Chapter 11
Which part of the neuron is its fiber? How do nerve fibers differ from the fibers of connective tissue (see Chapter 4) and the fibers in muscle (see Chapter 9)?
A nerve fiber is a long axon, an extension of the cell. In connective tissue, fibers are extracellular proteins that provide support. In muscle tissue, a muscle fiber is a muscle cell.
Which subdivision of the PNS is involved in (a) relaying the feeling of a "full stomach" after a meal, (b) contracting the muscles to lift your arm, and (c) increasing your heart rate?
(a) This "full stomach" feeling would be relayed by the sensory (afferent) division of the PNS (via its visceral afferent fibers). (b) The somatic nervous system, which is part of the motor (efferent) division of the PNS, controls movement of skeletal muscle. (c) The autonomic nervous system, which is part of the motor (efferent) division of the PNS, controls the heart rate.
List the basic functions of the nervous system.
1. Sensory input - uses sensory receptors to gather information/changes occurring both inside and outside the body 2. Integration - decides what to do with this information 3. Motor output - activates effector organs (muscles and glands) to cause a response (called a motor output)
ACh excites skeletal muscle and yet it inhibits heart muscle. How can this be?
ACh interacts with more than one specific receptor type, and this explains how it can excite at some synapses and inhibit at others.
Which is bigger, a graded potential or an action potential? Which travels farther? Which initiates the other?
Action potentials are larger than graded potentials and travel farther. Graded potentials generally initiate action potentials.
Which type of neuroglia controls the extracellular fluid environment around neuron cell bodies in the CNS? In the PNS?
Astrocytes control the extracellular environment around neuron cell bodies in the CNS, whereas satellite cells perform this function in the PNS.
What structure joins two neurons at an electrical synapse?
At an electrical synapse, neurons are joined by gap junctions.
Describe graded potentials and name several examples.
Graded potentials are short-lived, localized changes in membrane potential, usually in dendrites or the cell body - can be either depolarizations or hyperpolarizations. Their magnitude varies directly with stimulus strength. They are triggered by some change in the neurons envrinment that opens gated ion channels. A receptor potential is produced when a sensory receptor is excited by its stimulus, a postsynaptic potential is produced when the stimulus is a neurotransmitter released by another neuron
Define synapse.
Synapse - a junction that mediates information transfer from one neuron to the next or from a neuron to an effector cell
What pattern of neural processing occurs when we smell freshly baked apple pie and remember Thanksgiving at our grandparents' house, the odor of freshly cooked turkey, sitting by the fire, and other such memories?
The pattern of neural processing is parallel processing.
What pattern of neural processing occurs when your finger accidentally touches a hot grill? What is this type of pathway called?
The pattern of neural processing is serial processing. The response is a reflex arc.
For which cation are there the largest number of leakage channels in the plasma membrane?
There are more leakage channels for K+ than for any other cation.
Describe the relationship between current, voltage, and resistance.
Voltage - measure of potential energy generated by separated electrical charges Current - flow of electrical charge from one point to another Resistance - hindrance to charge flow provided by substances through which the current must pass Ohm's law gives us the relationship between voltage, current, and resistance: 1. current is directly proportional to voltage 2. there is no net current flow between points that have the same potential 3. current is inversely related to resistance
Describe the steps of information transfer at the neuromuscular junction.
1. Action potential arrives at axon terminal - AP arrives at the presynaptic axon terminal 2. Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal - depolarization of membrane by AP causes Na+ channels and voltage-gated Ca2+ channels to open as well. 3. Ca2+ entry causes synaptic vesicles to release neurotransmitter by exocytosis - synaptic vesicles release contents into synaptic cleft via exocytosis, Ca2+ is removed from terminal either by active Ca2+ pump or by the uptake of mitochondria 4. Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane 5. Binding of neurotransmitter opens ion channels, creating graded potentials 6. Neurotransmitter effects are terminated - reuptake, degradation, diffusion
Explain how action potentials are generated and propagated along neurons.
