Principles of Anatomy and Physiology, 2nd Asia-Pac PDF

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Even though the RAS receives input from the eyes, ears, and other sensory receptors, there is no input from receptors for the sense of smell; even strong odours may fail to cause arousal. People who die in house fires usually succumb to smoke inhalation without awakening. For this reason, all sleeping areas should have a nearby smoke detector that emits a loud alarm. A vibrating pillow or flashing light can serve the same purpose for those who are hearing impaired. The functions of the brain stem are summarised in table 14.2. ....................................................................................................................................................................................................................................................... CHECKPOI NT 6. Where are the medulla, pons, and midbrain located relative to one another? 7. What body functions are governed by nuclei in the brain stem? 8. List the functions of the reticular formation.

14.4 The cerebellum

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LEARNING OBJECTIVE 14.4 Describe the structure and functions of the cerebellum.

The cerebellum, second only to the cerebrum in size, occupies the inferior and posterior aspects of the cranial cavity. Like the cerebrum, the cerebellum has a highly folded surface that greatly increases the surface area of its outer grey matter cortex, allowing for a greater number of neurons. The cerebellum accounts for about a tenth of the brain mass yet contains nearly half of the neurons in the brain. The cerebellum is posterior to the medulla and pons and inferior to the posterior portion of the cerebrum (see figure 14.1). A deep groove known as the transverse fissure, along with the tentorium cerebelli, which supports the posterior part of the cerebrum, separates the cerebellum from the cerebrum (see figures 14.2b, 14.11b). In superior or inferior views, the shape of the cerebellum resembles a butterfly. The central constricted area is the vermis (= worm), and the lateral ‘wings’ or lobes are the cerebellar hemispheres (figure 14.8a, b). Each hemisphere consists of lobes separated by deep and distinct fissures. The anterior lobe and posterior lobe govern subconscious aspects of skeletal muscle movements. The flocculonodular ¯ lobe (flok-u-l ¯ o-NODu¯ -lar; flocculo- = wool-like tuft) on the inferior surface contributes to equilibrium and balance. The superficial layer of the cerebellum, called the cerebellar cortex, consists of grey matter in a series of slender, parallel folds called folia (= leaves). Deep to the grey matter are tracts of white matter called arbor vitae (AR-bor VI¯-t¯e = tree of life) that resemble branches of a tree. Even deeper, within the white matter, are the cerebellar nuclei, regions of grey matter that give rise to axons carrying impulses from the cerebellum to other brain centres. Three paired cerebellar peduncles (pe-DUNG-kuls) attach the cerebellum to the brain stem (see figures 14.7a and 14.8b). These bundles of white matter consist of axons that conduct impulses between the cerebellum and other parts of the brain. The superior cerebellar peduncles contain axons that extend from the cerebellum to the red nuclei of the midbrain and to several nuclei of the thalamus. The middle cerebellar peduncles are the largest peduncles; their axons carry impulses for voluntary movements from the pontine nuclei (which receive input from motor areas of the cerebral cortex) into the cerebellum. The inferior cerebellar peduncles consist of (1) axons of the spinocerebellar tracts that carry sensory information into the cerebellum from proprioceptors in the trunk and limbs; (2) axons from the vestibular apparatus of the inner ear and from the vestibular nuclei of the medulla and pons that carry sensory information into the cerebellum from proprioceptors in the head; (3) axons from the inferior olivary nucleus of the medulla that enter the cerebellum and regulate the activity of cerebellar neurons; (4) axons that extend from the cerebellum to the vestibular nuclei of the medulla and pons; and (5) axons that extend from the cerebellum to the reticular formation.

708 Principles of anatomy and physiology

Tortora, G. J., Derrickson, B., Burkett, B., Peoples, G., Dye, D., & Cooke, J. (2018). Principles of anatomy and physiology, 2nd asia-pacific edition. ProQuest Ebook Central http://ebookcentral.proquest.com Created from auckland on 2020-08-03 02:26:36

FIGURE 14.8

Cerebellum. The cerebellum coordinates skilled movements and regulates posture and balance.

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CEREBELLAR PEDUNCLES: Superior

ANTERIOR View Fourth ANTERIOR ventricle LOBE CEREBELLAR HEMISPHERE

POSTERIOR LOBE

VERMIS

Inferior

VERMIS

POSTERIOR

POSTERIOR

(a) Superior view

(b) Inferior view

Midsagittal plane

Middle

CEREBELLAR HEMISPHERE

Pineal gland Superior colliculus View

ANTERIOR

Inferior colliculus Cerebral peduncle Aqueduct of the midbrain (cerebral aqueduct) Mammillary body

FLOCCULONODULAR LOBE POSTERIOR LOBE

Cerebral peduncle

Pons Fourth ventricle ARBOR VITAE (WHITE MATTER) FOLIA CEREBELLAR CORTEX (GREY MATTER) CEREBELLUM

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POSTERIOR

Medulla oblongata Central canal of spinal cord ANTERIOR

(c) Midsagittal section of cerebellum and brain stem

(d) Midsagittal section

....................................................................................................................................................................................................................................................... QUES T I ON Which structures contain the axons that carry information into and out of the cerebellum?

The primary function of the cerebellum is to evaluate how well movements initiated by motor areas in the cerebrum are actually being carried out. When movements initiated by the cerebral motor areas are not being carried out correctly, the cerebellum detects the discrepancies. It then sends feedback signals to motor areas of the cerebral cortex, via its connections to the thalamus. The feedback signals help correct the errors, smooth the movements, and coordinate complex sequences of skeletal muscle contractions. CH APT ER 14 The brain and cranial nerves

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Tortora, G. J., Derrickson, B., Burkett, B., Peoples, G., Dye, D., & Cooke, J. (2018). Principles of anatomy and physiology, 2nd asia-pacific edition. ProQuest Ebook Central http://ebookcentral.proquest.com Created from auckland on 2020-08-03 02:26:36...