A&P - Chapter 12
Threshold membrane potential
(-55 mV) minimum voltage change required to generate an action potential; the sensitivity of voltage-gated channels to open in response to a minimum voltage change in membrane potential is the determining factor if an action potential is initiated.
Generation of an IPSP
(1) Neurotransmitter is released: The neurotransmitter, following its release, crosses the synaptic cleft and binds either to a postsynaptic neuron chemically gated K+ channel or a chemically gated Cl- channel, depending upon the neurotransmitter and channels present. (2) Chemically gated K+ or Cl- channels open: If the neurotransmitter binds to a receptor that is a chemically gated K+ channel, this channel opens and K+ moves out of the neuron down its concentration gradient, causing a loss of positively charged ions. In contrast, if the neurotransmitter binds to a receptor that is a chemically gated Cl- channel, opening of this channel allows Cl- to flow down its concentration gradient to move into the neuron, causing the gain of negatively charged ions. The amount of neurotransmitter determines the number of either chemically gated K+ or chemically gated Cl- channels that open. (3) IPSP is established: The outflow of positively charged K+ ions or the inflow of negatively charged Cl- ions that causes the inside of the cell to become slightly more negative. This temporary, more negative state is called an inhibitory postsynaptic potential (IPSP) (4) IPSP moves toward the initial segment: The local current of ions becomes weaker as it moves along the neuron plasma membrane toward the initial segment and decreases in intensity with the distance traveled form the site of neurotransmitter binding.
Generation of an EPSP
(1) Neurotransmitter is released: The neurotransmitter, following its release, crosses the synaptic cleft and binds specifically to a postsynaptic neuron receptor that is a chemically gated cation channel. (2) Chemically gated cation channels open: The opening of these channels allows both Na+ and K+ to move down their respective concentration gradients. However, more Na+ moves into the neuron than K+ moves out. The amount of neurotransmitters determines the number of chemically gated ion channels that open. (3) EPSP is established: The inflow of positively charged Na+ ions causes the inside of the neuron to become slightly more positive (less negative). This temporary, less negative state is called an excitatory postsynaptic potential (EPSP) (4) EPSP moves toward the initial segment: The local current of Na+ becomes weaker as it moves along the neuron plasma membrane toward the initial segment and decreases in intensity with the distance traveled from the site of neurotransmitter binding.
Depolarization and its propagation
(1) Reaching the threshold: Initially, the voltage-gated Na+ channels are closed and the membrane potential is -70 mV. Sodium ions flow within the cytosol into the region from adjacent areas. The membrane potential becomes more positive moving away from -70 mV. Voltage-gated Na+ channels (specifically, the activation gates) are triggered to open when sufficient Na+ flows into the region to change the membrane potential from -70 mV to -55 mV (the threshold value). (2) Depolarization: Voltage-gated Na+ channels remain open to allow rapid Na+ entry into the axon to cause depolarization. Sufficient Na+ enters the axon to reverse the membrane potential from negative (-55 mV) to positive (+30 mV). (Note that the exact value can vary from 0 mV to +50 mV.) This movement of Na+ is extremely small, representing a change of approximately 0.01% in the Na+ concentration, but is sufficient to cause depolarization at the plasma membrane.
Events of an Action Potential
(1) Threshold is reached when graded potentials reach the initial segment and are added together and the membrane potential changes. (2) Depolarization occurs when the threshold (-55 mV) is reached; voltage-gated Na+ channels open and Na+ enters rapidly, reversing the polarity from negative to positive (-55 mV -> +30 mV). Temporary inactivation state of voltage-gated Na+ channels occurs. (3) Repolarization occurs due to opening of voltage-gated K+ channels. K+ moves out of the cell and polarity is reversed from positive to negative (+30mV -> -70 mV). (4) Hyperpolarization occurs when voltage-gated K+ channels stay open longer then the time needed to reach the resting membrane potential; during this time the membrane potential is less than the resting membrane potential (-70 mV -> -80 mV). (5) RMP is reestablished after voltage-gated K+ channels are closed and the RMP has been reestablished at the plasma membrane by activity of leak channels and Na+/K+ pumps (-80 mV -> -70 mV).
Repolarization and Its Propagation
(3) Repolarization: Reaching the threshold (-55 mV) also triggers voltage-gated K+ channels to open. These channels are relatively slow to open and are not completely open until about the point that depolarization has ended. Voltage-gated K+ channels remain open to allow rapid K+ exit from the axon to cause repolarization. Sufficient K+ exits the axon to change the membrane potential from positive (+30 mV) to its negative RMP (-70 mV). (4) Hyperpolarization: The voltage-gated K+ channels typically remain open longer than the time needed to reestablish the resting membrane potential (-70 mV). The inside of the neuron during this brief time is more negative than the RMP, or hyperpolarized (decreasing to approximately -80 mV). (5) Reestablishing the RMP: Voltage-gated K+ channels close and the RMP is then reestablished at -70 mV by leak channels and Na+/K+ pumps.
