Chapter 10 and 11 Review Flashcards
Neural tissue consists of:
neurons and larger number of neuroglial cells. Neurons are amitotic, neuroglial are mitotic.
Neurons
functional unit of the nervous system
Neuroglia Cells
supportive & nutritive cells
Gross division of the Nervous System
Central Nervous System (CNS) and Peripheral Nervous System (PNS)
CNS
Central Nervous System consists of the brain & spinal cord and function with integrating signals.
PNS
Peripheral Nervous System: spinal nerves exiting from and returning to the CNS. Nerves emerging from the brain/brainstem are known as cranial nerves.
Efferent neurons
are conducting cells that carry information from the central nervous system (the brain and spinal cord) to muscles and organs throughout the body.
Afferent neurons
are sensory neurons that carry nerve impulses from sensory stimuli towards the central nervous system and brain, while efferent neurons are motor neurons that carry neural impulses away from the central nervous systme and towards muscles to cause movement.
Receptors
Functional division of the nervous system. Sensory structures (may be neural or specialized epithelial cells) which detect stimuli (environmental changes).
Afferent division of the PNS:
/or sensory. Neurons carrying signals from receptors to the CNS.
Integration of sensory signals are achieved in:
the CNS and an appropriate response is initiated.
Efferent division of the PNS
/or motor. eurons which arises from the CNS bringing motor commands to target organs (e.g.- glands or muscles).
Two types of efferent neutrons are:
- Somatic Nervous System (SNS)
2. Autonomic Nervous System (ANS)
Somatic Nervous System (SNS)
sends motor signals to Skeletal Muscles which are largely voluntary movements, and receives sensations which we perceive.
Autonomic Nervous System (ANS)
sends motor signals to and receives sensory signals from visceral organs in which regulation is automatic (e.g.- smooth muscle, cardiac and glandular secretion). The ANS has two basic divisions which are antagonistic in action: Sympathetic & Parasympathetic.
Anatomy of neurons:
consisting of the following features:
- Soma (cell body)
- Axon
- Synapse
Soma (Cell Body)
is the wide region of the neuron which contains a central Nucleus. Congregation of nuclei in CNS is referred to as nuclei, in the PNS is called ganglia.
Perikaryon
is cytoplasm, which is filled w/neurofilaments & neurotubules, mitochondria and Nissl bodies (ribosomes).
Grated Channels
Transmembraneous proteins are present on the neurolemma (cell membrane)
- Passive Channels
- Chemical-regulated channels
- Mechanical-regulated channels
Dendrites
slender, sensitive processes which receives signals from other neurons or receptors and transmitting them to the soma. They contain chemical & mechanical-regulated channels.
Passive Channels
are always open to allow the flow of some molecules.
Chemical-regulated channels
activated by ligands (eg- neurotransmittors)
Mechanical-regulated channels
activated by mechanical stress (eg- stretch or presure).
Axon
cytoplasmic extension from soma, carrying signal away from soma to telendendria.
Axolemma
Axon membrane which contains voltage-regulated Na+ channels serving as the basis for the propagation of a wave of depolarization known as an action potential.
Axoplasm
cytoplasm of the axon.
Axon Hillock
is the thickened portion of soma at the base of the axon.
Initial Segment
region of hillock that contains voltage gates to initiate an action potential.
Collaterals
several branches that emerge from the axon to produce several terminal branches.
Telodendria
fine extensions at the end of the axon which contain swellings referred to as synaptic knobs (terminal) which release vesicles of neurotransmitters by exocytosis.
Synapse
site where neurotransmitters are released from synaptic terminals and diffuse across a cleft to bind a receptor on an opposing membrane (muscle or neuron).
Physiological considerations of synaptic events:
- Presynaptic vesicles
- Postsynaptic receptors
- Clearing of Neurotransmitters from synapse:
- Axoplasmic transport
Presynaptic vesicles
once action potential reaches synaptic terminal, voltage-regulated Ca+2gates open resulting in intracellular Ca+2 initiating exocytosis of neurotransmitter vesicles.
Postsynaptic receptors
bind to ligand in synaptic cleft and will either open an ion channel that will either result in depolarization or hyperpolarization. Hence the effect of a neurotransmitter is dependent on the receptor, more so than the neurotransmitter itself.
Clearing of Neurotransmitters from synapse:
enzymes in the synaptic cleft metabolize the free neurotransmitters, after which neurotransmitters may be absorbed by the neuron.
Axoplasmic transport
refers to movement of substance through the axon:
- Antegrade axoplasmic transport
- Retrograde axoplasmic transport
Antegrade axoplasmic transport
substances flow from soma to telodendria (eg- neurotransmitters).
Retrograde axoplasmic transport
substances flow from telodendria to soma (eg- return of neurotransmitter metabolites which were reabsorbed for recycling.
