Body Systems I: Skin, Bones, Muscles, Nerves, Senses and Hormones

This is Unit Three of Medical Assistant Prep. Unit Two built the vocabulary. This unit puts it to work on six systems: the skin, the skeleton, the muscles, the nerves, the senses and the hormones. Carry one idea through this lecture. The body's systems are linked, so one complaint can begin in a system far from where the patient feels it, and the medical assistant records every concern without deciding which one matters. A constructed case at the end shows that idea at work. By the end you will be able to describe the skin and its warning signs. To name the parts of the skeleton and the muscle types. To outline the nervous system and the senses. And to explain how hormones regulate the body.

Four entries on the skin: its jobs, its glands and hair, the names for lesions, and a mole's warning signs.

Protection, sensing and heat.

MAS 051 The Skin Is an Organ

The skin is an organ, part of the integumentary system with the hair and nails. It keeps out germs and chemicals, prevents dehydration, senses touch and temperature, and helps regulate body heat. Broken skin loses its protective role. A wound is an open door, which is why later units treat it with care.

Hair, nails and sweat.

MAS 052 Hair, Nails and Sweat Glands

Hair, nails and sweat glands belong to the skin's system, and each has a job. Hair senses air movement through nerves at its roots; sweat glands release water and salt; glands in hairy areas such as the armpits add compounds that bacteria break down into odor. Changes in hair or nails can reflect wider illness, so note them and report them.

Description, not diagnosis.

MAS 053 Words for Skin Lesions

Skin findings have precise names, and records use them. An abscess is a localized collection of pus; a carbuncle is a deep abscess formed from several boils; a crust is dried fluid on a lesion's surface; a bulla is a fluid-filled blister. Naming a lesion is description, not diagnosis. Record what the provider names.

A memory aid for moles.

MAS 054 The Warning Signs in a Mole

Patients are taught five letters for early warning signs of melanoma in a mole. A is asymmetry, halves that do not match. B is an irregular border. C is several shades of brown or black. D is a diameter wider than six millimeters, a little under a quarter of an inch. E is evolving, a shape that has changed over time. Any one sign is a reason to see a provider. You may reinforce this teaching once the provider has given it, and you report any mole a patient worries about. The aid helps patients notice change. It never tells you a mole is harmless, so never say a spot looks fine.

Five entries on the frame and its motion: bone shapes, skeletal divisions, joints, muscle types and contraction.

Long, short, flat, irregular.

MAS 055 Four Shapes of Bone

Bones come in four main shapes: long, short, flat and irregular. Long bones are longer than they are wide, like the thigh bone; short bones are cube-like, as in the wrist and ankle; most skull bones are flat; vertebrae are irregular. Shape matters in injury, because long bones carry weight and leverage, and fracture differently.

Center and limbs.

MAS 056 Two Divisions of the Skeleton

The skeleton has two divisions: the axial skeleton along the body's center and the appendicular skeleton of the limbs. The axial skeleton, about eighty bones, includes the skull, spine and ribs; the appendicular skeleton, about one hundred twenty-six bones, includes the limbs and the girdles that attach them. The shoulder and pelvic girdles belong to the appendicular skeleton, not the axial.

Grouped by movement.

MAS 057 Three Kinds of Joint

A joint is where two bones meet, and joints are grouped by how much they move: immovable, slightly movable and freely movable. Skull sutures are immovable; ribs joined to the breastbone by cartilage are slightly movable; most adult joints, such as the knee, move freely. Freely movable joints are also called synovial joints, after their lining membrane.

Voluntary and automatic.

MAS 058 Three Types of Muscle

The body has three types of muscle: skeletal, smooth and cardiac. Skeletal muscle moves the bones under voluntary control; smooth muscle lines hollow organs such as blood vessels and the bladder; cardiac muscle forms the heart wall. Smooth and cardiac muscle are controlled automatically, so a patient cannot will them to change.

What a fiber needs to contract.

MAS 059 How a Muscle Attaches and Contracts

A skeletal muscle is an organ wrapped in connective tissue that extends into a tendon or a flat sheet attaching it to bone. Before a muscle fiber can contract, it must receive an impulse from a nerve, which is why muscles are rich in nerves and blood vessels. Weakness can start in the nerve, not the muscle, so report it as observed.

Nine entries on the nervous system and the senses: its divisions, the brain, the cord, nerves, and sight and hearing.

Central and peripheral.

MAS 060 Central and Peripheral Nervous Systems

The nervous system has two divisions: the central nervous system, the brain and spinal cord, and the peripheral nervous system, the nerves that branch out from them. Bone protects the central system: the skull around the brain, the vertebrae around the cord. Peripheral nerves are the body's communication lines, so numbness in a hand can start far from the hand.

Cerebrum to cerebellum.

MAS 061 Four Parts of the Brain

The brain has four main parts: the cerebrum, the diencephalon, the brain stem and the cerebellum. The cerebrum is the largest, split into two hemispheres; the diencephalon includes the hypothalamus, key to homeostasis; the brain stem joins the spinal cord; the cerebellum sits below the back of the cerebrum. A brain problem can show up in any body system.

Pathway and reflex center.

