Brain and Cranial Nerves

     There are 3 primary areas of the brain:

1. Cerebrum consists of:
   • 2 Cerebral hemispheres that are incompletely separated by the great longitudinal fissure. Each hemisphere consists of 4 lobes:
     • Frontal Lobes
       • Perform high level cognitive functions, such as reasoning, abstraction, concentration, and executive control
       • Provide for information storage, or memory
       • Control voluntary eye movement
       • Influence somatic motor control of activities such as respiration, GI activity, and blood pressure
       • Perform motor control of speech in the dominant hemisphere, usually the left
       • Contain the motor cortex that controls voluntary motor function
       • Contain the premotor cortex or motor association areas that are involved with generalized movement

     • Parietal Lobes interpret sensory information to define size, shape, weight, texture, consistency, and awareness of body parts. The non-dominant parietal lobe processes visual-spatial information and controls spatial orientation. The dominant lobe is involved with the ideomotor praxis

       
       
       
       

     • Temporal Lobes
       • Contain the primary auditory receptive areas, the auditory association area, and the interpretive area
       • Involved with auditory, visual, olfactory, and somatic perception and integration, which extend to learning, memory, emotional effect, and intellectual ability that involve these senses

     • Occipital Lobes contain the primary visual cortex and visual association areas, the primary functions of which are visual perception, some visual reflexes such as fixation, and involuntary smooth eye movements (smooth pursuit system)

   • Corpus Callosum which connects every part of each hemisphere with the corresponding part in the other hemisphere

   • Basal Ganglia which controls fine body movements, particularly of the hands and lower extremities

   • Diencephalon which contains:
     • Thalamus, which contains all sensory pathways, except the olfactory pathways, and which plays a role in conscious pain awareness, focusing attention, and in the reticular activating and limbic systems
     • Epithalamus, which is composed of the pineal body and is involved with the "food-getting" reflex
     • Hypothalamus, which controls temperature, water metabolism, hypophyseal secretions (such as FSH), visceral and somatic activities (such as heart rate, peristalsis, and pupillary dilation and constriction), visible physical expressions (such as response to emotions and blushing), and the sleep-wakefulness cycle
     • Subthalamus, which is closely related to the function of the basal ganglia

   • Internal Capsule which controls major sensory and motor function

   • Hypophysis (Pituitary Gland) which produces growth-stimulating hormone (GSH), ACTH, FSH, LH, thyroid-stimulating hormone (TSH), vasopressin (ADH), and oxytocin. The pituitary gland is connected to and controlled by the hypothalamus, the control center for the autonomic nervous system and the neuroendocrine system

2. Brain Stem consists of:
   • Midbrain, which serves as the pathway for the cerebral hemispheres and lower brain and as the center for auditory and visual reflexes. The midbrain processes visual stimuli, integrates visual and auditory motor reflexes, and relays auditory information

   • Pons, the bridge between the midbrain and the medulla, connects higher cerebral regions with the lower levels of the nervous system. The pons has some control of respiratory function and contains the nuclei of Cranial nerves V through VIII

   • Medulla, which transmits information for the coordination of head and eye movement, contains cardiac, vasomotor, and respiratory centers. Cranial nerves IX through XII emanate from the medulla

3. Cerebellum which controls fine movement, coordinates muscle groups, and maintains balance by regulating postural reflexes, controlling the coordination of voluntary muscle activity and muscle tone, providing muscle synergy throughout the body, and coordinating location in space, movements, and all sensory modalities.

     There are 12 pairs of cranial nerves.  They arise from  the cerebral hemispheres (I and II), from the midbrain (III and IV), from the pons (V, VI, VII, and VIII), and from the medulla (IX, X, XI, and XII). VII and VIII also arise from the medulla and V has branches in the midbrain and medulla.

1. Olfactory Nerve (I) is the sensory nerve for the sense of smell

2. Optic Nerve (II) is the sensory nerve for the receipt of primary visual stimuli

3. Oculomotor Nerve (III) innervates 4 of the 6 muscles involved in eye movement, is responsible for elevating the upper eyelid, and involved with pupillary constriction

4. Trochlear Nerve (IV) is responsible for downward and inward eye movement

5. Trigeminal Nerve (V) is a mixed nerve which is involved in sensations of pain, temperature, and light touch of the entire face, scalp, nose, and mouth and which innervates the muscles of mastication

6. Abducens Nerve (VI) is responsible for rotating the eye laterally

7. Facial Nerve (VII) is a mixed nerve which:
   • Innervates the muscles of facial expression and is responsible for closing the eyes, smiling, whistling, showing the teeth, wrinkling the nose and brow, and grimacing
   • Controls tearing and salivation
   • Mediates taste to the anterior two-thirds of the tongue
   • Mediates sensation from the skin lining to the external auditory meatus

8. Acoustic Nerve (VIII) is involved in the sense of hearing, balance, and orientation in space

9. Glossopharyngeal Nerve (IX) is a mixed nerve which innervates the pharynx, taste receptors on the tongue, parotid gland, and back of the ear

10. Vagus Nerve (X) is a mixed nerve which innervates the soft palate, pharynx, larynx, thoracic and abdominal organs (including the heart, lungs, and viscera), and external auditory meatus.

11. Spinal Accessory Nerve (XI) is responsible for shrugging the shoulders and rotating the head

12. Hypoglossal Nerve (XII) is responsible for normal speech and swallowing

Pathophysiology

     As a result of TBI, there can be injuries to any part of the brain and cranial nerves, as well as molecular and cellular changes, altered cerebral hemodynamics, cerebral edema, cerebral ischemia, increased ICP, and potential for herniation which can cause secondary neuronal demise. Click on Secondary Injuries for further information on the physiological changes that can occur following TBI and lead to complications that can result in secondary injury to the brain.