Anaxonic
from brain & special senses, it contains many branches from soma, axon not discernable from dendrites.
Structural Classification of Neurons
- Anaxonic
- Bipolar neurons
- Unipolar neurons
- Multipolar neurons
Bipolar neurons
rom special sense organs for vision, smell, taste & hearing- long dendritic pole from one end of soma, and long axonal pole at other end.
Unipolar neurons
the main sensory nerve of the SNS- one branch emerges from soma and then extends in two directons. In effect the dendrite conducts through the axon to the synapse, with one branch communicating with the soma.
Multipolar neurons
the main motor neuron of the SNS- many dendrites receive converging signals before passing stimulation down long slender axon to telondendria.
Functional Classification of Neurons:
- Sensory (Afferent) Neurons
- Motor (Efferent) Neurons
- Interneurons (association fibers)
Sensory (Afferent) Neurons
deliver information to CNS for integration.
a. Interoceptors
b. Exteroceptors
c. Proprioceptors
Interoceptors
monitor changes occurring internally.
Exteroceptors
monitor changes occurring externally.
Proprioceptors
monitor changes occurring in skeletal muscles & joints (position sense).
Motor (Efferent) Neurons
integrated response from CNS to skeletal muscle or viscera.
a. Somatic Motor Neurons
b. Visceral Motor Neurons
Somatic Motor Neurons
carries volitional commands to innervate skeletal muscle.
Visceral Motor Neurons
from ANS innervates viscera (eg- heart, lungs, glands). These neurons arise from the CNS as preganglionic nerves and synapse in ganglia (cluster of neuronal cell bodies) following which postganglionic nerves synapse w/viscera.
Interneurons (association fibers)
short neurons of the CNS (& autonomic ganglia), that quickly synapse with other neurons distributing sensory & coordinating muscular activity.
Neuroglia
non-conductive neural supportive tissue of nervous which out-number neurons.
Neuroglia of CNS types:
- Ependymal cells
- Astrocytes
- Oligodendrocytes
- Microglia
Ependymal cells
create epithelial linings of the ventricles of brain and central canal of spinal cord. They are responsible for secreting Cerebral Spinal Fluid (CSF) which both cushions neural tissue and transports nutrients & wastes.
Astrocytes
most common glial cell. It is packed with myofibrils and have several functions:
a. Blood-Brain Barrier
b. Structural Framework
c. Repair of neural tissue
d. Controls neural interstitial environment
Oligodendrocytes
have long slender cytoplasmic extensions that can wrap around different axons producing a membranous insulation known as a Myelin Sheath.
a. Internodes
b. Nodes of Ranvier
c. Saltatory conduction
d. White Matter of CNS
Microglia
CNS macrophages of mesodermal origin which serves to engulf & eliminate waste products and pathogens within CNC.
Blood Brain Barrier
wrap around capillaries controlling exchange of nutrients.
Repair of neural tissue:
repair in CNS is limited, astrocytes fill lesion w/scarring to prevent further damage.
Structural framework of Astrocytes
support neurons for advancing growth and synapse.
c. Repair of neural tissue
Controls neural interstitial environment
by adjusting blood flow and contacting both capillaries and neurons.
Internodes
section of axon containing myelin sheath (impervious to ion exchange)
Nodes of Ranvier
axonal spaces in between myelin which allows for ion exchange and propagation of a wave of depolarization.
Saltatory conduction
whereby wave of depolarization is quicker when it jumps for node to node on a myelinated axon.
White Matter of CNS
due to preponderance of myelinated axons, whereby grey matter of CNS is devoid of myelin (mainly containing cell bodies).
Neuroglia of PNS
Two types:
- Satellite Cells
- Schwan Cells
Satellite Cells
found in ganglia regulating neural environment for soma.
Schwan Cells
send out flat cytoplasmic extensions which wrap continual wrap around a region of axon to produce an internode of myelin it takes several Schwann cells to myelinate an axon:
a. Neurilemma b. Several non-myelinated neurons are still supported by Schwan cells, even though they do not provide electrical insulation.
Neurilemma
is the outermost portion of the remaining schwan cell (w/its nucleus) after forming a myelin sheath about the axon.
Transmembrane potential
exists owing to a difference in electrical charges across the membrane, thus creating potential energy which fluctuates depending on membrane permeability.
Resting Membrane Potential
at rest the neuron is more negative on the inside, -70 mv.
Active Na+/K+ pump (RMP)
removes 3 Na+ and admits 2 K+ into cell, while negatively charged proteins also contribute to negativity.
Chemical gradient (RMP)
favors the influx of Na+ and the efflux of K+; however as the membrane is more permeable to K+, the cell gets more negative with K+ efflux.
Electrical gradient (RMP)
favors influx of both Na+ & K+, but again Na+ cannot enter, electrical gradient slow K+ efflux. Membrane is said to have electrical resistance so potential remains.
