The pons (bridge in Latin) or METENCEPHALON is a BRAINSTEM structure that is located rostral to the MEDULLA and caudal to the MIDBRAIN. The dorsal portion is known as the PONTINE TEGMENTUM, which is the rostral extension of the MEDULLARY TEGMENTUM. The ventral portion consists of the PONTINE NUCLEI and the motor pathways, including the CORTICOSPINAL TRACT, and is highly developed in the human brain. Four CRANIAL NERVES (fifth, sixth, seventh and eighth) enter or exit the pons, and their sensory and motor nuclei are located in the pontine, as well as medullary, tegmentum. The pontine tegmentum also contains sensory and motor pathways.
The VESTIBULOCOCHLEAR NERVE (the eighth cranial nerve) enters the caudal pons. The nerve has a cochlear (see COCHLEA) branch, concerned with hearing, and a VESTIBULAR branch, concerned with equilibrium or balance. These primary afferent fibres have their cell bodies in the spiral and vestibular ganglia (see GANGLION), respectively, in the inner ear. The cochlear afferents terminate in the cochlear nuclei in the upper medulla, which give rise to central auditory pathways. Most of the vestibular afferents terminate in the vestibular nuclear complex in the caudal pons, while some enter the CEREBELLUM. These structures, in conjunction with the cranial nerves and their nuclei involved in EYE MOVEMENT control and the vestibulospinal pathways, play a key role in maintaining both static and dynamic balance. The FACIAL NERVE or the seventh cranial nerve also enters the caudal pons. The largest, motor branch enters the facial nucleus, which contains the cell bodies of facial MOTOR NEURONS. The facial motor nerve is involved in the control of superficial facial muscles that are important in facial expression. The taste fibres in the facial nerve descend to the medulla and terminate in the NUCLEUS OF THE SOLITARY TRACT, the origin of central gustatory (see GUSTATION) pathways. The cell bodies of taste fibres in the facial nerve are located in the geniculate ganglion in the temporal bone. The autonomic fibres of the facial nerve are involved in the secretion of saliva and tears, and originate from the superior salivatory nucleus in the pontine tegmentum.
The ABDUCENS NERVE (the sixth cranial nerve) is a motor nerve innervating the lateral rectus muscle of the eye involved in rolling the eyeball laterally (abduction). The abducens nucleus is located dorsally in the pontine tegmentum, and the nerve exits the caudal pons from its ventral aspect.
The TRIGEMINAL NERVE (the fifth cranial nerve) is among the largest cranial nerves. It consists of both motor and sensory branches, and enters the rostral pons from its ventrolateral aspect. The motor trigeminal nerve originates from the motor trigeminal nucleus. It innervates jaw muscles and is involved in the opening and closing of the jaws and mastication. The sensory trigeminal nerve carries information on TOUCH, PAIN and TEMPERATURE sensations on the skin of the face, oral cavity and the teeth, as well as PROPRIOCEPTION of jaw muscles. Proprioceptive afferents have their cell bodies in the mesencephalic trigeminal nucleus. All of the other afferents have their cell bodies in the trigeminal ganglion, and terminate in the sensory trigeminal complex, which is distributed in a longitudinal column through the lower brainstem.
In addition to the cranial nerves, the pons contains various descending and ascending tracts carrying motor and sensory fibres originating or terminating in nuclei in the pons, or coursing through the pons en route to the upper brainstem, FOREBRAIN, medulla, or SPINAL CORD. The pons is also connected with the cerebellum via three large fibre bundles: the superior, middle and inferior cerebellar peduncles. These fibre bundles interconnect the cerebellum with various brainstem and thalamic nuclei (see THALAMUS) involved in balance and motor co-ordination. The PONTINE RETICULAR FORMATION rcfers to the areas of the pontine tegmentum that are neither sensory nor motor. These so-called non-specific nuclei (see NON-SPECIFIC PROJECTIONS) include the oral and caudal reticular nuclei, as well as several neurochemically coded nuclei. All these nuclei have widespread projections to the forebrain as well as to other brainstem regions, in particular the reticular formation at other levels. The PEDUNCULOPONTINE TEGMENTAL NUCLEUS and the LATERODORSAL TEGMENTAL NUCLEUS contain CHOLINERGIC neurons that project to the thalamus and other forebrain structures, as well as more caudal regions of the BRAINSTEM RETICULAR FORMATION. The LOCUS COERULEUS in the dorsolateral pontine tegmentum is the major source of NORADRENALINE in the brain and spinal cord. The pons also contains the RAPHE NUCLEI along the midline in the tegmentum, including the caudal part of the DORSAL RAPHE NUCLEUS and the NUCLEUS RAPHE PONTIS. The raphe nuclei contain SEROTONIN neurons. Collectively, these reticular neurons appear to be involved in integrated functions such as maintenance of CONSCIOUSNESS and awakening from SLEEP, VIGILANCE and ATTENTION, and the modulation of sensory and motor functions according to the BEHAVIOURAL STATE.
KAZUE SEMBA
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