overview of anatomy and physiology

LESSON 1
Source: Marieb & Keller’s Essentials of Human Physiology, Twelfth Edition & Powerpoint Presentation
Lecture by: Dr. Jenina L. Canoy, RMT, MD
An Overview of Anatomy and Physiology

ANATOMY VS. PHYSIOLOGY

ANATOMY RELATIONSHIP PHYSIOLOGY
The study of the structure and shape of the body The parts of the body form a well-organizes unit, The study of how the body and its parts work or
and its parts and their relationships to one and each of those parts has a job to do to make function in order to maintain homeostasis.
another. the body operate as a whole.
Gross Anatomy – study of large, easily Structure determines what functions can take
observable structures. place (Form follows function).
Microscopic Anatomy – study of body
structures that are too small to be seen with the
naked eye (cells and tissues).

LEVELS OF STRUCTURAL ORGANIZATION

(1) Atoms Tiny building blocks of matter that combine together to form
Chemical Level (2) Molecules molecules
(3) Organelle Subcellular structure of the cell
Cellular Level (4) Cell The smallest units of all living things
Tissue Level (5) Tissue Groups of similar cells that have a common function
Organ Level (6) Organ Structure composed of two or more tissue types that performs
a specific function
Organ System Level (7) Organ System Group of organs that work together to accomplish a common
purpose
Organismal Level (8) Organism Represent the highest level of structural organization.

ORGAN SYSTEM OVERVIEW

SYSTEMS ORGANS NOTES
Layers of the Skin:
(1) Epidermis – waterproof barrier; creates skin tone
Epidermal and dermal regions (2) Dermis – contains connective tissue, hair follicles and sweat glands
(3) Hypodermis (Subcutis) – fat and connective tissue
Function:
Integumentary System (1) Protects deeper organs from mechanical, chemical and bacterial injury and drying
Cutaneous sense organs and glands out
(2) Excretes salts and urea
(3) Aids in regulation of body temperature
(4) Produces vitamin D
(5) Permits the sensation of touch, heat and cold
Division of Skeletal System:
(1) Axial – total of 80 bones
(1) Bones (2) Appendicular – 126 bones; attached to axial skeleton
(2) Cartilage
(3) Tendons Function:
Skeletal System (4) Ligaments (1) Support (framework of body and to maintain shape
(5) Joints (2) Storage of minerals
(3) Protection
(4) Production of ells
(5) Movement (joints allow movement)
(6) Endocrine Regulation
Types of Muscles:
(1) Cardiac
(2) Skeletal
(3) Smooth
Muscular System Muscles attached to the skeleton Function:
(1) Permits movement of the body by contracting
(2) Maintains posture
(3) Circulates blood throughout the body
(4) For generation of heat
Types of Nerves
(1) Motor or efferent nerves – transmit signals from brain
Main Parts of Nervous System (2) Sensory or afferent nerves – transmit information from body to CNS
Nervous System (1) CNS – brain and spinal chord Subsystems of PNS:
(2) PNS – nerves (1) Somatic
(2) Autonomic
(3) Enteric
(1) Hypothalamus – control center Hypothalamic-Pituitary-Adrenal Axis – axis where glands signal each other in
(2) Pituitary Gland sequence
 (3) Pineal Gland
Endocrine System (4) Thyroid Gland Function;
(5) Parathyroid glands (1) Helps maintain homeostasis, promotes growth and development
(6) Thymus (2) Produces chemical messengers called hormones that travel in the blood to exert
(7) Pancreas their effect on various target organs of the body
(8) Ovary & Testicles
(9) Adrenal Glands
2 Components
(1) Pulmonary Circulation – loops through the lungs
(2) Systemic Circulation – loop through the rest of the body to provide oxygenated
Cardiovascular (1) Heart blood
System (2) Blood vessels Function:
(3) Blood-essential components (1) transport system that carries oxygenated blood, CO2, nutrients, waste, ions,
hormones, and other substances
(2) Blood is propelled through pumping action of the heart
(3) Antibodies and other protein molecules in the blood protect the body
(1) Lymphatic vessels Lymphatic system is a part of circulatory and immune system
(2) Lymph nodes
(3) Spleen Function:
Lymphatic System (4) Thymus (1) Picks up fluid leaked from the blood vessels and returns it to the blood
(5) Tonsils (2) Cleanses blood of pathogens and other debris
(6) Scattered collection of lymphoid (3) Houses lymphocytes that act via the immune response to protect the body
tissues
Respiratory System (1) Nasal passages Function:
(2) Pharynx (1) Keeps the blood continuously supplies with oxygen while removing carbon
(3) Larynx dioxide
(4) Trachea (2) Contributes to the acid-base balance of the blood via its carbonic acid-bicarbonate
(5) Bronchi buffer system
(6) Lungs
(1) Oral Cavity 3 Phases
(2) Esophagus (1) Cephalic – site and smell
Digestive System (3) Stomach (2) Gastric – occurs in stomach; breakdown
(4) Small and Large Intestine (3) Intestinal – enzymes are produced for further breakdown
(5) Accessory structures
(1) Kidneys Function:
Urinary System (2) Ureters (1) Rids the body of nitrogen-containing wastes including urea, uric acis, and
(3) Bladder ammonia, which result from the breakdown of proteins and nucleic acids
(4) Urethra (2) Maintains water, electrolyte, and acid-base balance of blood
Male:
(1) testes
(2) prostate gland Provides germ cells called sperm for perpetuation of the species
(3) scrotum
(4) penis
Reproductive System (5) duct system
Female:
(1) ovaries
(2) uterine tubules Provide germ cells called eggs; the female uterus houses the developing fetus until
(3) uterus birth; mammary glands provide nutrition for the infant
(4) mammary glands
(5) vagina

