Tissues in Action

TISSUES IN ACTION - CLASS 9 CBSE COMPREHENSIVE NOTES

Introduction to Tissues and Hierarchy of Organization

**Definition of Tissue**: A **tissue** is a group of cells similar in structure and function that work together to perform a specific function in the body.

**Hierarchy of Organization in Multicellular Organisms**:

**Cells** → **Tissues** → **Organs** → **Organ Systems** → **Organism**

**Key Points**:

In **unicellular organisms** (like amoeba), a single cell performs all life functions

In **multicellular organisms** (plants and animals), different cell groups perform different functions

This specialization of tissues leads to **division of labour**, which increases efficiency and enables complex life processes

**Example**: In animals, muscle tissue enables movement; nervous tissue carries messages. In plants, xylem transports water and minerals; phloem transports food

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Why Plant and Animal Tissues Are Different

**Structural Differences**:

**Plant Cells vs Animal Cells**:

**Plant cells** have a rigid **cell wall** that provides support and rigidity, allowing plants to remain fixed in one place

**Animal cells** lack a cell wall and are flexible, allowing them to change shape easily for locomotion

Plants are **stationary** (sessile) and need structural support; animals are **mobile** and need flexibility

**Functional Differences Related to Nutrition**:

**Plants**: Have tissues for photosynthesis (use solar energy to synthesize food) and possess conducting tissues (xylem and phloem) for transport

**Animals**: Have tissues for digestion and breaking down various food sources; possess different conducting tissues for nutrient distribution

**Growth Pattern Differences**:

**Plant growth**: Continues throughout life due to presence of meristematic tissues at specific locations

**Animal growth**: Mostly limited to specific periods; growth tissues are not permanent

**Real-Life Example**:

A seedling grows into a tall tree throughout its life (indeterminate growth)

A child grows to a certain height and then stops growing (determinate growth)

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Tissues for Growth in Plants - Meristematic Tissues

Definition of Meristematic Tissue

**Meristematic tissue** is an actively dividing tissue responsible for growth in plants. It consists of cells capable of continuous cell division, adding new cells to the plant body.

Types of Meristematic Tissues

#### 1. Apical Meristem - Growth in Length

**Location**: At the tips of roots and shoots

**Function**: Increases height of stem and depth of roots through cell division

**Activity 3.1 Observations** (Onion Root Experiment):

When onion roots are placed in water and measured daily, they continue to grow

When root tips are cut by 1 cm, growth stops completely

This proves that roots grow only from their tips

**Inference**: Root tips contain actively dividing cells; root tips consist of apical meristem

**Why This Happens**: The cells at the root and shoot tips contain apical meristem which undergoes mitosis (observed in onion root tip experiments from Chapter 2), providing continuous cell division for length growth

**Example**: A grass seedling grows taller; tree roots penetrate deeper into soil

#### 2. Lateral Meristem - Growth in Girth (Thickness)

**Location**: Arranged in a ring along the circumference of stem

**Function**: Increases the diameter or thickness of the stem

**Mechanism**:

These cells divide and produce new cells both inward and outward

This creates a concentric pattern of growth

Results in increase in stem diameter

**Annual Growth Rings**:

Visible in cross-section of tree trunks

**Wide rings** = favorable growth conditions that year

**Narrow rings** = unfavorable conditions that year

**Age determination**: By counting annual rings, scientists can estimate tree age and understand climatic history

**Example**: Stems of dicot plants grow thicker over time; wood formation in trees

#### 3. Intercalary Meristem - Regrowth After Cutting

**Location**:

At the base of internode (part of stem between two nodes)

Just above the nodes

**Node**: Point on plant stem where branches or leaves arise

**Function**: Helps plants regenerate and grow new branches after cutting or grazing

**Mechanism**:

When the tip of a young stem is cut, the stem stops growing in length

However, new branches arise from the nodes due to intercalary meristem activity

This creates bushy appearance

**Examples**:

**Hedge trimming**: Cut hedges develop more branches and become bushier

**Grass mowing**: Lawns regrow after mowing because of intercalary meristem at grass nodes

**Grazing animals**: Grass regrows after being eaten by cattle or sheep due to this tissue

**Important Note**: Intercalary meristem is particularly important in grasses and monocots

Characteristics of Meristematic Tissue Cells

Meristematic cells have specific structural features enabling continuous division:

**Small size**

**Thin cell walls**

**Large and prominent nucleus**

**Dense cytoplasm** with many organelles

**Vacuoles absent** (or very small) - allows tightly packed cells

**Little or no intercellular space** - cells are closely packed

**Why vacuoles are absent**: Large vacuoles would take up space and prevent tight packing, hindering rapid cell division

Differentiation - Conversion to Permanent Tissue

**Definition**: **Differentiation** is the process by which meristematic cells lose the ability to divide and undergo structural and functional changes to become specialized permanent tissues.

**Process**:

Some cells from meristematic tissue remain meristematic (continue dividing)

Other cells lose ability to divide

These cells change in structure and function

They become specialized for specific functions (support, transport, storage)

**Result**: Meristematic tissue → Permanent tissue

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Permanent Tissues in Plants

**Definition**: **Permanent tissues** are tissues formed from meristematic tissue after differentiation. Cells lose the ability to divide but are specialized for specific functions.

**Classification**:

1. **Simple tissues**: Composed of only one type of cell

2. **Complex tissues**: Composed of more than one type of cell

Protective Tissue - Epidermis

**Definition**: **Epidermis** is the outermost protective layer of all plant parts (roots, stems, and leaves).

**Structure**:

Single layer of flat, rectangular, tightly packed cells

Forms continuous covering with little intercellular space

Outer wall covered with waxy layer called **cuticle** (made of cutin)

**Functions**:

1. **Mechanical protection**: Protects from injury and damage

2. **Reduces water loss**: Cuticle prevents water loss, especially in dry habitats

3. **Protection from parasites**: Cuticle prevents invasion by parasites and harmful microorganisms

4. **Thermoregulation**: Protects from extreme environmental conditions

**Special Modifications in Epidermis**:

**Root Epidermis**:

Contains **root hairs** (hair-like projections)

Function: Increase surface area for absorption of water and minerals from soil

Located in the root hair zone

**Leaf Epidermis**:

Contains **stomata** (sing. stoma) - pores in the epidermis

Structure: Bounded by two guard cells that control opening and closing

Functions of stomata:

Gaseous exchange (CO₂ entry for photosynthesis, O₂ exit)

**Transpiration**: Evaporation of water vapors through stomata

Creates **transpiration pull** in xylem, aiding water transport upward

Elimination of waste water from plant body

**Cuticle in Different Habitats**:

**Desert plants**: Thick cuticle to minimize water loss (advantageous)

**Underwater plants**: Thin or absent cuticle because water loss is not a concern; thick cuticle would prevent gas exchange (disadvantageous)

Supporting Tissues - Simple Permanent Tissues

Supporting tissues provide mechanical support and maintain plant structure. There are three types:

#### 1. Parenchyma

**Structure**:

Living cells with **thin cell walls**

Cells are **loosely packed** with large **intercellular spaces**

Contains prominent vacuoles

**Functions**:

**Storage**: Stores food materials, water, and other substances

**Photosynthesis**: In green parts of the plant, performs photosynthesis

**Flotation**: In aquatic plants, specialized parenchyma (aerenchyma) forms air spaces, enabling plants to float

**Flexibility**: Loose packing allows flexibility

**Examples**:

Pulp of fruits (stores sugars)

Pith of stem

Mesophyll of leaf (photosynthesis)

Air spaces in aquatic plant stems

#### 2. Collenchyma

**Structure**:

Living cells with **unevenly thickened walls**

**Thickening occurs at corners** due to deposition of pectin

Pectin gives flexibility (rubber-like property)

Intercellular spaces present

**Functions**:

**Support**: Provides structural support

**Flexibility**: Due to pectin, allows bending and flexing without breaking

**Tensile strength**: Withstands pulling forces

**Location and Examples**:

In stems and leaf stalks (petioles)

In tendrils (enables coiling and climbing)

Supporting tissue in young stems

**Real-Life Analogy**: Like rubber in a structure - provides support but can bend

**Comparison**: Coriander leaf stalks are soft and flexible (collenchyma); coconut husk fibers are hard and brittle (sclerenchyma)