Electrochemical gradient (RMP)
is the net sum of both gradients working together. Note however that the neuron is only semipermeable to ions, & less permeable to Na+. If the cell was completely permeable to Na+, membrane potential would be about +66mv.
Graded (local) Potential
produced from chemical &/or mechanical gates about the dendrites and soma which open ion gates to produce a local graded current. The gate may let Na+ or Ca+2 in making the cell more positive (depolarizing) or the gate may allow Cl- in or K+ out making the cell more negative (hyperpolarizing). The greater the stimulus, the greater the electrical change.
Local currents
the electrical changes produced by chemical or mechanical gates merely spread locally, but are not propagated down the neuron.
Repolarization
once chemical or electrical stimulus is removed, the cell returns to resting potential.
Summation of graded potentials
several chemical & mechanical gates can be activated producing either hyperpolarization or depolarization, the effects are summated.
Spatial Summation
Many gates are stimulated at same time.
Temporal Summation
One gate is stimulated with greater frequency.
Action Potential
If summation of the local current creates a threshold potential at the axon hillock, Na+ voltage-gated channels will initiate a propagated wave of depolarization. This is an All-or-None response once threshold voltage is reached to open the voltage gate.
Sequence of action Potential
- Depolarization to threshold
- Sodium influx
- Inactivation of voltage-gated channel
- Return to normal permeability
Depolarization to threshold
- first step
- graded potentials result in depolarization to threshold at the initial segment w/in the hillock.
Sodium influx
- second step
- at threshold voltage (about -60mv), Na+ activation gates open allowing large influx of Na+ such that cell depolarizes to about +30mv.
Inactivation of voltage-gated channel
- third step
- Once depolarization rises above 0mv’s, Na+ inactivation gates close, halting at this gate, but the neighboring voltage-gate now opens.
Return to normal permeability
- last/fourth step
- Once inactivation gate closes, Na+/K+ pump and leaking K+ channels open causing the repolarization (hyperpolarization) of the neuron potential.
Refractory Period
refers to the time in which a neuron will not respond to another stimulus:
a. Absolute Refractory Period
b. Relative Refractory Period
Absolute Refractory Period
neuron will not respond to stimuli while activation gates are already open or when inactivation gates are closed.
Relative Refractory Period
neuron will only respond to a suprathreshold stimuli, because K+ channels are still open and neuron is becoming hyperpolarized.
Propagation of Action Potential
occurs as the neighboring voltage gate reaches threshold
a. Continuous Propagation
b. Saltatory Propagation
Continuous Propagation
wave of depolarization along unmyelinated axon in which voltage gates are very close to each other.
Saltatory Propagation
wave of depolarization along myelinated fibers in which the voltage gates are located at each node of ranvier, so propagation jumps across internodes greatly increasing speed of conduction.
Axon diameter
also effects speed of transmission. Thicker axons pose less resistance.
Three types: Type A, Type B, Type C
Type A fibers
large myelinated fibers w/fastest transmission used for somatic sensory information to CNS and also Motor Neurons from CNS to skeletal muscle fibers.
Type B fibers
smaller myelinated fibers which are slower than type A. Type B fibers are found in preganglionic motor axons of the ANS.
Type C fibers
are the smallest fibers and are unmyelinated, so they are the slowest fibers. They conduct sensory impulses from pain and also postganglionic motor axons of ANS.
Synaptic Activity
after propagation of action potential along an axon, the message must continue to another neuron or effector cell. Vesicles are released by exocytosis owing to intracellular Ca+2 which is elevated owing to stimulation of voltage-regulated Ca+2 gates in synaptic knobs.
Chemical Synapse
refers to the release of neurotransmitters from terminal axon synaptic bulbs which bind receptors on other neurons or effector cells to result in a graded hyperpolarization or depolarization.
Acetylcholine
The most common neurotransmitter is acetylcholine (Ach)
Cholinergic synapse
Acetylcholine vesicles are released at the synaptic knob by exocytosis as Cholinergic synapses are found:
a. Neuromuscular junction of SNS (always depolarizing).
b. Neuron to neuron in PNS
c. Preganglionic neurons of ANS (always depolarizing).
d. Postganglionic Parasympathetic neurons (can be hyperpolarizing or depolarizing).
e. Many synapses w/in CNS.
Cholinergic receptors
are associated w/chemically-regulated ion channels which briefly allow permeability of ions (eg- Na+); thus creating a graded potential.
Removal of Acetylcholine
Acetylcholinesterase (AchE) enzyme will catabolize Ach in the synapse. The choline is often reabsorbed by the synaptic knob for recycling.