ORIENTATION AND DIRECTIONAL TERMS

TERM DEFINITION ILLUSTRATION EXAMPLE

Toward the head end or upper part of a

The forehead is superior to
Superior (cranial or cephalic) structure or the body; above nose

Away from the head end or toward the lower

Inferior (caudal) part of a structure or the body; below
The navel is inferior to the
breastbone.
 Anterior (ventral) Toward or at the front of the body; in front of
The breastbone is anterior to the
spine.

Posterior (dorsal) Toward or at the backside of the

body; behind
The heart is posterior to the
breastbone.

Medial Toward or at the midline of the body; on the

inner side of
The heart is medial to the arm.

Lateral Away from the midline of the body; on the

outer side of
The arms are lateral to the chest.

Between a more medial and a more lateral The collarbone is intermediate

Intermediate structure
between the breastbone and
the shoulder.

The elbow is proximal to the

Close to the origin of the body part or the
Proximal point of attachment of a limb
wrist (meaning that the elbow
to the body trunk is closer to the shoulder or
attachment point of the arm
than the wrist is).

Distal Farther from the origin of a body

part or the point of attachment of a limb to The knee is distal to the thigh.
the body trunk

The skin is superficial to the

Superficial (external) Toward or at the body surface
skeleton.

The lungs are deep to the rib

Deep (internal) Away from the body surface; more internal
cage.

ANATOMICAL TERMS AND REGIONAL POSITION

BODY PLANES/SECTION

BODY CAVITIES
 Dorsal Body Cavity Ventral Body Cavity Other Body Cavities
(1) Cranial Cavity – brain (1) Thoracic Cavity (1) Oral and Digestive
(2) Spinal Cavity – spinal cord (2) Abdominal Cavity Cavity
(3) Pelvic Cavity (2) Nasal Cavity
(3) Orbital Cavity
(4) Middle Ear Cavity

ABDOMINOPELVIC QUADRANTS ABDOMINOPELVIC REGIONS

homeostasis
LESSON 2
Source: Powerpoint Presentation
Lecture by: Dr. Jenina L. Canoy, RMT, MD

Homeostasis – central theme of physiology; describes the body’s ability to maintain relatively stable internal condition; if homeostasis is not achieved the
result is disease.
Receptor – type of sensor that monitors and responds to changes in the environment. It responds to such changes, called stimuli, by sending information (input)
to the second component, the control center. Information flows from the receptor to the control center along the afferent pathway.
Control center – determines the level (set point) at which a variable is to be maintained. This component analyzes the information it receives and then
determines the appropriate response or course of action.
Effector – provides the means for the control center’s response (output) to the stimulus. Information flows from the control center to the effector along the
efferent pathway.

