Chapter 14 The Brain and Cranial Nerves part

Chapter 14: The Brain and Cranial Nerves part I Lecture 19

I. An Introduction to the Organization of the Brain, p. 452 • The human brain ranges in size from 750 cc to 2100 cc and contains 98% of the body’s neural tissue. • The average brain weighs about 1. 4 kg (3 lb. ).

I. An Introduction to the Organization of the Brain, p. 452 • A Preview of Major Regions and Landmarks, p. 452 • Figure 14 -1 • The largest part of the brain is the cerebrum, which controls the higher mental functions such as thought, memory and conscious movement.

Fig. 14 -1, p. 453

I. An Introduction to the Organization of the Brain, p. 452 • The cerebrum is divided into left and right cerebral hemispheres, and covered by a surface layer of gray matter or neural cortex (cerebral cortex).

I. An Introduction to the Organization of the Brain, p. 452 • The surface is folded to increase surface area, forming – elevated ridges (gyri), – shallow depressions, (sulci), – and deep grooves (fissures).

I. An Introduction to the Organization of the Brain, p. 452 • The second largest part of the brain is the cerebellum, which coordinates repetitive body movements. • The cerebellum also has 2 hemispheres and is covered in cerebellar cortex.

Fig. 14 -1, part 1, p. 453

I. An Introduction to the Organization of the Brain, p. 452 • Underneath the cerebrum and cerebellum is the diencephalon, which links the cerebrum with the brain stem. • The diencephalon is divided into the left thalamus and right thalamus, which relay and process sensory information, and the hypothalamus, which is involved in hormone production, emotion and autonomic function.

Fig. 14 -1, part 2, p. 453

I. An Introduction to the Organization of the Brain, p. 452 • The hypothalamus is connected to the pituitary gland (a major endocrine gland) via a small stalk called the infundibulum. • The hypothalamus and pituitary gland are the interface between the nervous system and the endocrine system.

I. An Introduction to the Organization of the Brain, p. 452 • The brain stem, which processes information between the spinal cord and the cerebrum or cerebellum, includes – the mesencephalon, – the pons, – and the medulla oblongata

Fig. 14 -1, part 2, p. 453

I. An Introduction to the Organization of the Brain, p. 452 – The mesencephalon (midbrain) processes sight and sound (and their associated reflexes) and maintains consciousness. – The pons connects the cerebellum to the brain stem, and is involved in somatic and visceral motor control. – The medulla oblongata connects the brain to the spinal cord. In addition to relaying information, the medulla oblongata regulates autonomic functions such as heart rate, blood pressure and digestion.

Embryology of the Brain, p. 452 • Table 14 -1 • The organization of brain structures is determined by their embryological development. • The origin of the brain is the neural tube, which enlarges into 3 areas called primary brain vesicles (the prosencephalon, mesencephalon and rhombencephalon).

Embryology of the Brain, p. 452 • The prosencephalon and rhombencephalon subdivide to form 5 secondary brain vesicles.

Table 14 -1, p. 454

Ventricles of the Brain, p. 453 • Figure 14 -2 • The neural tube encloses a fluid-filled cavity called the neurocoel. • During development, the neurocoel expands to form chambers called ventricles, which are lined with cells of the ependyma.

Fig. 14 -2, p. 454

Ventricles of the Brain, p. 453 • The cerebral hemispheres each contain a large lateral ventricle, separated from each other by a thin medial partition called the septum pellucidum. The ventricle of the diencephalon is called the third ventricle. The lateral ventricles communicate with the third ventricle via the interventricular foramen (foramen of Monro).

Fig. 14 -2 a, p. 454

Ventricles of the Brain, p. 453 • The mesencephalon contains a narrow canal called the mesencephalic aqueduct (cerebral aqueduct), which connects the third ventricle with the fourth ventricle. • The fourth ventricle extends into the medulla oblongata and becomes continuous with the central canal of the spinal cord.

Fig. 14 -2 b, p. 454

Key • The brain is a large, delicate mass of neural tissue containing internal passageways and chambers filled with cerebrospinal fluid. • Each of the five major regions of the brain has specific functions. • As you ascend from the medulla oblongata (which connects to the spinal cord) to the cerebrum, those functions become more complex and variable. • Conscious thought and intelligence are provided by the neural cortex of the cerebral hemispheres.

II. Protection and Support of the Brain, p. 455 • The tissues of the brain are supported and protected by: – the bones of the cranium – the cranial meninges – cerebrospinal fluid • The brain is biochemically isolated from general circulation by the blood-brain barrier.

The Cranial Meninges, p. 455 • Figure 14 -3 • The cranial meninges are made up of 3 layers (dura mater, arachnoid mater and pia mater) continuous with the spinal meninges.

