Nervous System II: Divisions of the Nervous System

UNIT 3 - CHAPTER 11: NERVOUS SYSTEM II: DIVISIONS OF THE NERVOUS SYSTEM

11-1 LEARNING OUTCOMES

11.1 General Characteristics of the Central Nervous System

  • Describe the relationship among the brain, brainstem, and spinal cord.

11.2 Meninges

  • Describe the coverings of the brain and spinal cord.

11.3 Ventricles and Cerebrospinal Fluid

  • Discuss the formation and function of cerebrospinal fluid.

11.4 Brain

  1. Describe the development of the major parts of the brain and explain the functions of each part.

  2. Distinguish among sensory, association, and motor areas of the cerebral cortex.

  3. Discuss hemisphere dominance.

  4. Explain the stages in memory storage.

  5. Explain the functions of the limbic system and the reticular formation.

11.5 Spinal Cord

  1. Describe the structure of the spinal cord and its major functions.

  2. Describe the structure of a reflex arc.

11.6 Peripheral Nervous System

  1. Distinguish between the major parts of the peripheral nervous system.

  2. Describe the structure of a peripheral nerve and how its fibers are classified.

  3. Identify the cranial nerves and list their major functions.

  4. Explain how spinal nerves are named and their functions.

11.7 Autonomic Nervous System

  1. Characterize the autonomic nervous system.

  2. Distinguish between the sympathetic and the parasympathetic divisions of the autonomic nervous system.

  3. Compare sympathetic and parasympathetic nerve pathways.

  4. Explain how the different autonomic neurotransmitters affect visceral effectors.

11.8 Life-Span Changes

  1. Describe aging-associated changes in the nervous system.

11-3 11.1 General Characteristics of the Central Nervous System

A. Overview of CNS

  • The central nervous system (CNS) organs include the brain and spinal cord.

    1. Brain

    • Largest and most complex part of the nervous system, includes:

      • Two cerebral hemispheres

      • Diencephalon

      • Brain stem (attaches brain to spinal cord)

      • Cerebellum

    • Contains approximately 100 billion multipolar neurons.

    • Oversees functions of the entire body.

    • Provides characteristics like personality.

    1. Spinal Cord

    • Provides two-way communication between the CNS and the peripheral nervous system (PNS).

B. Protection of CNS

  • The CNS is protected by bones, membranes, and fluids.

    1. Bones

    • The brain is encased by eight skull bones.

    • The spinal cord is encased by 26 bones comprising the vertebral column.

    1. Membranes

    2. Fluid

11-4 11.2 Meninges

A. Layers of Meninges

  • The meninges are membranes around the brain and spinal cord, consisting of three distinct layers:

    1. Dura Mater

    • Outermost membrane attached to the inner periosteum of the skull.

    • Tough, white fibrous connective tissue containing many blood vessels and nerves.

    • Splits into two layers enclosing the dural sinuses (which collect venous blood from the brain).

    1. Arachnoid Mater

    • Middle layer, thin and net-like.

    • Contains the subarachnoid space filled with cerebrospinal fluid (CSF), serving as a cushion for the brain.

    1. Pia Mater

    • Innermost layer that clings to the brain surface.

    • Very thin and delicate connective tissue with many nerves and blood vessels providing nourishment.

    • Dips into grooves and contours of the brain.

    Note: Clinical relevance includes subdural hematoma and meningitis.

B. Meninges Around Spinal Cord

  • The dura mater is not attached to the bone of the vertebra, forming an epidural space filled with loose connective tissue and fat. CSF fills the subarachnoid space and central canal.

11-5 11.3 Ventricles and Cerebrospinal Fluid (CSF)

A. Role of CSF

  • CSF not only fills the subarachnoid space but also fills the ventricles (interconnected cavities) within the cerebral hemispheres and brainstem.

B. Structure of Ventricles

  1. Continuous with the central canal of spinal cord.

  2. Filled with CSF.

  3. Lined by ependymal cells (neuroglial cells in the CNS).

C. Secretion and Circulation of CSF

  1. Secretion

    • CSF is secreted by specialized capillaries called choroid plexuses, primarily within the lateral ventricles (ventricles 1 & 2).

  2. Circulation

    • CSF circulates from the lateral ventricles to the 3rd ventricle, then to the 4th ventricle, and into:

      • The central canal of the spinal cord.

      • The subarachnoid space of the meninges.

  3. Reabsorption

    • CSF is reabsorbed back into the bloodstream through arachnoid granulations that project into dural sinuses.

  4. Movement

    • CSF movement is facilitated by cilia of ependymal cells.

D. Properties of CSF

  1. Total Volume

    • Approximately 150 mL fills these spaces, with about 1 liter secreted daily to replenish the circulating volume every 3-4 hours.

  2. Functions

    • Mechanical Protection - serves as a cushion.

    • Chemical Protection - provides ions, hormones, and neurotransmitters.

    • See Clinical Application 11.2 regarding CSF Pressure.

11-4 Brain

  • The brain is the largest and most complex part of the nervous system, occupying the cranial cavity.

  • Composed of approximately one hundred billion multipolar neurons, overseeing the function of the body and influencing personality characteristics.