Structure of a nerve and ganglion
(a) A nerve consists of multiple fascicles, which are composed of bundles of axons. The entire nerve is unsheathed in a tough outer connective tissue called the epineurium. Each fascicle is wrapped in a tough connective tissue called the perineurium. Within fascicles, each axon is surrounded by a delicate connective tissue layer termed the endoneurium. (b) A ganglion is a collection of cell bodies along the length of a nerve.
Structures in a typical neuron
(a) Terminal extensions of an axon, illustrating the synaptic knobs of a presynaptic neuron, and (b) a postsynaptic neuron, including dendrites, cell body, and axon. the flow of electrical signals is from dendrites to the cell body of the axon until it reaches the synaptic knob, which releases neurotransmitters. (c) A synapse between the two neurons is shown
Elimination of neurotransmitter
1. Degradation by the enzyme (e.g., ACh esterase) 2. Reuptake by the transport (release and recycle)
Ganglia (ganglion)
A cluster of neuron cell bodies within the peripheral nervous system (PNS). The cluster of cell bodies results in a swelling, or enlarged portion, along the length of a nerve, which is often large enough to be observes with the naked eye. Specific types of ganglia include the posterior (dorsal) root ganglia associated with sensory neurons that extend into the spinal cord and the ganglia associated with motor neurons that extend to autonomic effectors
Endoneurium
A delicate layer of areolar connective tissue that surrounds each axon. These more delicate coverings function to separate and electrically insulate each axon.
Perineurium
A layer of dense irregular connective tissue that wraps each fascicle. These tough, fibrous connective tissue sleeves also provide protection and support, but to each bundle of axons. This layer supports blood vessels.
Choroid plexus
A network formed by both ependymal cells and nearby blood capillaries. The choroid plexus helps produce cerebrospinal fluid (CSF), a clear liquid that bathes the external surfaces of the CNS and fills its internal cavities. The cilia of ependymal cells help circulate the CSF.
What it takes to reach the threshol
A single EPSP is incapable of causing the postsynaptic neuron to reach threshold. Additionally, IPSPs negate the effect of EPSPs. The occurrence of both EPSPs and IPSPs together result in a "tug-of-war" as to whether the threshold is reached. Thus, numerous EPSPs must be generated in the receptive segment and arrive at the initial segment simultaneously, or nearly at the same time, if the threshold is to be reached.
Epineurium
A thick layer of dense irregular connective tissue that encloses the nerve. This fibrous tissue unsheathes the entire nerve to protect and support it like a tough leather sleeve.
Summation
Accumulation of stimuli occurring in neurons or muscle cells; e.g., wave summation
Example of neurotransmitters
Acetylcholine (ACh): excites/stimulates skeletal muscle, but inhibits cardiac muscle. Norepinephrine (NE) (stress): excites/stimulates cardiac muscle, but inhibits the digestive tract.
Transmissive segment
Action potential causes release of neurotransmitter
General functions of the nervous system: Processing and evaluating information
After processing sensory information, the brain and spinal cord determine what response, if any, is required.
Neurolemmocytes
Also called Schwann cells; elongated and flattened cells that wrap around and insulate axons within the PNS to form a myelin sheath through myelination; this allows for faster propagation of action potentials along the axon.
Sensory neuron
Also known as afferent neurons; responsible for conducting sensory input from both somatic sensory (e.g., touch receptors) and visceral sensory receptors (e.g., stretch receptors within the urinary bladder) toward the CNS. Most sensory neurons are unipolar, but a few somatic sensory neurons are bipolar (retina of the eye and olfactory epithelium of the nose).
Interneuron
Also known as association neurons; lie entirely within the CNS. They receive stimulation from many other neurons and carry out the integrative function of the nervous system- that is, they receive, process, and store information and "decide" how the body responds to stimuli. Facilitate communication between sensory and motor neurons; outnumber all other neurons (99%); either multipolar or anaxonic.
Motor neuron
Also known as efferent neurons; conduct motor output away from the CNS to both somatic effectors (i.e., skeletal muscle) and autonomic effectors (i.e., cardiac muscle, smooth muscle, and glands. All motor neurons are multipolar.
Three states of Voltage-gated Na+ channels: Inactivation state
Although the activation gate is open, the inactivation gate is temporarily closed (for several milliseconds) following activation of the Na+ channel- during this time, it cannot be stimulated to reopen, and entry of Na+ is prevented.
Three states of Voltage-gated Na+ channels: Resting-state
Although the inactivation gate is open, the activation gate is closed, and entry of Na+ is prevented. The resting state of voltage-gated Na+ channels is reestablished when the inactivation gate opens and the activation gate closes. Note that repolarization triggers the voltage-gated Na+ channels to make this change (i.e., inactivation state to resting state).
Generation of an Action Potential: Depolarization and its Activation
An action potential involves both depolarization and repolarization. Depolarization occurs as Na+ moves into the axon through open voltage-gated Na+ channels as Na+ moves into the axon through open voltage-gated Na+ channels to change the membrane potential from -70 mV to +30 mV.
Generation of an Action Potential: Repolarization and its Propagation
An action potential involves both depolarization and repolarization. Repolarization occurs as K+ moves out of the axon through open voltage-gated K+ channels to reverse the polarity from +30 mV to −70 mV.