Many axons can release several neurotransmitters:
a. Biogenic Amines
b. Amino Acids
c. Dissolved Gases
d. Neuropeptides
Biogenic Amines
- Norepinephrine (NE)
- Dopamine
- Serotonin
Amino Acids
two main amino acid neurotransmitters:
- Glutamate
- Gamma Aminobutyric Acid (GABA)
Dissolved Gases
- Nitric Oxide (NO)
- Carbon Monoxide (CO)
Neuropeptides
peptides w/in CNS:
- Substance P
- Opioids
Norepinephrine (NE)
- released at adrenergic synapses found in the brain and at sympathetic postsynaptic neurons at the ANS.
- Biogenic Amine
Dopamine
- can be inhibitory or excitatory. It is inhibitory in regions of brain regulating skeletal muscle tone and lack of dopamine is implicated in Parkinson’s.
- Biogenic Amine
Serotonin
- has widespread appearance throughout CNS, low levels have deleterious effect on attention & emotion. Antidepressants block serotonin catabolism.
- Biogenic Amine
Glutamate
Usually excitatory by opening Ca+2 channels.
-Amino Acids
Gamma Aminobutyric Acid (GABA)
Usually inhibitory effect (hyperpolarizes).
-Amino Acids
Nitric Oxide (NO)
relaxes smooth muscles associated w/blood vessels.
-Dissolved Gases
Carbon Monoxide (CO)
found in some brain synapses
-Dissolved Gases
Substance P
- is released from pain neurons in the CNS and also from neurons in the gut which influences digestive reflexes.
- Neuropeptide
Opioids
- are found throughout the CNS and they largely act as neuromodulators (morphine & endorphins) which act by blocking the release of Substance P from terminal knobs of pain fibers.
- Neuropeptide
Synaptic Delay
the time in which a neurotransmitter is released, until there is an effect on the postsynaptic membrane. (0.2- 0.5 msec). Messages travel faster with less synapsis.
Synaptic Fatigue
w/intense stimulation where neurotransmitter is utilized faster than it can be produced or recycled. In this case stimulation halts until neurotransmitter is replenished.
Neuromodulators
are released into a neural synapse to modulate the response by either inhibiting or enhancing pre-synaptic release of neurotransmitter or altering the post-synaptic response of a neurotransmitter.
Mechanism of Action of neurotransmitter:
- Direct effect via chemical-gated channels: Ionotropic effect altering ion concentrations.
- Indirect effect on membrane potential: neurotransmitter binds receptor that alters enzyme activity intracellularly producing a secondary messenger (eg- Cyclic-AMP) which alters activity of the cell. Some neurotransmitters can target Direct & Indirect receptors.
- Lipid soluble neurotranmitters can bind intracellular receptors (eg-NO).
Post -Synaptic Potentials
graded potentials can either be:
- Excitatory Post-Synaptic Potentials (EPSP)
- Inhibitory Post-Synaptic Potentials (IPSP)
Excitatory Post-Synaptic Potentials (EPSP)
graded potentials that depolarize cells (typically letting Na+ or Ca+2 into the cell):
a. Temporal Summation
b. Spatial Summation
Temporal Summation
rapid succession of stimuli which summate response. (EPSP)
Spatial Summation
many neurons simultaneously firing to summate response. (EPSP)
Inhibitory Post-Synaptic Potentials (IPSP)
graded potentials that hyperpolarize cells (typically letting Cl- in or K+ out).
Action Potential
ensues if the EPSP summates to reach threshold (all or none response).
Rate of Action Potential Generation
will influence the intensity of a sensation or the magnitude of motor command.
Neural Circuitry
is the basis by which integration occurs w/in the CNS:
a. Diverging Circuits
b. Converging Circuits
c. Reverberating Circuits
d. Parallel After-Discharge
Diverging Circuits
One neuron innervates multiple neurons- typical of sensory circuits.
Converging Circuits
Several neurons innervate a single neuron- typical of motor circuits.
Reverberating Circuits
an arrangement in which neurons send out collateral branches that go back to stimulate prior neurons in positive-feedback manner (eg- breathing or waking up).
Parallel After-Discharge
arrangement whereby neurons give off a series of collaterals that all converge on the same neuron (used for learning complex concepts).
Wallerian Degeneration
response of neurons of the PNS which ultimately leads to repair. Neural crush injuries which only lasts for one to two hours may repair following Wallerian degeneration
Process of Wallerian Degeneration:
A. Changes w/in Soma: Nissl bodies disperse & nucleus moves form central position as increases protein synthesis.
B. The axon & myelin distal to the injury degenerate
C. Macrophages migrate to the area to remove debris
D. Regeneration tube: Schwann cells proliferate to from a solid cellular cord through which the repairing axon go grow through (if neurilemma does not remain intact, repair cannot occur).
E. Axon growth: with help of neural cell body, the axon grows about 1.5 cm/day through the regeneration tube to land in the identical region of the original neuron.
