Chapter 11 Nervous System

Front 1

Nervous System

Back 1

-Is master controllig and communicating system of the body.

- Cells communicate via electrical and chemical signals

-Rapid and specific.

-Ussyally cause almost imnediate responses.

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Nervous System has 3 Overlapping Function.

Back 2

-Sensory input

-Integration

-Motor output

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Sensory Input

Back 3

Information gathered by sensory receptors about internal and external changes.

Front 4

Integration

Back 4

Processing and interpretation of sensory input.

Front 5

Motor Output

Back 5

Activation of Effector organs (muscles and glands) produces a reponse.

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Nervous system is divided into two principals parts:

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-Central Nervous System (CNS)

-Peripheral Nervous System (PNS)

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Central Nervous System

Back 7

-Brain and spinal cord.

-Integration and control center.

(Interprets sensory input and dictates motor output)

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Peripheral Nervous System

Back 8

-Cranial nerves and spinal nerves.

-Communication lines between the CNS and the rest of the body

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(PNS) has two functional divisions:

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-Sensory (Afferent) division.

-Motor (Efferent) division.

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Sensory (Afferent) Division

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-Somatic and viscera sensory nerves fibers.

-Conducts impulses from receptions to the CNS.

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Motor (Efferent) Division

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-Transmits impulses from CNS to effector organs (muscles and glands)

-Two divisions:

*Somatic nervous system

*Autonomic nervous system

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-Somatic Nervous System

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-Conduct impulses from CNS to skeletal muscle.

-Voluntary nervous system (Conscious control of skeletal muscle)

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-Autonomic Nervous System

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-Consist of visveral motor nerve fibers.

-Regulates smooth, cardiac muscle and glands.

-Involuntary nervous system.

-Two functional subdivisions:

*Sympathetic: Mobilizes body during emergency.

*Parasympathetic: Conserve energy non-energy functions. Promote housekeeping functions during rest.

Work in opposition to each other.

Front 14

Nervous System Histology.

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Nervous system consist of two principal cell types:

-Neuroglia (glial cells)

-Neurons (nerve cells)

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Neuroglia (Glial Cells)

Back 15

Small cells that surround and wrap delicate neurons.

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Neurons (Nerve Cells)

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Excitable cells that transmit electrical signals.

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4 Main Neuroglia Support CNS Neurons

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-Astrocytes

-Microglia cells

-Ependymal cells

-Oligodendrocytes

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Astrocytes

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-Most abundant, versatile, and highly branched of glial cells.

-Cling to neurons, synaptic endings, and capillaries.

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Astrocytes Functions:

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-Support and brace neuros.

-Paly role in exchanges between capillaries and neurons.

-Guide migration of young neurons.

-Control chemical enviroment aroung neurons.

-Respond to nerve inpulses and neurotransmitters.

-Influence neuronal functioning.

-Participate in information processing in brain.

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Microglia Cells

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-Small, ovoid cells with thorny processes that touch and monitor neurons.

-Migrate toward injured neurons.

-Can transform to phagocytize microorganisms and neuronal debris.

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Ependymal Cells

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-Range in shape from squamous to columnar.

-May be ciliated (cilia beat to circulate CSF)

-Line the cental cavities of the brain and spinal column.

-Form permeable barrier between cerebrospinal fluid (CSP) in cavities and tissue fluid bathing CNS cells.

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Oligodendrocytes

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-Branched cells.

-Processes wrap CNS nerve fibers, forming insulating myelin sheaths in thicker nerve fibers.

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Two Major Neuroglia seen in PNS

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-Satellite cells

-Schwann cells

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Satellite Cells

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-Surround neuron cell bodies in PNS

-Function similar to astrocytes of CNS

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Schwann Cells (Neurolemmocytes)

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-Surround all peripheral nerve fibers and form myelin sheaths in thicker nerve fibers. (similar function as oligodendrocytes.

-Vital to regeneration of damaged peripheral nerve fibers.

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Neurons (Nerve Cells)

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-Structural units of nervous system.

-Large, highly specialized cells that conduct impulses.

-All have cell body and one or more processes.

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Neurons Special Characteristics

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-Extreme longevity (last a person's lifetime)

-Amitotic, with few exceptions.

-High metabolic rate: requires continuous supply of oxygen and glucose.

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Nueron Cell Body

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-Also called the perikaryon or soma.

-Biosynthetic center of neuron:

*Synthesizes proteins, membranes, chemicals.

*Rough ER (chromatophilic substance, or Nissl bodies)

-Contains spherical nucleus with nucleolus.

-Some contain pigments.

-In most, plasma memrane is part of receptive region that recieces input from other neurons.

-Most neuron cellbodies are located in CNS.

*Nuclei: Clusters of neuron cell bodies in CNS.

*Ganglia: Clusters of neurons cells bodies in PNS.

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Neuron Processes

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-Armlike processes that extend from cell body

*CNS contains both neuron cell bodies and their processes.

*PNS contains chiefly neuron processes.