1. Resting state: All voltage-gated Na+ and K+ channels are closed - only leakage channels are open maintaining the resting membrane potential, each Na+ has two gates: a voltage-sensitive activation gate and an inactivation gate, depolarization opens and then inactivates sodium channels, K+ has one voltage-sensitive gate 2. Depolarization: Voltage-gated Na+ channels open - local currents depolarize axon membrane, voltage-gated sodium channels open and Na+ rushes into cell. This influx of positive charges depolarize that local patch of membrane further, opening more Na+ channels so the cell interior becomes progressively less negative. Threshold stimulus: depolarization hits critical level and becomes self-generating by positive feedback. 3. Repolarization: Na+ channels are inactivating, and voltage-gated channels open - Na+ inactivation gates begin to close and membrane permeability to Na+ declines. Slow voltage-gated K+ channels open and K+ rushes out of the cell. Both abrupt decline in Na+ permeability and the increased permeability to K+ contribute to repolarization. 4. Hyperpolarization: Some K+ channels remain open, and Na+ channels reset - K+ permeability lasts longer than needed to restore resting state, K+ efflux before potassium channels close
What are the steps to a reflex arc.
1. receptor 2. sensory neuron 3. integration center 4. motor neuron 5. effector
Differentiate between (1) a nucleus and a ganglion, and (2) a nerve and a tract.
A nucleus is a collection of neuron cell bodies in the CNS A ganglion is a collection of neuron cell bodies in the PNS A tract is a bundle of axons in the CNS A nerve is a bundle of axons in the PNS
How does a nucleus within the brain differ from a nucleus within a neuron?
A nucleus within the brain is a cluster of neuron cell bodies, whereas the nucleus within each neuron is a large organelle that acts as the control center of the cell.
Define absolute and relative refractory periods
Absolute Refractory Period - Incapable of generating another action potential (ensures all-or-none event and enforces one-way transmission of the AP) Relative Refractory Period - Requires greater than normal amount of stimulation to generate another action potential
Compare and contrast graded potentials and action potentials.
Action potential is a brief reversal of membrane potential through the use of voltage-gated channels on the axon. Action potentials send messages long distances without the magnitude decaying.
An action potential does not get smaller as it propagates along an axon. Why not?
An action potential is regenerated anew at each membrane patch.
Describe anterograde movement and retrograde movement of axons.
Anterograde movement of axons is movement away from the cell body - substances moved in this direction include mitochondria, cytoskeletal elements, membrane components (vesicles) used to renew the axon plasma membrane, and enzymes needed to synthesize certain neurotransmitters Reterograde movement of axons is movement toward the cell body - substances moved in this direction are mostly organelles returning to the cell body to be degraded or recycled - important means of intracellular communication, it allows the body to be advised of conditions at the axon terminals
Describe the two factors that largely affect the rate of impulse propagation.
Axon diameter - the larger the axon's diameter, the faster it conducts impulses Degree of myelination - presence of a myelin sheath dramatically increases the speed of propagation
Which structural and functional type of neuron is activated first when you burn your finger? Which type is activated last to move your finger away from the source of heat?
Burning a finger will first activate unipolar (pseudounipolar) neurons that are sensory (afferent) neurons. The impulse to move your finger away from the heat will be carried by multipolar neurons that are motor (efferent) neurons.
Explain the structural and functional divisions of the nervous system.
Central Nervous System - brain and spinal cord (in dorsal body cavity) - integrating and control center of NS, interprets input and dictates motor output Peripheral Nervous System - outside the CNS (nerves and ganglia), spinal nerves carry impulses to and from spinal cord, cranial nerves carry impulses to and from brain, peripheral nerves link all parts of the body to the CNS
Describe the action of neurotransmitters at channel-linked and G protein-coupled receptors.
Channel-linked receptors - ligand-gated ion channels that mediate direct neurotransmitter action, as ligand binds to one or more receptor units, the proteins change shape, this opens the central channel and allows ions to pass, membrane potential of target cell changes. Immediate, simple, and brief. G protein-coupled receptors - neurotransmitter binds to receptor protein and G protein is activated, normal G protein process occurs, second messengers change membrane permeability by regulating the opening or closing of ion channels. Indirect, complex, slow.
