Muscle System, Nervous System, General & Special Senses: Comprehensive Study Notes

Muscle System

Types of Muscle Tissue & Differences

Muscle tissue is classified into three main types, each with distinct structural and functional characteristics.

• Skeletal Muscle

  • Striated, voluntary

  • Long cylindrical fibers, multinucleated

  • Attached to bones

  • Fast contractions

  • Functions: movement, posture, heat production

• Cardiac Muscle

  • Striated, involuntary

  • Branched, single nucleus per fiber

  • Intercalated discs (gap junctions & desmosomes)

  • Autorhythmic (generates own APs)

  • Found only in heart

• Smooth Muscle

  • Non-striated, involuntary

  • Spindle-shaped, single nucleus

  • Slow, sustained contractions

  • Found in organs, vessels, respiratory and reproductive tracts

Connective Tissues of Muscle

Muscle fibers are organized and protected by layers of connective tissue.

• Epimysium: Surrounds entire muscle

• Perimysium: Surrounds fascicles (bundles of fibers)

• Endomysium: Surrounds individual muscle fibers

• All merge to form tendons

Formation of Muscle Fibers

Muscle fibers develop from embryonic cells called myoblasts.

• Myoblasts fuse → multinucleated muscle fibers

• Remaining myoblasts become satellite cells (repair)

Structure of Skeletal Muscle Fiber

Skeletal muscle fibers have specialized structures for contraction.

• Sarcolemma: cell membrane

• Sarcoplasm: cytoplasm

• Myofibrils: contractile protein structures

• Actin (thin filaments) & Myosin (thick filaments)

• Sarcoplasmic Reticulum (SR): stores Ca2+

• T-tubules: transmit AP deep into fiber

Sliding Filament Theory – How Contraction Happens

Muscle contraction occurs when actin and myosin filaments slide past each other.

1. Nerve impulse arrives at NMJ

2. ACh released, binds receptors

3. Sarcolemma depolarizes → AP travels down T-tubules

4. SR releases Ca2+

5. Ca2+ binds troponin, exposes myosin binding sites

6. Crossbridge cycle begins

Crossbridge Cycle (Detailed)

The crossbridge cycle is the molecular basis of muscle contraction.

1. Myosin head energized (ATP → ADP + Pi)

2. Ca2+ opens actin binding sites

3. Myosin binds actin → crossbridge formation

4. Powerstroke: myosin pulls actin

5. New ATP binds myosin → crossbridge detaches

6. ATP is hydrolyzed → process repeats

Continues while ATP + Ca2+ are available.

Types of Muscle Contractions

Muscle contractions are classified by whether they produce movement.

• Isotonic (movement)

  • Muscle changes length

  • Concentric: muscle shortens

  • Eccentric: muscle lengthens

• Isometric (no movement)

  • Muscle generates tension

  • Same length (e.g., pushing on a wall)

Myogram

A myogram graphs muscle tension vs. time, showing phases:

• Latent period

• Contraction phase

• Relaxation phase

Twitch, Treppe, Summation, Tetanus

These terms describe patterns of muscle response to stimulation.

• Twitch: Single stimulus → single contraction-relaxation cycle

• Treppe ("Staircase effect"): Repeated stimuli after full relaxation; each contraction slightly stronger

• Wave Summation: Second stimulus arrives before full relaxation; tension increases

• Incomplete Tetanus: Rapid stimuli, partial relaxation

• Complete Tetanus: Maximum tension, no relaxation at all

Recruitment

Recruitment increases the number of motor units activated to increase muscle force.

• Small fibers recruited first

• Large, stronger units last

Neuromuscular Junction (NMJ)

The NMJ is the synapse between a motor neuron and a muscle fiber.

• Includes: axon terminal, synaptic cleft, motor end plate (sarcolemma)

• Steps:

  1. AP arrives

  2. ACh released

  3. ACh binds receptors → depolarization

  4. Muscle AP generated → contraction begins

Motor Unit

A motor unit consists of one motor neuron and all the muscle fibers it innervates.