Nerve
An organ composed of a cablelike bundle of axons, connective tissue layers, and blood vessels, and it is a component of the peripheral nervous system (PNS).
Subthreshold value
Any change in voltage below the threshold value is not sufficient to open voltage-gated channels.
All-or-non law
Applies to action potentials propagated along the plasma membrane of neurons. If the threshold is reached, an action potential is initiated and propagated along the axon without decreasing in intensity (all). If only a subthreshold value is reached, an action potential is not initiated (none).
Subpopulation functions: Alter synaptic activity
Astrocytes add and eliminate synapses, as well as influence neuron communication at existing synapses to influence synaptic activity.
Subpopulation functions: Assist neuronal development
Astrocytes help direct the development of neurons in the fetal brain by secreting chemicals that regulate the formation of connections between neurons.
Subpopulation functions: Regulate interstitial fluid composition
Astrocytes help maintain an optimal chemical composition of the interstitial fluid (fluid around cells) within the brain. For example, astrocytes regulate potassium ion concentration by absorbing these ions to maintain constant potassium ion concentration that is critical to electrical activity of neurons.
Neuron transport
Axons move material to and from the cell body
Receptive Segment
Binding of a neurotransmitter released from presynaptic neurons; production of graded potentials
Three states of Voltage-gated Na+ channels: Activation state
Both the inactivation gate and the activation gate are open (activation gate opens in response to a voltage change); Na+ moves into the cell through the open channel.
Fascicle
Bundle of fibers or axons
Receptors
Cellular protein that binds a ligand (e.g., hormone, neurotransmitter); also structure that detects a stimulus.
Repolarization
Change in membrane potential from a depolarized value back to the resting value; the return of polarity from positive back to negative (the RMP). Repolarization is due to the opening of voltage-gated K+ channels and the subsequent movement of K+ out of the cell.
Depolarization
Change in membrane potential or voltage to a more positive value; This reversal of polarity is due to the opening of voltage-gated Na+ channels and the subsequent movement of Na+ into the cell
Hyperpolarization
Change in the membrane potential to a value more negative than the resting potential
Ependymal cells (CNS)
Ciliated simple cuboidal or simple columnar epithelial cells that line the internal cavities (ventricles) of the brain and the central canal of the spinal cord. These cells have slender processes that branch extensively to make contact to other glial cells in the surrounding nervous tissuse.
If Cl- channels open
Cl- diffuses into the neuron; the neuron becomes more negative; this process is called hyperpolarization.
Plasma membrane of functional segments in a neuron: The initial segment
Commonly considered to be the region of the axon hillock. This segment contains both voltage-gated Na+ channels and voltage-gated K+ channels. graded potentials are processed -> action potentials may be initiated.
Sensory nervous system: Somatic sensory
Components detect stimuli that we can consciously perceive; sensory input from the receptors of the five senses (e.g., eye) and proprioceptors.
Sensory nervous system: Visceral sensory
Components detect stimuli that we typically do not consciously perceive; sensory input from receptors of internal organs (e.g., heart) and blood vessels. For example, visceral receptors detect the stretch of an organ wall or the chemical composition of the blood.
Motor nervous system: Somatic motor
Components initiate and transmit motor output from the CNS to the only type of effector that can be voluntarily controlled- skeletal muscles. For example, you exert voluntary control over your leg muscles as you press on the accelerator of your car.
Motor nervous system: Autonomic motor
Components innervate and regulate the other types of effectors that can only be involuntarily controlled- cardiac muscle, smooth muscle, and glands. These effectors function without our conscious control. For example, we can neither voluntarily make our heart stop beating nor prevent our stomachs from growling.
Electrical synapse
Composed of a presynaptic neuron and a postsynaptic neuron physically bound together. Gap junctions are present in the plasma membranes of both neurons and facilitate the flow of ions between the cells. The cells act as though they share a plasma membrane. Thus, the electrical signal passes between the cells with essentially not synaptic delay; electrical synapses are considered to be fast because of this. Electrical synapses are located within limited regions of the brain and the eyes.
General characteristics of neurons: Conductivity
Conductivity involves an electrical change that is quickly propagated along the plasma membrane as voltage-gated channels open sequentially during an action potential. Thus, keep in mind that whereas excitability refers to the ability to initiate a local electrical change (a graded potential), conductivity refers to the ability to propagate (or move) an electrical change along the plasma membrane (an action potential).
The local currents associated with graded potentials are short-lived because the flow or current of ions along the plasma membrane experiences resistance
Consider that when cation channels open, and Na+ enters a cell and moves in the cytosol near the plasma membrane, Na+ experiences resistance from the cytosol contents. Consequently, the local current of Na+ becomes weaker and eventually this movement of Na+ ceases. Thus, a graded potential lasts only as long as the channels are open and until the local ion current ceases.
Functional classification of nerves: Mixed nerves
Contain both sensory and motor neurons. Most named nerves (including all spinal nerves and most cranial nerves) are mixed nerves. However, in mixed nerves, individual sensory or motor neurons still transmit only one type of information.
Functional classification of nerves: Motor nerves
Contain neurons that relay information away from the CNS (called motor neurons).