Identify different types of membrane ion channels.
Chemically gated channels/ligand-gated channels - open when the appropriate chemical binds Voltage-gated channels - open and close in response to changes in the membrane potential Mechanically gated channels - open in response to physical deformation of the receptor
Why does a myelinated axon conduct action potentials faster than a nonmyelinated axon?
Conduction of action potentials is faster in myelinated axons because myelin allows the axon membrane between myelin sheath gaps to change its voltage rapidly, and allows current to flow only at the widely spaced gaps.
Define saltatory conduction and explain how it differs from continuous conduction.
Continuous conduction - AP propagation in nonmyelinated axons, voltage-gated channels in the membrane are immediately adjacent to each other relatively slow Saltatory conduction - AP is generated in a myelinated fiber, current is maintained and moves rapidly to next myelin sheath gap where it triggers another AP
Why is cyclic AMP called a second messenger?
Cyclic AMP (cAMP) is called a second messenger because it relays the message between the first messenger (the original chemical messenger) outside of the cell and effector molecules that will ultimately bring about the desired response within the cell.
Describe depolarization and hyperpolarization.
Depolarization - decrease in membrane potential: inside of the membrane becomes less negative than the resting potential Hyperpolarization - increase in membrane potential: the inside of the membrane becomes more negative than the resting potential
Describe the types of neuroncircuits.
Diverging circuit - one input, many outputs - single neuron in brain can activate 100 or more motor neurons in the spinal cord and thousands of skeletal muscle fibers Converging circuit - many outputs, one input - different sensory stimuli can all elicit the same memory Reverberating circuit - signal travels through a chain or neurons, each feeding back to previous neurons - breathing, sleep-wake cycle, repetitive motor activities (like walking) Parallel after-discharge circuit - signal stimulates neurons arranged in parallel arrays that eventually converge on a single output - exacting mental processes such as mathematical calculations
Distinguish between electrical and chemical synapses by structure and by the way they transmit information.
Electrical synapses - gap junctions where their channel proteins connect the cytoplasm of adjacent neurons and allow ions and small molecules to flow directly from one neuron to the next, these neurons are electrically coupled Chemical synapses - converts electrical signal in axon to chemical signal in synapse (neurotransmitters). There are two parts to a chemical synapse: Axon terminal - presynaptic neuron contains many membrane bound sacs called synaptic vesicles which each contain thousands of neurotransmitters A receptor region - postsynaptic neuron's membrane, usually on a dendrite or cell body
Distinguish between excitatory and inhibitory postsynaptic potentials.
Excitatory Synapses - neurotransmitter binding opens chemically gated ion channels, allowing Na+ and K+ to pass simultaneously. The electrochemical gradient for Na+ is bigger than for K+ so more Na+ enters than K+ exits and the result is depolarization - this causes depolarizing graded potential called excitatory postsynaptic potentials (EPSPs) to occur at excitatory postsynaptic membranes. inhibitory synapses - Neurotransmitter binding opens chemically gated ion channels, allowing either K+ or Cl- to pass. The result is hyperpolarization.
Describe the two types of signals changes in membrane potential can produce.
Graded potentials - usually incoming signals operating over short distances that have variable strength Action potentials - long distance signals of axons that always have the same length
What are the types of nerve fibers?
Group A fibers - mostly somatic sensory or motor fibers, largest diameter, thick myelin sheaths, conduct impulses at speeds up to 150 m/s Group B fibers - lightly myelinated, intermediate diameter, transmit impulses at an average rate of 15 m/s Group C fibers - smallest diameter, nonmyelinated (incapable of saltatory conduction), conduct impulses at an average rate of 1 m/s B and C fiber groups include the autonomic nervous system motor fibers and the smaller somatic sensory fibers that transmit sensory impulses from the skin.
Which ions flow through chemically gated channels to produce IPSPs? EPSPs?
IPSPs result from the flow of either K+ or Cl− through chemically gated channels. EPSPs result from the flow of both Na+ and K+ through chemically gated channels.
If an axon receives two stimuli close together in time, only one AP occurs. Why?