F. Myelin Sheath: Schwann cells fold around developing neuron to produce a new myelin sheath.
Muscles generate tension to function to:
produce movement, stabilize body positions, generate heat, regulate organ volumes and move substances w/in the body. There are 3 basic muscle types:
- Skeletal Muscle
- Smooth Muscle
- Cardiac Muscle
Skeletal (voluntary or striated) Muscle
under control of somatic nervous system and typically attaches to bone.
Skeletal Muscle functions:
Functions include:
Produce body movement: contraction overcomes a load to approximate origin & insertion.
Maintain posture & body position: maintains tension w/o movement.
Support soft tissue: pelvic floor and abdominal wall.
Guard entrances & exits: acting as sphincters to regulate emptying.
Maintain body temperature: contraction is exergonic, releasing energy.
Store nutrient reserves: if inadequate nutritional intake, protein broken down for energy.
Functional anatomy of skeletal muscle:
muscle fiber is the basic contractile cell of muscle and it is arranged in bundles wrapped in CT with blood vessels and nerves.
CT of muscle organ:
- Endomysium
- Perimysium
- Epimysium
Endomysium
elastic CT which surrounds & isolates each muscle fiber, it contains:
Capillaries
Satellite cells
Nerve fibers
Capillaries
supplying each muscle fiber w/blood supply
Satellite cells
muscle stem cells for muscle repair of small injury.
Nerve fibers (muscle)
each muscle fiber is innervated by motor nerve fiber & CT insulates that fiber so that it can contract on its own.
Perimysium
divides muscles into compartments with fascicles (bundles) of 10 to 100 muscle fibers.
Epimysium
CT which covers the whole muscular organ separating it from other tissues.
Tendon/Aponeurosis
At the end of each muscle fiber, collagen fibers of the endomysium merges w/the perimysium and then with epimysium covered by deep fascia to emerge as a tendon or long flat aponeurosis. Sharpey’s fibers blend collagen fibers to periosteum.
Skeletal muscle fibers
are large multinucleated contractile cells. Embryologically created from about 100 myoblasts (stem cells). Muscle fibers contain the following features:
a. Sarcolemma
b. Sarcoplasm
c. Multinucleated: with nuclei off to periphery
d. Mitochondria: many mitochondria for energy.
e. Sarcoplasmic Reticulum
f. Transverse tubules (t-Tubules)
g. Myofibrils
h. Sarcoplasmic Triad
i. Sarcomeres
j. Myoglobin
k. Glycogen granules
Sarcoplasm
refers to skeletal muscle cytoplasm.
Sarcolemma
cell membrane of the skeletal muscle cell (fiber). Depolarization of sarcolemma is basis for muscular contraction.
Sarcoplasmic Reticulum
endoplasmic reticulum of muscle which stores calcium. Contains an enzyme (sequestrium) which binds and holds calcium.
Sarcoplasm cistern
is swelling of the tips of sarcoplasmic reticulum.
Transverse tubules (t-Tubules)
narrow tubules which extends from sarcolemma into the sarcoplasm (cytoplasm of muscle cell). Carries wave of depolarization into cell.
Myofibrils
cylindrical structures which span the length of the muscle fiber and contain myofilaments responsible for muscle contraction. Myofibrils are surrounded by sarcoplasmic reticulum. The main contractile myofilaments are actin & myosin
Thin filimaments
referred to as actin. Note: 6 thin filaments surround each thick filament. Thin filament consists of:
- F Actin
- Nebulin
- Tropomyosin
- Troponin
F actin
twisted chain of globulin actin proteins
Nebulin
strand between actin globules holding them together.
Tropomyosin
protein which covers the active sites on actin.
Troponin
regulatory protein that holds tropomyosin in place covering actin, but if troponin binds calcium, it moves tropomyosin off actin (exposing active sites of actin).
Thick filaments
primarily from two long myosin protein chains which twist around each other (as a tail), with active sites protruding (head). It is the active sites of myosin which forms cross-bridges with active sites on actin enabling the molecules to slide past one another (contraction).
Titan myofilaments
thin strong elastic fibers which resist damage from stretch.
Sarcoplasmic Triad
refers to two sarcoplasmic cisterns abutting one central T-tubule. Anatomical basis for calcium release on myofibrils.
Sarcomeres
refers to repetitive and specific arrangement of myofilaments w/in myofibril.
Sarcomere consists of:
- A-Band (anisotropic)
i. M-line
ii. H-zone
iii. Zone of overlap - I-Band (Isotropic)
i. Z-disc - Titan Filament
A-Band (anisotropic)
Is the dark middle portion of sarcomere consisting mainly of thick filaments:
- M-line
- H-zone
- Zone of overlap
M-line
is myomesin protein which serves as site of attachment for thick filaments
H-zone
central region abutting M-line consisting only of thick filaments.