Functional classification of nerves: Sensory nerves
Contain only neurons that relay information toward the CNS (called sensory neurons)
Plasma membrane of neurons
Contains transport proteins for moving substances across the plasma membrane. These include pumps and various types of channels.
Nerve signal transmission: (1) Action potential occurs at the neurofibril node
Depolarization is due to opening of voltage-gated Na+ channels at the neurofibril node; Na+ diffuses into the axon. (this is followed by repolarization as voltage-gated K+ channels open, and K+ diffuses out.)
Structural classification: Multipolar neuron
Description: Multiple processes extend directly from the cell body; typically many dendrites and one axon; the most common type of neuron Example of function types: All motor neurons; most interneurons
Structural classification: Anaxonic neuron
Description: Processes are only dendrites; no axon present Example of function types: Interneurons
Structural classification: Unipolar neuron
Description: Single short process extends directly from the cell and looks like a T as a result of the fusion of two processes into one long axon Example of function types: Most sensory neurons These neurons are also called pseudounipolar because they start out as bipolar neurons during development, but their two processes fuse into a single process.
Structural classification: Bipolar neuron
Description: two processes extend directly from the cell body; one dendrite and one axon; relatively limited in where they are located Example of function types: Some special sense neurons (e.g., the retina of the eye, olfactory epithelium in the nose)
General characteristics of neurons: Amitotic
During fetal development, most neurons lose the ability to form new cells through cell division (i.e., mitotic activity). Specific exceptions include the neurons in certain areas of the brain and in the olfactory epithelium of the nose.
When a neurotransmitter opens a chemically gated ion channel that allows sodium to enter the postsynaptic cell, the result is an a. IPSP. b. EPSP.
EPSP.
General characteristics of neurons: excitability
Excitability is responsiveness to a stimulus (e.g., chemical, stretch, pressure change). The stimulus causes a local electrical change in the resting membrane potential in the excitable cell by initiating the movement of ions across the plasma membrane of the excitable cell. These local electrical changes are called graded potentials.
Astrocytes (CNS)
Exhibit a starlike shake due to projections from their surface. These numerous cell processes have contact with both neurons and blood capillaries. Astrocytes are the most abundant glial cells in the CNS. Several subpopulations of glial cells have been identified, all with distinct functions in the CNS: Help form the blood-brain barrier, Regulate interstitial fluid composition, Form structural support, Assist neuronal development, Alter synaptic activity, Occupy the space of dying neurons.
Structural classification of nerves: Cranial nerves
Extend from the brain
Structural classification of nerves: Spinal nerves
Extend from the spinal cord
Voltage-gated potassium channels in a neuron's axon are triggered to open when membrane potential becomes more negative. T/F
False
Neurofibrils
Filamentous protein in a neuron composed of neurofilaments and microtubules.
Satellite cells (PNS)
Flattened cells arranged around neuronal cell bodies in a ganglion. the satellite cells physically separate cell bodies from their surrounding interstitial fluid. Function: Electrically insulate the cell body and regulate the continuous exchange of nutrients and waste products between neuron cell bodies and their environment.
Postsynaptic potentials
Graded potentials that occur in the postsynaptic neuron
Neurofilament
Group of intermediate-sized filaments in a neuron.
Neuron cell body (soma)
Houses both the nucleus and the cytoplasm. The nucleus contains chromatin and a prominent nucleolus, which synthesizes the cell body's large number of ribosomes. The cytoplasm within the cell body, which is more specifically called the perikaryon, is composed of a typical cellular organelles such as the endoplasmic reticulum, Golgi apparatus, ribosomes, and mitochondria.
Plasma membrane of functional segments in a neuron: The receptive segment
Includes both dendrites and the cell body (soma), which are the regions of the neuron that receive stimuli to excite the neuron. Chemically-gated channels (cation channels, K+ channels, and Cl- channels) are located in this segment; no specific numbers of voltage-gated channels are present. Note that cation channels allow the passage of both Na+ into the neuron and K+ out of the neuron. graded potentials are generated
The peripheral nervous system (PNS)
Includes nerves, which are bundles of axons of neurons and ganglia, which are clusters of neuron cell bodies located along nerves.
Plasma membrane of functional segments in a neuron: The conductive segment
Includes the axon and its terminal extensions (but not the synaptic knobs). Like the initial segment, it contains both voltage-gated Na+ channels and voltage-gated K+ channels. Conducts (= propagates action potential).
The central nervous system (CNS)
Includes the brain and spinal cord; the brain is protected and enclosed within the skull, whereas the spinal cord is housed and protected within the vertebral canal.
Plasma membrane of functional segments in a neuron: The transmissive segment
Includes the synaptic knobs and contains both voltage-gated Ca2+ channels and Ca2+ pumps. Action potential arrival -> neurotransmitter release -> +/- effect on a postsynaptic cell.
Myelin
Insulating covering around an axon composed of concentric layers of plasma membrane of glial cells (Neurolemmocytes or oligodendrocytes)
If K+ channels open
K+ diffuses out of the neuron; the neuron becomes more negative; this process is called hyperpolarization.
Oligodendrocytes (CNS)
Large cells with a bulbous (round) body and slender cytoplasmic extensions or processes. the extensions of oligodendrocytes wrap around and insulate axons within the CNS to form a myelin sheath through a process called myelination. This insulation allows for faster propagation of action potentials along the axon.