-Tracts: Bundles of neurons processes in CNS

-Nerves: Bundles of neuros processes in PNS

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Two Types of Neuron Processes

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-Dendrites

-Axon

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Dendrites

Back 31

-Motor neurons can contain 100s of these short, tapering, diffusedly branched processes. (contain same organelles as in cell body).

-Receptive (input) region of neuron.

-Convey incoming messages toward cell body as graded potentials (short distance signals).

-In many brain areas, finer dentrites are highly specialized to collect information.

Front 32

Axon: Structure

Back 32

-Each neuron has one axon that starts at coneshaped area called Axon Hillock.

-In some neurons, axons are short or absent; in others, axon comprises almost entire lenght of cell. (some axons can be over 1 meter long)

-Long axons are called nerve fibers.

-Axons have occasional branches called Axon Collaterals.

-Axons branch profusely at their end (terminus).

-Distal ending are called Axon Terminals or Terminal Boutons

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Axon: Functional Characteristics

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-Axon is the conducting region of neuron.

-Generates nerves impulses and tersmits them along Axolemma (neuron cell membrane) to axon terminal.

.Terminal: region that secretes neurotransmitters, which are released into extracellular space.

.Can excite or inhibit neurons it contacts.

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Axon: Functional Characteristics

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-Carries on many conversations with different neurons at the same time.-Axon rely on cell bodies to renew proteins and menbranes-Quickly decay if cut or damaged.

-Axon have efficient internal transport mechanisms. (molecules and organelles are moved along axons by motor proteins and sytoskeletal elements).

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Axon Movements Occurs in both Direction

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-Anterograde

-Retrograde

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Anterograde

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Away from cell body.

i.e. Mitichondria, cytoskeletal elements, membrane components, enzymes.

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Retrograde

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Toward cell body

i.e. Organalles to be degraded, signal molecules, virus, and bacteria toxins.

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Myelin Sheath

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Composed of myelin, a whitish, protein-lipid substance.

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Function of Myelin

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-Protect and electrically insulate axon.

-Increase speed of nerve impulse transmisision.

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Myelinated Fibers

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Segmented sheath surrounds most long or large-diameter axons

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Nonmyelinated Fibers

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Do not contain sheath. (conduct impulses more slowly).

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Myelination in the PNS

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-Formed by Schwann cells.

.Wraps around axon in jelly roll fashion.

.One cell forms one segment of myelin sheath.

-Outer collar of perinuclear cytoplasm (formerly called neurilemma): peripheral bulge containing nucleus and most of cytoplasm.

-Plasma membranes have less protein.

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Myelin Sheaths in the CNS

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-Formed by processes of oligodendrocytes, not whole cells.

-Each cell can wrap up to 60 axons at once
-Myelin sheath gap is present
-No outer collar of perinuclear cytoplasm
-Thinnest fibers are unmyelinated, but covered by long extensions of adjacent neuroglia

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White Matter/ Grey Matter

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-Regions of brain and spinal cord with dense collections of myelinated fibers (usually fiber tracts)/

-Mostly neuron cell bodies and non-myelinated fibers

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Structural Classification

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-Multipolar

-Bipolar

-Unipolar

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Multipolar

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-Three or more processes (1 axon,
others dendrites)

-Most common and major neuron type in CNS

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Bipolar

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Two processes (one axon, 1one dendrite)
Rare (ex: retina and olfactory mucosa)

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Unipolar

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-One T-like process (two axons)
-Also called pseudounipolar
-Peripheral (distal) process: associated with sensory receptor
-Proximal (central) process: enters CNS

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Functional Classification of Neurons

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Three types of neurons grouped by direction in which nerve impulse travels relative to CNS:

-Sensory

-Motor

-Interneurons

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Sensory

Back 50

-Transmit impulses from sensory receptors toward CNS
-Almost all are unipolar
-Cell bodies are located in ganglia in PNS

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Motor

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-Carry impulses from CNS to effectors
-Multipolar
-Most cell bodies are located in CNS (except some autonomic neurons)

Front 52

Interneurons

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-Also called association neurons
-Lie between motor and sensory neurons
-Shuttle signals through CNS pathways
-Most are entirely within CNS
-99% of body’s neurons are interneurons

Front 53

Basic Principles of Electricity

Back 53

-Opposite charges are attracted to each other
-Energy is required to keep opposite charges separated across a membrane
-Energy is liberated when the charges move toward one another
-When opposite charges are separated, the system has potential energy

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Voltage

Back 54

-A measure of potential energy generated by separated charge
-Measured between two points in volts (V) or millivolts (mV)
-Called potential difference or potential (charge difference across plasma membrane results in potential)
-Greater charge difference between points=higher voltage

Front 55

Current

Back 55

-Flow of electrical charge (ions) between two points
-Can be used to do work
-Flow is dependent on voltage and resistance

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Resistance

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-Hindrance to charge flow
-Insulator: substance with high electrical resistance
-Conductor: substance with low electrical resistance

Front 57

Ohm’s law

Back 57

-Gives relationship of voltage, current, resistance
-Current (I) = voltage (V)/resistance (R)
-Current is directly proportional to voltage
-Greater the voltage (potential difference), greater the current
-No net current flow between points with same potential
-Current is inversely proportional to resistance
-The greater the resistance, the smaller the current

