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UTEP / Engineering / BIOL 231127357 / Which characteristic of muscle tissue refers to its ability to stretch

Which characteristic of muscle tissue refers to its ability to stretch

Which characteristic of muscle tissue refers to its ability to stretch

Description

School: University of Texas at El Paso
Department: Engineering
Course: Human Anatomy and Physiology I
Professor: Dr. zaineb al-dahwi
Term: Spring 2017
Tags:
Cost: 50
Name: Ch. 9 & Ch. 11
Description: Muscles & Neurons
Uploaded: 04/10/2018
29 Pages 88 Views 3 Unlocks
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Chapter 9. Muscles & Muscle Tissue


Which characteristic of muscle tissue refers to its ability to stretch?



∙ Muscle Functions

o Movement of bones or fluids

o Maintaining body posture & position

o Stabilizing joints

o Generating heat to maintain body temp. (especially  skeletal muscle)

∙ Special Characteristics of Muscle Tissue

o Excitability (responsiveness or irritability): the ability to  receive & respond to stimuli – from nervous system o Contractility: ability to shorten when stimulated o Extensibility: ability to be stretched

o Elasticity: ability to recoil to resting length

∙ 3 Types of Muscle Tissue

1. Skeletal

 Attached to bones & skin


What are the features of sarcomere?



 Striated, multinucleate

  Voluntary 

 Powerful

2. Cardiac

  Only in the heart 

 Striated

 Involuntary

3. Smooth

 In the walls of hollow organs If you want to learn more check out How is the sara model performed?

  Not striated 

 Involuntary

∙ Skeletal Muscle

o Each muscle is served by one artery, one nerve, & one  or more veins

o Connective tissue sheaths: EPEndo

 Epimysium: dense regular connective tissue  

surrounding entire muscle

 Perimysium: fibrous connective tissue surrounding  fascicles (groups of muscle fibers)


What are the principles of muscle mechanics?



 Endomysium: fine areolar connective tissue  surrounding each muscle fiber 

o Muscles attach:

 DIRECTLY: epimysium of muscle id fused to the  periosteum of bone or perichondrium of cartilage  INDIRECTLY: connective tissue wrappings extend  beyond the muscle as a ropelike tendon or sheet like aponeurosis (layers of flat broad tendons) o Microscopic Anatomy If you want to learn more check out How do conductors and insulators differ?

 Cylindrical cell 10-100 μm in diameter, up to 30  cm long

 Multiple peripheral nuclei

 Many mitochondria (due to high metabolic  demand)

 Glycosomes for glycogen storage

 Myoglobin for O2 storage

 Contain myofibrils, sarcoplasmic reticulum, & T  tubules

o Myofibrils

 Densely packed, rod-like elements composed of  numerous myofilaments (basic unit of muscle)  80% of cell volume

 exhibit striations: perfectly aligned repeating  series of dark A bands & light I bands

 interaction of thick & thin filaments produces:  MUSCLE CONTRACTION

o Sarcomere

 Smallest contractile unit (functional unit) of a  muscle fiber

 Region of myofibril between 2 successive Z discs  Composed of thick & thin myofilaments made of  contractile proteins

 Features:

 Thick filaments (composed of myosin 

filaments): run length of A band

 Thin filaments (composed of actin filaments):  run length from I band partway into A band

 Z Disc: coin-shaped sheet of proteins that  anchors thin filaments & connects myofibrils  to one another If you want to learn more check out Why is dhikr important among muslim mystics?

 H Zone: lighter mid-region where filaments  do not overlap

 M line: protein myosin, holds adjacent thick  filaments together

o Ultrastructure of Thick Filament

 MYOSIN TAILS: 2 interwoven, heavy polypeptide  chains

 MYOSIN HEADS:  

 only at overlapping area

 2 smaller, light polypeptide chains (act as  cross bridges during contraction)

 actin binding sites

 ATP binding sites

 ATPase enzymes

o Ultrastructure of Thin Filaments

 Twisted double strand of fibrous protein F actin  F actin consists of G (globular) actin subunits  G actin bears active sites for myosin head  attachment during contraction

 Tropomyosin & troponin: regulatory proteins bound to actin

o Sarcoplasmic Reticulum (SR)

 Network of smooth ER surrounding each myofibril  Pairs of terminal cisternae form perpendicular  cross channels Don't forget about the age old question of How many countries signed the helsinki accords?

