Muscular System and Introduction to the Nervous System: Study Guide

Muscular System

Muscle Identification

This section covers the anatomical location and actions of major muscles in the human body, grouped by region and function.

• Facial Expression Muscles:

  • Frontalis: Located on the forehead; raises eyebrows and wrinkles forehead.

  • Corrugator supercilii: Deep to frontalis; draws eyebrows medially and downward (frowning).

  • Orbicularis oculi: Encircles the eye; closes eyelids (blinking, squinting).

  • Zygomaticus major & minor: Extends from cheekbone to corners of mouth; elevates corners of mouth (smiling).

  • Orbicularis oris: Encircles the mouth; closes and protrudes lips (kissing, whistling).

  • Depressor anguli oris: Extends from mandible to mouth; depresses corners of mouth (frowning).

  • Platysma: Superficial neck muscle; tenses skin of neck, depresses mandible.

• Extrinsic Eye Muscles (6): Superior, inferior, medial, and lateral rectus; superior and inferior oblique. Control eye movement in all directions.

• Muscles of Mastication (4): Masseter, temporalis, medial pterygoid, lateral pterygoid. Responsible for chewing movements.

• Muscles of Swallowing: Three muscles of the tongue (genioglossus, hyoglossus, styloglossus) manipulate food and aid swallowing.

• Neck Muscles:

  • Sternocleidomastoid: Flexes neck, rotates head to opposite side.

  • Trapezius: Elevates, retracts, and rotates scapula; extends neck.

• Vertebral Column: Erector spinae group extends and laterally flexes the vertebral column.

• Muscles of Ventilation (3): Diaphragm (primary muscle of inspiration), external intercostals (elevate ribs), internal intercostals (depress ribs).

• Abdominal Muscles (4): Rectus abdominis, external oblique, internal oblique, transversus abdominis. Flex and rotate trunk, compress abdominal contents.

• Pectoral Girdle and Upper Limb:

  • Pectoralis minor, Serratus anterior, Trapezius: Move and stabilize scapula.

  • Deltoid, Pectoralis major, Latissimus dorsi: Move the arm at the shoulder joint.

  • Biceps brachii, Brachialis: Flex the forearm at the elbow.

  • Triceps brachii: Extends the forearm at the elbow.

  • Brachioradialis: Flexes forearm in mid-pronated position.

  • Wrist flexors/extensors: Flex and extend the wrist and fingers.

• Hip and Lower Limb:

  • Iliopsoas, Sartorius: Flex the thigh at the hip.

  • Gluteus maximus: Extends and laterally rotates the thigh.

  • Gluteus medius, minimus: Abduct and medially rotate the thigh.

  • Quadriceps femoris (4 muscles): Extend the knee.

  • Hamstrings (3 muscles): Flex the knee, extend the hip.

  • Adductor magnus, longus, brevis, gracilis: Adduct the thigh.

  • Tibialis anterior: Dorsiflexes the foot.

  • Gastrocnemius, Soleus: Plantarflex the foot.

  • Fibularis longus: Everts and plantarflexes the foot.

Example: The biceps brachii flexes the elbow and supinates the forearm, important for lifting objects.

Muscle Physiology

This section explains the structure and function of muscle fibers, mechanisms of contraction, and energy sources.

Structure and Function of Muscle Fibers

• Skeletal muscle: Composed of bundles of muscle fibers (cells) surrounded by connective tissue.

• Muscle fiber: Contains myofibrils, which are made of myofilaments (actin and myosin).

• Myofilaments: Thick filaments (myosin) and thin filaments (actin, troponin, tropomyosin).

• Sarcomere: Functional unit of contraction; defined by Z-discs. Contains A band (myosin), I band (actin), H zone (myosin only), M line (center).

• Contraction: Sarcomere shortens as thin filaments slide past thick filaments.

Example: During contraction, the I band and H zone decrease in width, while the A band remains constant.

Electrical Excitability

• Resting membrane potential: Maintained by ion gradients (Na+ outside, K+ inside).

• Ion channels: Opening allows ions to move down gradients, changing membrane potential.

• Action potential: Rapid depolarization and repolarization of the membrane; triggers contraction.

Example: Opening of voltage-gated Na+ channels causes depolarization.

Contraction and Relaxation

• Neuromuscular junction (NMJ): Synapse between motor neuron and muscle fiber; acetylcholine (ACh) is the neurotransmitter.

