and the rest of the body. Information from sensory nerves of the PNS will travel to the CNS where that information is integrated and a decision is made to perform an action. That action is sent to the appropriate muscles or glands on efferent nerves in the PNS.
neuron characteristics of excitability
Excitability is the property of the neuronal membrane that allows it to generate local and transient or large and long lasting changes in membrane potential in response to a chemical signal. The chemical signal is often in the form of a neurotransmitter binding to a chemically gated ion channel. Opening these channels allow ions to flow across the membrane and change membrane potential. Voltage-gated ion channels also give the neuron excitability, they also allow the cell to change its permeability to different ions and thus alter membrane potential
neuron characteristic of conductivity
Conductivity is the property of the neuron that allows current to move (propagate) along the membrane. This property allows graded potentials to travel from the dendrites to the axon hillock or the action potential to travel along the axon.
neuron characteristic of secretion
Secretion is the property of neurons that allows them to release neurotransmitters under controlled conditions at the axon terminals. This response is controlled by the electrical activity of the neuron. If an action potential invades the axon terminal then vesicle fusion can occur. However, in the absence of an action potential, no vesicle fusion events will occur.
function of dendrites
Dendrites are short processes that branch off the cell body creating a ‘dendritic tree’. A neuron may have one dendrite or many, depending on its role in the nervous system. Axons from other neurons will form synapses with the dendrites, thus the dendrites are the major receptive region for input into the neuron. Increasing the number of dendrites will increase the amount of information a given neuron will receive.
function of axon
The axon is a singular process emanating from the cell body and is the main structure the neuron will use to communicate (i.e. send action potential) with other cells. Although there is only one axon per neuron, the axon can branch extensively, creating axon collaterals that will increase the number of cells that neuron can form synapses with. The initial part of the axon is called the axon hillock; this region is continuous with the cell body and is the site of action potential generation in a typical multipolar neuron. The terminal end of the axon is the axon terminal (also called synaptic terminal, synaptic knob, synaptic bulb) and will form the presynaptic side of a synapse with another neuron’s dendrite, a muscle cell, or a gland cell.
function of synaptic vesicles
Synaptic vesicles are small lipid bilayer membrane sacs that cluster in the axon terminals. These vesicles contain neurotransmitters. Following a depolarization of the axon terminal by an action potential, synaptic vesicles will fuse with the membrane of the axon terminal and release there packaged neurotransmitters into the synaptic cleft.
function of neurofilaments
Neurofilaments are a type of intermediate filament that is specific for neurons. Much like other intermediate filaments, neurofilaments are critical in providing tensile strength throughout the neuron, including the dendrites and axon. These proteins allow the cell to maintain its structure, and without these filaments the cell would not be able to function properly
what are the two types axonal transport
fast and slow axonal transport
what proteins are involved fast axonal transport
Fast axonal transport occurs in both an anterograde (cell body toward axon terminal) and retrograde (axon terminal to cell body) fashion. To keep cargo moving in the right direction specialized motor proteins (called kinesins and dyneins) attach to the cargo being moved and ‘walk’ down the microtubule scaffold that is present in the axon. Anterograde fast axonal transport will move substances including mitochondria, cytoskeletal elements, membrane components, and enzymes.
These substances are important for the neuron to maintain normal function. Retrograde fast axonal transport generally moves used up organelles that are going to the cell body to be recycled. However, sometimes signaling molecules can also move to the cell body via this mechanism and may signal for new protein synthesis to happen in the nucleus.
what proteins are involved in slow axonal transport
Slow axonal transport only moves substances in an anterograde (from the cell body to the axon terminal) fashion. This process, moves similar kinds of substances including enzymes and cytoskeletal elements, but also moves axoplasm (the cytoplasm within the axon)
what are the three structural classes of neurons
pseudounipolar, multipolar, and bipolar
multipolar neuron
Multipolar neurons are the most common in the nervous system and the major neuron type of the CNS
bipolar neuron
Bipolar neurons have two extensions from the cell body, one is a fused dendrite and the other is the axon. Bipolar neurons are rare and seen only in sensory organs
pseudounipolar neuron
Pseudounipolar neurons have one process leaving the cell body forming the axon; however, at one end of this process, receptive endings are present and act like dendrites taking in sensory information. Pseudounipolar neurons are found mainly in the PNS, where their cell bodies can be found in ganglia located adjacent to the spinal cord
What are the three connective tissue wrappings in a nerve?