The distinctive characteristics of the cranial meninges are: – The cranial dura mater has an inner fibrous layer (meningeal layer) and outer fibrous layer (endosteal layer). The endosteal layer is fused to the periosteum. Venous sinuses between the 2 layers receive blood from veins of the brain and deliver it to the jugular veins of the neck.

The distinctive characteristics of the cranial meninges are: – The cranial arachnoid mater covers the brain and is in contact with the inner epithelial layer of the dura mater.

The distinctive characteristics of the cranial meninges are: – The pia mater is attached to the brain surface by astrocytes. The subarachnoid space is between the arachnoid mater and the pia mater.

Fig. 14 -3, p. 456

Fig. 14 -3 a, bottom, p. 456

Fig. 14 -3 a, top, p. 456

Dural Folds • The inner layer of the dura mater form dural folds that extend into the cranial cavity to stabilize and support the brain. The dural folds contain collecting veins called dural sinuses. • Figure 14 -3 b

Dural Folds • The 3 largest dural folds are the falx cerebri, the tentorium cerebelli, and the falx cerebelli. – The falx cerebri projects between the cerebral hemispheres. It contains the superior sagittal sinus and the inferior sagittal sinus. – The tentorium cerebelli separates the cerebellum and cerebrum, and contains the transverse sinus. – The falx cerebelli divides the cerebellar hemispheres below the tentorium cerebelli.

Fig. 14 -3 b, p. 456

Cerebrospinal Fluid, p. 456 • Cerebrospinal fluid (CSF) surrounds all exposed surfaces of the CNS and interchanges with the interstitial fluid of the brain. The major functions of CSF are: – Cushioning delicate neural structures. – Supporting the brain. – Transporting nutrients, chemical messengers, and waste products.

The Formation of CSF • The choroid plexus is a combination of specialized ependymal cells and capillaries that produce cerebrospinal fluid. • The ependymal cells secrete CSF into the ventricles, remove waste products from the CSF, and adjust the composition of CSF over time.

The Protective Function of the Cranial Meninges • The cranial meninges and CSF cushion and protect the brain from cranial trauma that results from contact with the bones of the cranium.

Circulation of CSF • The choroid plexus produces about 500 ml of CSF a day, replacing the entire volume of CSF about every 8 hours. • CSF circulates from the choroid plexus thorough the ventricles to the central canal of the spinal cord. • CSF enters the subarachnoid space through 2 lateral apertures and 1 median aperture to circulate around the brain, spinal cord and cauda equina. • Extensions of the subarachnoid space (arachnoid villi) extend through the dura mater to the superior sagittal sinus. • Large clusters of villi form arachnoid granulations which absorb CSF into the venous circulation.

Fig. 14 -4, p. 457

Fig. 14 -4 a, p. 457

Fig. 14 -4 b, p. 457

The Blood Supply to the Brain, p. 458 • The brain has a continuous, high demand for nutrients and oxygen which must be supplied by blood circulation. Blood is supplied to the brain by the internal carotid arteries and vertebral arteries, and drained from the dural sinuses by the internal jugular veins.

The Blood Supply to the Brain, p. 458 • Disorders that interfere with normal blood circulation to the brain are called cerebrovascular diseases. A stroke or cerebrovascular accident (CVA) occurs when the blood supply to a portion of the brain is shut off, and neurons die.

The Blood-Brain Barrier • Neural tissue in the CNS is isolated from general circulation by the blood-brain barrier (BBB) formed by an extensive network of tight junctions between endothelial cells lining the capillaries of the CNS.

The Blood-Brain Barrier • Only lipid-soluble compounds (e. g. O 2, CO 2, steroids, and prostaglandins) can diffuse into the interstitial fluid of the brain and spinal cord. Astrocytes release chemicals that control the permeability of the endothelium to other substances, effectively controlling the blood-brain barrier.

The Blood-Brain Barrier • A blood-CSF barrier is formed by specialized ependymal cells surrounding the capillaries of the choroid plexus, which is not a part of the neural tissue of the brain. • Transport across the blood-brain and blood-CSF barriers is selective and directional, limiting the movement of many compounds. As a result, the chemical composition, p. H and concentrations of major ions in blood and CSF are different.

The Blood Supply to the Brain, p. 458 • The blood-brain barrier is continuous except in 4 specific cases: – In portions of the hypothalamus, where hypothalamic hormones enter the systemic circulation. – In the posterior lobe of the pituitary gland, where the hormones ADH and oxytocin are released into the circulation. – In the pineal glands, where pineal secretions enter the circulation. – At the choroid plexus, where specialized ependymal cells maintain the blood-CSF barrier.

Key • The meninges stabilize the position of the brain within the cranial cavity. • Cerebrospinal fluid provides protection against sudden jolts and shocks. • CSF also provides nutrients and removes wastes generated by active neural tissues. • The blood-brain barrier and the blood-CSF barrier selectively isolate the brain from chemicals in blood that might disrupt neural function.