NEGATIVE FEEDBACK POSITIVE FEEDBACK

Effectors OPPOSE the change; variable is pushed back toward the set-point Effectors ENHANCE the change; variable is pushed even farther from set-
value. point value.
The net effet of the response to the stimulus is to either shut off the original Tend to increase the original disturbance(stimulus).
stimulus or reduce its intensity
KEY SUMMARY
 Anatomy is the study of structure. Observation is used to see the sizes, shapes, and relationships of body parts.
 Physiology is the study of how a structure (which may be a cell, an organ, or an organ system) functions or works.
 Structure determines what functions can occur; therefore, if the structure changes, the function must also change.
 There are six levels of structural organization. Atoms (at the chemical level) combine, forming the unit of life, the cell. Cells are grouped into
tissues, which in turn are arranged in specific ways to form organs. An organ system is a group of organs that performs a specific function for the
body (which no other organ system can do). Together, all of the organ systems form the organism, or living body.
 Eleven organ systems make up the human body: the integumentary, skeletal, muscular, nervous, endocrine, cardiovascular, lymphatic, respiratory,
digestive, urinary, and reproductive systems.
 To sustain life, an organism must be able to maintain its boundaries, move, respond to stimuli, digest nutrients and excrete wastes, carry on
metabolism, reproduce itself, and grow. Survival needs include food, oxygen, water, appropriate temperature, and normal atmospheric pressure.
Extremes of any of these factors can be harmful. Anatomical terminology is relative and assumes that the body is in the anatomical position
(standing erect, with palms facing forward).
 Directional terms
a. Superior (cranial, cephalic): above something else, toward the head.
b. Inferior (caudal): below something else, toward the tail.
c. Anterior (ventral): toward the front of the body or structure.
d. Posterior (dorsal): toward the rear or back of the body or structure.
e. Medial: toward the midline of the body.
f. Lateral: away from the midline of the body.
g. Proximal: closer to the point of attachment.
h. Distal: farther from the point of attachment.
i. Superficial (external): at or close to the body surface.
j. Deep (internal): below or away from the body surface.
 Regional terms. Visible landmarks on the body surface may be used to specifically refer to a body part or area.
 Body planes and sections
a. Sagittal section: separates the body longitudinally into right and left parts.
b. Frontal (coronal) section: separates the body on
a longitudinal plane into anterior and posterior parts.
c. Transverse (cross) section: separates the body on a horizontal plane into superior and inferior parts.
 Body cavities
a. Dorsal: well protected by bone; has two subdivisions.
(1) Cranial: contains the brain.
(2) Spinal: contains the spinal cord.
b. Ventral: less protected than dorsal cavity; has two subdivisions.
(1) Thoracic: The superior cavity that extends inferiorly to the diaphragm; contains heart and lungs, which are protected by the rib cage.
(2) Abdominopelvic: The cavity inferior to the diaphragm that contains the digestive, urinary, and reproductive organs. The abdominal
portion is vulnerable because it is protected only by the trunk muscles. The pelvic portion is somewhat protected by the bony pelvis. The
abdominopelvic cavity is often divided into four quadrants or nine regions
c. Smaller body cavities include the oral, nasal, orbital, and middle ear cavities. All are open to the
outside of the body except the middle ear cavity.
 Body functions interact to maintain homeostasis, or a relatively stable internal environment within the body. Homeostasis is necessary for survival
and good health; its loss results in illness.
 All homeostatic control mechanisms have three components:
(1) a receptor that responds to environmental changes (stimuli) and
(2) a control center that assesses those changes and produces a response by activating
(3) the effector.
 Most homeostatic control systems are negative feedback systems, which act to reduce or stop the initial stimulus. Some are positive feedback
systems, which act to increase the initial stimulus, as in the case of blood clotting.
cell physiology
LESSON 3
Source: Marieb & Keller’s Essentials of Human Physiology, Twelfth Edition
Guyton and Hall’s Textbook of Human Physiology
 Krause’s Essential Human Histology for Medical Students
Lecture by: Dr. Reden A. Patalinjug, MD

Cell – basic living unit of the body, from Latin cella, meaning “small room”—coined by Robert Hooke when he saw small structures on the cross section of the
cork plant. Entire body contains about 100 trillion of cells.
Protoplasm – collective term for whats inside the cell membrane
Organelles – “little organs”; specialized parts of a cell that have a unique job to perform