 Functions in the regulation of intracellular 2+¿ Ca¿ 

levels

o T Tubules

 Continuous with the sarcolemma = plasma  membrane

 Penetrate the cell’s interior at each A band - I band junction

 Associate with the paired terminal cisternae (of  the SR) to form triads (remember T has 3 ends) that encircle each sarcomere

 Triad relationships:

 T tubules conduct impulses deep into muscle  fiber

 T tubule proteins: voltage sensors

 SR foot proteins: gated channels that  

regulate 2+¿ 

Ca¿ release from the SR cisternae 

o Neuron Structure:

 Cell body: contains nucleus

 Dendrite: surrounds cell body & receives  

signal

 Axon: long tail that conducts nerve signal  Axon terminals: transmit signal to muscle or  other neurons across a synapse We also discuss several other topics like How does a theory function in close reading?
If you want to learn more check out What is the correlation between chemical and physical change?

o Physiology: Contraction

 Activation of myosin cross bridges generate force  Causes muscle to shorten & ALSO lengthen or  remain the same under tension

 Shortening occurs when tension generated by  cross bridges on the thin filaments exceeds forces  opposing shortening

 Contraction ends when cross bridges become  inactive & tension declines, inducing fiber muscle  relaxation

 In relaxed state: thin & thick filaments overlap  slightly

 During contraction: myosin heads bind to actin,  detach, & bind again propelling the thin filaments  toward M line

 As H zones shorten & disappear, sarcomeres  shorten, muscle cells shorten, & the whole muscle  shortens

 Neuron Structure:

 Cell body: contains nucleus

 Dendrite: surrounds cell body & receives  signal

 Axon: long tail that conducts nerve signal  Axon terminals: transmit signal to muscle or  other neurons across a synapse 

 Requirements for contraction:

1. Activation: neural stimulation at a  

neuromuscular junction  

a. Axons of somatic motor neurons  

originated from spinal cord & travel to

skeletal muscles

b. Each axon forms several branches as  it enters a muscle (at axon terminals)

c. Each axon ending forms a  

neuromuscular junction (NMJ) with a  

single muscle fiber (synapse)

d. NMJ is situated midway along the  

length of the muscle fiber

e. Axon terminal & muscle fiber ate  

separated by the synaptic cleft

f. Synaptic vesicles of axon terminal  

contain the neurotransmitter  

acetylcholine (ACh) 

g. Junctional folds of the sarcolemma  

contain Ach receptors

h. Nerve impulse arrives at axon  

terminal, ACH is released & binds with

receptors on sarcolemma, then  

electrical events leas to the  

generation of action potential 

2. Generation of Action Potential:

a. ACh binding opens ligand gated ion  

channels = ACh receptors  

b. Simultaneous influx of +¿ 

Na¿ & efflux  

of +¿ 

K¿ causing interior of  

sarcolemma to be less negative

c. Local depolarization

d. generation & propagation of an action 

potential along the sarcolemma

e. end plate potential spreads to  

adjacent membrane areas

f. voltage-gated +¿ 

Na¿ channels open

g.+¿ 

Na¿ influx decreases membrane  

voltage toward critical threshold

h. if threshold is reached an action  

potential is generated (first  

depolarization)

i. causes a brief rise in intracellular

2+¿

Ca¿ levels from SR in muscle 

3. Repolarization:

a.+¿ 

Na¿ channels close & voltage-gated

+¿

K¿ channels open

b. +¿ 

K¿ efflux rapidly restores the resting

polarity

c. fiber cannot be stimulated & is in  

refractory period until repolarization  

is complete

d. ionic conditions of the resting state  

are restored by the +¿ 

Na¿ -+¿ 

K¿ pump

4. Termination: prevents continued muscle  

fiber contraction in the absence of  

additional stimulation

a. ACh effects are quickly terminated by  

the enzyme acetylcholinesterase in  

synaptic cleft

o Excitation-Contraction (E-C) Coupling

 Sequence of events by which transmission of  an AP along the sarcolemma leads to sliding  of the myofilaments

