Week 3 Flashcards
Types of Muscle
Skeletal
-multinucleated, cylindrical, striated, voluntary
Cardiac
-Cells are smaller and branched, striated, involuntary
Smooth
-Cells are tapered, nucleus on the periphery, not striated, involuntary
A motor neuron synapses with skeletal muscle at the_______________
Neuromuscular junction (NMJ)
Neuron releases _____________ which binds to ______________, causes depolarization.
ACh, nicotinic receptors
Motor Unit
Individual motor neuron and all of the muscle fibers it innervates
Small motor unit
neuron innervates few muscle fibers
Provides precise control of very fine movements (eyes, fingers)
Large motor unit
- neuron innervates many muscle fibers
- useful for contraction of muscles in the leg, maintaining posture
Muscle fibers are surrounded by a layer called the ________________
Endomysium
Muscle fibers are arranged in _____________ surrounded by a _____________
Fascicles, perimysium
Tendons are an extension of the ____________ that surrounds the muscle
Epimysium
Reflex actions
involuntary movements controlled in spinal cord
Volitional actions
complex voluntary activities, organized in motor areas in cerebral cortex
Final common pathway is the …
Alpha motor neuron
Where does the alpha motor neuron originate and exit?
Originates in ventral horn of the spinal cord, exits through ventral root
What do alpha motor neurons innervate?
Skeletal muscle
Skeletal muscle contractions results in…
Movement
Tension development to stabilize the body
Muscle cells =
FIBERS. A “muscle” is many fibers in parallel
Two types of skeletal muscle fibers
Extrafusal
Intrafusal
Extrafusal muscle fibers
strongest, most common, usually attached to tendons at either end
Intrafusal muscle fibers
tension-sensing fibers. Sensory neurons are located here
How are skeletal muscles classified?
based on metabolism and speed of contraction
Slow Twitch Fibers
- Red muscle
- produce slow, sustained contractions
- require aerobic metabolism to generate ATP for contraction. Thus, need a constant supply of oxygen
- Use molecules called MYOGLOBIN to store oxygen. Myoglobin has a dark pigment
- Used for long duration activities such as maintaining posture. Slow to fatigue
Fast Twitch Fibers
- White muscle
- Produce fast, short-duration contractions
- Rely on anaerobic metabolism supplied by GLYCOGEN, which is stored in muscle (and liver)
- Fire fast, but fatigue quickly
- Used for rapid, finely controlled movement or short bursts of energy
Distribution of types of muscle fibers is ….
Genetic. Specific types of training can develop one set of fibers more than the other
Sensory Neurons
have processes in muscle. Transmit info about state of contraction and tension
Two types of sensory neurons
Muscle Spindles
Golgi Tendon Organs
-Both types of sensory neurons transmit info to somatosensory cortex for integration or to spinal cord for reflex
Muscle spindles
innervate intrafusal fibers, act as stretch receptors. When intrafusal muscle cells stretch, muscle spindle increases its discharge of APs. LENGTH
Golgi tendon organs
Innervate extrafusal fibers near the tendons, generate more APs as fibers develop tension. FORCE
Muscle cell or fiber =
myocyte
plasma membrane =
sarcolemma
Striations in muscle (alternating dark and light bands) result from…
two types of myofilaments: ACTIN (thin) and MYOSIN (thick)
Sarcomere
functional unit of muscle.
A-band
at the center of the sarcomere. Dark band containing myosin
I-band
light band at ends of sarcomere, contains actin
M-line
in sarcomere. Holds myosin filaments in place
Z-discs
Edges of sarcomere comprised of Z-discs, which connect ends of actin from one sarcomere to the next
functional unit of muscle
sarcomere
myofilaments are surrounded by ___________ which stores __________
sarcoplasmic reticulum, Ca2+
Embedded in the sarcoplasmic reticulum are ____________ which are invaginations of plasma membrane
T-Tubules
AP travels down the muscle fiber, transverses into the ______________ on the SR, voltage gated ________ channels on SR open, ___________ binds to myofilaments, causing muscle contraction
Tubules, Ca2+, Ca2+
actin
two actin filaments are wrapped around each other
Tropomyosin
actin is associated with long fibrous proteins called tropomyosin. Each tropomyosin molecule is associated with a protein complex called troponin.
Troponin
protein complex associated with tropomyosin
T-T complex
together, tropomyosin and troponin form a regulatory protein complex called the T-T complex
Myosin
each thick filament is comprised of 300 - 400 myosin molecules wrapped together.
- each myosin contains a long rod-shaped tail and head
- the head contains binding sites for actin and ATP
- The head region is associated with myosin “LIGHT CHAINS”
- Head regions project from the sides of the filament and form CROSS-BRIDGES during muscle contraction
How are action potentials created on the muscle fiber?
- NMJ consists of the motor axon terminal, the MOTOR END PLATE of the muscle fiber, and the synaptic cleft that separates the two
- AP travels down the motor neuron, triggering voltage-gated Ca2+ channels to open. Ca2+ enters neuron, causes exocytosis of ACh-containing vesicles into synaptic cleft
- ACh binds to nicotinic receptors on motor end plate. Na+ enters muscle fiber, depolarizes membrane, creates an AP
Role of the T-tubule in muscle contraction
AP travels along the sarcolemma and follows down T-tubules toward center of cell. This special conduction allows innermost portion of cell to receive AP quickly.
Note that muscle fibers are not myelinated
Role of the SR (sarcoplasmic reticulum)
- Stores calcium in sac-like areas called TERMINAL CISTERNAE
- RYANODINE RECEPTOR (RyR) controls realease of calcium from SR. In skeletal muscle, RyR1 acts as a voltage-gated calcium channel. When open, calcium is released and travels to myofilaments. This causes muscle contraction.
- After contraction, intracellular calcium is stored in the SR using an ATP-dependent calcium pump.
- Release and storage of calcium occurs quickly, allowing muscle contraction to occur up to 100x/second.
Sliding Filament Theory
- Action potential (AP)
- AP travels through T-tubules
- Release of calcium from SR
- Calcium binds to troponin
- Conformational shift in actin to expose binding sites
- Myosin heads bind to actin
- Power stroke (contraction)
