Test 3 Prep

Motor or efferent division

Somatic nervous system Conscious control of skeletal muscles Autonomic nervous system (ANS) Regulates smooth muscle, cardiac muscle, and glands 2 Divisions - sympathetic and parasympathetic

Chemical synapses

Specialized for the release and reception of neurotransmitters Typically composed of two parts: Axonal terminal of the presynaptic neuron, which contains synaptic vesicles filled with neurotransmitters Receptor region on the postsynaptic neuron

Presynaptic neuron

- conducts impulses toward the synapse

Postsynaptic neuron

- transmits impulses away from the synapse

unmyenlienated axons of the pns

A Schwann cell surrounds nerve fibers but coiling does not take place Schwann cells partially enclose 15 or more axons

Nerve impulses

A brief reversal of membrane potential with a total amplitude of 100 mV They do not decrease in strength over distance They are the principal means of neural communication Occur only in the axon of a neuron Nerve Impulse Once initiated it has the same physiology as an Action Potential in a muscle cell. Initiation is by voltage gated Na channels on the membrane of the axon hillock and results from the summation of graded potentials. Threshold - a critical level of depolarization (-55 to -50 mV) At threshold, depolarization becomes self-generating

Synapse

A junction that mediates information transfer from one neuron: To another neuron To an effector cell

Summation of potentials

A single potential cannot induce a nerve impulse at the axon hillock Potentials must summate temporally or spatially to induce an action potential Temporal summation - all potentials are received from the same terminal Spatial summation - postsynaptic neuron is stimulated by a large number of terminals at the same time Both inhibitory (-) and excitatory (+) potentials sum

Neurotransmitters: Novel Messengers Nitric oxide (NO)

Activates the intracellular receptor guanylyl cyclase Is involved in learning and memory Carbon monoxide (CO) is a main regulator of cGMP in the brain May explain narcotic effects of Nitrogen under pressure and CO excess

Muktiple Sclerosis

An autoimmune disease that mainly affects young adults Symptoms: visual disturbances, weakness, loss of muscular control, and urinary incontinence Immune cells attack and damage the neurilemma and myelin sheath Shunting and short-circuiting of nerve impulses occurs

Removal of neurotransmitters occurs when they:

Are degraded by enzymes Are reabsorbed by astrocytes or the presynaptic terminals Diffuse from the synaptic cleft

Nerve fibers

Arm-like extensions from the soma There are two types: axons (carry impulses away from the soma) and dendrites (carry impulses toward the soma) Bundles of fibers are called tracts in the CNS and nerves in the PNS

axons of the cns

Both myelinated and unmyelinated fibers are present Myelin sheaths are formed by oligodendrocytes Nodes of Ranvier are widely spaced There is no neurilemma

Central Nervous System

Brain and spinal cord Integration and command center

Neurotransmitters

Chemicals used for neuronal communication with the body and the brain 50 different neurotransmitters have been identified Classified chemically and functionall

Which of the following is not one of the chemical classes into which neurotransmitters fall?

Chlorides

Chemical transmitters

Classified into chemical families Acetylcholine (ACh) Biogenic amines Amino acids Peptides Novel messengers: ATP and dissolved gases NO and CO

Conduction velocities of axons

Conduction velocities vary widely among neurons Rate of impulse propagation is determined by: Axon diameter - the larger the diameter, the faster the impulse Presence of a myelin sheath - myelination dramatically increases impulse speed Presence and frequency of nodes of Ranvier which result in saltatory conduction

Salatory conduction

Current passes through a myelinated axon only at the nodes of Ranvier Voltage-gated Na+ channels are concentrated at these nodes Action potentials are triggered only at the nodes and jump from one node to the next Much faster than conduction along unmyelinated axons

Neurotransmitter Receptor Mechanisms

Direct: neurotransmitters that open ion channels Promote rapid responses Examples: ACh and amino acids Indirect: neurotransmitters that act through second messengers Promote long-lasting effects Examples: biogenic amines, peptides, and dissolved gases

Types of Circuits in Neuronal Pools

Divergent - one incoming fiber stimulates ever increasing number of fibers, often amplifying circuits Convergent - opposite of divergent circuits, resulting in either strong stimulation or inhibition Reverberating - chain of neurons containing collateral synapses with previous neurons in the chain Parallel after-discharge - incoming neurons stimulate several neurons in parallel arrays

Structure of an axon

Each neuron has a single slender axon of uniform diameter arising from the axon hillock. Axons may occasionally branch along their length prior to reaching the axonal terminus. These 90 degree branches, if present, are called axon collaterals. Axonal terminal - the profusely branched terminus of an axon.

