Anatomy Exam 3

Chapter 10: The Muscular System: Skeletal Muscle Tissue and Muscle Organization

There are three types of muscle tissue

– Skeletal muscle

▪ Pulls on skeletal bones

▪ Voluntary contraction

– Cardiac muscle

▪ Pushes blood through arteries and veins

▪ Rhythmic contractions

– Smooth muscle

▪ Pushes fluids and solids along the digestive tract, for example

▪ Involuntary contraction

Muscle tissues share four basic properties

– Excitability

▪ The ability to respond to stimuli

– Contractility

▪ The ability to shorten and exert a pull or tension

– Extensibility

▪ The ability to be stretched or to increase in length

– Elasticity

▪ The ability to rebound toward its original length

Functions of Skeletal Muscles 

• Produce skeletal movement

– Pull on tendons to move the bones

• Maintain posture and body position

– Stabilize the joints to aid in posture

• Support soft tissue

– Support the weight of the visceral organs

• Regulate entering and exiting of material

– Voluntary control over swallowing, defecation, and urination

– Encircles orifices of the digestive and urinary tracts

• Maintain body temperature

– Some of the energy used for contraction is converted to heat

Anatomy of Skeletal Muscles 

Gross Anatomy

– Connective tissue

▪ Epimysium: dense tissue that surrounds the entire muscle

▪ Perimysium: dense tissue that divides the muscle into parallel compartments of fascicles

▪ Endomysium: dense tissue that surrounds individual muscle fibers

– Myosatellite cells repair damaged muscle tissue

▪ Tendons and aponeuroses

– Epimysium, perimysium, and endomysium converge to form tendons

– Tendons connect a muscle to a bone

– Aponeuroses: thick, flattened sheets that connect a muscle to a muscle

– Nerves and Blood Vessels

▪ Nerves innervate the muscle by penetrating the epimysium

▪ There is a chemical communication between a nerve and a muscle

– The chemical is released into the neuromuscular synapse (neuromuscular junction [NMJ])

▪ Blood vessels often parallel the nerves that innervate the muscle

– They then branch to form coiled networks to accommodate flexion and extension of the muscle

Microanatomy of Skeletal Muscle

Fibers

– Sarcolemma

▪ Membrane that surrounds the muscle cell

– Sarcoplasm

▪ The cytosol of the muscle cell

– Skeletal muscle fiber (same thing as a muscle cell)

▪ Can be 30–40 centimeters in length

▪ Multinucleate (each muscle cell has hundreds of nuclei)

▪ Nuclei are located just deep to the sarcolemma

– Myoblasts

▪ Embryonic cells that form skeletal muscle fibers

• Muscle fibers made of multiple myoblasts together. If 100 nuclei in fiber, formed from 100 myoblasts

– Myosatellite cells

▪ Myoblasts that do not form skeletal muscle fibers

▪ Differentiate to assist in repair and regeneration of skeletal muscle fibers

– Transverse tubules (T tubules)

▪ Tubules that conduct electrical impulses for muscle fiber contraction

Myofibrils and Myofilaments

– The sarcoplasm contains myofibrils

▪ Responsible for the contraction of muscles

▪ Myofibrils are attached to the sarcolemma at each end of the muscle cell

▪ Made of myofilaments

– Actin (thin protein filaments)

– Myosin (thick protein filaments)

▪ Surrounding each myofibril is the sarcoplasmic reticulum (SR)

– Consists of terminal cisternae and triads

Sarcomere Organization

▪ Sarcomere

– Main functioning unit of muscle fibers

– Approximately 10,000 per myofibril

– Consists of overlapping actin and myosin

– This overlapping creates the striations that give the skeletal muscle its identifiable characteristic

– Myosin (thick filament)

– Actin (thin filament)

▪ Both are arranged in repeating units called sarcomeres

▪ All the myofilaments are arranged parallel to the long axis of the cell

▪ Each sarcomere consists of:

– Z line (Z disc): boundary between sarcomere units

• Made of actinin

• Titin connects myosin to the Z bands

– I band: Isotropic band. Consists of only actin

– A band: Anisotropic band. Consists of actin and myosin

• Overlapping actin and myosin in the A band region creates striations. This overlap is called the zone of overlap

