Chapter 13 – The Peripheral Nervous System

Divisions of the Peripheral Nervous System (PNS)

The PNS consists of all neural structures outside the brain and spinal cord. It is divided into afferent (sensory) and efferent (motor) pathways.

• Afferent Division: Transmits sensory information from receptors to the CNS.

• Efferent Division: Carries motor commands from the CNS to effectors (muscles and glands).

Nerve Structure & Organization

Nerves are organized into bundles of axons surrounded by connective tissue wrappings.

• Connective Tissue Layers:

  • Endoneurium: Surrounds individual axons.

  • Perineurium: Encloses bundles of axons (fascicles).

  • Epineurium: Encloses the entire nerve.

Cranial Nerves

There are 12 pairs of cranial nerves, each with specific functions.

• Classification: Sensory, motor, or mixed.

• Examples:

  • Olfactory nerve (I): Sensory (smell)

  • Optic nerve (II): Sensory (vision)

  • Oculomotor nerve (III): Motor (eye movement)

Structure of Spinal Nerves

Spinal nerves connect the CNS to the rest of the body and are formed by the union of dorsal and ventral roots.

• Components: Ventral root (motor), dorsal root (sensory), dorsal root ganglia (sensory neuron cell bodies).

Plexuses of Spinal Nerves

Plexuses are networks of intersecting nerves that serve specific regions.

• Cervical Plexus: Serves neck and diaphragm.

• Brachial Plexus: Serves shoulder and upper limb.

• Lumbar Plexus: Serves lower limb and pelvis.

• Sacral Plexus: Serves lower limb and pelvic organs.

Dermatomes

A dermatome is an area of skin supplied by a single spinal nerve.

• Clinical Application: Used to diagnose nerve injuries.

Spinal Reflexes

Reflexes are rapid, involuntary responses to stimuli.

• Types: Crossed-extensor, withdrawal (flexor), stretch reflex.

• Components: Receptor, sensory neuron, integration center, motor neuron, effector.

• Reciprocal Inhibition: When one muscle contracts, its antagonist is inhibited.

General vs. Special Senses

Sensory systems are classified as general (touch, pain, temperature) or special (vision, hearing, taste, smell, equilibrium).

• General Senses: Widely distributed throughout the body.

• Special Senses: Localized in specific organs.

Types of Sensory Receptors

Sensory receptors are classified by the type of stimulus they detect.

• Mechanoreceptors: Detect mechanical forces (touch, pressure).

• Thermoreceptors: Detect temperature changes.

• Photoreceptors: Detect light (in the eye).

• Adaptation: Some receptors adapt quickly (e.g., touch), others slowly (e.g., pain).

Special Senses: Eye and Ear

The eye and ear are specialized organs for vision and hearing/equilibrium.

• Eye:

  • Layers: Fibrous (sclera, cornea), vascular (choroid, ciliary body, iris), neural (retina).

  • Photoreceptors: Rods (dim light), cones (color vision).

  • Pupillary reflexes, accommodation, refraction, myopia, hyperopia.

• Ear:

  • Regions: Outer ear, middle ear, inner ear.

  • Inner ear structures: Semicircular canals, saccule, utricle, cochlea.

  • Functions: Hearing (cochlea), equilibrium (vestibular apparatus).

Chapter 14 – The Autonomic Nervous System (ANS)

Overview of the ANS

The ANS regulates involuntary functions such as heart rate, digestion, and respiratory rate. It is part of the PNS.

• Divisions: Sympathetic and Parasympathetic.

Sympathetic vs. Parasympathetic Divisions

These divisions have opposing effects on target organs.

• Sympathetic: Prepares body for 'fight or flight' responses.

• Parasympathetic: Promotes 'rest and digest' activities.

• Shared Characteristics: Both use a two-neuron relay (preganglionic and postganglionic neurons).

• Differences:

  • Structure of neurons (length of fibers).

  • Location of ganglia (sympathetic: near spinal cord; parasympathetic: near or in target organs).

  • Neurotransmitters: Sympathetic (norepinephrine, some acetylcholine); Parasympathetic (acetylcholine).

Receptors in the ANS

Receptors determine the response to neurotransmitters.

• Adrenergic Receptors: Bind norepinephrine and epinephrine (alpha and beta types).

• Cholinergic Receptors: Bind acetylcholine (nicotinic and muscarinic types).

• Effects: Vary depending on receptor type and tissue.

Dual Innervation and Tone

Most organs receive input from both divisions, allowing precise control.

• Dual Innervation: Both sympathetic and parasympathetic fibers innervate many organs.

• Sympathetic Tone: Maintains baseline activity in blood vessels.

• Parasympathetic Tone: Maintains baseline activity in digestive and urinary tracts.

Visceral Reflexes

Visceral reflexes control involuntary functions and can be short or long.

• Short Reflexes: Bypass CNS, involve local circuits.

• Long Reflexes: Involve CNS integration.

• Comparison: Somatic reflexes involve one motor neuron; visceral reflexes may involve two.

Higher Brain Center Control

The hypothalamus and other brain centers modulate ANS activity.

Chapter 15 – The Endocrine System

Basic Functions and Comparison to Nervous System

The endocrine system uses hormones to regulate body functions over longer periods compared to the rapid signaling of the nervous system.

• Types of Messengers: Autocrines, paracrines, hormones, neurotransmitters.

• Factors Determined by Hormone Structure: Solubility, transport, receptor location.

Hormone Action at Targets

Hormones bind to specific receptors to exert their effects.

• Mechanisms:

  • Second Messengers: e.g., cAMP, IP3, Ca2+.

  • Hormone-Receptor Binding: Specificity and possible effects.

• Up-Regulation: Increase in receptor number in response to low hormone levels.

• Down-Regulation: Decrease in receptor number in response to high hormone levels.

Regulation of Hormone Secretion

Hormone secretion is regulated by various mechanisms.

• Negative Feedback: Most common regulatory mechanism.

• Examples: Blood glucose regulation by insulin.

Example Table: Hormone Regulation

Hypophyseal Portal System

This system connects the hypothalamus to the anterior pituitary, allowing for direct hormone transport.

Tropic Hormones

Tropic hormones stimulate other endocrine glands to release their hormones.

• Examples: Thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH).

Endocrine Disorders

Improper regulation or secretion of hormones can lead to various disorders.

• Examples: Diabetes mellitus, hypothyroidism, Cushing's syndrome.

Key Equations

• Blood Glucose Regulation:

• Negative Feedback Example:

Additional info: Some details and table entries were inferred for completeness and clarity.