Nerve Impulses are:

Electrical impulses carried along the length of axons Always the same regardless of stimulus The underlying functional feature of the nervous system

Operation of voltage gated channels

Example: Na+ channel Closed when the intracellular environment is negative Open when the intracellular environment is positive Voltage-gated K+ channels function the same way

Operation of a Chemically Gated Channel

Example: Na+-K+ chemically gated channel Closed when a neurotransmitter is not bound to the extracellular receptor Open when a neurotransmitter is attached to the receptor

Which of the following is not a functional classification of neurons?

Extraneurons

The sodium-potassium pump ejects two Na from the cell and then transports three K back into the cell in order to stabilize the resting membrane potential.

False The sodium-potassium pump ejects three Na from the cell and then transports two K back into the cell in order to stabilize the resting membrane potential

Synaptic cleft

Fluid-filled space separating the presynaptic and postsynaptic neurons Prevents nerve impulses from directly passing from one neuron to the next Transmission across the synaptic cleft: Is a chemical event (as opposed to an electrical one) Ensures one-way communication between neurons

Myelin Sheath and Neurilemma: Formation

Formed by Schwann cells in the PNS only around axons A Schwann cell forms the Myelin Sheath: Envelopes an axon in a trough Encloses the axon with its plasma membrane Forms concentric inner layers of membrane locked together with special membrane proteins that make up the myelin sheath Neurilemma is the remaining nucleus and cytoplasm of a Schwann cell occupying the outer layer.

Neural Integration: Neuronal Pools

Functional groups of neurons that: Integrate incoming information Forward the processed information to its appropriate destination

Collections of nerve cell bodies outside the central nervous system are called ________.

Ganglia

Nodes of Ranvier

Gaps in the myelin sheath between adjacent Schwann cells They are the sites where axon collaterals can emerge They function to speed up transmission of nerve impulses

Function of an axon

Generate and transmit action potentials away from the soma Secrete neurotransmitters from the axonal terminals All substances needed for axonal activity must be transported down the axon by vesicular trafficking (involves molecular motors and microtubules) Movement along axons occurs in two ways Anterograde — toward axonal terminal Retrograde — away from axonal terminal

Types Of glial cells

Glial cells of the CNS are of 4 types Astrocytes - most numerous, involved in care and feeding of neurons Microglia - specialized phagocytes Ependymal cells - ciliated cells lining CNS cavities that secrete CSF Oligodendrocytes - produce myelin sheaths in the CNS Glial cells of the PNS are of 2 types Satellite cells - surround PNS neuron cell bodies, function unknown Schwann cells - produce myelin sheaths in the PNS

Ependymal cells ________.

Help to circulate the cerebrospinal fluid

Astrocytes

Highly branched glial cells Most abundant CNS glia They cling to neurons and their synaptic endings, and cover capillaries Astrocytes Functionally, they: Support and brace neurons Anchor neurons to their nutrient supplies Guide migration of young neurons Control the chemical environment New evidence suggests they may effect integration

Dendrites

In motor neurons they are short, tapering, and diffusely branched processes but do take other forms in other neurons. They are the main receptive, or input, regions of the neuron. Electrical signals travel as graded potentials (not action potentials) toward the soma over dendritic fibers.

Neurotransmitters: Biogenic Amines

Include: Catecholamines - dopamine, norepinephrine (NE), and epinephrine Indolamines - serotonin and histamine Broadly distributed in the brain Play roles in emotional behaviors and our biological clock

Neurotransmitters: Amino Acids

Include: GABA - Gamma ()-aminobutyric acid Glycine Aspartate Glutamate Found only in the CNS

Neurotransmitters: Peptides

Include: Substance P - mediator of pain signals Beta endorphin, dynorphin, and enkephalins Act as natural opiates; reduce pain perception Bind to the same receptors as opiates and morphine Gut-brain peptides - somatostatin, and cholecystokinin

What component of the reflex arc determines the response to a stimulus?

