Structure of Skeletal Structure of Skeletal MuscleMuscle

Chapter 8: Muscular SystemChapter 8: Muscular System

Unit 2: Support and Unit 2: Support and MovementMovement

IntroductionIntroduction

Muscles are organs composed of Muscles are organs composed of specialized cells that use the specialized cells that use the chemical energy stored in nutrients chemical energy stored in nutrients to contract.to contract.

– Muscle action provide muscle tone, Muscle action provide muscle tone, propel body fluids and food, generate propel body fluids and food, generate the heartbeat, and distribute heat.the heartbeat, and distribute heat.

Muscle Tissue TypesMuscle Tissue Types

The muscle tissue in the body can be The muscle tissue in the body can be divided into 3 distinct categories:divided into 3 distinct categories:

1. Skeletal Muscle1. Skeletal Muscle

2. Smooth Muscle2. Smooth Muscle

3. Cardiac Muscle3. Cardiac Muscle

Structure of a Skeletal Structure of a Skeletal MuscleMuscle The skeletal muscle structure is The skeletal muscle structure is

composed of four different types of composed of four different types of tissues:tissues:

1. Skeletal Muscle Tissue1. Skeletal Muscle Tissue

2. Nervous Tissue2. Nervous Tissue

3. Blood Tissue [connective]3. Blood Tissue [connective]

4. Connective Tissue4. Connective Tissue

Connective Tissue Connective Tissue CoveringsCoverings Layers of fibrous connective Layers of fibrous connective

tissue called tissue called FasciaFascia separate an separate an individual skeletal muscle from individual skeletal muscle from adjacent muscles and hold it in adjacent muscles and hold it in position.position.

– Usually projects beyond its end to Usually projects beyond its end to form a cordlike tendonform a cordlike tendon

Fibers in tendon often intertwine with Fibers in tendon often intertwine with those in bone’s those in bone’s periosteumperiosteum..

Layers of Connective Layers of Connective TissueTissue The layer of connective tissue that The layer of connective tissue that

closely surrounds the whole of a closely surrounds the whole of a skeletal muscle is called skeletal muscle is called EpimysiumEpimysium..

– The connective tissue layer that extends The connective tissue layer that extends inward from the epimysium and separates inward from the epimysium and separates the muscle tissue into small compartments the muscle tissue into small compartments is called is called PerimysiumPerimysium..

Bundles of skeletal muscle fibers are called Bundles of skeletal muscle fibers are called FasciclesFascicles..

– Each muscle fiber within a fascicle lies within a Each muscle fiber within a fascicle lies within a layer of connective tissue in the form of a thin layer of connective tissue in the form of a thin covering called covering called EndomysiumEndomysium..

Skeletal Muscle Skeletal Muscle MicrographMicrograph

Skeletal Muscle FibersSkeletal Muscle Fibers

A skeletal muscle fiber is a A skeletal muscle fiber is a single single cellcell that contracts in response to that contracts in response to stimulation and then relaxes stimulation and then relaxes when the stimulation ends.when the stimulation ends.

– Each fasiculus (fiber) is a thin, Each fasiculus (fiber) is a thin, elongated cylinder with rounded elongated cylinder with rounded ends.ends.

– Each fasiculus contains many small, Each fasiculus contains many small, oval nuclei and many mitochondria.oval nuclei and many mitochondria.

MyofibrilsMyofibrils

Every muscle fiber (muscle cell) is Every muscle fiber (muscle cell) is surrounded by a cell membrane called surrounded by a cell membrane called a a sarcolemmasarcolemma

The cytoplasm in a muscle fiber is The cytoplasm in a muscle fiber is called the called the sarcoplasmsarcoplasm

SarcoplasmSarcoplasm contains multiple contains multiple threadlike threadlike MyofibrilsMyofibrils that lie parallel that lie parallel to one another.to one another.

– Contains two kinds of protein filaments:Contains two kinds of protein filaments:

Thick ones composed of the protein Thick ones composed of the protein MyosinMyosin..

Thin ones composed of the protein Thin ones composed of the protein ActinActin..

Striation Patterns of Striation Patterns of Skeletal MuscleSkeletal Muscle The striation pattern of skeletal The striation pattern of skeletal

muscle fibers has two main muscle fibers has two main parts:parts:

1. 1. I BandsI Bands: Light Bands: Light Bands

2. 2. A BandsA Bands: Dark Bands: Dark Bands

BandsBands

I BandsI Bands are composed of thin are composed of thin ActinActin filaments directly attached filaments directly attached to structures called to structures called Z linesZ lines..

A BandsA Bands are composed of thick are composed of thick mysosin filaments overlapping mysosin filaments overlapping thin actin filaments.thin actin filaments.

A Bands ContinuedA Bands Continued

The A Band consists not only of a The A Band consists not only of a region where the thick and thin region where the thick and thin filaments overlap, but also a filaments overlap, but also a central region (H Zone) consisting central region (H Zone) consisting only of thick filaments.only of thick filaments.