Front 58

Role of membrane ion channels

Back 58

Large proteins serve as selective membrane ion channels
K+ ion channel allows only K+ to pass through

Front 59

Two main types of ion channels

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-Leakage

-Gated

Front 60

Leakage (nongated) channels,

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Which are always open

Front 61

Gated Channels

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-In which part of the protein changes shape to open/close the channel
-Three main gated channels: chemically gated, voltage-gated, or mechanically gated

Front 62

Chemically gated (ligand-gated) channels

Back 62

Open only with binding of a specific chemical (example: neurotransmitter)

Front 63

Voltage-gated channels

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Open and close in response to changes in membrane potential

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Mechanically gated channels

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Open and close in response to physical deformation of receptors, as in sensory receptors

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When gated channels are open, ions diffuse quickly:
Along chemical concentration gradients from higher concentration to lower concentration
Along electrical gradients toward opposite electrical charge

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Electrochemical gradient

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-Electrical and chemical gradients combined
-Ion flow creates an electrical current, and voltage changes across membrane
-Expressed by rearranged Ohm’s law equation:
V = IR

Front 67

Generating the Resting Membrane Potential

Back 67

-A voltmeter can measure potential (charge) difference across membrane of resting cell
-Resting membrane potential of a resting neuron is approximately –70 mV
-The cytoplasmic side of membrane is negatively charged relative to the outside
-The actual voltage difference varies from

–40 mV to –90 mV
-The membrane is said to be polarized

Front 68

Generating the Resting Membrane Potential (cont.) Potential generated by:

Back 68

-Differences in ionic composition of ICF and ECF-Differences in plasma membrane permeability

Front 69

Differences in ionic composition

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-ECF has higher concentration of Na+ than ICFBalanced chiefly by chloride ions (Cl)

-ICF has higher concentration of K+ than ECFBalanced by negatively charged proteins

-K+ plays most important role in membrane potential

Front 70

Differences in plasma membrane permeability

Back 70

-Impermeable to large anionic proteins

-Slightly permeable to Na+ (through leakage channels)Sodium diffuses into cell down concentration gradient.

-25 times more permeable to K+ than sodium (more leakage channels)Potassium diffuses out of cell down concentration gradient

-Quite permeable to Cl–

-More potassium diffuses out than sodium diffuses in.As a result, the inside of the cell is more negative.Establishes resting membrane potential

Front 71

Sodium-potassium pump (Na+/K+ ATPase)

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stabilizes resting membrane potential

-Maintains concentration gradients for Na+ and K+

-Three Na+ are pumped out of cell while two K+ are pumped back in

Front 72

Changing the Resting Membrane Potential

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Membrane potential changes when:

Concentrations of ions across membrane change

Membrane permeability to ions changes

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Changes produce two types of signals

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-Graded potentials (incoming signals operating over short distances)

-Action potentials (long-distance signals of axons)

Changes in membrane potential are used as signals to receive, integrate, and send information

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Depolarization

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Decrease in membrane potential (moves toward zero and above)

Inside of membrane becomes less negative than resting membrane potential.

Probability of producing impulse increases

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Hyperpolarization

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Increase in membrane potential (away from zero) Inside of membrane becomes more negative than resting membrane potential

Probability of producing impulse decreases

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GRATED POTENTIAL

Short-lived, localized changes in membrane potential

The stronger the stimulus, the more voltage changes and the farther current flows

Triggered by stimulus that opens gated ion channels

Results in depolarization or sometimes hyperpolarization

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-Named according to location and function

*Receptor potential (generator potential): graded potentials in receptors of sensory neurons *Postsynaptic potential: neuron graded potential

-Once gated ion channel opens, depolarization spreads from one area of membrane to next

-Current flows but dissipates quickly and decaysGraded potentials are signals only over short distances

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ACTION POTENTIAL

-Principal way neurons send signals

-Means of long-distance neural communicationOccur only in muscle cells and axons of neurons

-Brief reversal of membrane potential with a change in voltage of ~100 mV

-Action potentials (APs) do not decay over distance as graded potentials do

-In neurons, also referred to as a nerve impulse-Involves opening of specific voltage-gated channels

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Generating an Action Potential

Back 78

1.Resting state: All gated Na+ and K+ channels are closed.

2.Depolarization: Na+ channels open.

3.Repolarization: Na+ channels are inactivating, and K+ channels open.

4.Hyperpolarization: Some K+ channels remain open, and Na+ channels reset.

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Generating an Action Potential (cont.)

Back 79

-Repolarization resets electrical conditions, not ionic conditions

-After repolarization, Na+/K+ pumps (thousands of them in an axon) restore ionic conditions

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Threshold and the All-or-None Phenomenon

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-Not all depolarization events produce APs

-For an axon to “fire,” depolarization must reach threshold voltage to trigger AP

-At threshold:-Membrane is depolarized by 15 to 20 mV

*Na+ permeability increases

*Na+ permeability increasesNa+ influx exceeds K+ efflux

*The positive feedback cycle begins

-All-or-None: An AP either happens completely, or does not happen at all