– H band: Consists of only myosin

– M line: Middle of the H band

▪ Thin Filaments (Actin)

▪ Twisted filaments of:

– F-actin strands (nebulin hold the F-actin strands together)

– G-actin globular molecules

• G-actin molecules consist of an active site (binding site)

– Tropomyosin: a protein that covers the active sites when the muscle is relaxed

– Troponin: holds tropomyosin in position

▪ Thick Filaments (Myosin)

– Myosin filaments consist of an elongated tail and a globular head (cross-bridges)

– Stationary molecule held in place by:

• Protein forming the M line

• A core of titin connecting to the Z lines

• Myosin heads project toward the actin filaments

Muscle Contraction

• The Sliding Filament Theory

– Upon contraction:

▪ The H band and I band get smaller

▪ The zone of overlap gets larger

▪ The Z lines move closer together

▪ The width of the A band remains constant throughout the contraction

Neural Control of Muscle Fiber Contraction

• Axon terminal: tip of the axon at the neuromuscular junction

• Synaptic vesicles contain a neurotransmitter (acetylcholine [Ach])

• Acetylcholine is released from the end of the axon into the synaptic cleft

• Acetylcholinesterase (AChE): enzyme in the synaptic cleft that breaks down acetylcholine

Muscle Contraction: A Summary

– The nerve impulse ultimately causes the release of a neurotransmitter (ACh), which comes in contact with the sarcoplasmic reticulum

– This causes a change in the membrane potential

▪ The action potential spreads across the surface along the T tubules

– The sarcoplasmic reticulum releases its stored calcium ions

– Calcium ions bind to troponin

– The bound calcium ions cause the tropomyosin molecule to roll so that it exposes the active sites on actin

– The myosin heads now extend and bind to the exposed active sites on actin

– This cycle is repeated

▪ Cross-bridge binding—cross-bridge pivoting— cross-bridge detachment

▪ Hydrolysis of ATP is required

– Acetylcholine is broken down by acetylcholinesterase

▪ Action potential stops

– Sarcoplasmic reticulum reabsorbs calcium ions

– Troponin/tropomyosin complex returns to its normal position

– Active sites are blocked

– Cross-bridges cannot bind to the active sites

– Muscle relaxes

Motor Units and Muscle Control

• Precise control

– A motor unit controlling two or three muscle fibers

– Example: the control over the eye muscles

• Less precise control

– A motor unit controlling perhaps 2000 muscle fibers

– Example: the control over the leg muscles

• Muscle tension depends on:

– The frequency of stimulation

▪ A typical example is a muscle twitch

– The number of motor units involved

▪ Complete contraction or no contraction at all (all or none principle)

▪ The amount of force of contraction depends on the number of motor units activated (recruitment)

• Muscle Tone

– The tension of a muscle when it is relaxed is called muscle tone

– Stabilizes the position of bones and joints

▪ Example: the amount of muscle involvement that results in normal body posture

– Muscle spindles

▪ Specialized muscle cells that are monitored by sensory nerves to control muscle tone

• Muscle Hypertrophy

– Enlargement of the muscle

– Exercise causes:

▪ An increase in the number of mitochondria

▪ An increase in the activity of muscle spindles

▪ An increase in the concentration of glycolytic enzymes

▪ An increase in the glycogen reserves

• Muscle Atrophy

– Discontinued use of a muscle

▪ A decrease in muscle mass

▪ A decrease in muscle tone

▪ Muscle becomes flaccid

▪ Muscle fibers become smaller and weaker

Three major types of muscle fibers

– Fast fibers (white fibers)

▪ Associated with eye muscles (fast contractions)

• Features of fast fibers

– Large in diameter

– Densely packed myofibrils

– Large glycogen reserves

– Relatively few mitochondria

– Use large amounts of ATP

– Muscles contract using anaerobic metabolism during glycolysis

– Fatigue rapidly

– Slow fibers (red fibers)

▪ Contain myoglobin

▪ Associated with leg muscles (slow contractions)