• Small units = small motor units

• Large units = large motor units

Ions Involved in Muscle Contraction

• Na+: depolarizes sarcolemma

• K+: repolarizes sarcolemma

• Ca2+: required for troponin binding sites

• Mg2+: required for ATP activity (enzyme regulation)

Neurotransmitter Involved

• Acetylcholine (ACh): Released at NMJ, triggers muscle AP

Energy for Muscle Contraction

Muscle contraction requires ATP, supplied by several mechanisms:

• Immediate (seconds): Stored ATP, creatine phosphate (rapid ATP regeneration)

• Short-term (1–2 min): Anaerobic glycolysis, produces lactic acid

• Long-term (minutes–hours): Aerobic respiration, uses oxygen, produces large amounts of ATP

Lactic Acid Fermentation, Oxygen Debt, Recovery

• Lactic Acid Fermentation: Occurs when O2 is limited; produces ATP quickly, leads to muscle fatigue & burning

• Oxygen Debt: Extra oxygen needed after exercise to convert lactic acid to pyruvate, restore ATP & creatine phosphate, recover homeostasis

• Recovery Period: Breathing & heart rate remain elevated; body restores homeostasis

Muscle Tone

• Resting, baseline tension

• Maintains posture

• Increases metabolic rate

• Muscle never completely relaxed

Energy Reserves in Muscle

• ATP

• Creatine phosphate

• Glycogen (major stored fuel)

• Myoglobin (O2 storage in muscle)

Nervous System

Components of the Nervous System

The nervous system is divided into central and peripheral components.

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

• Peripheral Nervous System (PNS): All nerves outside CNS; carries sensory and motor info

• Enteric Nervous System (ENS): "Brain of the gut"; controls GI tract independently

Afferent vs Efferent Divisions (PNS)

• Afferent (Sensory): Carries sensory info to CNS

• Efferent (Motor): Carries motor commands from CNS to effectors (muscles, glands)

• Somatic Nervous System (SNS): Voluntary control (skeletal muscle)

• Autonomic Nervous System (ANS): Involuntary control (smooth/cardiac muscle, glands)

Neuron Structure

Neurons are specialized cells for receiving and sending signals.

• Cell Body (Soma): Contains nucleus, organelles

• Dendrites: Highly branched, receive input

• Axon: Conducts action potentials, may be myelinated

Neuron Classification

• Structural:

  • Unipolar: Axon & dendrites fused (most PNS sensory neurons)

  • Multipolar: One axon, 2+ dendrites (most CNS neurons)

  • Bipolar: One dendrite, one axon (rare, special senses)

  • Anaxonic: Axon not clearly distinguishable

• Functional:

  • Sensory (Afferent): Carry info to CNS

  • Motor (Efferent): Carry instructions from CNS to effectors

  • Interneurons: Integrate sensory & motor info (CNS)

Neuroglia (Glial Cells)

Neuroglia support, protect, and nourish neurons.

• CNS Neuroglia:

  • Astrocytes: Maintain blood-brain barrier, support neurons

  • Ependymal cells: Line ventricles, produce CSF

  • Oligodendrocytes: Myelinate CNS axons

  • Microglia: Phagocytosis (debris, pathogens)

• PNS Neuroglia:

  • Satellite cells: Surround ganglia, regulate interstitial fluid

  • Schwann cells: Myelinate PNS axons, aid regeneration

Action Potentials

Action potentials are rapid electrical signals generated by neurons.

• Threshold: -40 to -55 mV

• All-or-none principle: If threshold is reached, AP occurs

• Strength of stimulus does not change AP size

Stages of Action Potential:

1. Resting Membrane Potential (~ -70 mV): Na+/K+ pumps maintain gradient

2. Threshold (~ -55 mV): Stimulus opens Na+ channels

3. Depolarization: Na+ channels open, membrane becomes positive

4. Repolarization: K+ channels open, membrane returns toward -70 mV

5. Hyperpolarization: K+ channels close slowly, membrane becomes more negative

6. Return to Resting Potential: Na+/K+ pumps restore ion distribution

Equation:

$V_{m} = V_{rest} + ext{depolarization} - ext{repolarization}$

Synapses & Neurotransmitters

• Neurotransmitters stored in synaptic vesicles

• Released into synaptic cleft, bind receptors

• Broken down by enzymes, reabsorbed/recycled

EPSP vs IPSP

• EPSP (Excitatory): Graded depolarization, moves membrane toward threshold

• IPSP (Inhibitory): Graded hyperpolarization, moves membrane away from threshold

Summation & Facilitation

• Spatial Summation: Multiple synapses stimulated at different locations

• Temporal Summation: Rapid repeated stimulation at one synapse

• Facilitation: Membrane potential brought closer to threshold by accumulating EPSPs

