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Muscle Makes Behavior Possible
The nervous and muscular systems are entirely unique to animals.
Their functions are so intertwinted, that they are sometimes collectively called the
Muscle tissue is composed of specialized contractile cells/fibers
that may be either
smooth (involuntary control)
provides musculature of hollow and tubular organs
cardiac (involuntary control)
skeletal (voluntary control)
Vertebrate Skeletal Muscle Anatomy
Vertebrate skeletal muscle is composed of long,
multinucleate muscle cells (=muscle fibers).
Each muscle fiber consists of hundreds of parallel myofibrils.
Each myofibril is a long strand of proteins comprising end-to-end contractile subunits called sarcomeres.
Connective tissue sheaths multiple muscle fibers into a fasciculum.
There are 10-100 muscle fibers/fasciculum, depending on which muscle.
Connective tissue bundles multiple fasciculi together into a muscle.
Nerves and blood vessels are embedded within the muscle.
At either end of the muscle, the connective tissue sheaths form tendons.
These connect the muscle to attachment points on the skeleton.
Vertebrate Muscle Cell Anatomy
The sarcolemma is the cell membrane of a muscle cell.
Sarcoplasmic reticulum is a specialized endoplasmic reticulum that regulates cytoplasmic [Ca+].
t (transverse) tubules are deep invaginations of the sarcolemma that allow membrane depolarization to quickly enter the cell's interior.
Terminal cisternae are enlarged regions of sarcoplasmic reticulum on either side of the t tubules. They store Ca+.
A myofibril consists of hundreds of sarcomeres, end to end.
Upon staining, the sarcomeres appear striated.
Striation is due to alternating bands of thin actin (light) and thick myosin (dark).
Each sarcomere has anatomical landmarks:
A band - length of a myosin filament, including actin overlap
H band - myosin filaments only
I band - actin filaments only
M line - region of myosin interlacing/anchoring
Z line/disc - region of actin anchoring
The sarcomere is the contractile subunit of striated muscle.
The Cross Bridge Cycle in skeletal muscle.
Muscle Contraction: The Cross Bridge Cycle
Both skeletal and smooth muscle contract via similar interactions between two proteins:
actin - long, thin filaments
myosin - short, thick filaments
The cycle is generally divided into four steps.
1. Cross Bridge Formation
2. Power Stroke
3. Cross Bridge detachment
4. Re-activation of Myosin Head
Twitches and Tetanus
The point of contact between spinal motor neuron and muscle cell is the neuromuscular junction.
AP frequency at the neuromuscular junction determines muscle tension.
A muscle twitch is one contraction and relaxation of a skeletal muscle fiber.
When a muscle is stimulated repeatedly, successive twitches sum.
The overall response to repeated simuli is greater than a single stimulus response.
This additive effect is known as summation.
Amplitude of summed contractions depends on time interval between stimuli.
Low frequency stimulation results in summation
Higher frequency stimulation results in a constant (fused) contraction, tetanus.
A tetanic contraction is sustained muscle contraction evoked when the motor nerve innervating a muscle fiber emits APs at a very high rate.
--not to be confused with the disease--is the maximum contractile response the muscle can achieve.
A Public Service Message: Be Up to Date on your Tetanus Vaccination!
Too Much Tetanus
Tetanus toxin is produced by the anaerobic bacterium Clostridium tetani.
The LD50 (mice) of tetanus toxin is approximately 2.5 - 3.0 ng/kg
Of all known toxins, only botulinum (LD50 2.0 ng/kg) has higher potency.
If a Clostridium tetani spore enters a wound, it produces toxin that binds to peripheral nerves.
Toxin is transported transcytotically to CNS inhibitory neurons.
Modified toxin cleaves synaptobrevin, a critical component of the SNARE complex.
Remember the SNARE complex? Quiz Question!
Affected motor neurons to become hyperexcitable.
At full progression of infection, essentially all motor neurons generate APs.
The resulting constant tetanic contractions of the skeletal muscles are almost invariably fatal.
is composed of layers of interdigitating, spindle-shaped cells
composed of three types of filaments
thick myosin filaments
thin actin filaments
intermediate cytoskeletal filaments
lacks striations: no myofibrils or sarcomeres
myosin and actin form "cross bridges" that change cell shape upon contraction
located in the walls of hollow organs
Smooth Muscle Innervation
Varicosities (swellings) along the autonomic neuron axons innervating smooth muscle contain neurotransmitter vesicles.