• Features of slow fibers

– Smaller diameter than fast fibers

– Take three times longer to contract after stimulation

– Can contract for extended periods of time

– Fatigue slowly

– Contain abundant myoglobin (creates the red color)

– Muscles contract using aerobic metabolism

– Contain more mitochondria that fast fibers

– Intermediate fibers (pink fibers)

▪ Contract faster than slow fibers but slower than fast fibers

• Features of intermediate fibers

Similar to fast:

▪ Have low myoglobin content

▪ Have high glycolytic enzyme concentration

▪ Contract using anaerobic metabolism

Similar to slow:

▪ Have lots of mitochondria

▪ Have a greater capillary supply 

– Resist fatigue

• Distribution of Fast, Slow, and Intermediate Fibers

– Fast fibers

▪ High density associated with eye and hand muscles

▪ Sprinters have a high concentration of fast fibers

▪ Repeated intense workouts increase the fast fibers

– Slow and intermediate fibers

▪ None are associated with the eyes or hands

▪ Found in high density in the back and leg muscles

▪ Marathon runners have a high amount

▪ Training for long-distance running increases the proportion of intermediate fibers

Organization of Skeletal Muscle Fibers

• Muscles can be classified based on shape or by the arrangement of the fibers

– Parallel muscle fibers

– Convergent muscle fibers

– Pennate muscle fibers

▪ Unipennate muscle fibers

▪ Bipennate muscle fibers

▪ Multipennate muscle fibers

– Circular muscle fibers

Parallel Muscles

– Muscle fascicles are parallel to the longitudinal axis

– Examples: biceps brachii and rectus abdominis

▪ When parallel muscles contract, the body of the muscle becomes shorter in length and broader in diameter.

Convergent Muscles

– Muscle fibers form a broad area but come together at a common point (tendon, tendinous sheet, or raphe)

– Example: pectoralis major

Pennate Muscles

– Muscle fibers form an oblique angle to the tendon of the muscle

▪ Unipennate – All the muscle fibers are on the same side of the tendon

– Example: extensor digitorum

▪ Bipennate – Muscle fibers are on both sides of the tendon

– Example: rectus femoris

▪ Multipennate – The tendon branches within the muscle

– Example: deltoid muscle

Circular Muscles – Also known as sphincter muscles

– Muscle fibers form concentric rings

▪ Examples: orbicularis oris and orbicularis oculi

Muscle Terminology

– Origin

▪ Point of muscle attachment that remains stationary

– Insertion

▪ Point of muscle attachment that is movable

– Actions

– The function of the muscle upon contraction

– There are two methods of describing muscle actions

▪ The first makes reference to the bone region the muscle is associated with

– The biceps brachii muscle causes “flexion of the forearm”

▪ The second makes reference to a specific joint the muscle is associated with

– The biceps brachii muscle causes “flexion at the elbow”

– Muscles can be grouped according to their primary actions into four types

▪ Prime movers (agonists)

– Responsible for producing a particular movement

▪ Biceps brachii—flexes the lower arm

▪ Antagonists

– Actions oppose the action of the agonist

▪ Triceps brachii—extends the lower arm

▪ Synergists

– Assist the prime mover in performing an action

▪ Latissimus dorsi and teres major—contract to move the arm medially over the posterior body

▪ Fixators

– Agonist and antagonist muscles contracting at the same time to stabilize a joint

▪ Flexor and extensor muscles contract at the same time to stabilize an outstretched hand

Names of Skeletal Muscles

• Most muscle names provide clues to their identification or location

• Muscles can be named for:

• Specific body regions or location

  • Brachialis: associated with the brachium of the arm

  • Tibialis anterior: associated with the anterior tibia

• Shape of the muscle

  • Trapezius: trapezoid shape

  • Deltoid: triangular shape

• Orientation of the muscle fibers

  • Rectus femoris: straight muscle of the leg

  • External oblique: muscle on outside that is oriented with the fibers at an angle

