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7.4.2.1. Somatic Nervous System

Cranial nerves

The cranial nerves are those that enter or leave the encephalon, they are 12 pairs and can be sensitive, motor or mixed. They are responsible for connecting the encephalon with the head, the trunk and some internal organs.

Some cranial nerves are related to the Autonomous Nervous System, such as the cranial nerve X or the vagus nerve, due to the control it carries out over the viscera.

I. Olfactory nerve

Sensitive

It transmits the olfactory impulses.

II. Optical

Sensitive

It transmits visual information to the brain.

III. Oculomotor

Motor

It innervates the muscles: levator upper eyelid, upper rectus, medial rectus, inferior rectus, and inferior oblique, which collectively perform the majority of eye movements; it also innervates the sphincter of the pupil.

IV. Trochlear o Pathetic

Motor

It innervates the superior oblique muscle, which depresses, rotates laterally (around the optic axis), and internally rotates the eyeball.

V. Trigeminal

Mixed

It perceives sensitive information from the face and innervates the chewing muscles (masseter and temporal).

VI. External ocula motor

Motor

It innervates the superior oblique muscle, which depresses, rotates laterally (around the optic axis), and internally rotates the eyeball.

VII. Facial

Mixed

It carries motor innervation to the muscles responsible for facial expression, the posterior belly of the digastric muscle and the stapedium, receives the taste impulses from the anterior two thirds of the tongue and provides secret-motor innervation to the salivary (except for the parotid) and the lacrimal gland.

VIII. Vestibule-cochlear o auditory

Sensitive

Perception of sounds, rotation and gravity (essential for balance and movement). The vestibular branch carries impulses to coordinate balance and the cochlear arm carries auditory impulses.

IX. Glossopharyngeal

Mixed

It receives taste impulses from the posterior third of the tongue, provides secret-motor innervation to the parotid gland and motor innervation to the stylopharyngeus muscle and the styloglossus muscle. It also relays some information to the brain from the palatine tonsils. This is directed to the opposite thalamus and some nuclei of the hypothalamus.

X. Vagus.

Mixed

Provides innervation to most laryngeal muscles and all pharyngeal muscles except stylopharyngeal (innervated by glossopharyngeus); carries parasympathetic fibers to the vicinity of all abdominal viscera located below the splenic flexure; and receives the sense of taste from the epiglottis. It controls the muscles that help articulate sounds in the soft palate. Symptoms of damage generate dysphagia, velopharyngeal insufficiency.

XI. Accessory or spinal

Motor

It controls the sternocleidomastoid muscles and the trapezius, overlaps with functions of the vagus. Symptoms of damage include an inability to shrug the shoulders and a weakness in head movements.

XII. Hypoglossal nerve

Motor

It provides motor innervation to the muscles of the tongue (except the palatoglossus muscle, which is innervated by the vagus nerve, and the styloglossus muscle, which is innervated by the glossopharyngeal nerve) and other lingual muscles. Important in swallowing (bolus formation) and articulation of sounds.

Nervios craneales

By Brain human normal inferior view with labels en.svg: Patrick J. Lynch, medical illustrator & derivative work: Beao *derivative work: Angelito7 [CC BY 2.5], via Wikimedia Commons

Spinal nerves

The spinal or spinal nerves originate from the spinal cord. There are 31 pairs, all mixed, of which:

  • 8 are cervical,
  • 12 are dorsal,
  • 5 are lumbar and
  • 6 are sacred.

Each spinal nerve leaves the medulla dividing into two branches or roots, a ventral or motor root and another dorsal or sensory root, which later join to form a single nerve cord. Thus, all  sensory neurons enter the spinal cord through the dorsal (posterior) root, and motor neurons exit through the ventral (anterior) root.

Nervios espinales o raquídeos

By Jmarchn (Own work) [CC BY-SA 3.0], via Wikimedia Commons


         

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Biology and Geology teaching materials for Compulsory Secondary Education (ESO) and Baccalaureate students.