Skeletal Muscles: Anatomy and Function

Module 9.1: Overview of Skeletal Muscles

• A muscle fiber is surrounded by the endomysium
• A fascicle refers to a bundle of muscle fibers enclosed by the perimysium
• The epimysium wraps around the entire muscle, which consists of many fascicles
• Fascicle arrangement impacts skeletal muscle appearance and function
• Muscles orientations include parallel, convergent, pennate, circular, or spiral
• Muscles are named based on shape, appearance, size, position, or structure (number of heads, e.g., triceps)
• Skeletal muscles facilitate joint movement, attaching to bones, which is known as the muscle’s action
• Other functions are facial expression, breathing and regulating body temperature
• Muscles often work together with each having a specific role
• Muscle attachment points are either relatively stationary

Module 9.2: Muscles of the Head, Neck, and Vertebral Column

• Facial expression muscles:
• The frontalis, occipitalis, and corrugator supercilii muscles move the forehead and eyebrows
• The orbicularis oculi muscle closes the eye
• Smiling, grimacing, grinning, and sneering are due to the zygomaticus major/minor, levator labii superioris, risorius, and orbicularis oris muscles
• Sad and "doubtful" expressions are caused by the depressor anguli oris, depressor labii inferioris, and mentalis muscles
• Sucking motions result from the buccinator muscle pulling the cheeks interiorly
• The six extrinsic eye muscles move the eyeball
• These include superior, inferior, medial, lateral rectus as well as superior and inferior oblique muscles
• Mastication muscles include the masseter, temporalis, medial and lateral pterygoid
• Muscles of swallowing:
• The genioglossus, hyoglossus, and styloglossus muscles move the tongue, and push food back during chewing

Module 9.3: Muscles of the Trunk and Pelvic Floor

• Muscles of inspiration are the diaphragm and the external intercostals
• Air leaves the lungs during forced expiration due to internal intercostal muscles
• The rectus abdominis, external and internal obliques flex the trunk and compress the abdominal cavity
• The transversus abdominis muscle also compresses the abdominal cavity
• Pelvic floor muscles:
• The pelvic diaphragm is formed by the levator ani muscle group, that works like sphincters around the urethra, vagina, and anal canal, forming the floor of the pelvis
• The external urethral and anal sphincters provide voluntary control of urination and defecation
• The deep/superficial transverse perineal muscles support pelvic organs
• The bulbospongiosus and ischiocavernosus muscles support penis and clitoris erection

Module 9.4: Muscles of the Pectoral Girdle and Upper Limb

• Muscles of the pectoral girdle:
• The trapezius muscle elevates, retracts, depresses, and rotates the scapula superiorly
• The levator scapulae muscle elevates the scapula
• The rhomboid major and minor muscles retract/inferiorly rotate the scapula
• The serratus anterior muscle protracts/superiorly rotates the scapula
• The pectoralis minor muscle protracts and depresses the scapula
• Arm movement muscles:
• The pectoralis major/coracobrachialis muscles flex, adduct, and medially rotate the arm
• The latissimus dorsi/teres major muscles adduct, medially rotate, and extend the arm
• The deltoid muscles abduct the arm
• The rotator cuff muscles stabilize the shoulder joint, namely the teres minor, infraspinatus, supraspinatus, and subscapularis muscles
• Elbow flexion muscles are the biceps brachii, brachialis, and brachioradialis
• The elbow extension muscle is the triceps brachii

Module 9.5: Muscles of the Hip and Lower Limb

• The iliopsoas, sartorius, and rectus femoris muscles flex the thigh
• The adductor muscle group, pectineus and gracilis muscles adduct and medially rotate the thigh
• The quadriceps femoris muscle group extends the leg at the knee
• The gluteus maximus extends, abducts, and laterally rotates the thigh during climbing
• The gluteus medius and minimus abduct and medially rotate the thigh during walking
• Muscles of the hamstring muscle group (biceps femoris, semitendinosus, and semimembranosus) extend the thigh and flex the leg
• The main dorsiflexors of the foot are the tibialis anterior and extensor digitorum longus
• The fibularis longus and brevis muscles evert the foot
• The main plantarflexors of the foot are the gastrocnemius and soleus
• Toe muscles are usually named by their function and location

Module 10.1: Overview of Muscle Tissue

• The three types of muscle tissue are skeletal, cardiac, and smooth
• Muscle cells turn chemical energy into mechanical energy by contracting to create tension
• Muscle tissue contains muscle cells and the surrounding extracellular matrix (endomysium)
• Muscle tissue properties include contractility, excitability, conductivity, distensibility, and elasticity
• A muscle cell's plasma membrane is called the sarcolemma, and the cytoplasm is the sarcoplasm
• The sarcoplasm contains myofibrils and the sarcoplasmic reticulum (SR)