ORGANIZATION OF THE CELL

STRUCTURE OF THE CELL

FIVE BASIC SUBSTANCES OF PROTOPLASM
The principal fluid medium of the cell is
water, which is present in most cells,
except for fat cells, in a concentration of
70 to 85 percent. Many cellular chemicals
are dissolved in the water. Others are
Water suspended in the water as solid
particulates. Chemical reactions take
place among the dissolved chemicals or at
the surfaces of the suspended particles or
membranes.
The ions provide inorganic chemicals for
cellular reactions and also are necessary
for operation of some of the cellular
control mechanisms. For instance, ions
Ions/Electrolytes acting at the cell membrane are required
for transmission of electrochemical
impulses in nerve and muscle fibers.
After water, the most abundant substances
in most cells are proteins, which normally
constitute 10 to 20 percent of the cell
Proteins mass. These proteins can be divided into
two types: structural proteins and
functional proteins.
Are mainly insoluble in water and
therefore are used to form the cell
Lipids membrane and intracellular membrane
barriers that separate the different cell
compartments.
Carbohydrates have little structural EUKARYOTIC CELL SIMILARITIES PROKARYOTIC CELL
function in the cell except as parts of Has a nucleus and other Presence of cell No nucleus or membrane
Carbohydrates glycoprotein molecules, but they play a organelles; plants and membrane, cytoplasm, enclosed organelles;
major role in nutrition of the cell. animals and DNA unicellular
ORGANELLES
DNA Dictates what the cell is going to do
Nucleus Transmits genetic information; control center
Chromatin Tangled spread out form of DNA inside the nuclear membrane
Nucleolus Where ribosomes are made
Ribosomes Protein synthesis; may be attached to the ER or free within the
cytoplasm
Cytoplasm Where organelles are bounded; jelly-like structure
Rough ER Has ribosomes attached; proteins are bound in vesicles for transport
to Golgi apparatus; synthesizes phospholipids
Smooth ER No ribosomes attached; lipid and steroid synthesis, lipid
metabolism, and drug detoxification
Golgi Body Packages, modifies, and segregates proteins
Vacuole Sac-like structures that store different materials
Lysosome Filled with enzyme that breakdown cellular debris
Mitochondria ATP production
Cytoskeleton Contains microfilaments and microtubules; maintains cell structure

LYSOSOMES PEROXISOMES
Breaks down biological polymers like proteins and polysaccharides Oxidizes organic compounds, breaking down metabolic hydrogen peroxides
Consists of digestive (hydrolase) enzymes and intracellular digestive system Consists of oxidative enzymes
that allows the cell to digest (1) damaged cellular structures, (2) food
particles that have been ingested by the cell, and (3) unwanted matter such
as bacteria.
Formed by breaking off from the Golgi apparatus and then dispersing Formed by self-replication (or by budding off from the smooth ER)
throughout the cytoplasm

SMOOTH (AGRANULAR) ER ROUGH (GRANULAR) ER

No ribosomes attached Has ribosomes attached
 Synthesizes lipid substances and for other processes of the cells promoted Involved in some protein production, protein folding, quality control and
by intrareticular enzymes dispatch
It is “smooth” because it is not studded with ribosomes and is associated It is called “rough” because it is studded with ribosomes
with smooth slippery fats

4 CONCEPTS OF THE CELL THEORY

1. A cell is the basic structural and functional unit of living organisms. So, when you define cell properties, you are in fact defining the properties of
life
2. The activity of an organism depends on the collective activities of its cells
3. According to the principle of complementarity, the biochemical activities of cells are dictated by their shape or form and by the relative number of
their specific subcellular structures
4. Continuity of life has a cellular basis
SPECIALIZED CELL FUNCTIONS
 Cells that connect body parts
 Cells that covers and lines body organs
 Cells that move organs and body parts (Muscular tissues)
 Cells that stores nutrients
 Cells that fights disease
 Cells that gathers information and controls body functions
 Cells of reproduction

PASSIVE TRANSPORT
 Movement of substances without any energy input
 Net movement is always “down the concentration gradient”—high concentration to low concentration