– A thickening know as an A thickening know as an M LineM Line is is found in the middle of the A Band.found in the middle of the A Band.

The segment of a myofibril that extends The segment of a myofibril that extends from one Z line to the next Z line is from one Z line to the next Z line is called a called a SarcomereSarcomere..

More Structures of the More Structures of the SarcoplasmSarcoplasm A network of membranous channels that A network of membranous channels that

surroundssurrounds each myofibril and runs along its each myofibril and runs along its length is known as the length is known as the Sarcoplasmic ReticulumSarcoplasmic Reticulum..

– The SR acts as a reservoir of Ca2+ ionsThe SR acts as a reservoir of Ca2+ ions

– Calcium ions are used to stimulate muscle contractionCalcium ions are used to stimulate muscle contraction *more to come on this later*more to come on this later

– Another set of membranous channels, called Another set of membranous channels, called Transverse TubulesTransverse Tubules, extend inward from the fibers’ , extend inward from the fibers’ membrane and passes all the way through the fiber.membrane and passes all the way through the fiber.

Each tubule opens to the outside of the muscle fiber and Each tubule opens to the outside of the muscle fiber and contains extracellular fluid.contains extracellular fluid.

– Both membranous channels activate the muscle contraction Both membranous channels activate the muscle contraction mechanism where the fiber is stimulated.mechanism where the fiber is stimulated.

More Structures of the More Structures of the Sarcoplasmic Sarcoplasmic ReticulumReticulum Another set of membranous channels, Another set of membranous channels,

called called Transverse TubulesTransverse Tubules, extend , extend inward from the sarcolemma and inward from the sarcolemma and passes all the way through the fiber.passes all the way through the fiber.

Each T-tubule passes by many myofibrilsEach T-tubule passes by many myofibrils Each tubule opens to the outside of the muscle Each tubule opens to the outside of the muscle

fiber and contains extracellular fluid.fiber and contains extracellular fluid.

– T-tubules & SR work together to activate the T-tubules & SR work together to activate the muscle contraction mechanism when the fiber is muscle contraction mechanism when the fiber is stimulated.stimulated.

Neuromuscular Neuromuscular JunctionJunction Each skeletal muscle fiber Each skeletal muscle fiber

connects to an axon from a nerve connects to an axon from a nerve cell, called a cell, called a Motor NeuronMotor Neuron..

– This axon extends outward from the This axon extends outward from the brain or spinal cord, and the muscle brain or spinal cord, and the muscle fiber contracts only when the motor fiber contracts only when the motor neuron stimulates it.neuron stimulates it.

The connection between the motor The connection between the motor neuron and muscle fiber is called a neuron and muscle fiber is called a Neuromuscular JunctionNeuromuscular Junction..

Neuromuscular Neuromuscular Junction ContinuedJunction Continued A neuromuscular junction includes A neuromuscular junction includes

the end of a motor neuron and the the end of a motor neuron and the motor end plate of a muscle fiber.motor end plate of a muscle fiber.

– A A Motor End PlateMotor End Plate is a specialized is a specialized region of the muscle fiber membrane region of the muscle fiber membrane where muscle fiber, nuclei, and where muscle fiber, nuclei, and mitochondria are abundant.mitochondria are abundant.

The sarcolemma is extensively folded in The sarcolemma is extensively folded in this region to create more surface area.this region to create more surface area.

More on Motor More on Motor NeuronsNeurons The end of the motor neuron The end of the motor neuron

branches and projects into recesses branches and projects into recesses of the muscle fiber membrane.of the muscle fiber membrane.

– Cytoplasm at the distal ends of the Cytoplasm at the distal ends of the motor neuron axons are rich in motor neuron axons are rich in mitochondria and contains vesicles mitochondria and contains vesicles that store chemicals called that store chemicals called NeurotransmittersNeurotransmitters..

NeurotransmittersNeurotransmitters

Nerve impulse traveling from the Nerve impulse traveling from the brain or spinal cord reach the end brain or spinal cord reach the end of the motor neuron axon.of the motor neuron axon.

– Some of the vesicles release a Some of the vesicles release a neurotransmitter into the neurotransmitter into the Synaptic Synaptic CleftCleft..

The The Synaptic CleftSynaptic Cleft is the gap between is the gap between the neuron and the motor end plate of the neuron and the motor end plate of the muscle fiber.the muscle fiber.

Motor UnitsMotor Units

A muscle fiber usually has a A muscle fiber usually has a single motor end plate.single motor end plate.

– One motor neuron may connect to One motor neuron may connect to many muscle fibers.many muscle fibers.

When a motor neuron transmits an When a motor neuron transmits an impulse, all of the muscle fibers it links impulse, all of the muscle fibers it links to are stimulated to contract to are stimulated to contract simultaneously.simultaneously.

– A motor neuron and the muscle fibers that A motor neuron and the muscle fibers that it controls constitute a it controls constitute a Motor UnitMotor Unit..