• Specific or unusual features

  • Biceps brachii: two origins

  • Teres major: long, big, rounded muscle

• Its origin and insertion points

  • Sternocleidomastoid: attachment to sternum, clavicle, and mastoid process

  • Genioglossus: points of attachment are chin and tongue 

• Primary function

  • Flexor carpi radialis: a muscle that is near the radius and flexes the wrist

  • Adductor longus: a long muscle that adducts the leg

• References to occupational or habitual action

  • Buccinator (means “trumpet player”): the buccinator area moves when playing a trumpet

  • Sartorius: derived from the Latin term (sartor), which is in reference to “tailors.” Tailors used to cross their legs to form a table when sewing material 

Levers and Pulleys: A System Designed for Movement

• Most of the time, upon contraction, a muscle causes action

• This action is applied to a lever (a bone)

• This lever moves on a fixed point called the fulcrum (joint)

• Structures associated with the applied force are the anatomical pulleys

There are three classes of levers

1. First class

   1. The fulcrum (joint) lies between the applied force and the resistance force (opposed force)

   2. Example: tilting the head forward and backward

2. Second class

   1. The resistance is located between the applied force and the fulcrum (joint)

   2. Example: standing on your tiptoes

3. Third class

   1. The force is applied between the resistance and fulcrum (joint)

   2. Example: flexing the lower arm

• Sometimes, a tendon may loop around a bony projection

• This bony projection could be called an anatomical pulley

– Example: lateral malleolus and trochlea of the eye

Aging and the Muscular System

• Changes occur in muscles as we age

– Skeletal muscle fibers become smaller in diameter

▪ Due to a decrease in the number of myofibrils

▪ Contain less glycogen reserves

▪ Contain less myoglobin

▪ All of the above results in a decrease in strength and endurance

▪ Muscles fatigue rapidly

– Skeletal muscle fibers become less elastic

• Develop fibrosis – thickening and scarring of connective tissue

• Tolerance for exercise decreases

• Ability to recover from injury decreases

Chapter 11: Axial Musculature

Introduction

• The skeletal muscles of the body can be subdivided into:

– Axial musculature

▪ Muscles that position the head and vertebral column

▪ Muscles that move the rib cage

– Appendicular musculature

▪ Muscles that stabilize or move the appendicular skeleton

• The muscles are innervated by nerves

The Four Groups of Axial Muscles

• The axial muscles can be placed into four groups based on location or function

– Muscles of the head and neck

– Muscles of the vertebral column

– Muscles of the rib cage and lateral walls of the abdominal and pelvic cavities

– Muscles of the pelvic floor

Muscles of the Head and Neck 

• Several groups of muscles of the head and neck are:

– Muscles of facial expression

– Extra-ocular muscles

– Muscles of mastication

– Muscles of the tongue

– Muscles of the pharynx

– Anterior muscles of the neck

• Muscles of Facial Expression

– Facial expression muscles are divided into five groups

▪ Mouth/eyes/scalp/nose/neck

▪ All are innervated by CN VII

– Orbicularis oris

– Buccinator

– Temporoparietalis

– Occipitofrontalis

– Platysma

• Extra-ocular Muscles

– Extrinsic eye muscles (muscles that control eye movement)

▪ Medial and lateral rectus muscles

▪ Superior and inferior rectus muscles

▪ Superior and inferior oblique muscles

▪ Inferior rectus/medial rectus/superior rectus/Inferior oblique: innervated by CN III

▪ Lateral rectus: innervated by CN VI

▪ Superior oblique: innervated by CN IV

– Eye movements

▪ Lateral rectus: rotates the eye laterally

▪ Medial rectus: rotates the eye medially

▪ Superior rectus: rotates the eye upward

▪ Inferior rectus: rotates the eye downward

▪ Superior oblique: rotates the eye downward and laterally

▪ Inferior oblique: rotates the eye upward and laterally

• Muscles of Mastication

– Masseter

– Temporalis

– Pterygoids

– All are innervated by CN V

• Muscles of the Tongue

– Genioglossus

– Hyoglossus

– Palatoglossus

– Styloglossus

– Genioglossus/Hyoglossus/ Styloglossus: innervated by CN XII

– Palatoglossus: innervated by CN X

• Muscles of the Pharynx

– Pharyngeal constrictors: Superior/Middle/Inferior constrictors

– Laryngeal elevators: Palatopharyngeus/ Salpingopharyngeus/ Stylopharyngeus

– Palatal muscles: Tensor veli palatini/ levator veli palatini

– Constrictors are innervated by CN X

– Elevators are innervated by CN IX and CN X

– Palatals are innervated by CN V and CN X

• Anterior Muscles of the Neck

– Digastric: Anterior belly (CN V)/ Posterior belly (CN VII)