#### 3. Sclerenchyma

**Structure**:

Cells with **very thick walls**

**Thickening due to deposition of lignin** (woody material)

Cells are mostly **dead** at maturity

No intercellular spaces; compactly arranged

**Functions**:

**Structural support**: Provides hardness and strength

**Protection**: Hard coverings protect seeds and nuts

**Mechanical strength**: Forms woody structure of plants

**Locations and Examples**:

In stems (forms wood)

In leaf veins (provides rigidity)

In hard coverings: coconut husk, walnut shell, seed coats

Fibers in jute, hemp, flax (commercial use)

**Why Dead**: Once lignin deposition completes, cells die but remain functional due to thick walls

Conducting Tissues - Complex Permanent Tissues (Vascular Tissues)

**Definition**: **Complex tissues** are composed of more than one type of cell working together to perform a specific function.

**Two main vascular tissues**: **Xylem** and **Phloem** (together called vascular bundle)

#### Xylem (Wood)

**Function**: Transports water and dissolved minerals from roots to all parts of the plant (upward transport)

**Secondary function**: Provides mechanical support and strength to the plant

**Composition** (Four types of cells):

1. **Tracheids**:

Long, narrow, tubular cells

Thick-walled and lignified

Dead cells at maturity

Perforations allow water passage

Found in ferns and conifers

2. **Vessels**:

Long, cylindrical tubes formed by stacking of many cells

End walls are perforated (end-to-end connections)

Thick-walled and lignified

Dead cells

More efficient water transport than tracheids

Found in flowering plants (angiosperms)

3. **Xylem Parenchyma**:

Living cells

Stores food (starch)

Only living component of xylem

4. **Xylem Fibres**:

Dead, elongated cells

Thick-walled

Primarily sclerenchymatous

Provide additional mechanical support

**How Water Reaches Tall Trees** (against gravity):

Xylem vessels and tracheids form continuous tubes from root to leaves

**Root pressure** pushes water upward

**Transpiration pull** from leaves creates suction (water evaporates from leaves, pulling water column upward)

**Adhesion and cohesion** of water molecules maintain the water column

Dead cells of xylem work with living cells of leaves to transport water upward

#### Phloem (Bast)

**Function**: Transports food (sugars/carbohydrates) prepared in leaves to all parts of the plant (bidirectional transport)

**Characteristic**: Mostly composed of living cells (unlike xylem which is mostly dead)

**Composition** (Four types of cells):

1. **Sieve Tubes**:

Long, tubular cells joined end-to-end

End walls are perforated (called **sieve plates**)

Sieve pores allow passage of food

Living cells at maturity (though nucleus degenerates)

Main transporting element

2. **Companion Cells**:

Specialized parenchyma cells

Closely associated with sieve tubes (connected by plasmodesmata)

Live cells with prominent nucleus

**Main function**: Regulate cellular functions of sieve tubes; monitor loading and unloading of sugars in sieve tubes

Provide energy for transport

3. **Phloem Parenchyma**:

Living cells

Function: Store food materials, resins, tannins, and latex

Distribute these substances

4. **Phloem Fibres**:

Dead, elongated cells

Thick-walled (primarily sclerenchymatous)

Provide mechanical strength to phloem tissue

**Difference from Xylem**:

Xylem: Unidirectional (root to leaf), mostly dead cells

Phloem: Bidirectional (leaves to roots or storage organs), mostly living cells

Tissue Systems in Plants

Plant tissues are organized into **three tissue systems**:

#### 1. Dermal Tissue System (Epidermis)

**Function**: Forms the outer protective covering of the plant body

Protects all internal structures

Reduces water loss

Single-layered epidermis in young plants

As plant ages, outer cells may develop cork layer (cork cambium produces cork cells)

**Components**: Epidermis with all its modifications (root hairs, stomata)

#### 2. Ground Tissue System

**Function**: Forms the main body of the plant between dermal and vascular tissues

**Composition**: Includes all three simple permanent tissues:

Parenchyma (storage and photosynthesis)

Collenchyma (support and flexibility)

Sclerenchyma (hard support)

**Location**: Interior of stems, roots, leaves

#### 3. Vascular Tissue System

**Function**: Conducts water and food throughout the plant; provides structural support

**Composition**:

**Xylem** (transports water and minerals upward)

**Phloem** (transports food in all directions)

Often arranged as **vascular bundles** in stems and roots

**Organization**: Vascular tissues are embedded within ground tissue

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Animal Tissues

**Definition**: Similar to plants, animal cells group together and specialize in performing different functions, forming **animal tissues**.

