A bundle of nerve fibers and connective tissue compose a [1] nerve . The connective tissues include a sheath called the [2] endoneurium around each nerve fiber, a perineurium that encloses each [3] fascicle and separates them from each other, and a/an [4] epineurium on the surface of the nerve as a whole. Nerve fibers are described as [5] afferent (sensory) if they conduct signals to the CNS and [6] efferent (motor) if they conduct signals out of it. A nerve that has only the former type of fibers is called a [7] sensory nerve, whereas those that contain types 5 and 6 are described as [8] mixed nerves. [9] Somatic fibers innervate the skin, skeletal muscles, and joints, whereas visceral fibers innervate blood vessels, glands, and viscera. A knotlike mass of nerve cell bodies outside the CNS is called a [10] ganglion .

The Cranial Nerves

Cranial nerves arise from the base of the brain and exit the cranial cavity through the [11] foramina of the skull. The only ones that are purely sensory are cranial nerves I and II, named the [12] olfactory nerve and [13] optic nerve , respectively. Some others, however, are predominantly motor; their sensory fibers are limited to [14] proprioceptive fibers that monitor the muscle contractions stimulated by the motor component. Examples of these include cranial nerves III, IV, and VI—named the [15] oculomotor nerve , [16] trochlear nerve , and [17] abducens nerve , respectively—which control the movements of the eyes. The [18] trigeminal nerve (cranial nerve V) has an extensive distribution to the face and transmits most sensory signals from the face to the brain; it is divided into ophthalmic, maxillary, and mandibular divisions. Most signals that control the muscles of facial expression travel by way of the [19] facial nerve (cranial nerve VII). Taste signals are transmitted to the brain by way of this nerve as well as cranial nerve IX, named the [20] glossopharyngeal nerve . Signals for hearing and balance travel by way of cranial nerve VIII, the [21] vestibulocochlear nerve . The most extensive cranial nerve, whose name means "wandering," is the [22] vagus nerve , cranial nerve X. Unlike other cranial nerves, which do not extend beyond the head and neck, this nerve sends branches throughout the thoracic and abdominal cavities. The last two cranial nerves, named the [23] accessory nerve and [24] hypoglossal nerve respectively, control swallowing, among other functions.

The Spinal Nerves

Spinal nerves emerge from the intervertebral foramina. Proximal to this point, each divides into a [25] dorsal root that carries sensory impulses into the spinal cord and a [26] ventral root that carries motor impulses out. The former branch includes a swelling called the [27] dorsal root ganglion where the somatosensory nerve cell bodies are located. Distal to the foramen, a spinal nerve divides into a small meningeal branch and a dorsal and ventral [28] ramus . Except in the thoracic region, the ventral 28s branch and rejoin to form nerve plexuses. From superior to inferior, these are the cervical, [29] brachial , lumbar, [30] sacral , and coccygeal plexuses. The major nerves that arise from these plexuses have names that indicate their functions or locations and should enable you to guess which plexus they arise from—for example, the phrenic nerves from the [31] cervical plexus, the radial and ulnar nerves from the [32] brachial plexus, the tibial and common peroneal nerves from the [33] sacral plexus, and the iliohypogastric and femoral nerves from the [34] lumbar plexus. The tibial and common peroneal nerves are bound together in a common connective tissue sheath and are often referred to collectively as the [35] sciatic nerve, a common focus of injury and pain. The skin can be divided into zones called [36] dermatomes , each of which generates sensory input into a given spinal nerve.

Somatic Reflexes

Reflexes are quick reactions to stimuli, described as [37] involuntary because they are usually not subject to conscious control and [38] stereotyped because they occur in nearly identical fashion every time. Reflexes typically involve a nerve pathway called a [39] reflex arc going from a receptor to the CNS and back to an effector. For some spinal reflexes, the receptor is a muscle [40] spindle —a cigar-shaped capsule containing a few noncontractile muscle fibers innervated by stretch-sensitive neurons. Primary and secondary [41] afferent nerve fibers carry stretch signals from these organs to the CNS, and [42] gamma motor neurons carry motor signals back to the intrafusal fibers to keep them taut and responsive.

