The Peripheral and Autonomic Nervous Systems

The Peripheral and Autonomic Nervous Systems

1. Peripheral Nervous System

   • The Peripheral nervous system (PNS) is a collection of nerves that are outside the skull and spinal column. These nerves carry sensory impulses originating in the peripheral sense organs to the brain, and motor impulses originating in the brain to the glands and muscles of the body.
   • The PNS contains three types of nerves: the cranial, spinal, and autonomic nerves that make up the autonomic nervous system. There are 12 pairs of cranial nerves and 31 pairs of spinal nerves. Of the three types of nerves, the cranial nerves are the most directly involved in speech, language, and hearing.
   • Cranial Nerves
     • Cranial Nerves emerge from brainstem and are attached to base of brain. They're part of lower motor system, and they receive much of their innervations from thecorticobulbar tract of the pyramidal system. Cranial nerves exit through foramina (holes) in based of skull. They exit at different levels of brainstem and top portion of spinal cord. 
     • Cranial nerves are numbered according to the vertical order in which they exit from the skull. They go out to connect to various sense organs and muscles of the neck and head.
     • There are 12 pairs of cranial nerves. First two cranial nerves are related to cerebral cortex. Cranial nerves III to XII originate from the brainstem and innervate the muscles of the pharynx, tongue, larynx, head, neck, and face.
     • Sensory nerves carry sensory information from a sense organ (e.g., the nose) to the brain. Motor nerves carry impulses from the brain to the muscles that move those muscles. Several cranial nerves are mixed nerves because that carry both sensory and motor impulses. 
     • Nerves unassociated with speech, language and hearing
       • Cranial Nerve I
         • (Olfactory nerve): A sensory nerve originating in the nasal cavity. It's involved with smell.
         • Cranial Nerve II
           • (Optic Nerve): Sensory nerve originating in the retina of the eye.
           • Cranial Nerves III and IV
             • Cranial Nerve III (Oculomotor nerve) and cranial nerve IV (Trochlear nerve) are motor nerves that originate in the midbrain area and innervate muscles corresponding to eye movement.
             • Cranial Nerve VI
               • (Abducens nerve): A motor nerve that controls eye movement.
     • Nerves associated with speech, language, and hearing
       • Cranial Nerve V
         • (Trigeminal nerve): A mixed nerve. Its sensory fibers are composed of three branches: opthalmic, maxillary, and mandibular. 1. Opthalmic branch: Has sensory branches in the nose, eyes, and forehead. 2. Maxillary branch: Has sensory branches to upper lip, maxilla, upper cheek area, upper teeth, maxillary sinus, and palate. 3. Mandibular branch: Has sensory branches to mandible, lower teeth, lower lip, tongue, part of cheek, and part of external ear.
         • Motor fibers of cranial nerve V innervate various jaw muscles, including temporalis, lateral and medial pterygoids, masseter, tensor veil palatini, mylohyoid, and anterior belly of digastric muscle.
         • Damage to cranial nerve V may result in inability to close mouth, difficulty in chewing, and trigeminalneuralgia (sharp pain in facial area).
         • Cranial Nerve VII
           • (Facial nerve): Also a mixed nerve.  Sensory fibers of facial nerve are responsible for taste sensations in anterior two thirds of tongue.
           • Motor fibers of facial nerve innervate muscles important to facial expression and speech. These muscles include buccinator, zygomatic, orbicular is oris,, platysma, stapedius, stylohyoid, frontal is, procures, nasals, depressor labii inferiors, depressor anguli oris, auricular muscles, various labial muscles, and posterior belly of the digastric.
           • A person with damage to facial nerve often has a mask like appearance or no facial expression.
           • Cranial Nerve VIII
             • (Vestibulococlear nerve): A sensory nerve for balance and hearing. Has two branches.
             • Vestibular branch is primarily responsible for maintenance of equilibrium or balance. Acoustic branch transmits sensory information from cochlea of inner ear to primary auditory center of brain, where it's interpreted.
             • Damage to vestibular branch results in hearing loss, problems with balance , or both.
             • Cranial Nerve IX
               • (Glossopharyngeal nerve): A mixed nerve. It has sensory, motor, and autonomic components.
               • Sensory component of glossopharyngeal nerve assists in processing sensations from the posterior third of the tongue. This component also provides general sensation from the tympanic cavity, ear canal, eustachian tube, faucial pillars, tonsils, soft palate, and pharynx.
               • Motor fibers of glossopharyngeal nerve innervate the stylopharyngeus, a muscle that raises and dilates the pharynx.
               • Experts differ on whether the motor fibers of the glossopharyngeal nerve also innervate the superior pharyngeal constrictor, the glossopharyngeal nerve, along with fibers of the vagus nerve (X), supplies motor fibers to the pharyngeal plexus, which innervate the upper pharyngeal constrictor muscles.
               • Lesions of the glossopharyngeal nerve may create difficulty in swallowing, unilateral loss of gag reflex, and loss of taste and sensation from posterior third of tongue.
               • Cranial Nerve X
                 • (Vagus nerve): A mixed nerve containing motor, sensory, and autonomic fibers. Called a wandering nerve because it extends into the chest and the stomach.
                 • Motor fibers of vagus nerve supply digestive system, heart, and lungs.  Sensory fibers convey information from digestive system, heart, trachea, pharynx, and larynx.
                 • The recurrent laryngeal nerve (RLN), a branch of vagus nerve, regulates intrinsic muscles of larynx, excluding cricothyroid, which is supplied by superior laryngeal nerve branch, RLN may be damaged during thyroid surgery, resulting in total or partial paralysis of vocal folds.
                 • Pharyngeal branch of vagus nerve supplies pharyngeal constrictors. Also supplies all muscles of velum except temper tympani which is innervated by trigeminal nerve.
                 • Damage  to vagus nerve includes a variety of sequelae such as difficulty swallowing, paralysis of the velum, and voice problems if RLN is damaged.
                 • Cranial Nerve XI
                   • (Spinal accessory nerve): A motor nerve. It's both a cranial and spinal nerve because of its cranial and spinal origin. Spinal root suplies trapezius and sternocleidomastoid muscles, which assist in head and shoulder movements. 
                   • In concert with the vagus nerve, the cranial fibers of accessory nerve innervate the uvula and legator veli palatini, muscles of soft palate. 
                   • Lesions of spinal accessory nerve may result in neck weakness, paralysis of the sternocleidomastoid, and consequent inability to turn the head, as well s an inability to shrug the shoulders or raise the arm above shoulder level.
                   • Cranial Nerve XII
                     • (Hypoglossal nerve): A motor nerve that runs under the tongue.
                     • Nerve supplies three extrinsic tongue muscles: styloglossus, hyoglossus, and genioglossus.
                     • Nerve also supplies all intrinsic muscles of tongue. 
                     • Lesions to hypoglossal nerve can result in tongue paralysis, diminished intelligibility, and swallowing problems.
   • Spinal nerves
     • Spinal nerves of PNS are closely related to autonomic nervous system. Together they control various bodily activities that are executed with little conscious effort or knowledge.
     • Spinal nerves can be motor, sensory, or mixed. They transmit motor information from CNS, to the muscles and carry sensory information from peripheral receptors to CNS.
     • Not all spinal nerves are directly involved with speech production. However, some nerves contribute to speech through innervation of the respiratory musculature. For example, the internal and external intercostals are innervated by the thoracic spinal nerves. Diaphragm is innervated by motor branches of C3-C5 spinal nerves.
     • Pairs of nerves
       • There are 31 pairs of spinal nerves. They're attached to the spinal cord through two roots: one is efferent and ventral, and the other is afferent and dorsal.
       • The 31 pairs are divided into segments. They're named after region of spinal cord to which they're attached: 8 pairs of cervical spinal nerves (C1-C8) 12 pairs of thoracic spinal nerves (T1-T12) 5 pairs of lumbar spinal nerves (L1-L5) 5 pairs of sacral spinal nerves (S1-S5) 1 pair of coccygeal nerves