Plasma membrane of neurons, channels: modality-gated channel
Located on the dendritic endings of sensory neurons and open (or close) in response to a specific type of sensory stimulus- they detect changes in the external or internal environment. For example, receptor cells of the skin contain modality-gated channels that re stimulated by mechanical pressure to open and receptor cells of the eye (photoreceptors) contain modality-gated channels that are stimulated by light to close.
Neurotransmitters
Molecules stored in vesicles and when released bind to an excitable cell to cause either an excitatory or an inhibitory effect on these target cells (other neurons or effectors).
General characteristics of neurons: Extreme longevity
Most neurons formed during fetal development are still functional in very elderly individuals
If Na+ channels open
Na+ diffuses into the neuron; the neuron becomes less negative; this process is called depolarization EX: -70 mV -> -60 mV
Distribution of pumps and channels in the plasma membrane of a neuron: Entire neuron
Na+/K+ pumps, Na+ leak channels, and K+ leak channels are found in the plasma membrane throughout the entire neuron. Additional types of channels and pumps are present in the plasma membrane only in specific functional segments of the neuron.
Nerves and vasculature
Nerves are vascularized by an extensive network of blood vessels, which branch and extend through both the epineurium and the perineurium to become capillaries (microscopic blood vessels) Capillaries are associated with the endoneurium and function as the site of exchange of substances (e.g., oxygen, glucose, waste products) between axons of neurons and the blood.
Nervous tissue
Nervous tissue is the primary tissue of the nervous system and is composed of two distinct cell types: neurons and glial cells
General characteristics of neurons
Neurons have several special characteristics including: Excitability, Conductivity, Secretion, Extreme longevity, and Amitotic
General characteristics of neurons: Secretion
Neurons release neurotransmitters in response to conductive activity
Graded potentials are established in the receptive segment by the opening of chemically gated channels
Neurotransmitter released from a presynaptic neuron binds with a specific type of chemically gated channel, triggering it to open, which temporarily allows passage of a small amount of a specific type of ion (or ions) across the plasma membrane. The ions then move along the plasma membrane in a local current. This movement occurs as the ions of like charges repel each other, resulting in their being pushed along.
Nervous tissue: Glial cells
Non-excitable cells that primarily support and protect the neurons
Slow axonal transport
Occurs at 0.1 to 3 millimeters per day; results from the flow of axoplasm called axoplasmic flow. The materials are only moved from the cell body toward the synaptic knob (anterograde). These substances include enzymes, cytoskeletal components, and new axoplasm for regenerating axons.
Fast axonal transport
Occurs at 400 millimeters per day; the rapid movement of membrane vesicles and their contents over long distances within a neuron. The power of this movement comes from specialized motor proteins (e.g., kinesin, dynein) that split ATP to supply the energy needed.
Relative refractory period
Occurs immediately after the absolute refractory period, during the hyperpolarization phase of the action potential. At this time, voltage-gated Na+ channels have returned to their resting state, but the neuron is hyperpolarized due to the slightly extended time that voltage-gated K+ channels remain open during repolarization. Thus, another action potential can now be initiated in an axon only if the stimulation of the plasma membrane is greater than the stimulus normally needed to generate an action potential.
continuous conduction
Occurs in unmyelinated axons and involves the sequential opening of voltage-gated Na+ channels and voltage-gated K+ channels located within the axon plasma membrane along the entire length of the axon.
Temporal summation
Occurs when a single presynaptic neuron repeatedly releases neurotransmitter to produce either multiple EPSPs or IPSPs in the postsynaptic neuron at the same location within a very short period of time.
Transmission process of chemical synapses
Occurs when neurotransmitter molecules stored in the synaptic vesicles are released from the synaptic knob of a presynaptic neuron into the synaptic cleft. Some of the neurotransmitter diffuse across the synaptic cleft to bind to receptors within the plasma membrane of the postsynaptic neuron to initiate a graded potential. Chemical synapses are considered to be slow (also common) because there is a synaptic delay associated with neurotransmitter release. The delay is the time between the neurotransmitter release from the presynaptic cell, its diffusion across the synaptic cleft, and neurotransmitter binding to receptors in the postsynaptic neuron plasma membrane.
Synaptic vesicles
Package of membrane enclosing neurotransmitter molecules in the synaptic knob.
effectors
Peripheral tissue or organ that responds to nervous or hormonal stimulation.
excitatory postsynaptic potential (EPSP)
Postsynaptic potentials that result int he neuron becoming more positive (depolarized)
Axon
Process of a neuron that propagates action potentials away from the cell body; typically a longer process emanating from the cell body to make contact with other neurons, muscle cells, or gland cells. Unlike the cell body, the axon is devoid of chromatophilic substance. May be insulated with a myelin sheath, which is formed by a certain type of glial cells (either neurolemmocytes or oligodendrocytes).
Dendrites
Process on a neuron where graded potentials are initiated; tend to be relatively short, small, tapering processes that branch off the cell body. Dendrites, like the cell body, are not insulated with myelin. Dendrites transmit graded potentials along the plasma membrane toward the cell body. the greater the number of dendrites, the more input a neuron may receive.