 Latent period: time when E-C coupling  

events occur between AP initiation & the  

beginning of contraction

 AP is propagated along sarcomere to T  

tubules

 Voltage-sensitive proteins stimulate 2+¿ 

Ca¿ 

release from SR which triggers contraction

o Major players in muscle contraction

 Calcium

 At low concentration:

o Tropomyosin blocks active sites on  

actin

o Myosin heads cannot attach to  

actin

o Muscle fiber relaxes

 At higher concentration:

o2+¿ 

Ca¿ binds to troponin 

o troponin changes shape & moves  

tropomyosin away from active sites

o myosin heads attach to actin

o events of cross bridge cycle occur

o when nervous stimulation ceases,

2+¿

Ca¿ is pumped back out &  

contraction ends

 Actin

 Myosin

 Tropomyosin

 Troponin

 ATP

o Cross Bridge Cycle

 Continues as long as the 2+¿ 

Ca¿ signal &  

adequate ATP are present

 Cross bridge formation: high-energy myosin  head attaches to thin filament

 Working (power) stroke: myosin head pivots  & pulls thin filament toward M line

 Cross bridge detachment: ATP attaches to  myosin head & the cross bridge detaches

 “Cocking” of the myosin head: energy from  the hydrolysis of ATP cocks the myosin head  into the high-energy state  

o Principles of Muscle Mechanics

1. Same principles apply to contraction of a  single fiber & a whole muscle

2. Contraction produces tension: the force  

exerted on the load or object to be moved

3. Contraction does not always shorten a  

muscle

a. Isotonic contraction

 Muscle changes in length &  

moves the load

 Concentric: the muscle shortens  

& does work

 Eccentric: the muscle contracts  

as it lengthens

b. Isometric contraction

 The load is greater than the  

tension the muscle is able to  

develop

 Tension increases to the  

muscle’s capacity, but the  

muscle does not change in  

length 

4. Force & duration of contraction vary in  

response to stimuli or different frequencies

& intensities

o Motor Unit

 The nerve-muscle functional unit

 A motor neuron in spinal cord & all the muscle  fibers it supplies

 More motor units activated = more muscle fibers  activated = stronger muscle contraction

 SMALL MOTOR UNITS: control fine movements  (fingers, eyes)

 LARGE MOTOR UNITS: in large weight-bearing  muscles (thighs, hips)

 Muscle fibers from motor unit are spread  

throughout the muscle so that a single motor unit  causes weak contraction of entire muscle

 Motor units usually contract asynchronously (not  at the same time) which helps prevent fatigue o Muscle Twitch

 Response of a muscle to single, brief threshold  stimulus

 3 phases:

1. latent period: events of E-C coupling

2. period of contraction: cross bridge  

formation; tension increases

3. period of relaxation: 2+¿ 

Ca¿ reentry into the  

SR; tension declines to zero

 different strength & duration: due to variations in  metabolic properties & enzymes in different  muscles

o Graded Muscle Responses

 Variations in the degree of muscle contraction  Required for proper control of skeletal movement  Responses are graded by:

 Changing the frequency of stimulation

 Single stimulus: single twitch

 Multiple stimuli: unfused or fused  

tetanus

 Temporal/wave summation: another  

stimulus before muscle relaxes  

completely, causing more tension

 More frequency = more force

 Changing the strength of the stimulus

 Threshold stimulus: stimulus strength at  

which the first observable muscle  

contraction occurs

 Muscle contracts more vigorously as  

stimulus strength is increased above  

threshold

 Contraction force is precisely controlled  

by recruitment (multiple motor unit  

summation), which brings more & more  

muscle fibers to action

 Size principle of recruitment: small  

highly excitable neurons are recruited,  

as stimulus intensity increases, moor  

units with larger fibers are recruited  

causing stronger contractions

o Muscle Tone

 Constant, slightly contracted state of all muscles  Due to spinal reflexes that activate groups of  motor units in response to input from stretch  receptors in muscles

 Keeps muscles firm, healthy, & ready to respond o Muscle Metabolism: Energy for Contraction  ATP is the only source used directly from  

contractile activities

 ATP is regenerated by:

 direct phosphorylation of ADP by creatine  phosphate (CP)

 1 ATP per CP

 anaerobic pathway (glycolysis)

 At 75 % of maximum contractile activity

 Bulging muscles compress blood vessels

 O2 delivery is impaired

 pyruvic acid is converted into lactic acid

lactic acid: diffuses into the  

bloodstream, is used as fuel by the  

liver, kidneys, & heart. Then  

converted back into pyruvic acid by

the liver

 2 ATP per glucose

 aerobic respiration

 produces 95% of ATP during rest & light  

to moderate exercise

 fuels stored in glycogen, blood borne  

glucose, pyruvic acid, & free fatty acids

 32 ATP per glucose

o Muscle Fatigue

 Physiological inability to contract: muscle cramps  Occurs when:

o Ionic imbalance interferes with E-C coupling o Prolonged exercise damages SR & interferes  with 2+¿ 