Stretch reflexes are mediated primarily through the brain, but when the stimulus is especially strong—as in the [43] tendon reflexes used for clinical tests of the nervous system—the spinal component of the response may be stronger. One of these, the patellar tendon reflex, is very quick because the reflex arc is [44] monosynaptic ; there is only one synapse between the afferent and efferent neuron, with no interneurons to slow down the signal. When one muscle of an antagonistic pair exhibits this kind of reflex, its antagonist must be suppressed so that it does not oppose the action of the agonist; this effect on the antagonist is called [45] reciprocal inhibition . The withdrawal, or [46] flexor , reflex is important for quickly protecting extended parts of the body from injury by heat, sharp objects, and so forth. It produces a more sustained muscle contraction because signals travel over many synapses on their way to the muscle, a pathway known as a [47] polysynaptic reflex arc. This is typically accompanied by a [48] crossed extensor reflex, which shifts and balances the body weight if one leg is quickly raised from the ground. Muscle contraction is inhibited by the [49] Golgi tendon reflex, which involves stretch receptors located in a tendon close to the muscle.

The Autonomic Nervous System: Introduction and Anatomy

The autonomic nervous system (ANS) has two functional subdivisions, the [50] sympathetic and the [51] parasympathetic . Its efferent pathways differ from those of the somatic nervous system because they involve two neurons, a [52] preganglionic neuron and a [53] postganglionic neuron . In the sympathetic nervous system, these neurons synapse with each other in a [54] sympathetic chain of ganglia alongside the spinal column. On average, each input neuron to this ganglion synapses with 17 output neurons; thus, the sympathetic nervous system tends to have widespread effects on many target organs at once, an effect called [55] mass activation . The sympathetic nervous system includes the [56] adrenal medulla , the core portion of a gland located atop each kidney. The cells of 56 behave like endocrine gland cells but are actually modified sympathetic neurons. When stimulated, they release mainly [57] epinephrine into the bloodstream, and this hormone complements the effect of direct sympathetic stimulation of many target organs.

The parasympathetic nervous system issues its fibers through certain [58] cranial and sacral nerves. About 90% of its fibers travel through the [59] vagus nerve. The ganglia of this system are located in or near the target organ; therefore this system has long [60] preganglionic fibers and short [61] postganglionic fibers.

A nerve fiber that secretes acetylcholine (ACh), or a receptor that binds it, is said to be [62] cholinergic ; a fiber that secretes norepinephrine (NE), or a receptor that binds it, is called [63] adrenergic . Receptors for ACh are classified as nicotinic or [64] muscarinic , after the drugs that were first used to identify them; receptors for NE are classified as alpha or beta receptors. Whether a neurotransmitter has excitatory or [65] inhibitory effects depends on which type of receptor it binds to. Many organs receive both sympathetic and parasympathetic fibers and are thus said to have [66] dual innervation . In such cases, the two autonomic divisions may have [67] antagonistic effects, in which they oppose each other’s actions (produce contrasting effects on the target organ), or [68] cooperative effects, in which they work together to achieve a single overall result. Blood vessels, however, can be controlled without 66. Many of them receive only sympathetic fibers, which have a steady background firing rate called [69] sympathetic tone that keeps the vessel partially contracted; this partial vasoconstriction is called [70] vasomotor tone . When the sympathetic firing rate rises, the vessel constricts, and when the firing rate drops, the vessel dilates. Although the autonomic nervous system is quite independent of conscious control, it does respond to input from other levels of the nervous system. The [71] hypothalamus of the brain, especially, controls many fundamental autonomic mechanisms of the body (such as vasomotion, thermoregulation, and salivation) and exerts its control through autonomic efferent fibers.