III. The Medulla Oblongata, p. 459 • Figure 14 -5 • The medulla oblongata is continuous with the spinal cord. It is here that the central canal opens into the fourth ventricle.

Fig. 14 -5, p. 460

Fig. 14 -5 a, p. 460

Fig. 14 -5 b, p. 460

Fig. 14 -5 c, p. 460

The medulla oblongata • Figure 14 -6 • The medulla oblongata contains all of the ascending and descending tracts that allow the brain and spinal cord to communicate, coordinates complex autonomic reflexes, and controls visceral functions.

The medulla oblongata includes 3 groups of nuclei: 1. Autonomic nuclei controlling visceral activities 2. Sensory and motor nuclei of cranial nerves 3. Relay stations along sensory and motor pathways

1. Autonomic nuclei controlling visceral activities: • The reticular formation (a mass of gray matter with embedded nuclei that extends from the medulla oblongata to the mesencephalon) within the medulla oblongata regulates autonomic functions.

1. Autonomic nuclei controlling visceral activities: • There are 2 major groups of reflex centers that control peripheral systems: – The cardiovascular centers (subdivided into the cardiac center and the vasomotor center), and – The respiratory rhythmicity centers

Fig. 14 -6, p. 461

Fig. 14 -6 a, p. 461

2. Sensory and motor nuclei of cranial nerves: • associated with 5 of the 12 cranial nerves (designated by Roman numerals VIII, IX, X, XI, and XII)

3. Relay stations along sensory and motor pathways: • The nucleus gracilis and the nucleus cuneatus pass somatic sensory information to the thalamus. • The solitary nucleus receives visceral sensory information. • The olivary nuclei (olives) relay information about somatic motor commands.

Fig. 14 -6 b, p. 461

IV. The Pons, p. 462 • Figure 14 -6 c • The pons links the cerebellum with the mesencephalon, diencephalon, cerebrum and spinal cord.

Fig. 14 -6 c, p. 461

IV. The Pons, p. 462 • The pons contains 4 groups: – Sensory and motor nuclei of cranial nerves V, VII and VIII. – Nuclei involved with the control of respiration: • The apneustic center and the pneumotaxic center modify the activity of the respiratory rhythmicity center. – Nuclei and tracts that process and relay information heading to or from the cerebellum. – Ascending, descending and transverse tracts • Transverse fibers (axons) link nuclei of the pons with the cerebellar hemisphere of the opposite side.

V. The Cerebellum, p. 462 • Figure 14 -7 • The surface of the cerebellum is composed of highly folded neural cortex (the folia).

Fig. 14 -7, p. 463

V. The Cerebellum, p. 462 • Figure 14 -7 • The anterior and posterior lobes are separated by the primary fissure. • At the midline, a narrow band of cortex (the vermis) separates the cerebellar hemispheres.

Fig. 14 -7 a, left, p. 463

Fig. 14 -7 a, right, p. 463

V. The Cerebellum, p. 462 • The flocculonodular lobe lies below the fourth ventricle. • The cerebellar cortex contains large, branched Purkinje cells that receive input from up to 200, 000 synapses.

Fig. 14 -7 b, left, p. 463

Fig. 14 -7 b, right, p. 463

V. The Cerebellum, p. 462 • The internal white matter of the cerebellum is highly branched, in a formation called the arbor vitae.

V. The Cerebellum, p. 462 • Cerebellar nuclei embedded in the arbor vitae relay information to the Purkinje cells. • The superior cerebellar peduncles, middle cerebellar peduncles, and inferior cerebellar peduncles are tracts that link the cerebellum with the brain stem, cerebrum and spinal cord. • Damage to the cerebellum, or alcohol intoxication, can disturb muscle coordination (ataxia).

VI. The Mesencephalon, p. 464 • Table 14 -4 summarizes the components and functions of the mesencephalon or midbrain. • Figure 14 -8

Fig. 14 -8, p. 465

VI. The Mesencephalon, p. 464 • Structures of the tectum (the roof of the mesencephalon posterior to the mesencephalic aqueduct) include 2 pairs of sensory nuclei called the corpora quadrigemina: – the superior colliculus receives visual input – the inferior colliculus receives auditory input

VI. The Mesencephalon, p. 464 • Structures of the tegmentum (anterior to the mesencephalic aqueduct) include: • the red nucleus (many blood vessels) • the substantia nigra (pigmented gray matter) • The cerebral peduncles (nerve fiber bundles on the ventrolateral surfaces of the mesencephalon) contain descending fibers to the cerebellum, and fibers carrying motor commands.

Fig. 14 -8 a, p. 465

Fig. 14 -8 b, p. 465