If a second stimulus occurs before the end of the absolute refractory period, no AP can occur because sodium channels are still inactivated.
How is a myelin sheath formed in the CNS, and what is its function?
In the CNS, a myelin sheath is formed by oligodendrocytes that wrap their plasma membranes around the axon. The myelin sheath protects and electrically insulates axons and increases the speed of transmission of nerve impulses.
What is meant by "integration," and does it primarily occur in the CNS or the PNS?
Integration involves processing and interpreting sensory information, and making a decision about motor output. Integration occurs primarily in the CNS.
Explain the importance of the myelin sheath and describe how it is formed in the central and peripheral nervous systems.
Myelin protects and electrically insulates fibers, and it increases the transmission speed of nerve impulses. Myelin sheaths in the PNS are formed by Schwann cells that indent and wrap around an axon, it then rotates around and the cytoplasm is forced from between the membranes. Myelin sheaths in the CNS are formed by oligodendrocytes that have multiple flat processes that can coil around as many as 60 axons at the same time.
List the types of neuroglia and cite their functions.
Neuroglia in the CNS: Astrocytes - make exchanges between capillaries and neurons, help determine capillary permeability, guide young neurons to form synapses, control chemical environment around neurons, respond to nearby nerve impulses, release neurotransmitters, controls extracellular fluid environment around neuron Microglial cells - monitor health of nearby neurons, phagocytize microorganisms or neuronal debris Ependymal cells - form permeable barrier between cerebrospinal fluid that fills the brain and spinal cord cavities, cushions brain and spinal cord Oligodendrocytes - wrap tightly around nerve fibers, produce myelin sheath Neurolgia in the PNS: Satellite cells - similar to astrocytes (but in PNS) Schwann cells - similar to oligodendrocytes (but in PNS), regeneration of damaged peripheral nerve fibers
Describe common patterns of neuronal organization and processing.
Neuronal pools - billions of neurons in the CNS, integrate incoming information from receptors or different neuronal pools and then forward the processed information to other destinations. Discharge zone - neurons most likely to generate impulses are those closely associated with the incoming fiber, they receive the bulk of the synaptic contacts Facilitated zone - neurons farther from the center are not usually excite to threshold, but they are facilitated and can easily be brought to threshold stimuli from another source
Define neurotransmitter and classify neurotransmitters by chemical structure and by function.
Neurotransmitter - signaling molecule by which neurons communicate, integrate, and send messages to the rest of the body Acetylcholine (ACh) - released by all neurons that stimulate skeletal muscles and by many neurons of the autonomic nervous system, synthesized from acetic acid and choline by the enzyme choline acetyltransferase and then transported into synaptic vesicles for later release. Biogenic Amines - includes catecholamines such as dopamine, norepinephrine, and epinephrine, and the indolamines, which include serotonin and histamine - play a role in emotional regulation and the biological clock Amino Acids - Glutamate, aspartate, glycine, and gamma aminobutyric acid (GABA) Peptides - the neuropeptides are a broad spectrum of molecules with a diverse effect. Neuropeptide called substance P is important mediator of pain signals, endorphins act as natural opiates Purines - nitrogen-containing bases that make up DNA and RNA, also ATP and adenosine Gases and Lipids - nitric oxide (NO) and carbon monoxide (CO) activate guanylate cyclase, the enzyme that makes the second messenger cyclic GMP Endocannanbinoids - most common G protein-coupled receptors in the brain
Define neuron, describe its important structural components, and relate each to a functional role.
Nuerons are the structural units of the nervous system. The neuron cell body consists of a spherical nucleus surrounded by cytoplasm (also called soma) - receives information from other neurons, biosynthetic center and metabolic center of a neuron The processes extend from the cell body of all neurons, the CNS contains both neuron and cell bodies and their processes, while the PNS contains chiefly neuron processes. Dendrites (of motor neurons)- short, tapering, diffusely branching extensions - main receptive or input regions (receive graded potentials) Axons - conducting region of the neuron
Which two types of neuroglia form insulating coverings called myelin sheaths?
Oligodendrocytes and Schwann cells form myelin sheaths in the CNS and PNS, respectively.