A. Brain Development

  1. Embryonic neural tube expands and hollows cranially (
    Refer to text for discussion on anencephaly).

  2. Development of three vesicles that split to become four adult ventricles.

  3. The walls of the three vesicles develop into specific adult brain areas:

    • Forebrain = Cerebrum, basal nuclei, and diencephalon.

    • Midbrain = Midbrain.

    • Hindbrain = Pons, medulla oblongata, and cerebellum.

B. Structure of the Cerebrum

  1. Cerebrum

    • Largest portion of the brain, divided into two cerebral hemispheres.

    • Hemispheres are connected by the corpus callosum, a deep bridge of nerve fibers.

    • Surface ridges known as convolutions (gyri) with grooves:

      • Sulci (shallow groves):

      • Central sulcus (between frontal and parietal lobes).

      • Lateral sulcus (between temporal lobe and others).

      • Fissure (deep groove):

      • Longitudinal fissure separating the two hemispheres.

      • Transverse fissure (between cerebrum and cerebellum).

    • Composition

      • Bulk is white matter (myelinated nerve fibers) transmitting impulses from the cortex to the spinal cord.

      • Cerebral cortex is gray matter containing bundles of interneuron cell bodies.

C. Functions of the Cerebral Cortex

  1. Functional Regions of the Cortex

    • Sensory Areas:

      • Responsible for conscious awareness of sensations, located in:

      • Primary somatosensory cortex: Receives general sensory info (temperature, touch, pressure, pain); located in postcentral gyrus of parietal cortex.

      • Visual Cortex Area: Receives info from photoreceptors in the retina; located in occipital cortex.

      • Auditory Cortex Area: Receives info from hearing receptors; located in temporal cortex.

      • Gustatory cortex: Concerns taste receptors; located in parietal cortex above temporal lobe.

    • Association Areas:

      • Involved in processing perceptions, memory, reasoning, judgment, and emotions. They are not directly tied to sensory or motor function and integrate information from all lobes.

    • Motor Areas:

      • Located in frontal cortex:

      • Primary motor cortex: Initiates voluntary muscle movements (precentral gyrus).

      • Broca's area: Related to motor speech, above the temporal lobe in the left hemisphere.

D. Hemisphere Dominance (Brain Lateralization)

  • Basic functions (sensory & motor) are equally controlled by both hemispheres through the corpus callosum.

  • Certain higher functions, such as language, are predominantly controlled by one hemisphere,

    • This hemisphere is termed the dominant hemisphere, usually the left in most individuals.

    • The non-dominant hemisphere (typically the right) governs orientation in space, art, musical appreciation, and emotions.

E. Memory

  1. Definition: Memory is the persistence of learning, allowing access to acquired knowledge.

  2. Types of memory:

    • Short Term Memory.

    • Long Term Memory.

    • Refer to text regarding hippocampus & amygdala's roles in memory.

F. Basal Nuclei

  • Masses of gray matter situated deep within the cerebral white matter.

  • Serve as relay stations for outgoing motor impulses from the brain, mediating between the primary motor cortex, basal ganglia, and brain stem.

  • Release dopamine, which inhibits excessive movements,

    • Refer to Clinical Application 11.3 regarding Parkinson Disease.

G. Diencephalon

  1. Contains important gray matter structures:

    • Thalamus: Central relay station for incoming sensory impulses (excluding smell), directing them to the appropriate cerebral cortex area for interpretation.

    • Hypothalamus: Main visceral control center, regulating:

      • Heart rate and blood pressure.

      • Body temperature.

      • Water and electrolyte balance.

      • Control of hunger and body weight.

      • Digestive movements and secretions.

      • Regulation of sleep-wake cycles.

      • Control of endocrine functions.

  2. Limbic System:

    • Controls emotional response, involved in emotional experience, expression, and recognition of life-threatening conditions.

    • Interacts with various deep brain structures and is vital in response to emotional stimulus.

H. Brain Stem

  • Composed of three major parts: midbrain, pons, and medulla oblongata.

  • The brain stem is the main pathway for sensory and motor fibers running to and from the cerebrum.

  1. Midbrain:

    • Located between diencephalon and pons; includes corpora quadrigemina (four dome-like protrusions).

    • Contains gray matter areas associated with reflex actions and the reticular formation.

  2. Pons:

    • Bulging portion of the brain stem, serving as a bridge for conduction tracts.

    • Houses the Pontine Respiratory Group, which regulates breathing.

  3. Medulla Oblongata:

    • The inferior portion of the brainstem, blending into the spinal cord.

    • Contains autonomic reflex centers for homeostatic functions, including:

      • Cardiac center (regulates heart rate).

      • Vasomotor center (controls blood pressure).

      • Respiratory center (regulates rhythm of breathing).

      • Additional centers manage involuntary actions such as vomiting and coughing.

  4. Reticular Formation:

    • Controls brain alertness; activity is decreased during sleep and inhibited by substances like alcohol.

    • Governs the phases of sleep including slow-wave sleep and REM sleep (characterized by brain activity akin to wakefulness).

I. Cerebellum

  1. A large structure located dorsally to the pons and medulla, below the occipital lobe (separated by the transverse fissure).

  2. The cerebellum is characterized by a pattern of white matter within gray matter known as