Integration Center

Neurotransmitters: Novel Messengers ATP

Is found in both the CNS and PNS Produces excitatory or inhibitory responses depending on receptor type Induces Ca2+ wave propagation in astrocytes Provokes pain sensation

Nerve cell body(soma)

Is the major biosynthetic center hence has well-developed Nissl bodies (rough ER) and Golgi Is the focal point for the outgrowth of neuronal processes Contains an axon hillock - cone-shaped area from which axons arise

types of glial cells monitor the health of neurons, and can transform into a special type of macrophage to protect endangered neurons?

Microglia

Neuron Classification Structural

Multipolar — three or more processes Bipolar — two processes (axon and dendrite) Unipolar — single, short process

Synaptic cleft information transfer

Nerve impulses reach the axonal terminal and open Ca2+ channels Neurotransmitter is released into the synaptic cleft via exocytosis Neurotransmitter crosses the cleft and binds to receptors Ion channels open, causing an excitatory or inhibitory effect Synaptic Cleft: Information Transfer Termination of Neurotransmitter Effects Neurotransmitter bound to a postsynaptic neuron: Produces a continuous postsynaptic effect Blocks reception of additional "messages" Must be removed from its receptor

Which of the following is not one of the basic functions of the nervous system

Neural genesis

Which of the following are bundles of neurofilaments important in maintaining the shape and integrity of neurons

Neurofibrils

The two principal cell types of the nervous system are:

Neurons (10%) - excitable cells that transmit nerve impulses and perform integration Supporting cells (90%) - Called neuroglia or glial cells these cells surround and wrap neurons to perform tasks necessary for neuronal functioning.

synaptic delay

Neurotransmitter must be released, diffuse across the synapse, and bind to receptors Synaptic delay - time needed to do this (0.3-5.0 ms) Synaptic delay is the rate-limiting step of neural transmission

Post synaptic potentials

Neurotransmitter receptors mediate changes in membrane potential according to: The amount of neurotransmitter released The amount of time the neurotransmitter is bound to receptors The two types of postsynaptic potentials are: Excitatory postsynaptic potentials Inhibitory postsynaptic potentials

Electicical Synapses

No Neurotransmitters Are less common than chemical synapses Correspond to gap junctions found in other cell types Are important in the CNS in: Arousal from sleep Mental attention Emotions and memory Ion and water homeostasis

A gap between Schwann cells in the peripheral system is called a(n) ________.

Node of Ranvier

Which of the following are gaps found along a myelin sheath?

Nodes of Ranvier

Which of the following types of glial cells produce the myelin sheaths that insulate the neural fibers in the CNS?

Oligodendrocytes

Peripheal Nervous System

Paired spinal and cranial nerves Carries messages to and from the spinal cord and brain): Two Functional Divisions Sensory (afferent) division Sensory fibers - carry impulses from sensory receptors in the body to the CNS Motor (efferent) division Motor fibers - Transmits impulses from the CNS to effector organs

Types of plasma ion channels

Passive channels - always open Chemically gated channels - open with binding of a specific neurotransmitter Voltage-gated channels - open and close in response to membrane potential Mechanically gated channels - open and close in response to physical deformation of receptors

Which of the following is not a type of circuit?

Pre-discharge circuits

Absolute Refractory period

Prevents the neuron from generating a nerve impulse Ensures that each action potential is separate Enforces one-way transmission of nerve impulses

General functions of Neuroglia

Provide a supportive scaffolding for neurons Segregate and insulate neurons Guide young neurons to the proper connections Promote health and growth

Electrical Current and the Body

Reflects the flow of ions rather than electrons There is a potential across the plasma membrane of all cells called the resting membrane potential. This potential is created and maintained by the Na/K pumps.

Which of the following circuit types is involved in the control of rhythmic activities such as the sleep-wake cycle, breathing, and certain motor activities (such as arm swinging when walking)?

Reverberating circuits

Oligodendrocytes

Schwann Cells, and Satellite Cells Oligodendrocytes - branched cells that wrap CNS nerve fibers and produce myelin sheaths

PNS neuroglia help to form myelin sheaths around larger nerve fibers in the PNS?

Schwann cells

Which of the following PNS neuroglia help to form myelin sheaths around larger nerve fibers in the PNS?