Define resting membrane potential and describe its electrochemical basis.
Resting membrane potential is the potential difference in a resting neuron - the membrane is said to be polarized. K+ flowing out of the cell causes the cell to become more negative inside. Na+ trickling into the cell makes the cell slightly more positive than it would be if only K+ flowed. So, resting membrane potential negativity is caused by the greater ability of K+ to diffuse out of the cell than for Na+ to diffuse into the cell.
Which types of neural circuits would give a prolonged output after a single input?
Reverberating circuits and parallel after-discharge circuits both result in prolonged output.
What are the two divisions of the peripheral nervous system?
Sensory/afferent division - convey impulses to the CNS from sensory receptors: somatic sensory fibers (skin, skeletal muscles, and joints) and visceral sensory fibers (organs within ventral body cavity) - keeps the CNS informed with events going on both inside and outside the body. Motor/efferent division - transmits impulses from the CNS to effector organs (muscles and glands)
Distinguish between serial and parallel processing.
Serial processing - input travels along one pathway to specific destination (reflexes). Reflexes are rapid, automatic response to stimuli, in which a particular stimulus always causes the same response. Parallel processing - input travels along several different pathways to be integrated in different CNS regions. Higher level mental functioning.
What are the two subdivisions of the motor division (of the PNS)?
Somatic nervous system - conducts impulses from CNS to skeletal muscles, referred to as the voluntary nervous system Automatic nervous system (ANS) - consists of visceral motor nerve fibers that regulate smooth muscle, cardiac muscle, and glands, referred to as the involuntary nervous system
Classify neurons by structure and by function.
Structural classification: Multipolar - three or more processes, one axon, and the rest dendrites - most common type in CNS Bipolar - two processes, an axon and a dendrite that extend from opposite sides of the cell body - retina of eye and olfactory mucosa Unipolar neurons - single short process that emerges from cell body - sensory neurons (found in ganglia in PNS) Functional classification: Sensory neurons (afferent) - transmit impulses from sensory receptors in the skin or internal organs toward or into CNS - unipolar, located in PNS Motor neurons (efferent) - carry impulses away from the CNS to effector organs - multipolar, located in CNS Interneurons - lie between motor and sensory neurons - multipolar, located in the CNS
What is the difference between temporal summation and spatial summation?
Temporal summation is summation in time of graded potentials occurring in quick succession at the postsynaptic membrane. It can result from EPSPs arising from just one synapse. Spatial summation is summation in space—a postsynaptic neuron is stimulated by a large number of terminals at the same time.
Describe the two components of the electrochemical gradient.
The concentration gradient - ions move from high to low concentration The electrical gradient - ions move toward an area of opposite electrical charge
For an open channel, what factors determine in which direction ions will move through that channel?
The concentration gradient and the electrical gradient — together called the electrochemical gradient — determine the direction in which ions flow through an open membrane channel.
Recall (from Chapter 9) that, at a neuromuscular junction, the AP in the muscle fiber is also triggered by a graded potential. Name that graded potential.
The graded potential at the junctional folds of the sarcolemma is called an end plate potential (EPP). It triggers an action potential in the adjacent sarcolemma.
Your patient, Ms. Young, has failing kidneys that cause an increase in her extracellular fluid K+ concentration. What would this do to the resting membrane potential of her neurons and muscle cells? Explain.
The resting membrane potential depends on both the concentration gradients of K+ and Na+ across the membrane and the permeability of the membrane to K+ and Na+. An increase in the extracellular K+ concentration will decrease the concentration gradient for K+ and this will decrease the resting membrane potential of neurons and muscle cells. In other words, the inside of these cells will become less negative (be depolarized).
What determines the size of a graded potential?
The size of a graded potential is determined by the strength of a stimulus.
Since some K+ is always leaking out of the cell and some Na+ is always leaking in, what prevents the concentration gradients from eventually being the same?
The sodium-potassium pump stabilizes the resting membrane potential by maintaining the concentration gradients for sodium and potassium - ejects 3 Na+ out and transports 2 K+ back into the cell.