Nervous and Muscular Tissue: Structure, Function, and Physiology

Nervous Tissue and the Nervous System

Classification of the Nervous System

The nervous system is a highly organized network responsible for coordinating body activities and maintaining homeostasis. It is divided into two main parts:

• Central Nervous System (CNS): Consists of the brain and spinal cord; serves as the main control center for processing information.

• Peripheral Nervous System (PNS): Composed of all neural tissue outside the CNS, including nerves and ganglia; connects the CNS to limbs and organs.

Organs of the CNS and PNS

• CNS: Brain and spinal cord.

• PNS: Cranial nerves (12 pairs), spinal nerves (31 pairs), ganglia, and sensory receptors.

Types of Nerves

• Cranial Nerves: 12 pairs, emerge from the brain.

• Spinal Nerves: 31 pairs, emerge from the spinal cord.

Characteristics and Structure of Neurons

Neurons are excitable cells specialized for communication via electrical and chemical signals.

• Excitability: Ability to respond to stimuli and generate action potentials.

• Longevity: Long-lived and generally amitotic (do not divide).

• High Metabolic Rate: Require continuous supply of oxygen and glucose.

• Cell Body (Soma): Contains the nucleus and organelles; metabolic center.

• Dendrites: Receive incoming signals from other neurons.

• Axon: Conducts electrical impulses away from the cell body.

• Axon Hillock: Initiates action potentials.

• Axon Terminals: Release neurotransmitters to communicate with other cells.

• Myelin Sheath: Insulating layer that increases the speed of impulse conduction.

• Nodes of Ranvier: Gaps in the myelin sheath where action potentials are regenerated.

Myelinated vs. Unmyelinated Axons

• Myelinated Axons: Conduct impulses rapidly via saltatory conduction.

• Unmyelinated Axons: Conduct impulses more slowly via continuous conduction.

Gray Matter vs. White Matter

• Gray Matter: Contains neuron cell bodies, dendrites, and unmyelinated axons; located on the outer surface (cortex) of the brain and inner part of the spinal cord.

• White Matter: Composed mainly of myelinated axons; found deeper in the brain and outer part of the spinal cord.

Repair in CNS vs. PNS

• CNS: Limited ability to repair due to inhibitory factors and lack of growth-promoting environment.

• PNS: Schwann cells promote axon regeneration, allowing some repair after injury.

The Central Nervous System (CNS)

Functions of the CNS

• Integrates sensory information and coordinates voluntary and involuntary responses.

• Responsible for higher functions such as thought, memory, and emotion.

Major Parts of the Brain

• Cerebrum: Largest part; responsible for higher brain functions.

• Diencephalon: Includes thalamus and hypothalamus; relays sensory information and controls autonomic functions.

• Brain Stem: Includes midbrain, pons, and medulla oblongata; controls basic life functions.

• Cerebellum: Coordinates movement and balance.

Protection of the CNS

• Bony Protection: Skull and vertebral column.

• Meninges: Three connective tissue layers (dura mater, arachnoid mater, pia mater).

• Cerebrospinal Fluid (CSF): Cushions and nourishes the CNS.

• Blood-Brain Barrier: Regulates passage of substances from blood into the brain.

Brain Surface Features and Lobes

• Gyri: Elevated ridges.

• Sulci: Shallow grooves.

• Fissures: Deep grooves separating large brain regions.

• Lobes: Frontal, Parietal, Temporal, Occipital, and Insula.

Sensory, Motor, and Association Areas

• Sensory Areas: Receive and interpret sensory information.

• Motor Areas: Control voluntary muscle movements.

• Association Areas: Integrate information for complex functions (e.g., reasoning, language).

Major Parts of the Diencephalon and Brain Stem

• Thalamus: Relay station for sensory information.

• Hypothalamus: Regulates homeostasis, endocrine functions, and autonomic nervous system.

• Epithalamus: Contains pineal gland; involved in sleep-wake cycles.

• Midbrain: Visual and auditory reflexes.

• Pons: Relays information and regulates breathing.

• Medulla Oblongata: Controls vital functions (heart rate, breathing, blood pressure).

Functions of the Spinal Cord

• Conducts sensory and motor information between the body and brain.

• Integrates reflexes.

The Peripheral Nervous System (PNS)

Spinal Nerve Plexuses

• Cervical Plexus: Supplies neck and diaphragm.

• Brachial Plexus: Supplies upper limbs.

• Lumbar Plexus: Supplies lower abdomen and thigh.

• Sacral Plexus: Supplies pelvis and lower limbs.

Reflexes

• Definition: Rapid, automatic response to a stimulus.

• Example: Knee-jerk (patellar) reflex.