MAS 062 The Spinal Cord's Two Jobs

The spinal cord does two jobs: it carries impulses to and from the brain, and it serves as a reflex center. Sensory signals travel up the cord; motor signals travel down it; reflexes happen without conscious thought, which is why they are faster than deliberate actions. A cord injury can cut both pathways below the damage.

Cranial and spinal.

MAS 063 Nerves of the Head and Body

Twelve pairs of cranial nerves leave the brain, and thirty-one pairs of spinal nerves leave the spinal cord. Cranial nerves serve mainly the head, neck and face and are numbered in Roman numerals; each spinal nerve pair matches a segment of the cord. The eighth cranial nerve carries hearing and balance, so it comes up again with the ear.

Fight or flight, rest and digest.

MAS 064 The Autonomic Nervous System

The autonomic nervous system runs automatic work, controlling smooth muscle, cardiac muscle and glands without conscious effort. Its two divisions usually work against each other. The sympathetic division, linked with fight or flight, readies the body for a threat, for example by speeding the heart. The parasympathetic division, called rest and digest, supports calmer functions such as digestion. Many organs receive fibers from both, one stimulating and the other slowing, and their balance keeps the body steady. Many common drugs act on these divisions, so one class can slow the heart and another speed it. Unit Five returns to those classes.

Neurons and their support.

MAS 065 Neurons and Their Support Cells

Nerve tissue has two main cell types: neurons, which carry impulses, and neuroglia, which support and protect them. A neuron has a cell body, dendrites that receive signals, and a single axon that sends them; neuroglia far outnumber neurons. Neurons do not divide, so a destroyed neuron cannot be replaced.

Distance, near, lens, nerve.

MAS 066 Common Eye Conditions

Several eye conditions are common in outpatient practice. Myopia blurs distant objects; presbyopia blurs near vision as the lens stiffens with age; a cataract is a cloudy lens; glaucoma damages the optic nerve, often through raised pressure inside the eye. Many conditions progress unnoticed, which is why vision screening matters.

Hearing and balance together.

MAS 067 How We Hear and Balance

Hearing begins when sound waves vibrate the eardrum, also called the tympanic membrane. Three tiny bones carry and amplify the vibration to the cochlea, where hair cells turn it into nerve signals; the semicircular canals of the inner ear sense head position for balance. Hearing and balance share the inner ear, so ear problems can cause dizziness.

Blocked or nerve-based.

MAS 068 Two Kinds of Hearing Loss

Hearing loss is conductive or sensorineural. Conductive loss happens when something in the outer or middle ear blocks sound, such as impacted earwax or a torn eardrum; sensorineural loss comes from the inner ear, the hearing nerve or the brain. Earwax blockage is one reason a provider may order an ear irrigation, covered in unit fourteen.

Four entries on hormones: receptors and feedback, the major glands, glucose control, and a case that crosses systems.

Receptors and feedback.

MAS 069 Hormones and Their Receptors

Hormones are chemical messengers carried in the blood, yet each affects only the cells with receptors for it, like a key fitting a lock. Many hormones are controlled by negative feedback: when a level rises, the gland reduces its output, and when it falls, output increases. Tiny amounts of hormone have large effects.

Glands that regulate the whole body.

MAS 070 The Major Endocrine Glands

Several glands release hormones that regulate the whole body. The pituitary, linked to the hypothalamus, directs other glands and releases growth hormone and a hormone that conserves water in the kidneys. The thyroid releases thyroid hormone, which needs iodine, and the parathyroid glands behind it regulate blood calcium. The adrenal glands release cortisol, which raises blood glucose, and epinephrine during stress. The pancreas holds islets that release insulin and glucagon. The testes and ovaries release sex hormones. Because each gland feeds back on others, a problem in one can look like another system's, which is why the provider asks broad questions.

Balancing blood glucose.

MAS 071 Insulin and Glucagon

The pancreas balances blood glucose with two opposing hormones. Beta cells release insulin when blood glucose is high, which lowers it; alpha cells release glucagon when glucose is low, which raises it. This balance fails in diabetes, which unit four introduces; that is why a glucose reading is part of so many visits.

Four concerns, one visit.

MAS 072 One Complaint, Many Systems

A constructed visit pays off the opening idea. An older woman says she has been feeling off. Asked why she came, she lists dizziness when she turns her head, trouble reading her medicine labels, weak hands with jars, and a mole her daughter thinks has grown. A careless interviewer might record only dizziness. This medical assistant records each item in the patient's words. Each points to a different system: the inner ear, the stiffening lens of age, a muscle or a nerve, and a mole's warning sign. She decides none of it and suggests no cause. She measures vital signs and tells the provider about several concerns, including a mole that may have changed. Older adults often have several problems at once, and the record is only as complete as the interview.

Look at what you can now do. You can describe the skin's jobs, name its lesions, and reinforce the five warning signs without ever calling a mole harmless. You can name bone shapes, the two skeletal divisions, joint types and the three muscle types. You can outline the central, peripheral and autonomic nervous systems, and the eye and ear conditions met in practice. And you can explain how hormones regulate the body, and record a complaint that crosses systems in full. Unit Four continues with the heart, lungs, digestion, kidneys and reproduction.