Central Nervous System, Peripheral Nervous System, Autonomic Nervous System, and Special Senses: Study Guide

Neurotransmitters and Neuronal Organization

Types and Classification of Neurotransmitters

Neurotransmitters are chemical messengers that transmit signals across synapses between neurons or from neurons to muscles. They can be classified based on their chemical structure and function.

• Acetylcholine (ACh): The primary neurotransmitter at neuromuscular junctions; also found in the autonomic nervous system.

• Amino Acids: Includes glutamate (excitatory), GABA (inhibitory), and glycine (inhibitory).

• Biogenic Amines: Includes norepinephrine, epinephrine, dopamine, serotonin, and histamine.

• Neuropeptides: Such as endorphins and substance P.

• Purines: Such as ATP and adenosine.

• Gases and Lipids: Such as nitric oxide (NO) and endocannabinoids.

Classification: Neurotransmitters can be classified as excitatory or inhibitory, depending on their effect on the postsynaptic membrane.

Patterns of Neuronal Organization

Neuronal organization refers to the arrangement of neurons into functional groups or circuits.

• Diverging Circuits: One input, many outputs (e.g., motor pathways).

• Converging Circuits: Many inputs, one output (e.g., sensory pathways).

• Reverberating Circuits: Signal travels through a chain of neurons, each feeding back to previous neurons (e.g., rhythmic activities like breathing).

• Parallel After-Discharge Circuits: Input is transmitted along several pathways to a single output (e.g., complex mental processing).

Chapter 12 – Central Control

Major Anatomy and Functions of the Brain

The brain is divided into several major regions, each with specialized functions:

• Cerebrum: Responsible for higher brain functions such as thought, memory, and voluntary movement.

• Diencephalon: Includes the thalamus (sensory relay), hypothalamus (homeostasis, endocrine control), and epithalamus.

• Brainstem: Includes the midbrain, pons, and medulla oblongata; controls basic life functions (breathing, heart rate).

• Cerebellum: Coordinates movement and balance.

Commissural, Association, and Projection Fibers

• Commissural Fibers: Connect corresponding gray areas of the two hemispheres (e.g., corpus callosum).

• Association Fibers: Connect different parts of the same hemisphere.

• Projection Fibers: Connect the cerebral cortex with lower brain or spinal cord centers.

Limbic System and Reticular Formation

• Limbic System: Involved in emotion, motivation, and memory; includes structures such as the hippocampus and amygdala.

• Reticular Formation: Regulates arousal and consciousness; filters incoming stimuli to discriminate irrelevant background stimuli.

Electroencephalogram (EEG) and Brain Waves

• EEG: A recording of electrical activity along the scalp produced by the firing of neurons within the brain.

• Types of Waves:

  • Alpha Waves: Awake but relaxed.

  • Beta Waves: Awake and alert.

  • Theta Waves: Common in children; may appear in adults during frustration or brain disorders.

  • Delta Waves: Deep sleep or brain injury.

Memory: Stages and Categories

• Short-Term Memory (STM): Temporary holding of information; limited capacity.

• Long-Term Memory (LTM): Limitless capacity; information stored for extended periods.

• Categories: Declarative (facts) and nondeclarative (skills, habits).

Protection of the CNS

• Meninges: Three connective tissue membranes (dura mater, arachnoid mater, pia mater) that cover and protect the CNS.

• Cerebrospinal Fluid (CSF): Cushions the brain and spinal cord; provides nutrients and removes waste.

• Blood-Brain Barrier: Selective barrier that protects the brain from harmful substances in the blood.

Formation and Pathway of Cerebrospinal Fluid

• Formation: Produced by the choroid plexuses in the ventricles of the brain.

• Pathway: Lateral ventricles → third ventricle → cerebral aqueduct → fourth ventricle → subarachnoid space → absorbed into venous blood via arachnoid villi.

Structure of the Spinal Cord

• Gross Structure: Extends from the foramen magnum to L1 or L2; consists of cervical and lumbar enlargements, conus medullaris, cauda equina, and filum terminale.

• Microscopic Structure: Central gray matter (butterfly-shaped) surrounded by white matter; contains dorsal (sensory) and ventral (motor) horns.

Chapter 13 – You're Getting on My Nerves!

General Structure of a Nerve

• Nerve: A bundle of axons in the peripheral nervous system (PNS).

• Connective Tissue Coverings: Endoneurium (around each axon), perineurium (around fascicles), epineurium (around the entire nerve).

