20.2 Integumentary, Musculoskeletal, Nervous & Sensory Systems
Key Takeaways
- The skin has three layers — epidermis, dermis, and subcutaneous (hypodermis) — and functions in protection, thermoregulation, sensation, excretion, and vitamin D synthesis.
- The adult skeleton has 206 bones divided into the axial skeleton (skull, vertebral column, ribs, sternum) and the appendicular skeleton (limbs and girdles).
- The nervous system divides into the central nervous system (brain and spinal cord) and the peripheral nervous system, which includes the somatic and autonomic divisions.
- The ABCDE rule screens pigmented lesions for melanoma: Asymmetry, Border irregularity, Color variation, Diameter over 6 millimeters, and Evolution or change.
- Osteoarthritis is degenerative wear producing asymmetric joint pain worse with use, while rheumatoid arthritis is autoimmune, symmetric, and associated with prolonged morning stiffness.
Integumentary System
Normal structure and function
The skin is the body's largest organ, composed of three layers:
| Layer | Composition | Function |
|---|---|---|
| Epidermis | Stratified squamous epithelium; avascular; contains melanocytes and keratinocytes | Barrier; pigmentation; continuously regenerates from the basal layer |
| Dermis | Connective tissue with collagen and elastin; contains blood vessels, nerves, hair follicles, and sebaceous and sweat glands | Strength, elasticity, sensation, thermoregulation |
| Subcutaneous (hypodermis) | Adipose and connective tissue | Insulation, energy storage, shock absorption; the layer targeted by subcutaneous injection |
Accessory structures include hair, nails (keratinized epidermal cells), sebaceous glands (sebum, lubrication), and sweat glands — eccrine glands distributed widely for thermoregulation, and apocrine glands in the axillae and groin activated at puberty.
Functions: protection from pathogens, ultraviolet radiation, and fluid loss; thermoregulation through vasodilation, vasoconstriction, and sweating; sensation through receptors for touch, pressure, temperature, and pain; excretion of water and electrolytes; and vitamin D synthesis from ultraviolet exposure.
Abnormal structure and function
| Condition | Description |
|---|---|
| Dermatitis / eczema | Inflammation with erythema, pruritus, and scaling; atopic dermatitis is associated with asthma and allergic rhinitis |
| Psoriasis | Autoimmune accelerated keratinocyte turnover producing sharply demarcated plaques with silvery scale, typically on extensor surfaces |
| Cellulitis | Bacterial infection of dermis and subcutaneous tissue; warm, erythematous, tender, spreading; requires systemic antibiotics |
| Impetigo | Superficial bacterial infection with honey-colored crust; highly contagious, common in children |
| Tinea | Fungal infection — tinea corporis (ringworm), pedis (athlete's foot), capitis, cruris |
| Acne vulgaris | Follicular obstruction with sebum and keratin, colonized by bacteria |
| Basal cell carcinoma | Most common skin cancer; pearly papule with telangiectasia; rarely metastasizes |
| Squamous cell carcinoma | Scaly, crusted, or ulcerated lesion; can metastasize |
| Melanoma | Most lethal; arises from melanocytes; screened by the ABCDE rule |
| Pressure injury | Ischemic tissue damage over bony prominences from sustained pressure, staged 1 through 4 |
ABCDE for a pigmented lesion: Asymmetry, Border irregularity, Color variation, Diameter greater than 6 mm, Evolution or change. Any evolving lesion is reported to the provider, and change over time is the most sensitive single feature.
Musculoskeletal System
Normal structure and function
The adult skeleton contains 206 bones:
- Axial skeleton (80): skull, vertebral column, ribs, and sternum — protects the central nervous system and thoracic organs.
- Appendicular skeleton (126): upper and lower limbs plus the pectoral and pelvic girdles — movement.
The vertebral column has 33 vertebrae: 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, and 4 fused coccygeal. A memory aid is breakfast at 7, lunch at 12, dinner at 5.
Bone functions are support, protection, movement (with muscles as levers), mineral storage of calcium and phosphorus, and hematopoiesis in red marrow.
Joints are classified by mobility: synarthroses (immovable, such as skull sutures), amphiarthroses (slightly movable, such as intervertebral discs and the pubic symphysis), and diarthroses (freely movable synovial joints — ball-and-socket at hip and shoulder, hinge at elbow and knee, pivot, gliding, saddle, condyloid).
Muscle occurs in three types: skeletal (striated, voluntary), smooth (non-striated, involuntary, in hollow organs and vessels), and cardiac (striated, involuntary, with intercalated discs). Tendons attach muscle to bone; ligaments attach bone to bone.
Abnormal structure and function
| Condition | Description |
|---|---|
| Osteoarthritis | Degenerative cartilage loss; asymmetric, weight-bearing joints, pain worse with use, brief morning stiffness under 30 minutes; Heberden and Bouchard nodes |
| Rheumatoid arthritis | Autoimmune synovial inflammation; symmetric, small joints of hands and feet, morning stiffness over an hour, systemic symptoms |
| Osteoporosis | Reduced bone density and microarchitecture; fragility fractures of hip, vertebrae, wrist; diagnosed by DEXA T-score of −2.5 or below |
| Gout | Uric acid crystal deposition; acute severe monoarticular pain, classically the great toe |
| Fracture | Break in bone continuity — closed, open (compound), greenstick, comminuted, spiral, impacted, pathologic |
| Sprain / strain | Sprain injures a ligament; strain injures a muscle or tendon |
| Carpal tunnel syndrome | Median nerve compression at the wrist; numbness of thumb, index, middle, and radial half of ring finger |
| Muscular dystrophy | Inherited progressive muscle fiber degeneration |
| Scoliosis | Lateral curvature of the spine |
The osteoarthritis-versus-rheumatoid distinction is the most heavily tested musculoskeletal contrast: asymmetric, use-related, brief stiffness versus symmetric, autoimmune, prolonged morning stiffness with systemic features.