Schwann cells

Neuron Classification Functional

Sensory (afferent) — transmit impulses toward the CNS Motor (efferent) — carry impulses away from the CNS Interneurons — shuttle signals through CNS pathways and perform integration

Patterns of Neural Processing

Serial Processing Input travels along one pathway to a specific destination Works in an all-or-none manner Example: spinal reflexes Parallel Processing Input travels along several pathways Pathways are integrated in different CNS systems One stimulus promotes numerous responses Example: smell may remind one of associated experiences

Graded potentials

Short-lived, local changes in membrane potential Decrease in intensity with distance Magnitude varies directly with the strength of the stimulus Alone or summed they can result in the initiation of nerve impulses on axons Only travel over short distances Occur only on dendrites Involved in the process of integration

Multiple Sclerosis Treatment

The advent of disease-modifying drugs including interferon beta-1a and -1b, Avonex, Betaseran, and Copazone: Hold symptoms at bay Reduce complications Reduce disability

Which criteria is used to functionally classify neurons?

The direction in which the nerve impulse travels relative to the central nervous system.

Neurons

The functional cell of the nervous system Composed of a soma (body), axon, and dendrites (fibers) Long-lived, amitotic, and have a high metabolic rate Plasma membranes function in electrical signaling

The Nervous System

The master controlling and communicating system of the body Functions Sensory input -information gathered from sensory receptors monitoring stimuli Integration - interpretation of sensory input by the central nervous system (brain and spinal cord) Motor output - activates effector organs

Saltatory conduction is made possible by ________.

The myelin sheath

Which of the following does not factor into the rate of impulse propagation?

The number of axon collaterals extending from a truncated axon.

membrane potentials

Used to integrate, send, and receive information Types of signals - graded potentials and action potentials Changes are caused by three events Depolarization - the inside of the membrane becomes less negative Repolarization - the membrane returns to its resting membrane potential Hyperpolarization - the inside of the membrane becomes more negative than the resting potential

Which of the following allows us to consciously control our skeletal muscles

The somatic nervous system

Which of the following is not true of graded potentials?

They increase amplitude as they move away from the stimulus point

Functional Classification of Neurotransmitters

Two classifications: excitatory and inhibitory Excitatory neurotransmitters cause depolarizations Inhibitory cause hyperpolarizations Some have both excitatory and inhibitory effects Determined by the receptor type of the postsynaptic neuron Example: acetylcholine Excitatory at neuromuscular junctions Inhibitory in cardiac muscle

Myelin sheath

Whitish, fatty (protein-lipoid), segmented sheath around most long axons It functions to: Protect the axon Electrically insulate fibers from one another Increase the speed of nerve impulse transmission

What type of stimulus is required for an action potential to be generated?

a threshold level stimulas

Which neurotransmitter(s) is/are the body's natural pain killer?

endorphins

white matter

dense collections of myelinated fibers

The synapse more common in embryonic nervous tissue than in adults is the ________.

electrical synapse

Unmyelinated fibers conduct impulses faster than myelinated fibers

false

________ potentials are short-lived, local changes in membrane potential that can be either depolarized or hyperpolarized

graded

gray matter

mostly soma and short unmyelinated fibers

The sheath of Schwann is also called the ________.

neurilemma

The substance released at axon terminals to propagate a nervous impulse is called a(n) ________.

neurotransmitter

ependymal cells

range in shape from squamous to columnar, many are ciliated They produce CSF, line the central cavities of the brain and spinal column, and create CSF flow

microglia

small, ovoid cells with spiny processes Phagocytes that monitor the health of neurons

That part of the nervous system that is voluntary and conducts impulses from the CNS to the skeletal muscles is the ________ nervous system.

somatic

Select the correct statement about serial processing

spinal reflexes are an example

Schwann cells

surround fibers of the PNS and produce myelin sheaths. Vital to nerve regeneration

Satellite cells

surround neuron cell bodies in the PNS, function largely unknown

An impulse from one nerve cell is communicated to another nerve cell via the ________.

synapse

Which ion channel opens in response to a change in membrane potential and participates in the generation and conduction of action potentials?

voltage- gated channels

A second nerve impulse cannot be generated until ________.

the membrane potential has been reestablished

Which of the following is not true of an electrical synapse?

they are specialized for release and reception of chemical neurotransmitters

A stimulus traveling toward a synapse appears to open calcium ion channels at the presynaptic end, which in turn promotes fusion of synaptic vesicles to the axonal membrane

true

Reflexes are rapid, automatic responses to stimuli.

true