(1) SIMPLE DIFFUSION – fat soluble molecules move directly through the phospholipid bilayer
(2) OSMOSIS – diffusion of water through a specific channel protein (aquaporin) or through the lipid bilayer
ISOTONIC SOLUTION HYPOTONIC SOLUTION HYPERTONIC SOLUTION
Solute=Water; 5% C6H12O6, 0.9% saline High Solute, Low Water Low Solute, High Water
Dynamic Equilibrium – water moves Cytolysis – water moves from the solution to Plasmolysis – water moves from inside the
equally in both directions and the cell remains inside the cell, cell swells and bursts open cell into the solution, cell shrinks (crenated)
same size

(3) FACILITATED DIFFUSION

 CARRIER-MEDIATED FACILITATED DIFFUSION – via protein carrier specific for one chemical; binding of substrate causes shape change
in transport protein
 CHANNEL-MEDIATED FACILITATED DIFFUSION – through a channel protein, mostly ions, selected on basis of size and charge

ACTIVE TRANSPPORT
 Requires the cell to use energy in the form of ATP
 Net movement is against the concentration gradient—low concentration to high concentration
(1) PROTEIN PUMPS – transport proteins that require energy to do work
(2) ENDOCYTOSIS – takes bulky material into a cell; uses energy
 Phagocytosis
 Pinocytosis
 Receptor-Mediated Endocytosis
(3) EXOCYTOSIS – forces material out of cell in bulk.

MITOSIS
 Early Chromatin threads coil and shorten so that the barlike chromosomes become visible under a microscope. Because DNA has already been
Prophase replicated, each chromosome is actually made up of two identical strands called sister chromatids.
Late The centrioles separate from each other and begin to move toward the opposite sides of the cell, directing the assembly of a mitotic
Prophase spindle. By the end of prophase, the nuclear envelope and the nucleoli have broken down and temporarily disappeared, and the
chromosomes have attached randomly to the spindle fibers by their centromeres.
Metaphase Chromosomes line up at the metaphase plate; straight line of chromosomes is seen
Anaphase Centromeres that have held the chromatids together split. The chromatids (now called chromosomes again) begin to move slowly apart,
drawn toward the opposite end of the cell.
Telophase The chromosomes at the opposite ends of the cell uncoil to become threadlike chromatin again. The spindle breaks down and disappears, a
nuclear envelope forms around each chromatin mass, and nucleoli appear in each of the daughter nuclei.
Cytokinesis Begins during late anaphase and completes during telophase. A contractile ring made of microfilaments forms a cleavage furrow over the
midline of the spindle, and it eventually squeezes, or pinches, the original cytoplasmic mass into two parts.

MEIOSIS
 HISTOLOGY

Tissue – French tissue, meaning something that is woven, from the verb tisser, “to weave”; an endemble of similar cells and their extracellular matrix from the
same origin that together carry out a specific function.
Histology – study of Tissues
Endocrine Gland – secretes hormones into bloodstream or nearby cells
Exocrine Gland – secrete juices into tubes or ducts that lead to the outside of the body.

4 TYPES OF TISSUES
1. Epithelial Tissue
2. Connective Tissue
3. Muscular Tissue
4. Nervous Tissue

(1) EPITHELIAL TISSUE – avascular, rely on connective tissue; protect deeper layers of tissue from injury/infection
 Glandular Epithelium – forms endocrine and exocrine glands and secretes hormones and other substances
 Proper Epithelium – covers and lines the outer and inner body

Cell shape NUMBER OF LAYERS

One layer: simple epithelial tissues More than one layer: stratified epithelial tissues
Squamous Diffusion and filtration & Secretion in serous membranes Protection
Cuboidal Secretion and absorption; ciliated types propel mucus or Protection; these tissue types are rare in humans
Columnar reproductive cells
Transitional Simple transitional epithelium does not exist. Protection; stretching to accommodate distension of urinary structures
(2) CONNECTIVE TISSUE – most abundant and widely distributed
 Adipose Tissue
 Cartilage
 Bones
 Blood
 Loose Connective Tissue
 Fibrous Connective Tissue

(3) MUSCULAR TISSUE – Highly specialized to contract or shorten to produce movement; well vascularized
  Cardiac – branching structure, uninucleate, and striated
 Skeletal – long multinucleate, striated
 Smooth – short, no striations, packed together

(4) NERVOUS TISSUE – Specialized to receive and conduct electrochemical impulses; irritability and conductivity are their two major functional
characteristics
Life support of neurons