Module 10.2: Structure and Function of Skeletal Muscle Fibers

• Skeletal muscle fibers feature striations, multiple nuclei and voluntary control
• The sarcolemma has inward extensions that surround myofibrils called T-tubules
• The Ca2+-storing SR swells where it meets T-tubules to form terminal cisternae
• Myofibrils consist of contractile, regulatory, and structural proteins
• Myofilaments are composed of myofilaments
• Myofilament types include:
• Thick filaments of myosin, a contractile protein
• Thin filaments of contractile actin proteins and the regulatory proteins troponin and tropomyosin
• Elastic filaments composed of the structural protein titin
• Skeletal muscle tissue striations result from myofilament arrangement
• The light I bands consist of thin filaments only, and have a Z-disc in the middle
• The dark A bands contain overlapping thick and thin filaments and have a central H zone, bisected by the M line
• The sarcomere is the functional unit

Module 10.3: Skeletal Muscle Fibers as Electrically Excitable Cells

• The sarcolemma has a charge separation, or electrical gradient
• Membrane potentials are electrical potentials across plasma membranes
• A resting muscle fiber has a resting membrane potential of around -90 mV
• Ions cross the sarcolemma through leak and gated channels
• Na+ concentration is higher outside the cell, while K+ concentration is higher inside
• The Na+/K+ pump helps maintain the concentration gradient
• The negative resting membrane potential is from K+ leaking out through leak channels
• Ion movement is driven by concentration and electrical gradients
• The electrochemical gradient is the sum of these two forces
• An action potential is a temporary reversal involving:
• Na+ entering the fiber during depolarization
• K+ exiting during repolarization

Module 10.4: The Process of Skeletal Muscle Contraction and Relaxation

• Skeletal muscle fibers are innervated by motor neurons
• The neuron meets the fiber at the neuromuscular junction (NMJ)
• The NMJ includes the axon terminal, synaptic cleft, and motor end plate
• Skeletal muscle contraction includes excitation, excitation-contraction coupling, and contraction
• Excitation starts with the axon terminal releasing ACh into the synaptic cleft, which binds to ACh receptors on the motor end plate
• The resulting end-plate potential triggers an action potential in the sarcolemma that travels down the T-tubules during excitation-contraction coupling
• This triggers the opening of Ca2+ channels in the SR, flooding the cytosol
• In preparation for contraction, Ca2+ in the cytosol binds troponin, which allows tropomyosin to move away from the active sites of actin
• During the contraction phase, ATP hydrolysis "cocks" the myosin head
• Myosin then binds to actin and generates a power stroke, pulling it toward the sarcomere's center
• Muscle relaxation includes:
• ACh being broken down in the synaptic cleft
• Ca2+ concentration in the cytosol returning to normal

Module 10.5: Energy Sources for Skeletal Muscle

• Immediate energy sources are stored ATP in the cytosol and creatine phosphate
• Glycolytic catabolism is an anaerobic process where the cytosol splits glucose, producing ATP
• Oxidative catabolism is an aerobic process done in the mitochondria where glycolysis products, fatty acids, and amino acids are oxidized to generate ATP

Module 10.6: Muscle Tension at the Fiber Level

• A muscle fiber's single contraction-relaxation cycle is called a twitch
• A twitch consists of the latent, contraction, and relaxation periods
• Muscle fibers are classified as fast-twitch or slow-twitch
• Contraction force depends on frequency of stimulus by the motor neuron and the resulting Ca2+ concentration
• Unfused tetanus results if a muscle fiber is stimulated before the relaxation period is completed
• Fused tetanus results if a muscle fiber is stimulated 80–100 times per second before the relaxation period begins
• A contraction can produce maximal tension at an optimal sarcomere length
• Type I muscle fibers are slow-twitch fibers using primarily oxidative catabolism
• Type II muscle fibers are fast-twitch fibers using primarily glycolytic catabolism

Module 10.7: Muscle Tension at the Organ Level

• A motor unit has a single motor neuron and the muscle fibers it innervates
• Recruitment occurs when more motor units are activated for stronger contractions
• Muscle tone is produced by small, involuntary contractions of alternating motor units
• Muscle contraction includes isotonic concentric (shortening), isotonic eccentric (lengthening), and isometric (unchanging length)

Module 10.8: Skeletal Muscle Performance

• Endurance and resistance training changes skeletal muscle fiber structure/biochemistry
• Disuse leads to muscle fiber atrophy
• Muscular fatigue is caused by depleted key metabolites, inadequate oxygen delivery, metabolite accumulation, and environmental factors
• Excess postexercise oxygen consumption (EPOC) occurs after exercise to correct homeostatic imbalances