Neurotransmitter is released when an AP passes the varicosity.
Varicosities from one axon may contact several muscle cells.
A single muscle cell may be innervated by both sympathetic and parasympathetic neurons.
Multiple muscle cells can be influenced by a single neuron.
Single muscle cells can be influenced by multiple neurons.
Preganglionic neurons release acetylcholine.
Postganglionic neurons release epinephrine and norepinephrine.
The same neurotransmitter may produce opposite effects by binding to different receptors.
For example, noepinephrine
enhances vascular muscle contraction when bound to α adrenergic receptors (excitatory)
relaxes bronchiole smooth muscle when bound to β-2 adrenergic receptors (inhibitory)
The nature of the response in smooth muscle thus depends on the combination of
type of receptor
second messenger molecules
Smooth Muscle Contraction
Actin filaments are anchored to dense bodies composed of
Smooth muscle Cross Bridge Activation
Calcium activates calmodulin, a regulatory protein.
The vertebrate spinal cord has two types of circuits:
reside within a single spinal cord segment
e.g., simple spinal reflex circuit
ascending (sensory) circuits send information from spinal cord to brain
descending (motor) circuits send information from brain to spinal cord
Muscle spindles are sensory receptors within the belly of a muscle.
A muscle spindle fiber
detects changes in muscle length
is embedded in extrafusal (garden variety) muscle fibers
(from the Latin fusus meaning "spindle")
has both sensory and motor components
Muscle spindles play a role in
regulating muscle contraction
resisting muscle stretch
Muscle Spindle Components and Organization
Each spindle is encapsulated in connective tissue.
Spindles are aligned parallel to extrafusal muscle fibers.
The system consists of
extrafusal muscle fibers (alpha, α) - bulk of the muscle
intrafusal muscle fibers (gamma, γ) - fibers inside the spindle
alpha (α) motor neurons
the most common type of muscle motor neuron
synapse onto extrafusal (α) muscle fibers
transmit APs from CNS to extrafusal muscle fibers
(we will use "motor neuron" (no Greek letter) synonymously)
afferent sensory neurons (1a are the largest type)
sensory terminals coil around noncontractile intrafusal (γ) muscle fibers
carry stretch sensory information from the spindle to the CNS
in the diagram, these are are shown in blue
gamma (γ) motor neurons
synapse on either side of the noncontractile center
send APs from the CNS to regulate muscle spindle fiber contraction
in the diagram, these are shown in red
Muscle spindles are too few and too small to sense muscle tension themselves.
Instead, they sense and send information about
rate of change of muscle length
...to the CNS.
The Principle of Reciprocity
Muscles (and groups of muscles) are usually arranged in antagonist pairs.
One muscle group's action opposes that of the other
flexor muscles bend the body part
extensor muscles straighten the body part
agonist muscles work together with each other.
antagonist muscles work in opposition to each other.
A motor command for a particular movement must coordinate
contraction of agonists (excitatory signals)
relaxation of antagonists (inhibitory signals)
1a afferent neurons synapse onto sets of motor neurons that send
excitatory signals to a particular muscle
inhibitory signals to that muscle's antagonist
Such reciprocal muscle control ensures that muscle groups
do not counteract each other and suppress movement.
Involuntary Movement of Skeletal Muscle: Reflexes
A reflex is a local action performed without brain involvement.
Two well studied reflexes are the
The Stretch Reflex
The simplest reflex involves only
1a afferent/sensory neurons
α motor neuron
...which synapse directly onto each other in the spinal cord.
1. Intrafusal muscle fibers are stretched.
2. AP is generated in 1a sensory neurons.
3. 1a sensory neurons APs travel to the spinal cord.
4. If threshold is reached, an AP is generated in the α motor neuron.
5. Motor neuron APs travel directly back to the muscle.
6. Muscle flexes without brain involvement.
Other Functions of the Stretch Reflex
The stretch reflex also contributes to load compensation.
If a large animal jumped onto your back, the added weight would buckle your knees.
The stretch reflex helps prevent this.