Cranial Nerves: Numbers, Names, Types, and Functions

Number	Name	Type	Function
I	Olfactory	Sensory	Smell
II	Optic	Sensory	Vision
III	Oculomotor	Motor	Eye movement, pupil constriction
IV	Trochlear	Motor	Eye movement
V	Trigeminal	Both	Facial sensation, chewing
VI	Abducens	Motor	Eye movement
VII	Facial	Both	Facial expression, taste
VIII	Vestibulocochlear	Sensory	Hearing, balance
IX	Glossopharyngeal	Both	Taste, swallowing
X	Vagus	Both	Visceral sensation, parasympathetic control
XI	Accessory	Motor	Head and shoulder movement
XII	Hypoglossal	Motor	Tongue movement

Autonomic Nervous System (ANS)

Overview and Divisions

The ANS regulates involuntary physiological processes, including heart rate, blood pressure, respiration, digestion, and sexual arousal. It is divided into:

• Sympathetic Division: "Fight or flight"; thoracolumbar origin (T1-L2); uses norepinephrine as main postganglionic neurotransmitter.

• Parasympathetic Division: "Rest and digest"; craniosacral origin (brainstem, S2-S4); uses acetylcholine as main neurotransmitter.

Feature	Sympathetic	Parasympathetic
Origin	Thoracolumbar (T1-L2)	Craniosacral (Brainstem, S2-S4)
Ganglia Location	Near spinal cord (paravertebral)	Near or within target organs
Neurotransmitters	Acetylcholine (preganglionic), Norepinephrine (postganglionic)	Acetylcholine (both)
General Function	Fight or flight	Rest and digest

Cholinergic and Adrenergic Receptors

• Cholinergic Receptors: Bind acetylcholine; include nicotinic (always excitatory) and muscarinic (excitatory or inhibitory) types.

• Adrenergic Receptors: Bind norepinephrine and epinephrine; include alpha (α) and beta (β) types.

Special Senses

Olfaction (Smell) and Gustation (Taste)

• Olfactory receptors: Located in the olfactory epithelium of the nasal cavity.

• Taste buds: Located on papillae of the tongue, soft palate, and pharynx; detect sweet, sour, salty, bitter, and umami.

Vision

• Accessory Structures: Eyelids, eyelashes, lacrimal apparatus, extrinsic eye muscles.

• Three Layers of the Eyeball: Fibrous (sclera, cornea), vascular (choroid, ciliary body, iris), nervous (retina).

• Photoreceptors: Rods (dim light), cones (color and detail).

• Common Disorders: Myopia, hyperopia, astigmatism, cataracts, glaucoma.

Hearing and Equilibrium

• Ear Anatomy: External (auricle, canal, tympanic membrane), middle (ossicles, auditory tube), inner (cochlea, vestibule, semicircular canals).

• Equilibrium: Static (vestibule) and dynamic (semicircular canals).

Muscle Tissue Types

Overview of Muscle Tissue

Muscle tissue is specialized for contraction and is essential for movement, posture, and various bodily functions. There are three main types of muscle tissue: skeletal, cardiac, and smooth muscle, each with distinct structures and functions.

• Skeletal Muscle: Attached to bones and skin, responsible for voluntary movements, striated, and multinucleated.

• Cardiac Muscle: Found only in the heart, responsible for pumping blood, striated, involuntary, and contains intercalated discs.

• Smooth Muscle: Located in walls of hollow organs (except the heart), non-striated, involuntary, and spindle-shaped cells.

Functions and Characteristics of Muscle Tissue

• Excitability (Responsiveness): Ability to receive and respond to stimuli.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched.

• Elasticity: Ability to recoil to resting length after stretching.

• Movement of bones or fluids (e.g., blood)

• Maintaining posture and body position

• Stabilizing joints

• Heat generation (especially skeletal muscle)

• Additional: Protects organs, forms valves, controls pupil size, causes "goosebumps"

Connective Tissue Wrappings of Skeletal Muscle

Skeletal muscles are organized into bundles and surrounded by connective tissue layers that provide support and transmit force.

• Endomysium: Areolar connective tissue surrounding each muscle fiber (cell).

• Perimysium: Fibrous connective tissue wrapping around a fascicle (bundle of muscle fibers).

• Epimysium: Dense irregular connective tissue covering the entire muscle.

• Tendons: Cordlike structures connecting muscle to bone, mostly collagen fibers.

• Aponeuroses: Sheetlike structures attaching muscles indirectly to bones or other muscles.

Skeletal Muscle Structure and Microscopic Anatomy

Each skeletal muscle fiber is a long, cylindrical cell containing multiple nuclei and specialized organelles for contraction.

• Sarcolemma: Specialized plasma membrane of muscle fiber.

• Myofibrils: Long organelles inside muscle cell, responsible for striations.

• Sarcomere: The contractile unit, extending from one Z disc to another.