The connective tissue wrappings in a nerve are very similar to those that you would find in the muscle. The epineurium surrounds the outside of the nerve creating a protective sheath. Inside the nerve, fascicles are again present. The fascicles are surrounded by a perineurium. Inside each fascicle are many axons of individual neurons. Filling in the space around each axon is the endoneurium. So the 3 connective tissue wrappings in order from superficial to deep would be the epineurium, perineurium, and endoneurium
If a person has a brain tumor, is it more likely to have developed from neurons or from glial cells? Why
A brain tumor is most likely to develop from glial cells. The major reason for this is that neurons are generally amitotic and, outside of a few special regions, are not capable of cell division. Glial cells, however, are constantly dividing like many other cells outside the brain. Cancers (including brain tumors) result directly from hyperactivity of cell division. As a result glial cells are the only cell in the nervous system that are capable of acquiring mutations of cell division that would allow them to divide at a higher rate and create pathologies like brain tumors. In early childhood, while the brain is completing development, brain tumors may originate from dividing neurons
Which specific type of glial cells ensheaths (wraps around) axons in the CNS
In the CNS, oligodendrocytes are responsible for myelination of axons. Each oligodendrocyte sends out many branches from its cell body. Each branch will wrap around a small section of an axon and create a myelin sheath (insulating cover) for that region of the axon. One oligodendrocyte can help myelination many different axons, but is not responsible for completely myelinating the axon of one individual neuron.
Which specific type of glial cells ensheaths (wraps around) axons in the PNS
In the PNS, Schwann cells are responsible for myelination of axons. Each individual Schwann cell will myelinate a small region of a neuron. The entire cell is involved in wrapping each section of axon, with the organelles and cytoplasm pushed to the periphery to allow for the tight membrane wrappings. Many Schwann cells are needed to myelinate a single axon, and each Schwann cell is participating in myelinating only one axon
What is the function of the myelin sheath?
The myelin sheath serves two important functions. The first is to provide protection for the axon. The second is to provide insulation; this property is directly related to the speed of conduction of action potentials down the axon. The insulation of the myelin provides an increased resistance at the membrane and inhibits electrical current from escaping. This property allows generated potentials to travel quickly under regions of myelination without much decay in the potential amplitude.
How does myelination of axons occur in the PNS?
In the PNS myelination occurs when a Schwann cell begins to envelop an axon. The Schwann cell will then begin to rotate around the axon wrapping its plasma membrane around the axon in concentric layers. As the number of layers increases the wrappings get tighter and the cytoplasm and organelles of the Schwann cell are forced out to the periphery. This creates a many layer thick sheet of plasma membrane tightly wrapped around the axon. This arrangement provides the protection and insulation described above
How does the process of nerve regeneration occur in the PNS
Nerve regeneration can be observed in the PNS under some conditions. Large-scale damage from injuries where many axons are severed or axons are severed close to their cell bodies in the spinal cord will inhibit regeneration. However, smaller injuries such as severing a single axon can be recovered from. The process of regeneration begins with breaking down the axon distal to the site of injury and removing any debris via phagocytosis. When the path is clear of debris, the axon will form a growth process or growth cone that will begin to look for chemical signals telling it which way to grow. The target tissue and Schwann cells will release chemical signals trying to attract the axon. As the axon grows, Schwann cells will form a regeneration tube to help guide the axon to the target
The portion of the nervous system that conducts impulses from the skin, joints, skeletal muscles, and special senses is the _________ division
somatic sensory
Which statement best describes the distinguishing features of neurons?