Ca¿ regulation & release 

 Total lack of ATP causes contractures (states of  continuous contractions) due to cross bridges  being unable to detach (rigor mortis)

o Oxygen Deficit

 Defined as extra O2 needed after exercise for:  Replenishment of oxygen, glycogen, ATP, &  CP reserves

 Conversion of lactic acid to pyruvic acid, glucose,  & oxygen

o Heat Production during Muscle Activity

 40% of the energy released in muscle activity is  useful as work

 remaining 60% given off as heat

 dangerous heat levels are prevented by radiation  of heat from skin & sweating

o Contractile Force is Increased by:

 Large number of muscle fibers activated

 Large (in size) muscle fibers activated

 High frequency stimulation

 Muscle & sarcomere are stretched slightly over  100% of resting strength

o Velocity & Duration of Contraction Influenced by: 1. Muscle fiber type

2. Load: more load = longer latent period, less  muscle shortening, shorter duration, & slower  contraction

3. Recruitment: more recruitment = longer  

duration of contraction

o Muscle Fiber Type

 Defined by 2 characteristics:

1. Speed of contraction:  

a. Speed at which myosin ATPases split ATP

b. Pattern of electrical activity of the motor  

neurons

2. Metabolic pathways for ATP synthesis:

a. Oxidative fibers: use aerobic pathways

b. Glycolytic fibers: use anaerobic glycolysis  

 3 types

 slow oxidative fibers: endurance type  

activities, slow fatigue

 fast oxidative fibers: sprinting, walking

 fast glycolytic fibers: short-term intense  

movements, fast fatigue (e.g. hitting a  

baseball)

o Effects of Exercise

 Aerobic (endurance) exercise:

 Increase in muscle capillaries, number of  

mitochondria, & myoglobin synthesis

 Greater endurance, strength, & resistance to  fatigue

 Converts fast glycolytic fibers into fast  

oxidative fibers

 Anaerobic (resistance) exercise:

 Muscle hypertrophy (increase in volume), due to increase in fiber size

 Increase in mitochondria, myofilaments,  

glycogen stores, & connective tissue

 Overload principle

 Forcing a muscle to work hard promotes  

increase in muscle strength & endurance

 Muscles adapt to increased demands

 Muscles must be overloaded to produce  

further gains

∙ Smooth Muscle

o Usually in 2 layers: longitudal (outer) & circular (inner) o Microscopic Structure

 Spindle-shaped fibers: thin & short compared with  skeletal

 Connective tissue: endomysium only

 SR: less developed than skeletal

 Caveolae: pouch-like infoldings that contain  calcium in sarcolemma that sequester 2+¿ 

Ca¿ 

 No sarcomeres, myofibrils, or T tubules & not  striated

o Peristalsis

 Wavelike constant, rhythmic alternating  

contractions & relaxations of smooth muscle  layers that mix & squeeze substances through the  lumen of hollow organs

o Innervation of Smooth Muscle  

 Autonomic nerve fibers innervate smooth muscle  & diffuse junctions

 Varicosities (bulbous swellings) of nerve fibers  store & release neurotransmitters (acetylcholine,  epinephrine, & norepinephrine)

o Myofilaments in Smooth Muscle

 Ratio of thick to thin filaments (1:13) is much  lower than in skeletal muscle (1:2)

 Thick filaments have heads along entire length  (skeletal only in overlapping areas)

 Myofilaments are spinally arranged, causing  smooth muscle to contract in corkscrew manner  Dense bodies: proteins that anchor noncontractile  intermediate filaments to sarcolemma at regular  intervals

o Contraction of Smooth Muscle

 Slow synchronized contractions

 Cells are electrically couples by gap junctions  Some cells are self-excitatory & act as pacemakers for sheets of muscle

 Rate & intensity of contraction is modified by  neural & chemical stimuli

 Sliding filament mechanism for actin & myosin  interaction

 Final trigger for contractions: increase intracellular 2+¿

Ca¿ 

2+¿ 

Ca¿ from the SR & extracellular space

o Role of Calcium Ions

2+¿ 

Ca¿ binds to & activates calmodulin

 activated calmodulin activates myosin light  chain kinase (MLCK)

 activated kinase phosphorylates & activates  myosin

 cross bridges interact with actin

o Contraction of Smooth Muscle

 Very energy efficient (slot ATPases)

 Myofilaments maintain a latch state for  

prolonged contractions (low ATP consumption =  fatigue resistant)

 Relaxation requires:

∙2+¿ 

Ca¿ detachment from calmodulin 

∙ active transport of 2+¿ 

Ca¿ into SR & EFC 

∙ dephosphorylation of myosin to reduce  

myosin ATPase activity by MLC  

phosphatase

o Regulation of Contraction

1. Neural regulation:

a. Neurotransmitter binding increases 2+¿ 

Ca¿ 

in sarcoplasm; either graded (multi-unit)  

potential or action (single-unit) potential

b. Response depends on neurotransmitter  

released & type of receptor molecules

2. Hormones & local chemicals:

a. Bind to G-protein linked receptors

b. Either enhance or inhibit 2+¿ 

Ca¿ entry 

o Special Features of Contraction

1. Stress-relaxation response:

a. Responds to stretch only briefly, then  

adapts to new length

b. Retains ability to contract on demand

c. Enables organs such as the stomach &  

bladder to temporarily store contents

2. Length & tension relationship

a. Tension over a much wider range of muscle

lengths

b. Can contract when it is between half &  

twice its resting length

3. Hyperplasia:

a. Cells can divide & increase their numbers  

o Types of Smooth Muscle

1. Single-unit (visceral):

a. In walls of hollow visceral organs except  

heart

b. Sheets contract rhythmically as a unit

c. Often exhibit spontaneous action  

potentials

d. Arranged in opposing sheets & exhibit  

stress-relaxation response

2. Multiunit:

a. Located in large airways, large arteries,  

arrector pili muscles, & iris of eye

b. Gap junctions are rare

c. Arranged in motor units

d. Graded contractions occur in response to  

neural stimuli

∙ Disorders & Developmental Aspects

o Muscular Dystrophy

 Group of inherited muscle-destroying diseases  Muscles enlarge

 Muscle fibers atrophy (wasting or decrease in size)  Duchenne muscular dystrophy (DMD):

∙ Most common

∙ Inherited, sex-linked, carried by females &  expressed in males due to lack of dystrophin  protein

∙ Death due to cardia or respiratory failure ∙ No cure, but viral gene therapy or infusion of  stem cells with correct dystrophin genes  shows promise

 All muscle tissues rom embryotic development  Multinucleated skeletal cells form by fusion  Growth factor fibroblast stimulates clustering of  

ACh receptors at neuromuscular junctions  Smooth muscle myoblasts develop gap junctions  Skeletal muscle cells have limited regenerative  ability

 Smooth muscle regenerates throughout life  Female skeletal muscle makes up 36% of body  mass

 Male skeletal muscle makes up 42% of body mass, primarily due to testosterone

 Body strength per unit muscle mass is the same in both sexes

 With age, connective tissue increases & muscle  fibers decrease

 By age 30 loss of muscle mass (sarcopenia) begins  Regular exercise reverses sarcopenia

Chapter 11. Fundamentals of the Nervous System & Nervous Tissue

∙ Function

o Sensory input: information gathered by sensory  receptors about internal changes

o Integration: interpretation of sensory input

o Motor output: activation of effector organs (muscles &  glands) produces a response

∙ Division of Nervous System

o Central nervous system (CNS)

  Brain & spinal cord 

 Covered with meninges (membranes; 3 continuous sheets of connective tissue) & bones

 Integration & command center

o Peripheral nervous system (PNS)

 Connects the CNS to the limbs & organs

 Paired spinal & cranial nerves carry messages to & from CNS

 Sensory neurons: afferent neurons (nerve impulses toward to CNS)

 Motor neurons: efferent neurons (nerve impulses  away from CNS)  

 Somatic nervous system: conscious control

 Autonomic nervous system: unconscious  

control

∙ Sympathetic division: mobilizes body  

systems during activity

∙ Parasympathetic division: conserves  

energy & promotes house-keeping  

functions during rest

o Histology of Nervous Tissue

 2 principal cell types

1. neurons: excitable cells that transmit  

electrical signals

2. glial cells (neuroglia): supporting cells

a. astrocytes (CNS) - BBB

b. microglia (CNS) -immune

c. ependymal cells (CNS) – cerebrospinal

fluid

d. oligodendrocytes (CNS) - myelination

e. satellite cells (PNS) - cell body wrap

f. schwann cells (PNS) myelination

 Astrocytes (Astro-looks like sparkly star)

o Most abundant, versatile & highly branched  

glial cells

o Cling to neurons, synaptic endings, &  

capillaries (blood brain barrier)

o Support & brace neurons

o Help determine capillary permeability

o Guide migration of young neurons

o Control chemical environment

o Participate in information processing of the  

brain

 Microglia (immune cells)

