The nervous systems include the central nervous system (CNS) (the brain and spinal cord) and the peripheral nervous system (PNS) (all the neurons the agonist, because it carries out the intended
movement, and the triceps is called the antagonist, because it opposes that movement. During exten- sion, the triceps is the agonist and the biceps is the antagonist.
Disorders of Skeletal Muscle
Disorders affecting skeletal muscle include trauma, genetic diseases of muscle proteins, metabolic dis- eases, autoimmune diseases, and motor neuron in- fection or degeneration (Box 6-2). Muscle disorders may be treated by an orthopedist, rheumatologist, or neurologist. Common laboratory tests ordered for these disorders are listed in Table 6-2.
BOX 6-2 Disorders of the Muscular System Trauma
• Contusions
• Tendon injuries
• Tendonitis: From overuse, overexertion, or repetitive strain injuries
Genetic Disease
• Muscular dystrophies: Inherited defects in the proteins of the muscle cell; Duchenne’s muscular dystrophy is the most common type
Metabolic Disease
• Inherited defects in metabolic function: Especially defects of the mitochondria; most are rare
Autoimmune Disease
• Myasthenia gravis: Antibodies form against the receptor for acetylcholine, preventing nerve–muscle communication
• Polymyositis and dermatomyositis Motor Neuron Disease
• Amyotrophic lateral sclerosis (ALS; also called Lou Gehrig’s disease): Degeneration of motor neurons; cause is unknown
• Poliomyelitis: Viral infection that has been virtually eradi- cated; new cases are caused by vaccine virus; can cause paralysis
Test Disorder or Purpose
Aldolase Muscle disease
Aspartate aminotransferase (AST) Muscle disease Creatine kinase-MM (CK-MM) Muscle damage Creatine kinase-MB (CK-MB) Cardiac muscle damage
Myoglobin Muscle damage
Troponin Cardiac muscle damage
TABLE 6-2 Common Laboratory Tests for Muscle Disorders
Epidermis Pore
Free nerve endings Hair shaft
Sebaceous gland Nerve ending
Artery Vein
Adipose tissue
Nerve (pain) Sweat gland Connective tissue Touch
receptors
Pressure receptor
Dermis
Subcutaneous layer
Capillaries Erector pili
FIGURE 6-12 The integumentary system covers and protects the body. It permits sensation of the environment and regulates the internal temperature. (Modified from Herlihy B: The human body in health and illness, ed. 5, St. Louis, 2015, Saunders.)
BOX 6-3 Disorders of the Integumentary System
Infection
• Fungal infection
• Athlete’s foot
• Ringworm
• Bacterial infection
• Staphylococcus infection: Impetigo
• Streptococcus infection: Necrotizing fasciitis (“flesh-eating” bacteria)
Neoplastic Disease
• Carcinoma
• Melanoma Inflammation
• Acne
• Psoriasis
Test Disorder or Purpose
Culture and sensitivity (C&S) Bacterial or fungal infection Potassium hydroxide (KOH) prep Fungal infection
Skin biopsy Malignancy
TABLE 6-3 Common Laboratory Tests for Integumentary Disorders
outside the CNS) (Figure 6-13). Sensory informa- tion from the periphery is received by the CNS, where it is integrated and processed to form an un- derstanding of experience. The CNS directs move- ment by sending commands to the muscles through
the motor portion of the PNS. The CNS also directs some actions of other organs, including secretion by some glands.
Peripheral Nervous System
The PNS is composed of two major divisions: the sensory and somatic systems. Sensory neurons, called afferents, receive stimulation from special- ized cells within their sensory organ—the eyes, nose, mouth, ears, skin, and subcutaneous tissue such as joints, muscle, and internal organs. Afferents transmit information to spinal cord neurons, allowing sensory information to ascend to the brain. Motor neurons, called efferents, receive information from spinal
cord neurons and carry it to the target organ, either a muscle or a gland. (A third type of neuron, the inter- neuron, conveys information between afferents and efferents and is located in the CNS.)
Neurons are bundled together in the periphery for protection. These bundles are called nerves. A nerve may contain only motor or only sensory neu- rons, or it may be a mixed nerve, containing both.
Nerves also contain blood vessels, which nourish the neurons within.
Motor System
The motor system is divided into two branches: the somatic motor system and the autonomic motor system. Somatic motor neurons innervate skeletal muscles and can be consciously controlled. Auto- nomic motor neurons innervate cardiac and smooth muscle and normally cannot be consciously con- trolled (although indirect control is possible through focused relaxation techniques such as meditation and biofeedback). Digestive secretions are also
Spinal cord
Intercostal nerves
Radial nerve Median nerve
Ulnar nerve
Femoral nerve
Sciatic nerve Posterior cutaneous nerve
Cervical plexus
Brachial plexus
Lumbar plexus
Sacral plexus
FIGURE 6-13 The nervous system receives sensations and controls motor responses, in addition to helping maintain homeostasis.
partly controlled by the autonomic system, as are secretions from the adrenal medulla.
Motor commands of both types originate in the brain and descend through the spinal column.
Spinal neurons make synapses with lower motor neurons, which branch out through the gaps between vertebrae to reach their target.
Central Nervous System
Within the CNS, bundles of neurons are called tracts. The spinal cord has both ascending and descending tracts. The top of the spinal cord merges into the brainstem, a region at the base of the skull that is vital for basic life processes, including respi- ration (Figure 6-14). The brainstem also connects with the thalamus, a major relay station for incom- ing sensory information. Posterior to the brainstem is the cerebellum, a principal site for fine-tuning of motor commands. Superior to the brainstem and partially covering the cerebellum is the cerebrum, which contains the cerebral cortex, divided into
left and right hemispheres (not shown Figure 6-14).
The cerebral cortex is the site of thought, emotion, and memory. Motor control of the left side of the body begins in the right cerebral hemisphere, and the right side of the body is controlled by the left cerebral hemisphere.
In addition to the nutrients received from blood, the brain is nourished and cushioned by cerebrospi- nal fluid (CSF), which is secreted by the brain and circulates within the ventricles (see Figure 6-14).
The brain is surrounded by three membranes—
the pia mater, the arachnoid, and the dura mater—that lubricate and protect the brain. To- gether, these constitute the meninges. The spinal cord is also surrounded by meninges.
Disorders of the Nervous Systems Disorders affecting the nervous systems include trauma, stroke, infection, neoplastic diseases, de- generation, autoimmune diseases, developmental disorders, and psychiatric illnesses (Box 6-4).
Hypothalamus Third ventricle
Fourth ventricle
Central canal of spinal cord Pituitary gland
Cerebellum Choroid plexus
Cerebrum with pia mater
Subarachnoid space Dura mater
Arachnoid villi
FIGURE 6-14 This sagittal section of the brain and cranium shows the brain’s internal organization, plus the ventricles and three protective membranes.
Neurologic disorders are treated by a neurologist, and psychiatric disorders are treated by a psychia- trist or other mental health professional. Common laboratory tests ordered for these disorders are listed in Table 6-4. Disorders of the CNS are most often diagnosed using imaging techniques such as magnetic resonance imaging (MRI) and computed tomography (CT) scanning.