A 54-year-old patient reports pain and stiffness in the small joints of both hands, symmetric on the right and left, with morning stiffness lasting about 90 minutes and accompanying fatigue. Which condition does this pattern most strongly suggest?
Nervous System
Normal structure and function
The neuron is the functional unit, consisting of dendrites (receive), a cell body, and an axon (transmit), with many axons insulated by a myelin sheath that speeds conduction. Signals cross synapses by neurotransmitter release. Neuroglia support and protect neurons.
| Division | Components |
|---|---|
| Central nervous system (CNS) | Brain and spinal cord |
| Peripheral nervous system (PNS) | 12 pairs of cranial nerves, 31 pairs of spinal nerves |
| — Somatic | Voluntary skeletal muscle control and sensory input |
| — Autonomic | Involuntary control of glands, smooth muscle, cardiac muscle |
| —— Sympathetic | "Fight or flight" — increases heart rate, dilates bronchi and pupils, shunts blood to muscle |
| —— Parasympathetic | "Rest and digest" — slows heart rate, stimulates digestion, constricts pupils |
Major brain regions: the cerebrum with four lobes — frontal (executive function, personality, motor, Broca speech area), parietal (sensation), temporal (hearing, memory, Wernicke comprehension), occipital (vision); the cerebellum (coordination, balance); the brainstem with midbrain, pons, and medulla oblongata, which houses the vital cardiac, respiratory, and vasomotor centers; and the diencephalon with the thalamus (sensory relay) and hypothalamus (temperature, hunger, thirst, and endocrine control through the pituitary).
The CNS is protected by bone, three meninges (dura, arachnoid, pia mater), and cerebrospinal fluid.
Abnormal structure and function
| Condition | Description |
|---|---|
| Cerebrovascular accident (stroke) | Ischemic (about 87 percent) or hemorrhagic interruption of cerebral blood flow; sudden focal deficit; screened by BE-FAST |
| Transient ischemic attack | Temporary deficit that resolves fully; a warning of impending stroke |
| Epilepsy / seizure disorder | Abnormal synchronous cortical discharge |
| Parkinson disease | Dopaminergic degeneration; resting tremor, rigidity, bradykinesia, postural instability |
| Multiple sclerosis | Autoimmune demyelination of the CNS; relapsing-remitting neurologic deficits |
| Alzheimer disease | Progressive neurodegenerative dementia with amyloid plaques and neurofibrillary tangles |
| Meningitis | Inflammation of the meninges; fever, headache, nuchal rigidity, photophobia |
| Peripheral neuropathy | Distal sensory loss and burning, commonly from diabetes |
| Bell palsy | Unilateral facial nerve (CN VII) paralysis, usually self-limited |
| Migraine | Recurrent headache, often unilateral and throbbing, with photophobia, phonophobia, and nausea |
Special Senses
The eye
Light passes through the cornea (primary refraction), the aqueous humor, the pupil (aperture controlled by the iris), and the lens (accommodation), through the vitreous humor to the retina, where rods handle dim light and peripheral vision and cones handle color and detail. The optic nerve (CN II) carries the signal to the occipital lobe.
| Condition | Description |
|---|---|
| Myopia / hyperopia | Nearsightedness / farsightedness — refractive errors |
| Presbyopia | Age-related loss of lens accommodation for near vision |
| Astigmatism | Irregular corneal curvature |
| Cataract | Clouding of the lens; painless progressive blurring and glare |
| Glaucoma | Optic nerve damage usually with elevated intraocular pressure; peripheral vision lost first; screened by tonometry |
| Macular degeneration | Loss of central vision; leading cause of vision loss in older adults |
| Conjunctivitis | Inflammation of the conjunctiva; bacterial, viral, or allergic |
| Diabetic retinopathy | Retinal microvascular damage; requires annual dilated examination |
The glaucoma-versus-macular-degeneration contrast — peripheral loss versus central loss — is a recurring exam item.
The ear
The external ear (auricle and external auditory canal) conducts sound to the tympanic membrane. The middle ear contains the ossicles — malleus, incus, stapes — which amplify vibration, and the eustachian tube, which equalizes pressure with the nasopharynx. The inner ear contains the cochlea (hearing, via the vestibulocochlear nerve, CN VIII) and the semicircular canals (equilibrium).
| Condition | Description |
|---|---|
| Otitis media | Middle ear infection; common in children because the eustachian tube is shorter and more horizontal |
| Otitis externa | External canal infection ("swimmer's ear") |
| Conductive hearing loss | Sound transmission blocked in outer or middle ear — cerumen impaction, effusion, otosclerosis |
| Sensorineural hearing loss | Cochlear or CN VIII damage — presbycusis, noise exposure, ototoxic drugs |
| Tinnitus | Perception of ringing without external sound |
| Vertigo / Ménière disease | Inner ear disorder producing vertigo, tinnitus, and fluctuating hearing loss |
Presbycusis — age-related sensorineural loss affecting high frequencies first — is the reason the correct communication accommodation for an older adult is to lower pitch rather than raise volume.
A patient reports that she has been bumping into furniture on either side and has difficulty seeing while driving at night, but her central reading vision remains sharp. Which condition does this pattern of loss most suggest?
A medical assistant is preparing to give a subcutaneous injection. Which skin layer is the target, and what characterizes it?