o Small, ovoid cells with thorny processes

o Migrate toward injured neurons

o Phygocytize microorganisms & neuronal debri  Ependymal Cells

o Range in shape from squamous to columnar

o Ciliated:  

 line the central cavities (ventricles) of  

the brain & spinal column (central canal)

 separate the CNS interstitial fluid from  

the cerebrospinal fluid in the cavities

 move cerebrospinal fluid

 Oligodendrocytes

o Branched cells  

o Processes wrap CNS nerve fibers, forming  

insulating myelin sheaths

 Satellite & Schwann Cells

o Satellite cells surround neuron cell bodies in  

the PNS

o Schwann cells (neurolemnocytes) surround  

PNS fibers & form myelin sheaths; vital to  

regeneration of damaged PNS fibers

∙ Neurons (Nerve Cells)

o Long-lived (individual’s life span)

o Amitotic (do not go through mitosis) with few  exceptions

o High metabolic rate: continuous supply of oxygen &  glucose

o Plasma membrane functions in:

 Electrical signaling

 Cell-to-cell interactions during development  Structure:

 Cell body

 Processes: dendrites & axons

 Bundles in CNS – tracks

 Bundles in PNS – nerves

o Cell Body (Perikaryon or Soma)

 Biosynthetic center of a neuron

 Spherical nucleus with nucleolus

 Well-developed Golgi Apparatus

 Rough ER calls Nissl bodies

 Network of neurofibrils (neurofilaments)

 Axon hillock: cone shaped area from which axon  arises

 Clusters of cell bodies are called nuclei in CNS &  ganglia in PNS

 Dendrites

o Short, tapering & diffusely branches

o Receptive (input) region of a neuron

o Convey electrical signals toward the cell body  Axon

o One axon per cell arising from axon hillock o Long axons (nerve fibers)

o Occasional branches

o Numerous terminal branches

o Knoblike axon terminals (synaptic knobs or  boutons)

 Secretory region of neuron

 Release neurotransmitters to excite or  

inhibit other cells

o Functions:

 Conducting region of a neuron

 Generates & transmits nerve impulses  (action potentials) away from the cell  body

 Molecules & organelles are moved along axons by motor molecules in 2 different  directions  

1. Anterograde: toward axon  

terminal (mitochondria,  

membrane components,  

enzymes)

2. Retrograde: toward cell body  

(organelles to be degraded,  

signal molecules, viruses, &  

bacterial toxins)

o Myelin Sheath

 Segmented protein-lipoid sheath around most long large diameter axons

 Functions:

 Protect & electrically insulate axons  Increase speed of nerve impulse  

transmission

 In PNS

 Schwann cells wrap many times  

around the axon forming myelin  

sheath

 Neurilemmal: peripheral bulge of  

Schwann cell cytoplasm

 Nodes of ranvier:

∙ Myelin sheath gaps, between  

adjacent schwann cells

∙ Un-insulated axon membrane

∙ Sites where axon can branch

 Unmyelinated fiber

∙ One schwann cell  

incompletely

∙ Enclose 15 or more axons

∙ Typically thin nerve fibers

∙ Low velocity nerve impulse

 In CNS

 Formed by processes of  

oligodendrocytes, not by whole  

cells

 Nodes of Ranvier are present &  

conduct electrical activity

 No neurilemmal

 Thinnest fibers are unmyelinated

∙ White Matter & Gray Matter

o White matter

 Dense collections of myelinated fibers

o Gray matter

 Mostly neuron cell bodies & unmyelinated fibers ∙ Structural Classification of Neurons

o 3 types:

1. multipolar:  

a. 1 axon & several dendrites

b. MOST ABUNDANT

c. Motor neurons & interneurons

2. Bipolar:  

a. 1 axon & 1 dendrite

b. RARE (retinal & olfactory neurons)

3. Unipolar:

a. Single, short process with 2 branches

b. Peripheral process: often associated with  

sensory receptor

c. Central process: branch entering CNS

d. Mainly in PNS

∙ Functional Classification of Neurons

o 3 types

1. sensory (afferent): transmit impulses from sensory  receptors toward the CNS

2. motor (efferent): carry impulses from the CNS to  effectors  

3. interneurons (association neurons): shuttle signals  through CNS pathways; most entirely within the CNS ∙ Membrane Potentials of Neurons

o Irritability: capable of responding to stimulation by  generating action potential (nerve impulse)

o Conductivity: capable of conducting impulses  o Membrane ion channels: protein complexes; responsible for resting potential & graded potential