**Key Difference from Plant Tissues**:

Animal tissues lack cell wall, allowing flexibility

Growth is typically determinate (stops at certain age)

No meristematic tissues in adults

Epithelial Tissues - Structure and Functions

**Definition**: **Epithelial tissue** is a covering tissue that forms the outer surface of the body (skin) and lines internal organs and body cavities.

**General Characteristics**:

**Closely packed cells** with very little space between them

Forms continuous sheet-like structure

Cells are held together by intercellular cement

**Functions**:

1. **Protection**: Prevents entry of pathogens and germs

2. **Reduces water loss**: Protects from dehydration

3. **Absorption**: Absorbs nutrients and substances

4. **Secretion**: Secretes substances like sweat, mucus

5. **Sensation**: Some epithelial cells detect stimuli

6. **Movement**: Facilitates movement of substances

**Principle**: **Structure and function relationship** - different epithelial tissues have different shapes and modifications suited to their specific functions

**Types of Epithelial Tissues** (Based on Structure and Function):

#### 1. Squamous Epithelium

**Structure**:

Single layer of flat, thin, plate-like cells

Cells tightly fitted together like floor tiles

Minimal intercellular space

Very thin tissue

**Location**:

Outer layer of skin (epidermis)

Lining of blood vessels (endothelium)

Lining of alveoli in lungs

Lining of body cavities

**Functions**:

Protection from mechanical injury

Rapid diffusion and absorption (due to thin structure)

Allows gaseous exchange in lungs

**Advantage**: Thin structure allows quick diffusion and exchange of substances

**Real-Life Example**: Lining of lungs (alveoli) - thin structure allows efficient oxygen-carbon dioxide exchange

#### 2. Cuboidal Epithelium

**Structure**:

Single layer of cube-shaped cells

Cells appear square in cross-section

Equal dimensions in length, width, and height

Cells are more robust than squamous cells

**Location**:

Lining of ducts of sweat glands, salivary glands

Lining of kidney tubules (collecting ducts)

Covering of ovaries

Lining of intestinal glands

**Functions**:

**Absorption**: Absorbs useful substances

**Secretion**: Secretes various substances

**Transportation**: Moves substances through ducts

**Advantage**: Cube shape provides structural strength while maintaining absorption/secretion capability

**Real-Life Example**: Kidney tubules - absorb filtered useful substances from urine

#### 3. Columnar Epithelium

**Structure**:

Single layer of tall, column-like (rectangular) cells

Cells are elongated vertically

Height much greater than width

Cells may have additional modifications

**Sub-types**:

**Simple Columnar**:

Single layer without modifications

Located in: Small intestine lining, stomach lining

**Ciliated Columnar**:

Cells with cilia (hair-like projections) on free surface

Cilia beat in coordinated manner

Location: Respiratory tract (trachea, bronchi), fallopian tubes

**Location**:

Lining of small intestine (most common site)

Lining of stomach

Lining of large intestine

Respiratory passages (ciliated form)

Fallopian tubes (ciliated form)

**Functions**:

**Absorption**: Maximum surface area for nutrient absorption

**Secretion**: Secretes digestive enzymes and mucus

**Movement**: Ciliated form moves substances (mucus with trapped particles in respiratory tract; egg in fallopian tube)

**Protection**: Secreted mucus protects underlying tissues

**Modifications**:

**Microvilli**: Finger-like projections on free surface increase absorption area

**Cilia**: Hair-like structures that beat and move substances

**Goblet cells**: Among columnar cells, secrete mucus

**Advantage**:

Large surface area (especially with microvilli) for efficient absorption

Secretory cells produce protective mucus

**Real-Life Examples**:

**Small intestine epithelium**: Columnar cells with microvilli absorb nutrients maximally

**Respiratory tract**: Ciliated columnar cells beat cilia to move mucus and trapped particles outward, protecting lungs

**Stomach**: Columnar cells secrete gastric juices for digestion

#### 4. Stratified Epithelium

**Structure**:

Multiple layers of cells (not single layer)

