Ch 11 Fundamentals of the Nervous System and Nervous Tissue

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

Microglia

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

Integration Center

Ependymal cells ________.

Help to circulate the cerebrospinal fluid

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

Schwann cells

The sheath of Schwann is also called the ________.

neurilemma

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

a threshold level stimulas

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

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

synapse

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

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

endorphins

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

somatic

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

electrical synapse

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

graded

Select the correct statement about serial processing

spinal reflexes are an example

Which of the following is not true of graded potentials?

They increase amplitude as they move away from the stimulus point

Saltatory conduction is made possible by ________.

The myelin sheath

A second nerve impulse cannot be generated until ________.

the membrane potential has been reestablished

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

Ganglia

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

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

neurotransmitter

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

Node of Ranvier

Reflexes are rapid, automatic responses to stimuli.

true

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

Central Nervous System

Brain and spinal cord
Integration and command center

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

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

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.

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

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

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

microglia

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

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

Oligodendrocytes

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

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

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

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

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

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.

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.

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

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

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.

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

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

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

white matter

dense collections of myelinated fibers

gray matter

mostly soma and short unmyelinated fibers

Neuron Classification
Structural

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

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

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

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.

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

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

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

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

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

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

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

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

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

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

Synapse

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

Postsynaptic neuron

- transmits impulses away from the synapse

Presynaptic neuron

- conducts impulses toward the synapse

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

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

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

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

Removal of neurotransmitters occurs when they:

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

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

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

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

Chemical transmitters

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

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

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

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

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

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

Neural Integration: Neuronal Pools

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

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

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

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