2. Autonomic Nervous System

   • Autonomic nervous system (ANS) is generally viewed as part of PNS.
   • ANS controls and regulates the internal environment of our bodies; for example, heartbeat is controlled by ANS. Heartbeat is controlled through sympathetic and parasympathetic divisions of ANS, which supply the body's smooth muscles and various glands that secrete hormones. There are two branches of ANS: sympathetic and parasympathetic.
   • ANS doesn't have a direct effect upon speech, language, or hearing. However, emotionally relaxed and aroused states created by ANS actions may have some effect on various parameters of communication.
   • For example, people who stutter may become more dysfluent when the sympathetic branch is aroused. In another example, people who habitually speak too quickly and loudly, with consequent trauma to vocal folds, might benefit from feelings of relaxation provided from activation of parasympathetic branch
   • Sympathetic Branch
     • Sympathetic branch of ANS mobilizes body for "fight-or-flight" situations. Activation of sympathetic branch accelerates heart rate, dilates pupils, raises blood pressure, and increases the blood flow to the peripheral body structures (e.g., legs, which run from danger). Humand feel emotionally aroused when sympathetic branch of ANS is activated.
   • Parasympathetic Branch
     • Parasympathetic branch of ANS helps bring body back to state of relaxation. After body has mobilized for highly charged situations, the parasympathetic branch lowers blood pressure, slows heart rate, increases activity within stomach, and generally relaxes the body. Humans feel relaxed and calm during activation of parasympathetic branch of ANS.

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