Outer layers are flat (squamous), inner layers are cuboidal or columnar

Provides extreme durability

**Location**:

Outer layer of skin (epidermis)

Lining of mouth, pharynx, and esophagus

Lining of vagina and anal canal

**Functions**:

**Maximum protection**: Mechanical protection from friction and injury

**Waterproofing**: Prevents water loss and entry of germs

**Durability**: Multiple layers provide redundancy

**Advantage**: Multiple layers provide protection against wear and tear

**Real-Life Example**: Skin epidermis - undergoes continuous wear and tear, so multiple layers provide continuous protection as outer layers shed

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Connective Tissues

**Definition**: **Connective tissues** bind and support other tissues and organs. They have specialized cells and abundant extracellular matrix.

**Types and Functions**:

1. Bone (Skeletal Tissue)

**Structure**:

Hard, rigid tissue

Matrix of calcium salts and protein fibers (collagen)

Cells called **osteocytes** embedded in matrix

Highly vascularized

**Functions**:

**Support**: Framework of the body

**Protection**: Protects vital organs (skull protects brain, ribs protect heart and lungs)

**Movement**: Serves as attachment for muscles

**Storage**: Stores calcium and phosphorus

**Blood formation**: Bone marrow produces blood cells

**Location**: Skeleton of the body - skull, vertebral column, ribs, limbs

**Example**: Femur (thighbone) - supports body weight and moves during walking

2. Cartilage (Elastic Connective Tissue)

**Structure**:

Firm but flexible tissue

Matrix of proteins and elastic fibers

Cells called **chondrocytes** embedded in matrix

Less vascularized than bone (avascular in some areas)

Bluish-white translucent appearance

**Functions**:

**Support with flexibility**: Provides structural support while allowing movement

**Reduces friction**: Smooth surface reduces friction at joints

**Cushioning**: Absorbs shocks and impacts

**Flexibility**: Allows bending and movement

**Location**:

Joints (articular cartilage)

Ear (pinna)

Nose

Trachea and bronchi (maintains airway)

Intervertebral discs (between vertebrae)

**Example**: Cartilage in knee joint - allows bending while protecting bone; ear cartilage allows ear to bend

**Difference from Bone**:

Cartilage is more flexible

Cartilage is less mineralized

Cartilage has slower regeneration

3. Loose Connective Tissue (Areolar Tissue)

**Structure**:

Cells scattered in abundant extracellular matrix

Contains fibers (collagen and elastic)

Contains white blood cells for defense

**Functions**:

**Binding**: Binds muscles to bone, organs together

**Support**: Provides structural support to organs

**Defense**: Contains immune cells

**Transport**: Transports oxygen, nutrients, waste

**Location**:

Beneath skin (subcutaneous tissue)

Between muscles

Around organs

Fills spaces between tissues

**Example**: Tissue beneath skin - binds skin to muscles and provides cushioning

4. Dense Connective Tissue (Fibrous Tissue)

**Structure**:

Dense arrangement of collagen fibers

Few cells compared to matrix

Strong and relatively inflexible

**Functions**:

**Strength**: Provides maximum tensile strength

**Attachment**: Attaches muscles to bone and bone to bone

**Location**:

**Tendons**: Connect muscles to bone

**Ligaments**: Connect bone to bone at joints

**Skin**: Deep layer (dermis)

**Example**: Tendons in fingers - very strong to withstand pulling forces during movement

5. Adipose Tissue (Fat Tissue)

**Structure**:

Cells specialized for fat storage (adipocytes)

Each cell contains large fat droplet

Cells separated by delicate connective tissue

**Functions**:

**Energy storage**: Stores reserve energy

**Insulation**: Insulates body and maintains temperature

**Cushioning**: Provides cushioning and protection to organs

**Waterproofing**: Reduces water loss

**Location**:

Beneath skin (subcutaneous fat)

Around kidneys and heart

In bone marrow

**Example**: Subcutaneous fat layer in skin - keeps body warm in winter; fat around kidneys protects them from damage

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Muscle Tissues

**Definition**: **Muscle tissue** is specialized tissue composed of contractile cells (muscle fibers) that can contract and relax, enabling movement.

