The brain stem links the spinal cord to the brain. Messages between the brain and the rest of the body pass through the brain stem in large nerve bundles. However, the brain stem is more than a nerve impulse highway. According to Dr. Richard Snell, ten of the twelve paired cranial nerves are linked to the brain at the brain stem. These nerves deliver messages from the sensory organs located primarily in the head to the brain, and relay messages back to the muscles that control those organs.
The Midbrain
The midbrain is involved in reflexive responses to movement in the visual field or to noise, enabling one to quickly focus on the cause of commotion. The nuclei of the oculomotor and trochlear nerves are here. These nerves supply the eye muscles. The midbrain also includes the substantia nigra and the red nucleus, clusters of neurons that are involved in muscle control. Damage to the substantia nigra results in Parkinson's disease.
The Pons
The pons bulges from the front of the brainstem. The trigeminal, abducens and facial nerve nuclei lie within it. The trigeminal nerve delivers facial sensory information to the brain. It is implicated in trigeminal neuralgia, which causes severe facial pain. The abducens nerve supplies another eye muscle. Damage to it can result in double vision. The facial nerve mediates taste, facial expression, and tactile information from parts of the ear. Lesions here can cause drooping features and facial asymmetry on the affected side. Extensive connections to the cerebellum give the pons a role in posture and movement.
The Medulla
The acoustic, glossopharyngeal, vagus, accessory and hypoglossal nerve nuclei are in the medulla. These nerves connect the medulla to the ears, the face, the tongue, the throat, and the organs and muscles of the chest and abdomen. These links give the medulla roles in a variety of functions, including hearing, balance, respiration, circulation, blood pressure, blood chemistry, heart rate, posture, taste, chewing, swallowing, digestion and speech. The medulla is a regulatory center for many of the body's autonomic functions, primarily through the extensive links of the vagus nerve. Dr. John Kimball, of Kimball's Biology Pages, says the vagus nerve is the primary nerve of the parasympathetic nervous system, which sustains normal internal functioning in the body of an unstressed individual. This contrasts to the sympathetic nervous system, which responds to stress.
The Reticular Formation
The reticular formation courses through all three segments of the brain stem. This structure has widespread connections with the spinal cord, the sensory organs, the cerebellum and the cerebral cortex. Through its spinal connections, it can influence the heart rate, respiration, posture and equilibrium. Its cerebral connections have generated much scientific interest. The so-called reticular activating system plays a critical role in sustaining consciousness. It regulates sleep and wakefulness. If the connections between the reticular formation and the cerebrum are disrupted, a deep, potentially irreversible coma will occur.
References
- "Clinical Neuroanatomy. An Illustrated Review with Questions and Explanations," 3rd ed.; Richard Snell; 2001.
- "Atlas of Functional Neuroanatomy," 2nd ed.; Walter J. Hendelman; 2006.
- Kimball's Biology Pages; Peripheral nervous system, human; Organization of the Nervous System.
- Brain Explorer; Brain Atlas; Introduction.
- Neurokinesiology, A New Path to Human Health; Neurological Background; The Reticular Formation.