a) A person is born with all of the neurons they will ever have
b) Most neurons formed in fetal development last a lifetime, but some brain regions in adults can generate new neurons
c) Neurons are constantly dying and being replaced throughout all regions of the brain
d) Stem cells in the brain become glia, which can later become neurons if there is a need for them to do so.
b) Most neurons formed in fetal development last a lifetime, but some brain regions in adults can generate new neurons
Based on structure, the most common type of neuron is the ______ neuron
multipolar
The glial cell that myelinates and insulated axons within the CNS is the
oligodendrocyte
define ion channels
An ion channel is any integral membrane protein that allows ions to pass passively between the extracellular and intracellular space. These proteins do not require energy and they do not control the direction of ion flow. The leak channels that establish resting membrane potential are considered ion channels.
define receptor
A receptor is an integral membrane protein that produces a physiological change in a cell following a stimulus. There are two classes of receptors – ionotropic receptors and metabotropic receptors.
inotropic receptors
Ionotropic receptors are ion channels that require a stimulus to open. The stimulus comes in the form of the binding of a neurotransmitter to a receptive region on the integral protein.
metabotropic receptors
Metabotropic receptors, or G-protein coupled receptors, are also integral membrane proteins that are not directly coupled to ion channels. These receptors are coupled to G-proteins that can interact with other intracellular proteins. Once activated the G-protein initiates an intracellular cascade that results in the activation of a second messenger. Different G-protein coupled receptors will couple to different G-proteins that can go on to activate different second messengers. The key benefit of utilizing a metabotropic receptor is that the signal becomes amplified within the cell. This amplification of signal means that a single neurotransmitter binding event can result in large changes in cell physiology that can have long term effects
major difference between ion channels and receptors
One of the major differences between ion channels and receptors is how they are activated. Receptors must have a neurotransmitter binding event in order to become activated. In the case of ionotropic receptors, once the neurotransmitter binds the protein behaves just like an ion channel. Because of this you can say that an ion channel can be a type of receptor, but not all receptors are ion channels (i.e. metabotropic receptors)
3 mechanisms ion channels use to open and close
conformational change, structural change, and blocking gate
conformational change
where a single region of the pore changes shape allowing for ion flow
structural change
where the entire pore region changes shape
blocking gate
where a specialized protein responds to cues and closes the pore region from the intracellular side
the gating mechanisms used to allow channels to respond to different stimuli
The decision to open or close an ion channel is made by the gating mechanism specific to the type of channel. There are four different types of gating for ion channels. Ligand gating is observed when the ion channel contains a receptive region for a specific chemical (ligand), like a neurotransmitter. Once a ligand binds, the ion channel undergoes a conformational change allowing the pore region to pass ions. This is a common gating mechanism we will see throughout the nervous system. Another common gating mechanism is voltage gating. Ion channels that are voltage gated contain a series of charged amino acids that respond to voltage changes across the plasma membrane. When the membrane potential changes, voltage gated ion channels undergo a conformational change and open their pore region. Ion channels can also be gated by phosphorylation. This mechanism is common for G-protein coupled receptors. An activated second messenger can phosphorylate the ion channel triggering a conformational change and opening of the pore region. A final mechanism that is common in sensory receptors is the stretch or pressure gate. In this mechanism physical deformation of the plasma membrane causes a structural change in the ion channel which will allow the ion channel to open
What are the 4 “rules” to be considered a neurotransmitter?
There are many molecules that are active in the nervous system. In order to correctly classify them neuroscientists have established a series of ‘rules’ for a molecule to be considered a neurotransmitter. First, it must be synthesized and released from a neuron. Second, the substance should be released from nerve terminals in a chemically or pharmacologically identifiable form. Third, the substance should reproduce at the postsynaptic cell the same events that are seen following presynaptic electrical stimulation. In other words, the molecule that is released from the neuron must behave the same following normal physiological release and when it is introduced into the synapse experimentally. Finally, there must be an appropriate mechanism for termination of action.