**General Functions**:

**Movement**: Enables locomotion and movement of body parts

**Circulation**: Maintains blood circulation through heart contractions

**Movement of materials**: Moves food through digestive tract, moves urine

**Types of Muscle Tissue** (Based on Structure and Control):

1. Skeletal Muscle (Striated Muscle/Voluntary Muscle)

**Structure**:

Long, cylindrical, multinucleate fibers

Cells contain alternating light and dark bands (striations)

Nucleus located at periphery (multiple nuclei)

Organized in bundles with connective tissue

**Contraction Control**:

**Voluntary** - controlled by conscious thought

Controlled by somatic (voluntary) nervous system

**Functions**:

**Locomotion**: Enables walking, running, jumping

**Movement of body parts**: Lifting, bending, etc.

**Maintaining posture**: Keeps body upright

**Location**:

Attached to bones via tendons

Forms muscles of limbs, trunk, face

Examples: Biceps, triceps, quadriceps

**Characteristics**:

Fastest contraction

Tires easily (fatigues)

Rich blood supply (red appearance)

**Example**: Biceps muscle - contracts when you lift an object; relaxes when you put it down (under conscious control)

**Diagram to Draw**: Skeletal muscle fiber showing:

Long cylindrical shape

Alternating light (I) and dark (A) bands

Multiple nuclei at periphery

Striations (alternating bands)

Label: Muscle fiber, Nucleus, Striations, Sarcomere

2. Cardiac Muscle (Involuntary Muscle)

**Structure**:

Branched, short fibers

Cells are uninucleate (single nucleus) or binucleate

Nucleus located centrally

**Intercalated discs** - special connections between cells join them end-to-end

Striations present (light and dark bands)

**Contraction Control**:

**Involuntary** - not controlled by conscious thought

Controlled by autonomous nervous system and hormones

Has intrinsic rhythmicity (beats on its own)

**Functions**:

**Pumping blood**: Heart contraction circulates blood throughout body

**Circulation**: Maintains continuous blood flow

**Location**:

Heart wall only

Forms myocardium (cardiac muscle layer)

**Characteristics**:

**Rhythmic contractions**: Beats continuously and regularly

**Exceptional endurance**: Does not fatigue

**Strong contractions**: Each contraction is powerful

**Rich blood supply**: Highly vascularized

**Special Feature - Intercalated Discs**:

Specialized junctions between cardiac muscle cells

Composed of desmosomes and gap junctions

Desmosome: Holds cells together mechanically

Gap junction: Allows passage of ions and electrical signals, enabling synchronized contraction

**Why No Fatigue**: Rich blood supply, anaerobic respiration capability, continuous oxygen availability

**Example**: Heart beats 70-100 times per minute throughout life without getting tired

**Diagram to Draw**: Cardiac muscle fiber showing:

Branched structure

Striations (alternating bands)

Central nucleus

Intercalated disc between cells

Label: Cardiac muscle cell, Nucleus, Striations, Intercalated disc

3. Smooth Muscle (Non-striated Muscle/Involuntary Muscle)

**Structure**:

Long, spindle-shaped (fusiform) fibers

Uninucleate (single nucleus per cell)

Nucleus located centrally

Absence of striations (non-striated/smooth appearance)

Organized in sheets or bundles

**Contraction Control**:

**Involuntary** - not controlled by conscious thought

Controlled by autonomous (sympathetic and parasympathetic) nervous system and hormones