Conductive segment
Propagation of action potential
General functions of the nervous system: Collecting information
Receptors are specialized nervous system structures that monitor changes in both the internal and external environment called stimuli. For example, receptors in the skin detect stimuli associated with touch- this sensory information then is relayed along neurons to the spinal cord and brain.
_______________ the time between signals sent from the same presynaptic terminal increases the strength of the graded potential. This is an example of ________________ summation. a. Reducing; spatial b. Increasing; spatial c. Reducing; temporal d. Increasing; temporal
Reducing; temporal
Sensory nervous system
Responsible for receiving sensory information from receptors and transmitting this information to the CNS. This information from the receptors to the CNS is called sensory input
Inhibitory postsynaptic potentials (IPSP)
Results in the neuron becoming more negative (hyperpolarized)
Action potential
Self-propagating change in membrane potential occurring in excitable cells (e.g., neurons, muscle cells); involves two processes: depolarization and repolarization.
Graded potential
Small deflection in the resting membrane potential in excitable cells due to the movement of small amounts of ions across the plasma membrane; it may result in either a depolarization or hyperpolarization relatively small (less than 1 mV), short-lived changes in the resting membrane potential that are caused by the movement of small amounts of ion across the plasma membrane.
Structural classification of nerves
Structural classification is based upon the CNS component from which the nerve ends (cranial nerves and spinal nerves)
Graded potentials are added together at the axon hillock in a process known as _________.
Summation
Initial segment
Summation of graded potentials; initiation of action potenential
Nerve signal transmission: (2) Na+ diffusion (but no action potential) occurs at the myelinated region of an axon
The Na+ diffuses through axoplasm of the axon internal to the axolemma (which is insulated by myelin). Two critical aspects should be noted regarding this Na+ diffusion: (A) It is relatively fast, faster than the events at the neurofibril nodes, and (b) as Na+ movement occurs through the axoplasm, it experiences resistance and the local current decreases in intensity (becomes weaker) with distance.
Nerve signal transmission: (3) A new action potential occurs at the next neurofibril node
The arrival of the relatively weak Na+ current at the next neurofibril node is sufficient to cause the opening of voltage-gated Na+ channels located there. This results in the establishing of a new action potential as Na+ enters the axon and a new local current is established. This process repeats as the nerve signal continues down the length of an axons until it reaches the synaptic knobs.
Nervous tissue: Neurons
The basic structural unit of the nervous system; Excitable cells that initiate and transmit electrical signals
General functions of the nervous system: Initiating response to information
The brain and spinal cord initiate a response as motor information is relayed along neurons to structures called effectors. Effectors include all three types of muscle tissue and glands. The effect may be either muscle contraction (or relaxation) or a change in gland secretion activity.
Refractory Period
The brief time after an action potential has been initiated during which an axon is either incapable of generating another action potential or greater than normal amount of stimulation is required to generate another action potential. The excitable neuron plasma membrane recovers at this time and becomes ready to respond to another stimulus. The refractory period has two phases: The absolute refractory period and the relative refractory period.
The neurons control center
The cell body serves as the neurons control center because (a) it contains both the nucleus and the cytoplasm and (b) it functions in many of the cell's metabolic activities. It also transmits graded potentials (i.e., local electrical changes in the membrane potential that vary in size) along its plasma membrane to the axon. Graded potentials are both received from the dendrites and initiated at the cell body.
Axoplasm
The cytoplasm within an axon.
Subpopulation functions: Form structural support
The cytoskeleton in astrocytes strengthens these cells to provide a structural framework to support and organize neurons within the CNS.
Graded potentials vary in both the degree of change and the direction of change of the RMP
The degree of change of a graded potential is dependent upon the magnitude of the stimulus. A larger stimulus opens more chemically gated channels, and more ions flow across the plasma membrane than occurs during a weaker stimulus. The direction of change (whether the potential will become more positive or negative) is dependent upon the type of chemically gated channel that opens. The result of the direction is either depolarization or hyperpolarization
Subpopulation functions: Help form the blood-brain barrier
The ends of astrocyte process are called perivascular feet: They both cover and wrap around capillaries in the brain. the perivascular feet and the brain capillaries together contribute to a blood-brain barrier (BBB). The BBB strictly controls movement of substances from exiting the blood and entering the nervous tissue in the brain. The BBB protects the delicate neurons of the brain from harmful substances, but at the same time allows needed nutrients to pass through.
Cytoskeleton
The entire neuron has an extensive cytoskeleton, which is composed of microtubules, microfilaments (actin), and a type of intermediate filament called neurofilament. These proteins are within the cell body and extend into the dendrites and axons. Function: Maintain neuron shape and provide structural support. The neurofilaments aggregate to form parallel bundles called neurofibrils. Microtubules (embedded in parallel clusters with the neurofibrils of an axon) participate in cellular transport within axons.
Receptive segment to Initial segment
The local currents of ions associated with the graded potentials (Na+ ion currents with EPSPs and K+ and Cl- ion currents with IPSPs) that are established in the receptive segment move along the plasma membrane toward the initial segment. The outcome of these multiple local currents is determined when the arrive in the initial segment (axon hillock). The changes in the membrane potentials associated with these graded postsynaptic potentials are "added" in the initial segment to determine if an action potential is initiated. The process is called summation.