– Mylohyoid: CN V

– Geniohyoid: CN XII

– Stylohyoid: CN VII

– Sternocleidomastoid: CN XI

– Omohyoid: Cervical nerve C 1–C 3

– Sternothyroid: Cervical nerve C 1–C 3

– Sternohyoid: Cervical nerve C 1–C 3

– Thyrohyoid: CN XII

Muscles of the Vertebral Column

• The muscles of the back form three distinct layers

– Superficial layer (extrinsic back muscles): move the neck

– Intermediate layer (extrinsic back muscles): move the vertebral column

– Deep layer (intrinsic back muscles): interconnect the vertebrae

• The Superficial Layer of the Intrinsic Back Muscles

– Splenius capitis

– Splenius cervicis

– Both are innervated by cervical nerves

• The Intermediate Layer of the Intrinsic Back Muscles

– Erector spinae (group of three muscles)

▪ Spinalis (most medial of the three)

▪ Longissimus

▪ Iliocostalis (most lateral of the three)

• The Deep Layers of the Intrinsic Back Muscles

– Transversospinales (a group of five muscles)

▪ Semispinalis

▪ Multifidus

▪ Rotatores

▪ Interspinales

▪ Intertransversarii

• Spinal Flexors

– Longus capitis

– Longus colli

▪ The above two muscles rotate or flex the neck

– Quadratus lumborum

▪ Flexes the vertebral column laterally

Oblique and Rectus Muscles

• These muscles can be grouped in this manner:

– Cervical muscles

▪ Scalene muscles

– Thoracic muscles

▪ Intercostals/transversus muscles/serratus

– Abdominal muscles

▪ Oblique/Transversus abdominis

• Cervical muscles

– Scalene muscles

▪ Anterior

▪ Middle

▪ Posterior

– All scalenes will elevate the ribs (inhalation)

• Thoracic muscles

– Intercostal muscles

▪ External intercostal: elevates the ribs

▪ Internal intercostal: depresses the ribs

– Transversus thoracis: depresses the ribs

– Serratus posterior muscles

▪ Superior: elevates the ribs

▪ Inferior: depresses the ribs

• Abdominal muscles

– External oblique

▪ Compresses the abdomen/depresses ribs/laterally flexes the torso

– Internal oblique

▪ Compresses the abdomen/depresses ribs/laterally flexes the torso

• Rectus muscles

– Cervical region

▪ Geniohyoid/Omohyoid

▪ Sternohyoid/Sternothyroid

– Thoracic region

▪ Diaphragm

– contracts and expands thoracic cavity

– Abdominal region

▪ Rectus abdominis

– Depresses ribs/flexes vertebral column

– Consists of linea alba and tendinous inscriptions

• The Diaphragm

– Major breathing muscle

– When it contracts, the diaphragm lowers to increase the volume of the thoracic cavity

▪ Inspiration

– When it relaxes, the diaphragm rises to lower the volume of the thoracic cavity

▪ Expiration

Muscles of the Perineal Region and the Pelvic Diaphragm 

• The perineal region (pelvic floor)

– Divided into two triangles

▪ Urogenital triangle (anterior triangle)

▪ Anal triangle (posterior triangle)

– Pelvic diaphragm: forms the foundation

– The two triangles are separated by:

▪ Superficial transverse perineal muscle

• Muscles of the urogenital triangle

– Superficial urogenital triangle muscles

▪ Superficial transverse perineal

▪ Ischiocavernosus

▪ Bulbospongiosus

– Deep urogenital triangle muscles

▪ Deep transverse perineal

▪ External urethral sphincter

• Muscles of the anal triangle

– Coccygeus

– Levator ani

▪ Iliococcygeus

▪ Pubococcygeus

– External anal sphincter