Note that neurotransmitters can have very different chemical structures. Some are simple amino acids, others are chains of amino acids (“peptides”), some are derivatives of amino acids (indolamines, catecholamines), and even a gas can be classified as a neurotransmitter (nitric oxide)
Which function of the plasma membrane is directly affected by the opening of ion channels?
selective permeability
How would you classify an integral membrane protein that binds a ligand and results in the immediate opening of an ion channel?
inotropic receptor
current
Current (I) is the flow of electrical charge from one point to another point and is measured in amperes (A).
resistance
Resistance (R) is the hindrance to change flow by substances through which current must pass and is measured in ohms (Ω). In a cell, resistance most often takes the form of closed ion channels. These integral membrane proteins are directly responsible for resisting the flow of ions across the cell membrane
voltage
Voltage (V) is the potential energy generated by separated electrical charges and is measured in volts (V). A potential difference for a cell can be referred to as the membrane potential and represents the difference in charge on the intercellular and extracellular sides of the membrane
how are current, resistance, and voltage related?
These properties are related to each other by Ohm’s Law, which states that current is equal to voltage divided by resistance (I =V/R). In this relationship current is directly proportional to voltage, but inversely proportional to resistance
What are the major functions of the plasma membrane, and which of these functions are important for establishing excitable membranes?
There are 5 major functions of the plasma membrane: 1) to create a mechanical barrier, 2) selective permeability, 3) establish and electrochemical gradient, 4) facilitate cell-to-cell communication, 5) facilitate intercellular cell signaling.
Three of these functions are critical for establishing excitable membranes. The first is the mechanical barrier, which maintains the physical separation between intracellular and extracellular environments. The second is selective permeability, which allows ions to move between intracellular and extracellular spaces only when integral membrane proteins open selective channels. The third function is the electrochemical gradient, which dictates the direction ions will move when ion channels open.
What is a membrane potential?
The membrane potential is the electrical disequilibrium (imbalance) that exists between the extracellular fluid and intracellular fluid of a living cell. The membrane potential exists because of the uneven distribution of electrical charges between these two spaces, separated by the plasma membrane
what is the resting membrane potential
The resting membrane potential is simply the membrane potential for a given cell at rest. The specific value of the resting membrane potential can vary, ranging from -50 to -100 mV.
How can you have a membrane potential if the human body is electrically neutral?
The human body is eclectically neutral because the intracellular space has equal numbers of positive and negative charges, and the extracellular space has equal numbers of positive and negative charges. Thus, each space is neutral. However, the intracellular and extracellular spaces are not electrically neutral with each other; the mechanical barrier created by the plasma membrane creates an imbalance in charge across the plasma membrane is what creates the membrane potential
What is an electrical gradient?
An electrical gradient is the unequal distribution of charge across a given area that facilitates the movement of a changed particle toward an area of opposite electrical charge.
What is a chemical gradient?
A chemical (or concentration) gradient is the unequal distribution of chemicals across a given area that facilitates the movement of a chemical from and area of higher concentration to and area of lower concentration.
What is the electrochemical gradient?
The electrochemical gradient is the combination of the electrical and chemical gradients for a specific ion. Ions will have both a chemical and electrical property that will dictate its electrochemical gradient. The two gradients that make up the electrochemical gradient will establish how an individual ion will move when selective ion channels open in the membrane.
What is the equilibrium potential?
The equilibrium potential is defined as the membrane potential that exactly opposes a given concentration gradient for a given ion. In other words, the equilibrium potential exists where the forces driving the electrical gradients of an ion are directly opposed to the forces driving the chemical gradient of the same ion. We can use the Nernst equation to calculate the equilibrium potential of a given ion assuming we know the extracellular and intracellular concentrations of that ion.