• Steps of contraction:

  1. Action potential arrives at NMJ.

  2. ACh released, binds to receptors on muscle fiber.

  3. Na+ influx depolarizes membrane.

  4. Action potential spreads along sarcolemma and T-tubules.

  5. Ca2+ released from sarcoplasmic reticulum (SR).

  6. Ca2+ binds troponin, shifting tropomyosin, exposing actin sites.

  7. Myosin heads bind actin (cross-bridge formation).

  8. Power stroke (myosin pulls actin); ATP required for detachment.

  9. Relaxation: Ca2+ pumped back into SR, ACh broken down by acetylcholinesterase.

• ATP: Required for cross-bridge detachment and Ca2+ reuptake.

• Triad: T-tubule flanked by two terminal cisternae of SR; ensures rapid Ca2+ release.

• Troponin/tropomyosin: Regulate access to actin binding sites.

• Cross-bridge cycle: Attachment, power stroke, detachment (requires ATP), reactivation.

• Acetylcholinesterase: Enzyme that degrades ACh, ending contraction signal.

• Ca2+ in cytosol vs SR: High cytosolic Ca2+ = contraction; low = relaxation.

Example: Muscle relaxation occurs when Ca2+ is actively transported back into the SR.

Energy Sources

• ATP is required for cross-bridge cycling and Ca2+ pumps.

• Three processes for ATP production:

  1. Creatine phosphate: Rapid, short-term ATP supply.

  2. Anaerobic glycolysis: Produces ATP without oxygen; yields lactic acid.

  3. Aerobic respiration: Uses oxygen; produces most ATP, supports sustained activity.

Example: Sprinters rely on creatine phosphate and glycolysis; marathoners rely on aerobic respiration.

Muscle Tension

• Zone of overlap: Optimal overlap of actin and myosin maximizes tension.

• Muscle fiber types:

  • Type I (slow-twitch): Fatigue-resistant, aerobic, high myoglobin.

  • Type II (fast-twitch): Fatigue quickly, anaerobic, low myoglobin.

• Motor unit: One motor neuron and all muscle fibers it innervates; small units allow fine control.

Example: Eye muscles have small motor units for precise movement.

Smooth and Cardiac Muscle

• Smooth muscle: Non-striated, involuntary, found in walls of hollow organs; contracts slowly, can sustain tension.

• Cardiac muscle: Striated, involuntary, found only in heart; cells connected by intercalated discs, contract rhythmically.

Example: Cardiac muscle's automaticity allows the heart to beat without neural input.

Introduction to the Nervous System

Overview of the Nervous System

The nervous system is divided into central and peripheral components, with distinct sensory and motor functions.

• CNS (Central Nervous System): Brain and spinal cord; processes and integrates information.

• PNS (Peripheral Nervous System): All neural tissue outside CNS; transmits signals to and from CNS.

• Motor (efferent) division: Carries commands from CNS to effectors (muscles/glands).

• Sensory (afferent) division: Brings sensory information to CNS.

• Afferent: Toward CNS; Efferent: Away from CNS.

Example: Touching a hot surface sends afferent signals to the CNS, which sends efferent signals to withdraw the hand.

Nervous Tissue

Nervous tissue consists of neurons and supporting glial cells, each with specialized structures and functions.

• Neuron structure: Cell body (soma), dendrites (receive input), axon (transmits output).

• Neuron types:

  • Pseudo-unipolar: Single process splits into two branches; sensory neurons.

  • Bipolar: One axon, one dendrite; found in special senses.

  • Multipolar: One axon, multiple dendrites; most common type.

• Nuclei: Clusters of neuron cell bodies in CNS.

• Tracts: Bundles of axons in CNS.

• Ganglia: Clusters of neuron cell bodies in PNS.

• Nerves: Bundles of axons in PNS.

• Glial cells (6 types):

  • Astrocytes (CNS): Support, blood-brain barrier.

  • Oligodendrocytes (CNS): Myelinate CNS axons.

  • Microglia (CNS): Immune defense.

  • Ependymal cells (CNS): Line ventricles, produce cerebrospinal fluid.

  • Schwann cells (PNS): Myelinate PNS axons.

  • Satellite cells (PNS): Support neuron cell bodies in ganglia.

• Myelin: Insulating layer around axons; increases speed of signal transmission.

• White matter: Myelinated axons; Grey matter: Neuron cell bodies and unmyelinated fibers.

Example: Multiple sclerosis is a disease of CNS myelin loss (oligodendrocyte dysfunction).

Electrophysiology of Neurons

Neurons generate and transmit electrical signals through changes in membrane potential, mediated by ion channels.