Nerve Regeneration

• Process: Damaged axons in the PNS can regenerate if the cell body is intact; Schwann cells form a regeneration tube to guide axonal regrowth.

• Limitation: CNS axons have limited regeneration due to inhibitory factors and lack of supportive environment.

Cranial Nerves

• There are 12 pairs of cranial nerves, each with specific sensory, motor, or mixed functions.

• Examples:

  • Olfactory (I): Smell

  • Optic (II): Vision

  • Oculomotor (III): Eye movement

  • ... (and so on through Hypoglossal (XII))

Spinal Nerves and Rami

• Spinal Nerve Formation: Formed by the union of dorsal and ventral roots.

• Rami: Each spinal nerve splits into dorsal ramus (innervates back) and ventral ramus (innervates limbs and anterior trunk).

Plexuses

• Plexus: A network of intersecting nerves.

• Major Plexuses: Cervical, brachial, lumbar, and sacral plexuses.

Peripheral Nerves and Muscles Innervated

• Each plexus gives rise to peripheral nerves that innervate specific muscles (e.g., phrenic nerve from cervical plexus innervates the diaphragm).

Reflex Arc Components

• Receptor: Detects stimulus.

• Sensory Neuron: Transmits afferent impulses to CNS.

• Integration Center: Processes information (may be monosynaptic or polysynaptic).

• Motor Neuron: Conducts efferent impulses to effector.

• Effector: Muscle or gland that responds to the impulse.

Chapter 14 – Don’t Think About It!

Somatic vs. Autonomic Nervous Systems

Feature	Somatic	Autonomic
Effectors	Skeletal muscle	Cardiac muscle, smooth muscle, glands
Control	Voluntary	Involuntary
Neurons in Pathway	One	Two (pre- and postganglionic)
Neurotransmitter	Acetylcholine	Acetylcholine, norepinephrine

Parasympathetic vs. Sympathetic Divisions

Feature	Parasympathetic	Sympathetic
Origin	Craniosacral	Thoracolumbar
Function	"Rest and digest"	"Fight or flight"
Effect on Heart Rate	Decreases	Increases
Effect on Digestion	Increases	Decreases

Cholinergic and Adrenergic Fibers and Receptors

• Cholinergic Fibers: Release acetylcholine; include all preganglionic neurons and parasympathetic postganglionic neurons.

• Adrenergic Fibers: Release norepinephrine; most sympathetic postganglionic neurons.

• Receptors:

  • Cholinergic: Nicotinic and muscarinic receptors.

  • Adrenergic: Alpha and beta receptors.

Chapter 15 – Here’s Looking at You!

Structure of the Eye and Accessory Structures

• Fibrous Layer: Sclera (white of the eye) and cornea (transparent front part).

• Vascular Layer: Choroid, ciliary body, and iris.

• Inner Layer: Retina (contains photoreceptors).

• Accessory Structures: Eyelids, eyelashes, conjunctiva, lacrimal apparatus, and extrinsic eye muscles.

Pathway of Light and Focusing

• Light passes through the cornea → aqueous humor → lens → vitreous humor → retina.

• Accommodation: The lens changes shape to focus light for near or distant vision.

Photoreceptors: Structure and Function

• Rods: Sensitive to dim light; provide black-and-white vision.

• Cones: Detect color and provide sharp vision in bright light.

Common Eye Disorders

• Astigmatism: Irregular curvature of the cornea or lens.

• Cataract: Clouding of the lens.

• Glaucoma: Increased intraocular pressure damages the optic nerve.

• Hyperopia (Farsightedness): Image focused behind the retina.

• Myopia (Nearsightedness): Image focused in front of the retina.

• Color Blindness: Deficiency in one or more types of cones.

Structure and Function of the Ear

• External Ear: Auricle and external auditory canal.

• Middle Ear: Tympanic membrane and ossicles (malleus, incus, stapes).

• Inner Ear: Cochlea (hearing), vestibule, and semicircular canals (balance).

Sound Conduction Pathway

• Sound waves → tympanic membrane → ossicles → oval window → cochlear fluid → basilar membrane → hair cells → auditory nerve.

Pitch, Loudness, and Localization of Sound

• Pitch: Determined by the frequency of sound waves; different frequencies stimulate different regions of the basilar membrane.

• Loudness: Determined by the amplitude of sound waves; greater amplitude causes more vigorous vibrations.

• Localization: Determined by comparing the intensity and timing of sounds reaching both ears.