**Functions**:

**Peristalsis**: Moves food through digestive tract

**Vasoconstriction/vasodilation**: Controls blood vessel diameter and blood pressure

**Respiration**: Contraction of bronchial smooth muscle controls airflow

**Elimination**: Moves urine through ureter and bladder

**Location**:

Walls of internal organs

**Digestive tract**: Esophagus to anus (entire length)

**Blood vessels**: Arteries and veins

**Respiratory tract**: Bronchi and bronchioles

**Urinary system**: Ureters and bladder

**Reproductive system**: Fallopian tubes, uterus

**Characteristics**:

**Slow contraction**: Slower than skeletal muscle

**Continuous tone**: Maintains partial contraction (muscle tone)

**High endurance**: Continuous activity without fatigue

**Simultaneous contraction**: Fibers contract together as sheets

**Example**:

Digestive system: Smooth muscles contract in waves (peristalsis) to move food from mouth to anus

Blood vessels: Smooth muscle contraction narrows blood vessels (vasoconstriction) to increase blood pressure

**Diagram to Draw**: Smooth muscle fiber showing:

Spindle shape (fusiform)

Single central nucleus

Absence of striations (smooth appearance)

Relaxed and contracted states

Label: Smooth muscle cell, Nucleus, No striations

**Comparison Table - Muscle Tissues**:

**Skeletal**: Striated, multinucleate, voluntary, fast, fatigues easily, locomotion

**Cardiac**: Striated, uninucleate, involuntary, rhythmic, no fatigue, heart function

**Smooth**: Non-striated, uninucleate, involuntary, slow, no fatigue, internal organ movement

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Nervous Tissue

**Definition**: **Nervous tissue** is specialized tissue composed of neurons and supporting cells (glial cells) that detects stimuli and transmits electrical and chemical signals throughout the body.

**Main Function**:

**Communication**: Transmits messages (impulses) between different parts of body

**Control**: Coordinates and controls body functions

**Sensation**: Detects stimuli from environment and internal organs

**Composition**:

1. Neurons (Nerve Cells)

**Definition**: A **neuron** is the functional unit of nervous tissue capable of receiving and transmitting electrical and chemical signals.

**Structure of Neuron**:

**Cell Body (Soma)**:

Contains nucleus

Contains cytoplasm with organelles

Center of metabolic activities

**Dendrites**:

Multiple, short, branched processes

Receive signals (impulses) from other neurons

Function: Reception of impulses

**Axon**:

Single, long process arising from cell body

Conducts impulses away from cell body

Ends in axon terminals (synaptic terminals)

Function: Transmission of impulses

**Synapses**:

Junctions between neurons

Presynaptic terminal: End of axon of transmitting neuron

Synaptic cleft: Gap between neurons

Postsynaptic receptor: Membrane of receiving neuron

Neurotransmitters: Chemicals that transmit signal across synapse

**Overall Function**: Reception → Integration (in cell body) → Transmission

**Real-Life Example**: When you touch something hot, sensory neurons receive signal, transmit to spinal cord, signal processed, motor neurons transmit signal to muscles causing withdrawal

**Diagram to Draw - Neuron Structure**:

Cell body with nucleus in center

Multiple short dendrites branching from cell body

Single long axon extending from cell body

Axon terminals at end with synaptic vesicles

Label: Cell body, Nucleus, Dendrite, Axon, Axon hillock, Nodes of Ranvier, Myelin sheath, Axon terminal, Synaptic vesicle, Synapse

2. Supporting Cells (Glial Cells)

**Definition**: **Glial cells** (neuroglia) are non-neuronal supporting cells in nervous tissue.

**Types and Functions**:

**Astrocytes**: Provide structural support, nutrients to neurons

**Oligodendrocytes**: Form myelin sheath around axons in brain and spinal cord

**Microglia**: Immune defense in nervous tissue

**Ependymal cells**: Produce cerebrospinal fluid

**General Functions**:

**Structural support**: Hold neurons in place

**Nutrient supply**: Transport nutrients to neurons

**Waste removal**: Remove dead cells and debris

**Insulation**: Myelin sheath speeds up impulse conduction

**Protection**: Immune defense against pathogens

**Proportion**: Outnumber neurons in nervous tissue

**Types of Neurons** (Based on Function):

1. **Sensory Neurons (Afferent Neurons)**:

Carry impulses from sense organs to central nervous system

Detect stimuli

2. **Motor Neurons (Efferent Neurons)**:

Carry impulses from central nervous system to muscles and glands

Cause response

3. **Interneurons (Relay Neurons)**:

Present in central nervous