Saltatory Conduction

• Occurs in myelinated axons

• AP "jumps" node to node (Nodes of Ranvier)

• Faster and more energy-efficient than continuous conduction

Adrenergic Synapses

• Release norepinephrine (NE)

• Usually excitatory & depolarizing

• Common in brain & sympathetic ANS

Spinal Cord

Spinal Cord Overview

• Extends from medulla oblongata to L1-L2 vertebrae

• Enclosed within vertebral column

• Connects brain and PNS

• Responsible for reflexes and information transmission

Dorsal Roots vs Ventral Roots

Feature	Dorsal Root	Ventral Root
Type	Sensory (afferent) fibers	Motor (efferent) fibers
Cell bodies	In dorsal root ganglion	In spinal cord (anterior horn)
Information	Touch, pain, proprioception	Muscle control

White Matter vs Gray Matter

Feature	White Matter	Gray Matter
Composition	Myelinated axons	Neuron cell bodies, dendrites, unmyelinated
Function	Ascending & descending tracts	Integration & local processing
Tracts	Sensory & motor	Horns (anterior, lateral, posterior)

Gray Horns & Information Carried

• Anterior (Ventral) Horns: Somatic motor neurons (skeletal muscle)

• Lateral Horns: Visceral motor neurons (autonomic)

• Posterior (Dorsal) Horns: Sensory & interneuron input from periphery

Brachial Plexus

• Spinal nerves (C5-T1)

• Major nerves: radial, median, ulnar, axillary, musculocutaneous

• Formed by ventral rami

Choroid Plexus

• Located in ventricles of brain, near spinal cord

• Produces cerebrospinal fluid (CSF)

• Nourishes CNS and cushions the spinal cord and brain

Peripheral Effector

• Any organ, muscle, or gland responding to motor commands from CNS

• Example: Skeletal muscle (somatic), cardiac muscle, glands (autonomic)

Reflexes & Types

Reflexes are automatic, rapid responses to stimuli.

• Stretch Reflex (Myotatic Reflex): Monosynaptic (1 synapse), e.g., patellar reflex

• Flexor Reflex (Withdrawal Reflex): Polysynaptic, e.g., withdrawal from pain

• Crossed Extensor Reflex: Opposite limb extends when one limb withdraws

• Superficial Reflexes: E.g., plantar reflex

Reflex Arc Components

1. Receptor – senses stimulus

2. Sensory neuron – transmits signal to spinal cord

3. Integration center – spinal cord interneurons

4. Motor neuron – transmits signal to effector

5. Effector – muscle or gland responds

Information Processing in Spinal Cord

• Gray matter: integrates sensory & motor signals locally

• White matter: transmits signals up & down cord

• Reflexes can occur without brain involvement (simple responses)

Brain

Major Brain Structures & Functions

Structure	Location	Function
Cerebrum	Largest part, divided into lobes	Conscious thought, memory, sensory perception, voluntary movement
Medulla Oblongata	Brainstem, continuous with spinal cord	Controls autonomic functions (heart rate, respiration, blood pressure)
Pons	Brainstem, above medulla	Relays signals, controls breathing
Cerebellum	Posterior cranial fossa	Coordinates movement, balance
Hypothalamus	Diencephalon	Regulates hormones, thirst, hunger, temperature
Thalamus	Diencephalon	Relay station for sensory info

Cerebrospinal Fluid (CSF)

• Produced by choroid plexus (ventricles of brain)

• Functions: cushions brain/spinal cord, removes waste, circulates nutrients

Blood-Brain Barrier (BBB)

• Structure: Endothelial cells of CNS capillaries + tight junctions + astrocyte foot processes

• Function: Protects CNS from toxins/pathogens, controls entry of nutrients/ions/hormones

Meninges – Layers & Functions

Layer	Description	Function
Dura Mater	Tough, outer layer	Protection, forms venous sinuses
Arachnoid Mater	Middle, web-like	CSF circulation in subarachnoid space
Pia Mater	Thin, adheres to CNS surface	Supports blood vessels, protects CNS tissue