Myofilaments

• Thin Filaments: Composed of actin proteins, anchored to Z discs.

• Thick Filaments: Composed of myosin proteins, contain ATPase enzymes and myosin heads (cross bridges).

The arrangement of thin and thick filaments creates the characteristic banding pattern of skeletal muscle.

Sarcoplasmic Reticulum (SR)

• Specialized smooth endoplasmic reticulum that stores and releases calcium ions, essential for muscle contraction.

The Nerve Stimulus and Action Potential

Muscle contraction is initiated by stimulation from a motor neuron at the neuromuscular junction.

• Motor Unit: One motor neuron and all the muscle fibers it stimulates.

• Neuromuscular Junction: Site where the axon terminal of a motor neuron communicates with the sarcolemma of a muscle fiber.

• Neurotransmitter: Acetylcholine (ACh) is released to stimulate muscle contraction.

• Synaptic Cleft: Gap between nerve and muscle, filled with interstitial fluid.

Muscle Contraction: Sliding Filament Model and Cross Bridge Cycling

Muscle contraction occurs when myosin heads bind to actin, forming cross bridges and pulling thin filaments toward the center of the sarcomere. This process shortens the muscle fiber.

• In the relaxed state, thin and thick filaments overlap only at the ends of the A band.

• During contraction, the H zone and I bands shorten, Z discs move closer, and the muscle shortens.

• ATP is required for cross bridge detachment and re-cocking of the myosin head.

• Absence of ATP (as in rigor mortis) prevents detachment, causing muscle stiffness after death.

Graded Response and Muscle Tension

• Muscle fiber contraction is "all or none"—a fiber contracts fully or not at all.

• Graded responses occur when different numbers of fibers contract, producing varying degrees of muscle tension.

• Tetanus is a sustained, smooth contraction resulting from rapid stimulation.

• Isometric Contractions: Muscle tension increases, but the muscle does not shorten or produce movement (e.g., pushing against a wall).

• Isotonic Contractions: Muscle shortens and movement occurs (e.g., bending the knee).

• Muscle Tone: Continuous, partial contraction of muscles keeps them healthy and ready to respond.

Energy for Muscle Contraction

• Direct Phosphorylation: Creatine phosphate donates a phosphate to ADP to form ATP.

• Aerobic Respiration: Uses oxygen to generate ATP from glucose; efficient but slower.

• Anaerobic Glycolysis: Generates ATP quickly without oxygen, producing lactic acid as a byproduct.

ATP is the immediate source of energy for muscle contraction, but muscle fibers store only small amounts, requiring continuous regeneration during activity.

Muscle Fatigue and Oxygen Deficit

• Fatigue occurs due to ionic imbalances, lactic acid accumulation, and decreased ATP supply.

• After exercise, oxygen deficit is repaid by rapid, deep breathing to restore normal metabolic conditions.

Smooth Muscle Structure and Function

• Found in the walls of most hollow organs (except the heart), organized in two layers: longitudinal and circular.

• Spindle-shaped, uninucleate cells without striations.

• SR is less developed; most Ca2+ influx is from outside the cell.

• No sarcomeres, myofibrils, or T tubules.

• Innervated by autonomic nerve fibers at diffuse junctions; varicosities release neurotransmitters into the extracellular space.

• Thick to thin filament ratio is lower than in skeletal muscle; calmodulin binds Ca2+ instead of troponin.

• Dense bodies anchor filaments, analogous to Z discs in skeletal muscle.

Contraction and Regulation of Smooth Muscle

• Slow, synchronized contractions; cells are electrically coupled by gap junctions.

• Some cells are self-excitatory and act as pacemakers.

• Contraction is triggered by increased intracellular Ca2+, primarily from the extracellular space.

• ATP is required for contraction; relaxation requires Ca2+ detachment from calmodulin and active transport out of the cell.

• Regulated by nerves, hormones, and local chemical changes.

• Stress-relaxation response allows adaptation to stretching (e.g., in the bladder).

• Smooth muscle can undergo hyperplasia (increase in cell number), unlike skeletal muscle.

Developmental Aspects and Disorders of Muscle Tissue

• All muscle tissue develops from myoblasts.

• Cardiac and skeletal muscle become amitotic after birth; smooth muscle retains regenerative ability.

• With aging, muscle fibers are lost and replaced by connective tissue, leading to decreased strength and mass.

• Fibromyalgia: Chronic inflammation of muscles and connective tissues, cause unknown.

• Hernia: Protrusion of an organ through a muscle wall, often due to muscle weakness or strain.

• Duchenne Muscular Dystrophy: Inherited disease causing muscle degeneration due to lack of dystrophin.

• Myasthenia Gravis: Autoimmune disease where acetylcholine receptors are attacked, leading to muscle weakness and possible respiratory failure.