Module 10.9: Smooth and Cardiac Muscle

• Smooth muscle tissue functions in peristalsis, forming sphincters, and regulating flow through hollow organs
• Smooth muscle cells feature a single nucleus as well as lacking T-tubules, striations, and sarcomeres
• They also have a less extensive SR than skeletal muscle fibers
• The autonomic nervous system, stretch, and hormone activity stimulates Smooth muscle contraction
• binding to calmodulin activates smooth muscle cell contraction, activating myosin light-chain kinase (MLCK), starting crossbridge cycles
• Ca2+
• Smooth muscle cell relaxation comes from removing Ca2+ from the cytosol
• Smooth muscle tissue types include:
• Single-unit smooth muscle cells that contract together
• Multi-unit smooth muscle cells that contract independently
• Cardiac muscle cells are joined physically/electrically by intercalated discs
• Pacemaker cells coordinate their electrical activity

Module 11.1: Overview of the Nervous System

• Structurally, the nervous system divides into the central (CNS) and peripheral (PNS) nervous systems
• The brain and spinal cord make up the CNS
• The PNS contains cranial and spinal nerves
• Functionally, the nervous system has three divisions:
• The sensory (afferent) PNS division has somatic and visceral sensory branches
• Sensory input processed by the CNS is called integration
• The motor (efferent) PNS division has somatic motor and autonomic nervous system (ANS) branches

Module 11.2: Nervous Tissue

• Nervous tissue has neurons and neuroglial cells
• Neurons are excitable and send/propagate/receive action potentials
• They consist of a cell body along with receptive dendrites and an axon
• Neurons are classified by structure and function
• Structural classes include multipolar, bipolar, and pseudounipolar neurons
• Functional classes include sensory (afferent), interneurons, and motor (efferent) neurons
• Neuroglia within the CNS include:
• Astrocytes anchor neurons/blood vessels
• Oligodendrocytes form the myelin sheath
• Microglia are phagocytes
• Ependymal cells create and circulate cerebrospinal fluid
• Neuroglia in the PNS include the neurolemmocytes which form the myelin sheath and satellite cells surround neuron cell bodies
• A myelin sheath forms when CNS oligodendrocytes as well as PNS neurolemmocytes wrap around the axon as many as 100 times. This greatly speeds up action potential conduction
• Unmyelinated PNS axons are embedded in neurolemmocytes

Module 11.3: Electrophysiology of Neurons

• There is a charge separation (potential) across the neuron membrane
• Resting neurons are polarized, exhibiting a resting membrane potential of -70 mV
• Ions move across the axolemma through always open leak and gated channels
• Two important ionic gradients are Na+ and K+
• Na+ concentration is higher outside the cell
• K+ concentration is higher inside
• A local potential is a small, local change in neuron membrane potential
• It may depolarize the neuron (making it less negative) or hyperpolarize (making it more negative)
• Local potentials are graded, reversible, and decremental, useful for short-range signaling
• Na+ voltage-gated channels have three states: resting, activated, and inactivated
• K+ voltage-gated channels have two states: resting and activated
• An action potential is rapid depolarization/repolarization
• The depolarization phase causes Na+ to flood the axon, increasing the membrane potential to a positive value

Module 11.4: Neuronal Synapses

• A synapse occurs where a neuron meets its target cell, which is called synaptic transmission
• Electrical synapses feature neuron axolemmas electrically joined with gap junctions which are nearly instantaneous and bidirectional
• Chemical synapses rely on neurotransmitters, tend to be slower and unidirectional
• Chemical synapse events start with an action potential at the presynaptic axon terminal, causing exocytosis of neurotransmitters from vesicles
• Neurotransmitters bind to receptors on the postsynaptic membrane causing a local postsynaptic potential
• One of two things may take place during a postsynaptic potential:
• An excitatory postsynaptic potential (EPSP) depolarizes the postsynaptic neuron
• An inhibitory postsynaptic potential (IPSP) hyperpolarizes the postsynaptic neuron
• Neural integration is the assembling of excitatory and inhibitory stimuli to decide if a neuron action will take place or not

Module 11.5: Neurotransmitters

• Neurotransmitters impact the opening/closing of ion channels in the postsynaptic neuron's axolemma
• There are two neurotransmitter receptor types: ionotropic and metabotropic
• Neurotransmitter effects are described as excitatory (inducing EPSPs) or inhibitory (inducing IPSPs)
• Many neurotransmitters are capable of generating both
• The major neurotransmitters:
• Acetylcholine which degrades via acetylcholinesterase, is mostly excitatory
• Biogenic amines:
• norepinephrine
• dopamine
• epinephrine
• serotonin
• histamine
• Amino acid neurotransmitters:
• glutamate
• glycine
• 𝛾-aminobutyric acid (GABA)
• The brain's major excitatory neurotransmitter is glutamate
• Both GABA and glycine are major inhibitory neurotransmitters in the CNS

Module 11.6: Functional Groups of Neurons

• Interneurons are organized into neuronal pools that enable CNS specialization that helps with mental activity
• An input neuron starts signals in a neuronal pool
• The pattern of connection between neuronal pools is called a neural circuit
• Neural circuit types:
• Diverging circuit starts with at least one input neuron contacting an increasing number of postsynaptic neurons
• Converging circuit takes signals from multiple neurons and converge it onto fewer final postsynaptic neurons