Cranial Nerves & Associations

Cranial Nerve	Associated Brain Region	Function
I – Olfactory	Telencephalon	Smell
II – Optic	Diencephalon	Vision
III – Oculomotor	Midbrain	Eye movement, pupil constriction
IV – Trochlear	Midbrain	Eye movement
V – Trigeminal	Pons	Facial sensation, chewing
VI – Abducens	Pons	Eye movement
VII – Facial	Pons	Facial expression, taste
VIII – Vestibulocochlear	Pons/Medulla junction	Hearing, balance
IX – Glossopharyngeal	Medulla	Taste, swallowing
X – Vagus	Medulla	Visceral sensory/motor, autonomic functions
XI – Accessory	Medulla	Neck muscles
XII – Hypoglossal	Medulla	Tongue movement

General Senses

Reflexes Related to the Senses

• Corneal Reflex: Triggered by touching cornea or strong light; bilateral blink

• Direct Light Reflex: Same eye constricts pupil

• Consensual Light Reflex: Opposite eye also constricts

• Vestibulo-Ocular Reflex (VOR): Stabilizes vision during head movement

Acquired vs. Flexor Reflex

• Acquired Reflex: Learned, developed through repetition (e.g., driving)

• Flexor Reflex (Withdrawal Reflex): Innate, protective; rapid withdrawal from pain

Receptors and CNS Nuclei

• Root Hair Plexus: Mechanoreceptors wrapped around hair follicles

• Vestibular Nuclei: Located in pons and medulla; control balance and posture

• Solitary Nucleus: Located in medulla oblongata; receives visceral sensory info

Types of Receptors (General Senses)

• Nociceptors: Pain receptors; found in skin, joints, periosteum, walls of blood vessels

• Mechanoreceptors: Respond to mechanical deformation; tactile, pressure, vibration

• Baroreceptors: Detect pressure/stretch in vessels and organs

• Proprioceptors: Body position, muscle length, tension

Repair of Neural Receptors

• Peripheral Receptor Regeneration: Limited but possible; Schwann cells guide repair

• CNS Receptor/Neuron Repair: Very limited; oligodendrocytes inhibit axon regrowth

Special Senses

Taste (Gustation)

• Primary taste sensations: sweet, salty, sour, bitter, umami

• Taste buds: CN VII (anterior 2/3), CN IX (posterior 1/3), CN X (epiglottis)

Vision

• Lens Shape Control: Ciliary muscle + suspensory ligaments

• Regions of Eye & Retina: Optic disc (blind spot), fovea centralis (cones), outer/inner segments (photoreceptors)

• Myopia vs Hyperopia: Nearsighted (eye too long), farsighted (eye too short)

• Astigmatism: Irregular curvature of cornea/lens

Neurotransmitter Release in Different Lighting

• Photoreceptors (rods) release glutamate

• In dark: depolarized, Na+ channels open

• In light: hyperpolarized, Na+ channels close

How Light Activates Rods

1. Photon hits rhodopsin in outer segment

2. Rhodopsin splits → activates transducin

3. Transducin activates phosphodiesterase (PDE)

4. PDE breaks down cGMP

5. cGMP-gated Na+ channels close

6. Cell hyperpolarizes → signal to bipolar/ganglion cells → optic nerve

Hearing (Audition)

• Tympanic Membrane: Separates external & middle ear; transfers vibrations

• Auditory Ossicles: Malleus, incus, stapes; amplify vibrations

• Cochlea: Contains Organ of Corti (hearing organ); converts vibrations to nerve impulses

• Oval Window: Stapes vibrates, transmits to cochlea

• Tensor Tympani: Muscle attached to malleus; dampens loud sounds

• Basilar Membrane: Runs length of cochlea; different regions for different frequencies

• Hair Cells: Located in Organ of Corti; receptors for hearing

Action Potential – Labeled Diagram (Text Version)

Membrane Potential (mV):

• (1) Resting Membrane Potential (~ -70 mV): Na+/K+ pumps maintain gradient

• (2) Threshold (~ -55 mV): Na+ channels open

• (3) Depolarization: Na+ channels open, membrane becomes positive

• (4) Repolarization: K+ channels open, membrane returns toward -70 mV

• (5) Hyperpolarization (undershoot – -80 to -90 mV): K+ channels close slowly

• (6) Return to Resting Potential: Na+/K+ pumps restore ion distribution

Equation:

$V_{m} = V_{rest} + ext{depolarization} - ext{repolarization}$

Additional info: These notes cover key topics from chapters 10–17, 12–14, and 17 of a standard Anatomy & Physiology curriculum, including muscle tissue, nervous tissue, spinal cord, brain, general and special senses. Tables have been reconstructed for clarity and completeness.