Retrograde transport
The movement of materials from synaptic knobs toward the cell body
Anterograde transport
The movement of materials from the cell body towards synaptic knobs
Components of the nervous system
The nervous system is composed of the brain, spinal cord, nerves, and ganglia and its primary tissue is nervous tissue.
Organization of the nervous system
The nervous system is organized into both structural and functional categories. (a) Structural divisions include the central nervous system, which is composed of the brain and spinal cord, and the peripheral nervous system, which is composed of nerves and ganglia. (b) Functionally, the nervous system consists of the sensory nervous system and the motor nervous system, both of which are further divided into somatic and visceral (or autonomic) components.
Objective of the nervous system
The nervous system serves as the body's primary communication and control system. It provides a rapid means of integrating and regulating body functions through electrical signals transmitted along specialized nervous tissue cells called neurons to accomplish the objectives of (1) collecting information, (2) processing and evaluating information, and (3) initiating response on information.
Process of Myelination in PNS
The neurolemmocyte starts to encircle a 1-millimeter portion of an axon. As the neurolemmocyte continues to wrap around the axon, the cytoplasm and nucleus of the neurolemmocyte are squeezed to the periphery of the neurolemmocyte. The overlapping inner layers of the plasma membrane form the myelin sheath. The myelin sheath, because it is formed by the plasma membrane, is composed of the same components- lipids and proteins. The periphery of the neurolemmocyte contains the cytoplasm and nucleus and is called the neurilemma.
Distinctive feature of the neuron cell body
The neuron cell body contains a large number of ribosomes. The ribosomes are either attached to the endoplasmic reticulum (ER) as part of an extensive rough ER or are free ribosomes within the cytosol. Collectively, they readily absorb basic dyes when a nervous tissue sample is stained for viewing with a microscope; thus, they appear as a dark-staining bodes and are referred to as chromatophilic substance. It is the chromatophilic substance that accounts for the gray color of gray matter seen in gross dissections of the brain and spinal cord. (The axon hillock is the only portion of the cell body that lacks chromatophilic substance)
Axolemma
The plasma membrane of an axon.
Myelination
The process by which part of an axon is wrapped with myelin; completed by neurolemmocytes (PNS) and oligodendrocytes (CNS); consists of the plasma membrane of these glial cells and contains a large proportion of lipids and a smaller amount of proteins. The high lipid content of the myelin gives an axon distinct, glossy-white appearance and effectively insulates an axon.
Synapse
The specific location where a neuron is functionally connected to either another neuron or an effector (muscle or gland). There are two types of synapses in the human body: chemical synapses and electrical synapses; most synapses within the nervous system are chemical synapses.
Neurofibril nodes
The uninsulated regions of the axon between the myelin sheaths.
Inactivation state
The voltage-gated Na+ channels are opened for only a very short duration and then close, changing from the activation state to the temporary inactivation state as the inactivation gate closes. This temporarily prevents their reopening.
Plasma membrane of neurons, channels: Voltage-gated channels
These channels are also normally closed, but they are temporarily open in response to changes in electrical charge (potential) across the plasma membrane. When open, they allow a specific type of ion to diffuse across the membrane. Examples of voltage-gated channels include voltage-gated Na+ channels, voltage-gated K+ channels, and voltage-gated Ca2+ channels. Most channels have one gate that is either closed or open.
Plasma membrane of neurons, channels: Leak channels (passive)
These channels are always open, allowing continuous diffusion of a specific type of ion from a region of high concentration to a region of low concentration. Examples of leak channels are sodium ion (Na+) leak channels and potassium (K+) leak channels.
Plasma membrane of neurons, channels: Chemically gated channels
These channels are normally closed. They temporarily open in response to binding of a neurotransmitter. When open, they allow a specific type of ion (or ions) to diffuse across the plasma membrane. Examples of chemically gated channels include chemically gated K+ channels and chemically gated chloride ion (Cl-) channels.
Motor nervous system
This system is responsible for initiating and transmitting motor information from the CNS to the effectors. This information from the CNs to effectors is called motor output.
Absolute refractory period
Time period when an excitable cell cannot be restimulated to respond; is the time after an action potential onset when no amount of stimulus, no matter how strong, can initiate a second action potential. This period spans depolarization and almost all of repolarization. During this time, the voltage-gated Na+ channels are first opened to cause depolarization, and are then in the inactivation state; preventing further Na+ flow. It isnʼt until the voltage-gated Na+ channels change from an activated state to an inactivated state (toward the end of repolarization) that another action potential may be propagated. The absolute refractory period ensures that the action potential moves along the axon in only one direction toward the synaptic knobs.
Indicate the summative effect that brings the initial segment closest to threshold. a. One IPSP and one EPSP in proximity to each other b. Two EPSPs located a large distance apart c. Two IPSPs in proximity to each other d. Two EPSPs in proximity to each other e. Two IPSPs located a large distance apart
Two EPSPs in proximity to each other
Glial cells of the PNS
Two types of glial cells are found in the peripheral nervous system (PNS). these specialized glial cells function in insulating neurons and include (1) satellite cells and (2) neurolemmocytes.