 2 main types of ion channels

1. leakage (nongated) channels: always open

2. gated channels:

a. ligand-gated channels (chemically gated): 

open with binding of a specific  

neurotransmitter  

b. voltage-gated channels: open & close in  

response to changes in membrane  

potential

c. mechanically gated channels: open & close

in response to physical deformation of  

receptors

∙ Resting Membrane Potential (Vr)

o Potential difference across the membrane of resting cell is approximately -70mV in neurons (cytoplasmic side of  membrane is negatively charged relative to outside) o Generated by:

 Differences in ionic makeup of intracellular &  extracellular fluid (ICF & ECF)

 Differential permeability of plasma membrane o Differences in ionic makeup

 ICF has lower concentration of sodium & chlorine  than ECF

 ICF has higher concentration of potassium &  

negatively charged proteins than ECF

o Differential membrane permeability

 Impermeable to negative proteins

 Slightly permeable to sodium (through leakage  channels)

 75x more permeable to potassium (more leakage  channels)

 freely permeable to chlorine

o Negative interior of the cell is due to much greater  diffusion of potassium out of cell than sodium diffusion  into cell

o Sodium-potassium pump & ATPase stabilizes the resting membrane potential by maintaining concentration  gradients for sodium & potassium

o Membrane potential changes when:

 Concentration of ions across membrane change  Permeability of membrane ions changes

 Changes in membrane potential are signals used  to receive, integrate, & send info

o Depolarization: inside becomes more positive  Increases the probability of producing a nerve  impulse

o Hyperpolarization: inside becomes more negative  Decrease the probability of producing a nerve  impulse

∙ Membrane Potentials as Signals

o 2 types:

1. graded potentials: incoming short-distance  

signals, typically in dendrites

 short lived, localized changes in membrane

potential

 depolarization & hyperpolarization

 spread as local currents & change memb  

potential of adjacent regions

 occur when stimulus causes gated-ion  

channels to opne

 magnitude varies directly with stimulus  

strength

 decrease in magnitude with distance as  

ions flow & diffuse through leakage  

channels

2. action potentials: long-distance signals of axons  brief reversal of membrane potential with  

total amplitude of 100 mV

 occurs in muscle cells & axons of neurons

 does not decrease magnitude over  

distance

 resting state: only leakage channels for  

sodium & potassium are open; all gated  

sodium & potassium channels are closed

 properties for gated channels: voltage  

gated channels

i. each sodium channel has 2 voltage

sensitive gates: activation gates that  

open with depolarization &  

inactivation gates that block channel  

once its opened

ii. each potassium channel has one  

voltage-sensitive gate that opens  

slowly with depolarization

o Action Potential Phases

1. Depolarizing phase:

 Local currents open voltage-gated sodium  channels

 Sodium influx causes more depolarization  At threshold (-55 to -50 mV)positive feedback leads to opening of all sodium channels,  

leading to a reversed membrane polarity to  30 mV (spike of action potential)

3. Repolarizing phase:

 Slow inactivation gates close sodium  

channel

 Membrane permeability to sodium declines to resting levels

 Slow voltage-sensitive potassium gates  

open

 Potassium exits the cell & internal  

negativity is restored

4. Hyperpolarization:

 Some potassium channels remain open,  

allowing excess potassium efflux

 This causes after-hyperpolarization of the  membrane

∙ Role of Sodium-Potassium Pump

o Repolarization  

 Restores electrical conditions of the neuron but not resting ionic conditions

o Ionic redistribution back to resting conditions is restored by the thousands of pump

∙ At Threshold:

o Membrane is depolarized by 15-20 mV from resting  value

o Sodium permeability increases

o Odium influx exceeds potassium efflux

o Positive feedback cycle begins

o Subthreshold stiumus: weak local depolarization that  does not reach threshold

o Threshold stimulus: strong enough to push the  membrane potential toward & beyond threshold o AP is ALL OR NONE phenomenon: Aps either happen  completely or not at all