Cell Body

Carries messages to other neurons

and glands
Axon

Collects signals; sends back to

soma
Dendrites

Transfer electric activity

Synapse (information) from one cell to
another

the integumentary system

LESSON 4
Source:
Lecturer: Ms. Nina Betina Marie M. Igana

Body Membranes – covers surfaces, line body cavities and form protective sheets around organs
Keratin – fibrous protein makes the epidermis a tough protective layer and makes up hair and nails
Keratinization – formation of a layer of dead, protective cells filled with keratin; occurs on all exposed surfaces, skin, nails, and hair except eyes
Skin Life Cycle – takes 15-30 days for a cell to move from stratum germinativum to stratum corneum

2 MAJOR GROUPS OF BODY MEMRBANES

Epithelial Membrane Connective Tissue membrane
The skin
1. Outer epidermis – stratified squamous epithelium;
avascular
Cutaneous
2. Inner dermis – dense connective tissue; vascular
membrane
Hypodermis (subcutaneous tissue/superficial fascia) –
(1)not considered part of the skin, composed of adipose tissue; Made of soft areolar connective tissue
(2) site for fat and nutrient storage; (3) responsible for the
body’s curves
Lines tubular structures and all body cavities that open to the
exterior
Mucous Composed of epithelium resting on loose connective tissue Synovial
membrane Many secrete mucus, not all do membranes Contains no epithelial cells at all
Layer of simple squamous epithelium resting on a thin layer of
areolar connective tissue
Line body cavities that are closed to the exterior except dorsal Lines the fibrous capsules surrounding joints
body cavity and joint cavities.
Occurs in pairs:
1. Parietal layer – lines the wall of the cavity
Serous 2. Visceral layer – covers the outside of the organs in Provide smooth surface and secrete lubricating fluid
membrane the cavity
Examples of serous membranes are in the peritoneum, the
pleura, and the pericardium

Integumentary System
 Largest system of the body, accumulates 16% of body weight and covers 1.5 to 2m2 in area
 Uppermost layer of skin is full of keratin and is cornified
 Made up of 2 parts:
1. Cutaneous membrane – skin, integument
2. Appendages/derivative structures of the skin – hair and hair follicles, nails, and cutaneous glands
*Also functions as cutaneous sensory receptors, and blood reservoir

LAYERS OF EPIDERMIS
  “horny layer”
 Most superficial layer exposed to the
outside environment
 15-30 layers thick of keratinizes, water
Stratum corneum resistant cells
 ¾ of the epidermal thicknesss
 Cells in this layer can still be anchored to
each other by desmosomes
 The entire layer is replaced during a period
about 2-4 weeks
 Protects the skin against abrasion and
penetration
 “clear layer”
 Smooth, translucent layer found only in the
thick skin of theme palms, soles, and digits.
 Keratinocytes in this layer are dead and
Stratum lucidum flattened, densely packed with keratin and
eleidin, a clear protein rich in lipids,
derived from keratohyalin, which gives
these cells their transparent appearance and
provides barrier to water.
 “Grainy layer”
 4-6 layers of keratin, and keratohyalin, seen
as dense granules within the cells; these 2
proteins give the layer its grainy
appearance.
Stratum granulosum  These cells flatten, their nuclei and
organelles begin to disintegrate, and they
accumulate 2 types of granules: (1)
keratohyaline granules,which help forms
keratin for the upper layers, and (2)
lamellar granules which contain a water
resistant glycolipid which slows water loss
across the epidermis
 “Spiny layer” or “Prickly layer”
 8-10 layers of keratinocytes bound by
desmosomes (protein; specialized for
adhesion)
 The synthesis of keratin begins and they
release a water-repelling glycolipid that
makes the skin relatively waterproof
Stratum spinosum  Cells shrink until the cytoskeleton stick
out . both the protruding desmosomes and
cytoskeletons make this layer spiny
 Contains Langerhans cells, which are
dendritic cells active in immune response.

 “Germinating layer”
 Deepest layer of epidermis
 Resembles corrugated cardboard
 Basal cells are the most nourished
epidermal cells
 Daughter cells are continuously produced,
Stratum existing cells are pushed upward
basale/germinativum  Cells present: (1) Keratinocytes –
produces keratin, (2) Melanocytes –
produces pigment melanin, and star-shaped
epidermal dendritic cells, a protective
immune cell, (3) Merkel cells – act as light
touch receptors