MEDICAL BIOLOGY, HUMAN ANATOMY, PHYSIOLOGY AND PATHOLOGY - Ya.I. Fedonyuk 2010

ANATOMY, PHYSIOLOGY, PATHOLOGY

SECTION 3. ANATOMICAL AND PHYSIOLOGICAL ASPECTS OF BODY FUNCTION SELF-REGULATION

THE BRAIN

5. MIDBRAIN

The Midbrain develops from the third cerebral vesicle (mesencephalon). It comprises the cerebral peduncles and the midbrain tectum. Its cavity is the cerebral aqueduct (of Sylvius), which connects the Third ventricle with the fourth. The tectum consists of two superior and two inferior colliculi, which serve as subcortical visual and auditory centers, respectively (Fig. 3.17).

Class="center">

Fig. 3.17. Brainstem; superior and posterior view

A cross-section of the midbrain reveals the tectum and cerebral peduncles. The cerebral peduncles consist of the tegmentum and the basis pedunculi, separated by the substantia nigra (pigmented interneurons) (Fig. 3.18). The Pyramidal Tracts and pathways connecting the Cerebral Cortex with the nuclei of the Pons and Cerebellum pass through the basis pedunculi. The tegmentum contains both gray and White matter. The Gray matter is represented by the red Nucleus, reticular formation, and the nuclei of the III and IV pairs of Cranial Nerves. The white matter consists of ascending (sensory) nerve fibers that form the medial and lateral (auditory) lemnisci here.

Fig. 3.18. Cross-section of the midbrain at the level of

The midbrain performs a reflex function. Orienting visual and auditory Reflexes are integrated at the level of the superior and inferior colliculi of the midbrain tectum.

Of particular note when examining the reflex Functions of the midbrain is decerebrate rigidity—a sharp increase in the tone of the extensor Muscles of the limbs, HEAD, and trunk following transection of the Brain (in experimental settings) just below the red nuclei. If such an animal is placed on its paws, it will stand like a toy: the limbs are extended, the head is thrown back, and the tail is raised (Fig. 3.19).

Fig. 3.19. Decerebrate rigidity