∙ Stimulus Intensity

o All action potentials are independent of stimulus  intensity

o CNS determined stimulus intensity by the frequency of  impulses

 Strong stimuli generate Aps more often than  

weaker stimuli

o Time from opening sodium channels until resetting of  the channels is called the refractory period 

o The relative refractory period follows absolute  refractory period & elevates threshold for AP generation ∙ Conduction Velocity

o Conduction velocities of neurons vary widely

 Effect of axon diameter

o Larger diameter fibers have less resistance to local current flow & have faster impulse  

conduction

 Effect of myelination

o Continuous conduction in unmyelinated  

axons is slower than saltatory conduction in  

myelinated axons

o Myelin sheaths insulate & prevent leakage of  charge

o Salutatory conduction in myelinated axons is  about 30x faster

o Voltage-gated sodium channels are located at the nodes & Aps appear to jump rapidly from  

node to node

∙ Synapse

o A junction that mediates info transfer from one neuron  to another neuron or to an effector cell

o Presynaptic neuron: conducts impulses toward synapse o Postsynaptic neuron: transmits impulses away from  synapse

o Electrical or chemical synapses

o Types:

 Axodendritic: between axon of one neuron &  

dendrite of another

 Axosomatic: between axon of one neuron & cell  body (soma) of another

 Less common:

o Axoaxonic: axon to axon

o Dendrodendritic: dendrite to dendrite

o Dendrosomatic: dendrite to soma  

o Electrical Synapses

 Less common than chemical synapses

 Electrically coupled by connexin

 Important in embryotic nervous tissue &  

some brain regions

o Chemical Synapses

 Specialized for the release & reception of  

synaptic vessicles

 Composed of axon terminal of the  

presynaptic neuron which contains  

nurotransmitters & dendritic region on  

postsynaptic neuron

o Synaptic Cleft

 Fluid-filled space separating the presynaptic  

& postsynaptic neurons

 Prevents nerve impulses from directly  

passing from one neuron to the next

 Transmissions acros synaptic cleft:

 Is a chemical event

 Involves release, diffusion, & binding of  

neurotransmitters

o Infomration Transfer

 Action potential arrives at axon terminal &  opens voltage-gated calcium channels

  Synaptotagmin protein binds calcium &  

promotes fusion of synaptic vesicles with  

axon membrane

 Exocytosis of neurotransmitter occurs

 Neurotransmitter diffuses & binds to  

receptors (ligand-gated ion channels) on the  postsynaptic neuron

 Ion channels are opened causing an  

excitatory or inhibitory event (graded  

potential) 

o Termination of Neurotransmitter Effects

 Within a few milliseconds the  

neurotransmitter effect is terminated

 Degradation by enzymes

 Reuptake by transporters in astrocytes or  axon terminal

 Diffusion away from synaptic cleft

o Pstsynaptic Potentials

 Graded potentials (GP)

 Strength determined by:

∙ Amount of neurotransmitter released

∙ Time the neurotransmitter is in the area

 Types of postsynaptic potentials:

o Excitatory (EPSP)

 Neurotransmitter binds to &  

opens ligand-gated ion channels  

(simultaneous flow of sodium &  

potassium)

 Sodium influx greater than  

potassium efflux: depolarization

o Inhibitory (IPSP)

 Neurotransmitter binds to &  

opens channels for potassium or  

chlorine efflux or chlorine influx

 Causes hyperpolarization

o Integration: Summation

 A single EPSP cannot induce an AP

 EPSPs can summate to reach a threshold

 IPSPs can also summatewith EPSPs, cancelling each  other out

 Temporal summation: one or more presynaptic  neurons transmit impulses in rapid-fire order  Spatial summation: postsynaptic neuron is  stimulated by a large number of terminal at the same time (Autism)

o Chemical Classes of Neurotransmitters

 Acetylcholine (ACh)

 Released at NMJ & some ANS neurons &  

brain interneurons

 Degraded by the enzyme  

acetylcholinesterase

 Amino acids

 Glutamate: major excitatory  

neurotransmitter in CNS

 GABA: major inhibitory NT

 Aspartate

 Glycine

 Biogenic amines (monoamines):

 dopamine, norepinephrine (NE), epinephrine,  serotonin, histamine (addiction, ADHD,  

fight/flight, depression, allergy)

 broadly distributed in the brain

 play role in many aspects of the brain functions   Peptides (neuropeptides):

 Substance P: mediator of pain signals

 Endorphins: act as natural opiates, reduce pain  perception

 Gut-brain peptides: somatostatin and  

cholecystokinin (CCK)

 Fat cells- brain peptide - leptin

 Purines such as ATP:

 in both the CNS and PNS

 provoke pain sensation

 Gases and lipids

 Nitric oxide (NO) and carbon monoxide (CO)  involved in learning and memory

 Endocannabinoids  

 lipid soluble; synthesized on demand from  membrane lipids

 bind to G protein–coupled receptors in the brain

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