How does the equilibrium potential of individual ions contribute to the resting membrane potential of a cell?
At rest, the cell will be permeable to both potassium and sodium due to the presence of leak channels. Given that we have to consider the movement of 2 different ions, the Nernst equation will not be helpful in calculating resting membrane potential. To calculate the resting membrane potential (RMP), we must use the Goldman-Hodgkin-Katz (GHK) equation which considers the concentration and permeability of each ion. An easy way to think about the calculation of resting membrane potential is to first think about the equilibrium potential of each ion. The equilibrium potential of potassium is -90mV and the equilibrium potential of sodium is +30mV. So, if at rest the membrane is permeable to both of these ions, then the RMP must be some value between -90mV and +30mV. Given that there is a much larger number of potassium leak channels, there is also a much greater permeability of the cell to potassium. Since potassium has a greater permeability, the equilibrium potential of potassium will have a greater influence over the RMP. We know by using the GHK equation that the RMP for most neurons is around -70mV. This value reflects the greater permeability of potassium relative to sodium. If the permeability of the two ions changed, then the RMP would also change.
What is the primary function of Na+/K+ pumps and leak channels in maintaining the resting membrane potential?
The resting membrane potential is a function of the activity of the Na+/K+ ATPase and the flow of ions (in particular potassium) through leak channels. The passive diffusion of ions through leak channels causes the ionic imbalances, and the active transport of ions by the Na+/K+ ATPase maintains the ionic imbalance.
The most important leakage channels are those that are selective for potassium. The movement of potassium in and out of the cell through these channels directly establishes the resting membrane potential. Potassium is drawn out of the cell due to its chemical gradient, but the net effect of this movement is a slight membrane hyperpolarization which attracts the positively charged potassium ions back into the cell.
The Na+/K+ ATPase is an active transport pump that extrudes 3 sodium ions and brings in 2 potassium ions with every cycle. The movement of these ions by the pump maintains their unequal distribution across the plasma membrane and thus maintains the electrochemical gradient of each ion
What tissue types have excitable membranes and why do these cell types display these properties?
Muscle and nervous tissue have excitable membranes. The development of excitable membranes in these tissue types allows them to rapidly respond to changes in the environment and rapidly communicate. Our ability to take in sensory information from the environment, integrate that information, and produce a rapid and appropriate response is only possible due to the development of excitable membranes in muscle and nervous tissue.
According to Ohm’s Law, current is
Directly related to voltage and inversely related to resistance
If there were no sodium leak channels, the resting membrane potential of a neuron would be
more negative
A depolarization is when the inside of a neuron becomes__________relative to the resting membrane potential.
less negative
Under normal physiological conditions the Na+/K+ ATPase transports __________.
Na+ out of and K+ into the cell
What property of ion channels allows them to limit the type of ion that can pass through the plasma membrane when open?
Selective permeability
What is the difference between a chemically gated channel and a voltage-gated channel? How are they gated? Where are they located?
The major difference between a chemically gated ion channel and a voltage gated ion channel is how they are gated or opened. Chemically gated channels are opened following the binding of a chemical (most commonly a neurotransmitter). This binding causes a conformational change in the receptor opening the channel through which ions can pass. These types of ion channels are located in the dendrites and the neuronal cell body where axon terminals from other neurons are present. Voltage gated channels are opened following a change in membrane potential. The change in membrane potential causes the protein to undergo a conformational change that opens the channel allowing ions to pass. These types of channels are located in the axon hillock, throughout the axon, and in the axon terminals.
Which functional segments of a neuron contain chemically gated channels? Which functional segments contain voltage-gated channels?
The receptive segments o f a n e u r o n (dendrites and cell body) contain chemically gated channels. The axon hillock (trigger zone), conductive region (axon) and terminal, contain voltage-gated channels. The selectivity of voltage-gated channels will change in the different segments: voltage-gated sodium and potassium channels are found along the axon and terminal, while voltage gated calcium channels are located only in the terminal.