• Gradients: Na+ and K+ gradients set up by Na+/K+ ATPase pump.

• Membrane potential: Determined by ion flux through channels.

• Channel types:

  • Leak channels: Always open; set resting potential.

  • Ligand-gated channels: Open in response to neurotransmitter binding.

  • Voltage-gated channels: Open in response to changes in membrane potential.

• Graded potentials: Local, variable changes in membrane potential; decrease with distance.

• Action potentials: All-or-none, propagate without decrement.

• Voltage-gated Na+ channel: Rapid activation and inactivation; initiates action potential.

• Voltage-gated K+ channel: Opens more slowly; repolarizes membrane.

• Action potential steps:

  1. Resting state: $V_m \approx -70\ \mathrm{mV}$

  2. Depolarization: Na+ influx, $V_m$ rises toward $+30\ \mathrm{mV}$

  3. Repolarization: K+ efflux, $V_m$ returns toward rest

  4. Hyperpolarization: K+ channels remain open, $V_m$ below rest

  5. Return to resting potential

• Refractory periods:

  • Absolute: No new action potential possible (Na+ channels inactivated).

  • Relative: Stronger stimulus needed (K+ channels open).

• Propagation: Action potentials self-propagate along axon; unidirectional due to refractory period.

Example: Myelinated axons conduct action potentials faster via saltatory conduction.

Neuronal Synapses

Synapses are specialized junctions where neurons communicate via neurotransmitters, producing postsynaptic potentials.

• Synapse structure: Presynaptic terminal, synaptic cleft, postsynaptic membrane.

• Synaptic transmission steps:

  1. Action potential arrives at presynaptic terminal.

  2. Ca2+ influx triggers neurotransmitter release.

  3. Neurotransmitter binds postsynaptic receptors.

  4. Postsynaptic potential generated (EPSP or IPSP).

• EPSP (Excitatory Postsynaptic Potential): Depolarizes membrane, increases chance of action potential.

• IPSP (Inhibitory Postsynaptic Potential): Hyperpolarizes membrane, decreases chance of action potential.

• Ionotropic receptors: Ligand-gated ion channels; direct ion flow.

• Metabotropic receptors: G-protein coupled; indirect effects via second messengers.

• Summation:

  • Temporal: Multiple signals from one synapse over time.

  • Spatial: Signals from multiple synapses at once.

Example: Glutamate binding to an ionotropic receptor causes Na+ influx (EPSP); glycine binding opens Cl- channels (IPSP).

Neurotransmitters & Receptors

Neurotransmitters are chemical messengers with specific effects depending on their receptors.

• Ionotropic vs Metabotropic:

  • Ionotropic: Fast, direct ion flow (e.g., nicotinic ACh receptor).

  • Metabotropic: Slow, indirect, uses second messengers (e.g., muscarinic ACh receptor).

• Major neurotransmitters and functions:

  • Glutamate: Main excitatory neurotransmitter in CNS; learning and memory.

  • Glycine: Main inhibitory neurotransmitter in spinal cord.

  • GABA: Main inhibitory neurotransmitter in brain.

  • Dopamine: Motor control, reward, motivation.

  • Serotonin: Mood, appetite, sleep.

  • Histamine: Wakefulness, immune response.

  • Norepinephrine/Epinephrine: Alertness, fight-or-flight response.

  • Acetylcholine: Muscle activation, autonomic functions.

  • Substance P: Pain transmission.

  • Opioids: Pain modulation.

  • Neuropeptide Y: Appetite regulation.

• EPSP example: Glutamate receptor opens Na+ channels, depolarizing the postsynaptic cell.

• IPSP example: Glycine receptor opens Cl- channels, hyperpolarizing the postsynaptic cell.

Example: Dopamine deficiency in the basal ganglia is associated with Parkinson's disease.

Neurotransmitter	Main Function	Receptor Type	Effect
Glutamate	Excitatory, learning/memory	Ionotropic/metabotropic	EPSP
Glycine	Inhibitory (spinal cord)	Ionotropic	IPSP
GABA	Inhibitory (brain)	Ionotropic/metabotropic	IPSP
Dopamine	Motor, reward	Metabotropic	Varies
Serotonin	Mood, sleep	Metabotropic	Varies
Acetylcholine	Muscle activation	Ionotropic/metabotropic	EPSP/IPSP

Additional info: Some neurotransmitters can have both excitatory and inhibitory effects depending on the receptor subtype.