Zone of overlap
Darkest region which consists of both thick & thin filaments.
I-Band (Isotropic)
Is the light portion of the sarcomere which is devoid of thick filaments:
- Z-disc
Z-disc
mesh of protein (actinins) which serves as attachment site for thin filaments and denotes the boundary of a sarcomere (from z-dic to z-disc).
Dystrophin
protein which binds adjacent myofibrils attaching to extracellular CT to distribute tension so that sarcolemma does not tear.
Muscular Dystrophy
from lack of dystrophin protein (structural protein).
Titan filament
structural protein attaching the Z-disc to M-line assuring the arrangement of six thin filaments surround each thick filament even after stretch.
Myoglobin
binds and holds oxygen in muscle (for aerobic respiration).
Glycogen granules
for readily available glucose for energy.
Contraction of skeletal muscles
when muscle fibers contract, they pull on tendons creating tension (note: muscles cannot push). Contraction occurs with sequence of events:
- Neural Excitation
- Excitation-Contraction
- Contraction Cycle
- Relaxation
Excitation-Contraction
Step 2
- Once Ach binds receptors on motor endplate, sodium gates open starting a wave of depolarization that spread over the sarcolemma & down the T-tubules. Depolarization at t-tubule results in opening calcium channels of sarcoplasmic cisterns and a rise in intracellular calcium.
Neural Excitation
each muscle fiber is innervated by one terminal branch (synaptic knob) of a neuron from the somatic nervous system. Neuron lands at a Neuromuscular Junction
Synaptic terminal
region where terminal neuron releases Acetylcholine (Ach).
Synaptic cleft
narrow space between neuron & muscle where Ach diffuses across.
Motor End-Plate
region of muscle fiber in cleft which contains receptors for Ach. It also contains Acetylcholinesterase enzyme to catalyze unbound Ach.
Contraction Cycle
Step 3
At rest, myosin heads are energized by energy from ATP
a. Actin active sites are exposed: once calcium binds troponin thus moving tropomyosin off of actin.
b. Formation of cross-bridge: binding of energized myosin head with actin binding site.
c. Pivoting of myosin head: Once bound, myosin release its energy by swiveling toward M-line (power stroke), pulling the actin chain with it, thus increasing the zone of overlap bringing to two z-discs closer and shortening the sarcomere. ADP is released from head.
d. Reactivation of myosin: ATP must bind the myosin head before myosin head releases the actin cross-bridge and is available to bind a new actin site if calcium is present. Note: not all the myosin heads release at the same time (eg- hand over hand in a tug of war).
Relaxation
follows contraction in absence of calcium. Muscles resume their resting tension only if acted upon by external force (eg- gravity or antagonist muscle). Muscles do not actively stretch.
Relaxation occurs based on:
a. Duration of neural stimulation
b. Presence of intracellular Ca+
c. Availability of ATP
Duration of neural stimulation (relaxation)
contraction stops when release of Ach halts and Acetylcholinerase catalyzes Ach allowing membrane to repolarize.
Presence of intracellular Ca+ (relaxation)
once membrane repolarizes, calcium channels on cistern close and calcium is actively pumped back into sarcoplasmic reticulum
Availability of ATP (relaxation)
prolonged contractions can result in depletion of ATP, such that myosin head cannot be energized for further contraction.
Rigor Mortis
w/in 3-4 hours after death, ATP is no longer available and the calcium pump of sarcoplasmic reticulum fails resulting in:
High intracellular calcium
Exposing actin binding sites due to calcium
Cross-bridging with power stroke causing muscle contraction.
Failure to break cross-bridge bond w/o ATP present leads to rigor.
Autolysis w/in 15-25hrs from release of lysosomal enzymes catabolizing structural proteins thus relaxing the muscle.
Tension production
Shortening of sarcomeres produce tension on tendons, stimulation of a muscle fiber causes all their sarcomeres to shorten; however, tension from a given fiber will vary
Length-tension relationship
there is a narrow range of resting muscle length in which there is a maximal zone of overlap; and hence, maximal tension can be generated.
a. Preloading or stretching fiber
b. Compressing or shortening muscle
Frequency of stimulation
frequency of stimulation will increase force.
Preloading or stretching fiber
decreases zone of overlap and reduces contractile force.
Compressing or shortening muscle
causes myosin to hit z-disc, reducing contractile force.
Twitch
refers to a muscle fiber response from a single stimulation (~50ms)
Contraction Phase
Power stroke results in tension (~15ms)
Relaxation Phase
Ca+2 concentration declines and active sites of actin are once again blocked by tropomyosin and the fiber relaxes (~25ms)
Refractory Period
brief period (~5ms) after a stimulation, in which a muscle fiber is not capable of responding to another stimulus (it is already depolarized).
