Muscle Fiber Anatomy: Basic Sarcomere Subdivisions The basic function of skeletal muscle is to move various parts of the body via muscle contraction. Muscle structure is specifically related to its function. Muscles are composed of numerous multinucleated muscle cells called muscle fibers, or myofibers. Muscle fibers insert into tendons at their ends, at what on the microscopic level is referred to as the myotendinous junction. The…

Neuromuscular Junction The outflow of nearly all behavior depends upon the neuromuscular system, where nerves emanating from the spinal cord and brainstem make connections with skeletal muscles that allow us to move, stand, and express ourselves. The numbers of skeletal muscles in the human body is daunting: somewhere between 500 and 1000. The face alone has enormous numbers of muscles that allow us to express our…

Peripheral Nervous System: Overview The major function of the nervous system is the transmission and processing of information. This relies on propagation of electrical signals between the peripheral nervous system and the central nervous system. The somatic peripheral nervous system is composed of afferent (sensory) neurons (green in the figure) and efferent (motor) neurons (red in the figure). The first-order somatic sensory neurons transmit the initial…

Neonatal Hypotonia Neonatal hypotonia, often referred to as the “floppy infant,” is the main presenting clinical feature of most neuromuscular diseases of early life. However, disorders of the central nervous system may also manifest with hypotonia. Two types of muscle tone can be assessed clinically: postural and phasic. Postural (antigravity) tone is a sustained, low-intensity muscle contraction in response to gravity. It is mediated by both…

Pain Pathways Anatomy Ascending Pathways Endorphin System Pain propagation is initiated with activation of nociceptors, distributed within skin, muscle, joints, and viscera. These receptors include small-diameter Aδ and C-fiber free nerve endings representing distal primary afferent neurons. Cutaneous Aδ fibers (myelinated) mediate sharp sensation of first-phase or acute pain known to trigger withdrawal responses. These include two fiber groups; first are high-threshold mechanoreceptors fibers , responding…

General Topography of Autonomic Nervous System The nervous system is divided into somatic and autonomic divisions: the somatic division controls predominantly voluntary activities, while the autonomic system regulates involuntary functions. The two divisions develop from the same primordial cells; they comprise closely associated central and peripheral components and are both built up from afferent, efferent, and interneurons linked to produce ascending and descending nerve pathways and…

Peripheral Nerve The main function of the peripheral nerve is to rapidly conduct electrical signals between the central nervous system and end organs. The neuronal cell body is the main functional component that maintains the neuron and produces important structural proteins and neurotransmitters that are necessary for nerve function. The peripheral nerve axon is the major component of the neuron that, through a cascade of physiologic…

Compression Neuropathies Compression neuropathies occur acutely (e.g., proximal radial nerve palsy, peroneal neuropathy at the fibular hear) or more gradually (e.g., median neuropathy at the wrist, ulnar neuropathy at the elbow). Acute compressive neuropathies typically develop at sites where external pressure can compress the nerve against a harder surface, such as the radial nerve at the humerus' spiral groove. Chronic mononeuropathies (e.g., entrapment neuropathies) occur where…

Cervical Disk Herniation Cervical disk disease is a common disorder, accounting for 1% to 2% of all hospital admissions in the United States. Unlike lumbar disk disease, which is approximately six times more common, cervical disk disease is rarely caused by trauma. In fact, severe degenerative cervical disk disease often develops in indolent patients. Etiology. Cervical disk disease is likely multifactorial, with contributing factors ranging from…

Spinal Column The spinal column is built up from alternating bony vertebrae and fibrocartilaginous disks, which are intimately connected by strong ligaments and supported by powerful musculotendinous masses. The individual bony elements and ligaments are described in Plates 3-2 to 3-10 . There are 33 vertebrae (7 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 4 coccygeal), although the sacral and coccygeal vertebrae are usually fused…

Spinal Cord The spinal cord is the downward continuation of the medulla oblongata. It extends from the upper border of the atlas to end in a tapering extremity, the conus medullaris , opposite the lower border of the first lumbar vertebra, or at the level of the intervertebral disk between the upper two lumbar vertebrae (see Plate 2-1 ). From the conus, a slender, median, fibrous…

Overview of Cranial Nerves The brainstem is the source of all the cranial nerves and provides sensory, motor, and, through the vagus nerve, parasympathetic preganglionic innervation to the face, head, thorax, and most of the abdominal viscera. Distinct motor and sensory nuclei within the brainstem project to the various structures of the head to provide (1) general sensory information from the face, ears, and oropharynx and…

Skull: Anterior and Lateral Aspects The anterior, or facial, aspect of the skull is composed of the frontal part of the calvaria (skullcap) above and the facial bones below. The facial contours and proportions are largely determined by the underlying bones, and it is a commonplace observation that they show considerable variations associated with age, sex, and race. The outer surface of the frontal bone underlies…

Overview of Headaches Headache is one of the most common reasons for consulting a physician and is one of the top three reasons for lost work days. Rather than a disease, headache is a symptom, frequently providing a valuable warning of hidden pathology. Physicians treating patients for headache must decide whether the headache represents a primary or secondary headache syndrome. Primary headaches are most common and…

Clinical Presentations of Brain Tumors Brain tumors commonly present with symptoms of elevated intracranial pressure or focal neurologic dysfunction. Elevated intracranial pressure can directly result from an enlarging mass or can be secondary to the development of hydrocephalus stemming from obstruction of the ventricular system and cerebrospinal fluid (CSF) flow by the tumor. The specific neurologic dysfunction depends on the local mass effect of the tumor.…