Microglia (CNS)
Typically small cells that have slender branches extending from the main portion of the cell; represent the smallest percentage of CNS glial cells; classified as phagocytic cells (macrophages) of the immune system. Functions: Wander through the CNS and replicate in response to an infection, protect the CNS against microorganisms (e.g., bacteria) and other potentially harmful substances by engulfing and destroying them through phagocytosis, and remove debris from dead or damaged nervous tissue that results from infections, inflammation, trauma, and brain tumors.
Voltage-gated Na+ channels
Unique in that they have two gates (an activation gate and an inactivation gate) and thus can exhibit one of three states: resting (closed), activation (open) or inactivation (closed)
unmyelinated axons of PNS
Unmyelinated axons are partially surrounded by a neurolemmocyte but are not wrapped in a myelin sheath. They help protect and support the axon. The axon merely rests in a depressed portion of the neurolemmocyte, but its plasma membrane does not form repeated layers around the axon.
Subpopulation functions: Occupy the space of dying neurons
When neurons are damaged and die, the space they formerly occupied is often filled by astrocytes that replicate through cell division.
A graph of an EPSP would plot time against a voltage trace that would resemble a. a hill where the high point approaches the threshold value. b. a valley where the low point approaches the threshold value. c. a valley where the low point is the farthest away from the threshold value. d. a hill where the high point is the farthest away from the threshold value.
a hill where the high point approaches the threshold value.
presynaptic neuron
a neuron (nerve cell) that fires the neurotransmitter as a result of an action potential entering its axon terminal.
Postsynaptic neuron
a neuron to the cell body or dendrite of which an electrical impulse is transmitted across a synaptic cleft by the release of a chemical neurotransmitter from the axon terminal of a presynaptic neuron.
Glial cells of the CNS
astrocytes, ependymal cells, microglia, and oligodendrocytes. They can be distinguished based upon size, intracellular organization, and the presence of specific cytoplasmic processes
Substances that cause facilitation of a neuron ____________________________. a. continually create additional ligand-gated ion channels at the receptor region b. cause the neuron to fire that may not under the same circumstances in the absence of the facilitator c. disable the trigger zone of the axon hillock, thereby preventing it from reaching threshold d. alter the resting membrane potential of a presynaptic neuron by making it hyperpolarized
cause the neuron to fire that may not under the same circumstances in the absence of the facilitator
Hyperpolarization of a neuron results from a. the entry of any ion. b. either the entry of a cation or the exit of an anion. c. the entrance of either sodium or potassium. d. either the entry of an anion or the exit of a cation.
either the entry of an anion or the exit of a cation.
Functional classification of nerves
functional classification of nerves is based upon the functional type of neuron (sensory neuron or motor neuron) a nerve contains.
When chloride enters the neuron via chemically gated chloride channels, the membrane potential will become more negative than the resting membrane potential. This is called _____________.
hyperpolarization
Plasma membrane of neurons: Pumps
maintains specific concentration gradients by moving substances up (against) a concentration gradient (active). Pumps within the plasma membrane of the neuron contains both sodium-potassium (Na+/K+) pumps and calcium (Ca+) pumps.
Functional classification
neurons are classified functionally according to the direction in which action potentials are propagated relative to the CNS. The three categories are sensory neurons, motor neurons, and interneurons.
Assume in a laboratory you were able to isolate a neuron and remove the Acetylcholine receptors from the postsynaptic membrane. The substance _________________ would no longer cause facilitation of this neuron. caffeine nicotine
nicotine
Saltatory conduction
occurs in the myelinated axons. Here, action potentials do not occur in regions of the axon that are myelinated- rather, they are generated only at the neurofibril nodes. Saltatory conduction also is more efficient because less energy is required by Na+/K+ pumps to maintain the RMP.
Spatial summation
occurs when multiple presynaptic neurons release neurotransmitter at various locations onto the receptive segment, thus generating EPSPs and IPSPs, or both in the postsynaptic neuron.
An inhibitory postsynaptic potential results from the opening of____________________. a. sodium and/or potassium channels b. potassium and/or chloride channels c. chloride and/or sodium channels
potassium and/or chloride channels
Graded potentials are produced within the _______________________ segment of a neuron. a. receptive b. initial c. transmissive d. conductive
receptive
Chemical synapse
the junction between two communicating cells (neurons, and neurons and effectors); neurotransmitter released at this type of synapse. May stimulate or inhibit the postsynaptic cell. Composed of a presynaptic neuron, which is the signal producer (releases neurotransmitter), and a postsynaptic neuron, which is the signal receiver, or target (binds neurotransmitter). The two neurons are separated by an extremely narrow, fluid-filled gap called the synaptic cleft.
Axon collaterals
the side branches of an axon; most axons and axon collaterals branch extensively at their distal end into an array of fine terminal extensions.
Synaptic knobs
the slightly expanded regions at the extreme tips of the fine terminal extensions; also called synaptic bulbs, end bulbs, or terminal boutons. Ends at a functional junction called a synapse.