Relative Refractory Period
muscle will respond to a suprathreshold stimulus.
Treppe
refers to contraction due to a second stimulation right after a twitch was complete. This second contraction is stronger because extra calcium is present intracellularly and all elastic slack has been removed.
Wave Summation
refers to stronger contractions because stimulation occurs before the muscle fiber has not had the opportunity to fully relax or relax at all.
Incomplete Tetanus
wave summation by which relaxation phase is never completed so leads to a stronger contraction building to a plateau.
Complete Tetanus
wave summation by which muscle fiber is not offered any relaxation phase and leads to strongest contractions building to the strongest plateau for that particular muscle fiber.
Tension within muscle organ:
is the sum of all contracting muscle fibers
Size of muscle fiber
muscle fibers w/more myofibrils can generate greater tension.
Size of Motor Unit
Each muscle fiber is contacted from one neuron. A motor unit is one motor neuron and all the muscle fibers it contacts.
- small motor units innervates 4-6 muscle fibers are used for fine motor control.
- large motor units may innervate 1,00 – 2,000 muscle fibers for gross control of powerful muscle.
Motor Unit Recruitment
number of motor units recruited.
Asynchronous Motor Summation
for sustained long periods of sub-maximal contractions (postural muscles) different motor units fire & rest asynchronously, so muscle fibers do not fatigue and there is no net change in tension over time.
Muscle Tone
utilized to stabilize bone & joint position & maintain body position serving as shock absorbers for sudden impact. Greater muscle tone requires higher rate of metabolism (burning more calories, even at rest).
Isotonic Contraction
contraction resulting in change in muscle length while tension remains constant:
- Concentric contraction
- Eccentric contraction
Concentric contraction
refers to peak tension resulting in muscle shortening.
Eccentric contraction
refers to a constant peak tension while muscle lengthens.
Isometric Contraction
contraction in which the muscle does not change in length, but tension applied to the load may increase without overcoming the load.
Speed of Contraction
is inversely proportional to the load, as speed of power stroke decreases with magnitude of load being applied.
Return to resting length
muscle fibers will return to normal length depending upon:
- Elastic forces: especially from stretched tendons, may recoil slightly
- Opposing muscle contraction: form antagonistic muscles will re-stretch muscles.
- Gravity: may help a joint to return to its normal position after contraction.
Energy Reserves (Muscular activity)
Even small muscles have thousands of fibers, and each fiber has billions of thick filaments which utilize large amounts of ATP, so muscles most store energy reserves
ATP
While at rest, muscles catabolize fats & glucose to produce an ATP surplus; however, ATP is a large molecule taking up too much space, so energy is mostly transferred to CP.
Creatine Phosphate (CP)
main form of storing energy. The enzyme Creatine phosphokinase (CPK), a high energy phosphate can be transferred to and from ATP.
Glycogen granules
Serve as potential fuel by converting to glucose in which 2 ATP are quickly created through anaerobic metabolism. With low energy demands, the pyruvic by products can enter Kreb’s cycle in mitochondria to produce 34 ATP aerobically. Under high energy demands with peak level activities, muscles rely on anaerobic metabolism.
Aerobic metabolism
furnishes 95% of ATP in resting cell.
Muscle Fatigue
inability to perform a required level of activity:
a. Depletion of reserves: muscle cell uses up all its ATP, CP and glycogen reserves.
b. Damage to Sarcolemma &/or SR from strenuous activity.
c. Decline in pH: high levels of pyruvate is converted to lactic acid and the lower pH interferes with enzyme activity and Ca+2 binding to troponin.
d. Sense of weariness: with low pH and pain, brain is effected negatively.
Recovery Period
After periods of activity when energy reserves are depleted, there is a period of recovery that follows when head is generated while reserves are restored
- Lactic acid removal
- oxygen debt recovered
- heat production and loss
Lactic acid removal
lactic acid enters bloodstream and is converted to pyruvic acid in the liver.
30% of pyruvic acid enters TCA cycle to produce ATP.
Cori Cycle of liver can convert pyruvic acid to glucose to be stored in muscle.
Oxygen debt recovered
O2 demand remains elevated during recovery for creation of ATP.
Heat production & Loss
muscle contraction produces 85% of body’s heat requirement, peak exertional activity losses 70% of energy as heat during glycolysis. Afterward, body continues to cool down by sweating.
Hormones
can effect muscle metabolism.
- Growth hormones and testosterone
- Thyroid hormone
- Epinephrine
Growth hormones & testosterone
stimulate synthesis of contractile protein. Not muscle cells cannot multiply, but they do hypertrophy.
Thyroid hormone
increase metabolism and energy consumption in muscles.
Epinephrine
released under stressful conditions and can stimulate increase in force and duration of a contraction.
Muscle performance
Concerned with both force (developing tension) & endurance of muscular contraction.
