Chapter 42: Nervous System and Drugs

I. Learning Objectives

• Understand the primary functions of nervous systems.

• Compare and contrast nerve nets and bilateral nervous systems in invertebrates with examples.

• Discuss the structure and function of the vertebrate nervous system (CNS and PNS), including divisions of the PNS.

• Describe a simple reflex and the components of a reflex arc.

• Explore withdrawal reflexes, muscle spindle reflexes, and mixed reflexes. Distinguish between ipsilateral and contralateral reflexes.

• Recognize the origin of the forebrain, midbrain, and hindbrain from the neural tube and the adult structures they develop into.

• Identify anatomical regions of the adult human brain and their functions.

• Discuss the structure and functions of the human cerebrum, including surface anatomy and associated homunculus.

• Describe the general structure of the spinal cord and define tracts and columns.

• Name the protective barriers of the CNS.

II. Primary Functions of Nervous Systems

• Receive Information: Collecting data from internal and external stimuli.

• Integrate Information: Processing stimuli to evaluate for possible responses.

• Send Response: Initiating and controlling rapid reactions to stimuli around or within the organism.

III. Invertebrate Nervous Systems

A. Nerve Nets

• Structure: Composed of interconnected neurons lacking a central control.

• Function: Enables movement and response to stimuli in multiple directions.

• Example Animals: Cnidarians, such as jellyfish.

B. Bilateral Nervous Systems

• Structure: Features a central control point (brain or ganglia) with interconnected neurons.

• Function: Allows for integration of signals, directional responses, and localized detection of stimuli.

• Example Animals:

  • Planarians (Phylum Platyhelminthes): Have cerebral ganglia functioning as a primitive brain with nerve cords.

  • Annelida: Exhibit well-developed brains and nerve cords for adaptive responses.

  • Arthropoda: Possess specialized brain regions tailored for various functions.

  • Cephalopoda: Feature complex brains and sensory systems, enhancing learning capabilities.

IV. Vertebrate Nervous System

A. Central Nervous System (CNS)

• Components: Comprises the brain and spinal cord.

• Function: Connects cranial & spinal nerves to peripheral systems.

B. Reflexes

• Components of a Reflex Arc:

  • Withdrawal Reflex: Involves muscle contraction upon stimulation; typically monosynaptic.

  • Muscle Spindle Reflex: Regulates muscle stretch and tension via reflex arcs.

  • Withdrawal Reflex and Cross Extensor Reflex: These utilize excitatory and inhibitory interneurons for balanced movement.

  • Ipsilateral vs. Contralateral Reflexes: Ipsilateral reflexes occur on the same side, while contralateral reflexes involve response across opposite sides.

V. Peripheral Nervous System (PNS)

• Location and Components: Encompasses nerves outside the CNS, categorized into afferent (sensory) and efferent (motor) nerves.

• Divisions:

  • Somatic Division: Governs voluntary actions.

  • Autonomic Division: Manages involuntary functions (further divided into parasympathetic and sympathetic).

VI. Brain and Spinal Cord Development

• Differentiation from Neural Tube: The forebrain, midbrain, and hindbrain evolve during development, forming distinct regions.

VII. Major Brain Regions

A. Anatomical Regions of the Adult Human Brain

1. Cerebellum: Coordination of movement, balance, and motor skills.

2. Medulla Oblongata: Regulates life-sustaining functions such as respiration and heartbeat.

3. Pons: Connects brain centers and assists in respiration regulation.

4. Midbrain: Involves visual and auditory reflexes.

5. Forebrain: Comprises structures like the thalamus (sensory relay) and hypothalamus (visceral control).

6. Cerebrum: Main processing area, characterized by gray matter cortex and white matter connections.

VIII. Structure and Function of the Cerebrum

• Surface Anatomy: Consists of sulci, gyri, fissures, including the central sulcus, pre- and postcentral gyrus.

• Cerebral Lobes: Each lobe handles different cognitive functions, detailed through the associated homunculus.

IX. General Structure of the Spinal Cord

• Tracts and Columns: Composed of gray and white matter; tracts carry impulses to and from the brain.

X. Protection of the CNS

• Barriers: Structures include bone, meninges (dura mater, arachnoid mater, pia mater), cerebrospinal fluid (CSF) for cushioning, and the blood-brain barrier regulating exchange.

XI. Drugs and the Nervous System

A. Definitions and Classifications

• Drug: A substance impacting nervous system transmission and potentially altering psychological states.

• Terms:

  • Receptor Site: Specific areas where drugs act.

  • Agonist/Antagonist: Agonists activate receptors; antagonists inhibit them.

B. Dependence and Addiction

• Dependence: Physiological adjustments lead to withdrawal upon cessation.

• Addiction: Compulsive, damaging use despite negative consequences.

• Tolerance: Diminished response necessitating higher doses for effect.

C. Sources and Examples of Drugs

• Major Sources: Include natural, synthetic compounds, and plants.

• Examples from Plants: Atropine (cold relief), Digitalis (cardiac stimulation), Paregoric (from opium).

D. Drug Mechanisms & Interactions

• Cocaine Mechanism: Diagram and explanation needed regarding how it interacts with neurotransmitter systems.

• Other Effects: Examine actions of strychnine, tetanus, lead, black widow spider venom, curare, and myasthenia gravis.