Types of Skeletal muscle fibers:
a. Fast Fibers (Type IIB or Fast-Twitch Glycolytic)
b. Slow Fibers (Type I or Slow-Twitch Oxidative)
c. Intermediate Fibers (Type IIA or Fast-Twitch Oxidative/Glycolytic)
Fast Fibers
(Type IIB or Fast-Twitch Glycolytic) strongest fibers packed w/most myofibrils w/fastest ATPase and less blood vessels, myoglobin & mitochondria. These white anaerobic fibers are strongest, fastest muscle and quickest to fatigue.
Slow Fibers
(Type I or Slow-Twitch Oxidative): weaker muscles w/fewer myofibrils, slower ATPase, but more blood vessels, myoglobin & mitochondria. These red aerobic fibers are less strong, but the most fatigue resistant.
Intermediate Fibers
(Type IIA or Fast-Twitch Oxidative/Glycolytic): Is a blending of slow & fast fibers consisting of pink aerobic/anaerobic fibers of medium strength & medium fatigue resistance.
Physical conditioning
refers to altering power & endurance through training. Exercise can increase number of myofibrils but not number of muscle fibers. May produce more Intermediate fibers, but cannot change fast fibers to slow or slow fibers to fast.
Anaerobic conditioning
High intensity ballistic activity lasting 2 minutes utilizing IIB.
Energy reserves: relies on amounts of stored ATP, CP & glycogen (less blood).
Ability to tolerate lactic acid build-up during glycolysis.
Aerobic conditioning
low level activities over extended periods. Utilizes good vascularization of muscles and the TCA cycle of mitochondria to produce ATP:
Utilizes Type I fibers which can be converted to Type II A.
Improves cardiovascular performance: accelerated blood flow to muscles.
Interval training
combines aerobic & anaerobic exercises, serving to increase strength and cardiovascular performance and creation of more Type IIA fibers- greatest calorie burning.
Smooth Muscle
involuntary fibers influenced by Autonomic Nervous System which appear smooth because they lack myofibrils and arrangement of sarcomeres. These cells contain actin and myosin filaments and are capable of mitosis and repair.
Smooth muscle anatomical characteristics:
Long slender, spindle-shaped cells
Single central nucleus
No myofibrils so no striations
Thick filaments are scattered throughout cell, and myosin has many active heads
Thin filaments are attached to dense bodies associated w/intermediate filaments associated with the sarcolemma. Muscle fiber contracts like a corkscrew.
Dense bodies attach to neighboring cells to transmit forces from cell to cell.
CT surrounds Smooth muscles but does not form a tendon.
Functional Characteristics of smooth muscle:
- Excitation-Contraction coupling
- Length-Tension relationship
- Control of contractions
Smooth Muscle Excitation-Contraction coupling
is triggered by Ca+2 from extracellular sources and binds a myosin enzyme which activates the myosin head to then bind with actin.
Smooth Muscle Length-Tension relationship
smooth muscle does not have an optimal zone of overlap, so can contract well over a broad range of stretch (plasticity).
Smooth Muscle Control of contractions:
they need not receive neural stimulation to contract. They can respond to mechanical stretch, chemicals and hormones.
Neural innervation can be:
Multiunit Smooth Muscle Cells
Visceral Smooth Muscle Cells
Smooth muscle tone is always present.
Multiunit Smooth Muscle Cells
smooth muscle may be innervated by more than one motor neuron from the autonomic nervous system.
Visceral Smooth Muscle Cells
sheets of smooth muscle which tend to be linked with gap junctions and display auto-rhythmicity usually from physical stretch, but may have a few cells contacted by autonomic motor nerves.
Cardiac Muscle
muscle of heart, is thicker than smooth & thinner than skeletal muscle.
Structural consideration of Cardiac Muscle:
- Cardiocytes
- One central nucleus
- T-tubules
- SR lacks terminal cisternae (no triad), most Ca+2 comes in extracellularly.
- Mitochondria
- Intercalated discs
Cardiocytes
cardiac muscle fiber with extensive branching.
T-tubules
circle the sarcomere over the z-disc.
Mitochondria
are abundant as mainly uses aerobic metabolism of lipids & glycogen.
Intercalated discs
consists of gap junctions and desmosomes and serves as site of attachment between cardiocytes. Serves as basis for heart muscles acting as a functional syncytium.
Functional considerations of Cardiac Muscle:
- Auto-rhythmicity
- Neural innervation
3.
Auto-rhythmicity
pacemaker cells of the heart spontaneously depolarize to initiate contraction, without external input.
Neural innervation
Autonomic Nervous System can regulate rate & strength of contraction.
Long Refractory Period
Cardiac contraction lasts 10x that of skeletal fibers, and cardiac contraction cannot summate (tetanize). Relaxation is required for filling.
