Peripheral Nervous System (PNS)

Divisions of the PNS

The Peripheral Nervous System (PNS) connects the central nervous system (CNS) to limbs and organs. It is divided into afferent (sensory) and efferent (motor) pathways.

• Afferent Pathways: Carry sensory information from receptors to the CNS.

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

Nerve Structure & Organization

Nerves are bundles of axons in the PNS, organized into fascicles and surrounded by connective tissue wrappings.

• Fascicles: Bundles of nerve fibers (axons) within a nerve.

• Connective Tissue Wrappings:

  • Endoneurium: Surrounds individual axons.

  • Perineurium: Surrounds each fascicle.

  • Epineurium: Encloses the entire nerve.

Cranial Nerves

There are 12 pairs of cranial nerves, each with a specific number, name, and function. They may be sensory, motor, or mixed.

• Example: CN II (Optic Nerve) – Sensory, vision.

• Example: CN VIII (Vestibulocochlear Nerve) – Sensory, hearing and balance.

• Example: CN X (Vagus Nerve) – Mixed, controls heart, lungs, digestive tract.

Structure of Spinal Nerves

• Ventral Root: Contains motor (efferent) fibers.

• Dorsal Root: Contains sensory (afferent) fibers.

• Dorsal Root Ganglia: Contains cell bodies of sensory neurons.

• Plexus: Network of intersecting nerves.

Major Nerve Plexuses

• Cervical Plexus: Major nerve: Phrenic nerve (innervates diaphragm).

• Brachial Plexus: Major nerves: Axillary, Musculocutaneous, Median, Ulnar, Radial (innervate upper limb muscles and skin).

• Lumbar Plexus: Major nerves: Femoral, Obturator (innervate anterior and medial thigh).

• Sacral Plexus: Major nerve: Sciatic nerve (innervates posterior thigh, leg, and foot).

Dermatome

A dermatome is an area of skin supplied by a single spinal nerve. Dermatomes are important in diagnosing nerve injuries.

Spinal Reflexes

Reflexes are rapid, automatic responses to stimuli. The basic components of a reflex arc are:

1. Receptor

2. Sensory neuron

3. Integration center

4. Motor neuron

5. Effector

Types of spinal reflexes include:

• Stretch Reflex: e.g., knee-jerk reflex.

• Withdrawal (Flexor) Reflex: Withdrawal from painful stimulus.

• Crossed-Extensor Reflex: Maintains balance during withdrawal.

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

Sensory Receptors and Senses

• General Senses: Touch, pain, temperature, pressure, proprioception.

• Special Senses: Vision, hearing, equilibrium, taste, smell.

• Adaptation: Decreased sensitivity to a constant stimulus.

• Types of Sensory Receptors (by stimulus):

  • Mechanoreceptors: Respond to mechanical force (touch, pressure, vibration).

  • Thermoreceptors: Respond to temperature changes.

  • Photoreceptors: Respond to light (in the eye).

  • Chemoreceptors: Respond to chemicals (taste, smell, blood chemistry).

  • Nociceptors: Respond to pain.

• Which type do not adapt? Nociceptors (pain receptors) do not adapt.

Special Senses: Eye and Ear

• Eye:

  • Layers: Fibrous (sclera, cornea), Vascular (choroid, ciliary body, iris), Neural (retina).

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

  • Fovea Centralis: Area of sharpest vision.

  • Optic Disk: Blind spot, no photoreceptors.

  • Pupillary Constriction: Decreases pupil size (bright light).

  • Pupillary Dilation: Increases pupil size (dim light).

  • Accommodation: Lens changes shape to focus.

  • Refraction: Bending of light as it passes through the eye.

  • Myopia: Nearsightedness.

  • Hyperopia: Farsightedness.

  • CN II: Optic nerve, transmits visual information.

• Ear:

  • Regions: Outer ear (collects sound), Middle ear (transmits vibrations), Inner ear (hearing and balance).

  • Inner Ear Structures:

    • Cochlea: Hearing.

    • Semicircular Canals: Rotational equilibrium.

    • Saccule and Utricle: Linear equilibrium.

  • CN VIII: Vestibulocochlear nerve, transmits hearing and balance information.

Autonomic Nervous System (ANS)

Overview and Divisions

The ANS is a part of the PNS that regulates involuntary functions. It has two main divisions:

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

• Parasympathetic Division: Promotes 'rest and digest' activities.

Shared Characteristics

• Both divisions use a two-neuron relay (preganglionic and postganglionic neurons).

• Effectors regulated: smooth muscle, cardiac muscle, glands.

Differences Between Divisions

• Preganglionic Fibers:

  • Sympathetic: Short

  • Parasympathetic: Long

• Postganglionic Fibers:

  • Sympathetic: Long

  • Parasympathetic: Short

• Location of Ganglia:

  • Sympathetic: Near spinal cord (sympathetic chain)

  • Parasympathetic: Near or within target organs

• Origin in CNS:

  • Sympathetic: Thoracolumbar region

  • Parasympathetic: Craniosacral region

• Neurotransmitters:

  • Preganglionic: Both divisions use acetylcholine (ACh)

  • Postganglionic: Sympathetic uses norepinephrine (NE), parasympathetic uses ACh

Receptors

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

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

Effects and Regulation

• Dual Innervation: Most organs receive input from both divisions, often with opposite effects.

• Sympathetic Tone: Baseline sympathetic activity, maintains blood pressure.

• Parasympathetic Tone: Baseline parasympathetic activity, maintains normal digestive and urinary function.

Visceral Reflexes

• Similar elements as somatic reflexes, but involve two motor neurons.

• Long Reflexes: Involve CNS and two motor neurons.

• Short Reflexes: Bypass CNS, occur in autonomic ganglia.

Higher Brain Center Control

The hypothalamus and other brain regions can influence ANS activity, integrating emotional and physiological responses.

Endocrine System

Basic Functions and Comparison to Nervous System

The endocrine system regulates body activities by releasing hormones into the bloodstream. It acts more slowly but has longer-lasting effects than the nervous system.

Types of Chemical Messengers

• Autocrines: Act on the same cell that secretes them.

• Paracrines: Affect nearby cells.

• Hormones: Travel through blood to distant targets.

• Neurotransmitters: Released by neurons, act across synapses.

Hormone Structure and Mechanisms

• Hormone structure (amino acid-based vs. steroid) determines solubility, transport, and receptor location.

• Two Major Mechanisms:

  • Second Messenger Systems: Used by most amino acid-based hormones; hormone binds to receptor on cell membrane, activating a second messenger (e.g., cAMP).

  • Direct Gene Activation: Used by steroid hormones; hormone enters cell and binds to intracellular receptor, directly affecting gene transcription.

Possible Effects of Hormone-Receptor Binding

• Alter membrane permeability

• Stimulate synthesis of proteins or enzymes

• Activate or deactivate enzymes

• Induce secretory activity

• Stimulate mitosis

Second Messengers

• Examples: cAMP, IP3, DAG, Ca2+

• Amino acid-based hormones can be effective at low concentrations due to signal amplification by second messengers.

• Steroid hormones require higher concentrations as they act directly on DNA.

Regulation of Hormone Secretion

• Humoral Stimuli: Changes in blood levels of ions/nutrients (e.g., insulin release in response to blood glucose).

• Neural Stimuli: Nerve fibers stimulate hormone release (e.g., adrenal medulla).

• Hormonal Stimuli: Hormones stimulate other endocrine glands (e.g., pituitary hormones).

Up-Regulation and Down-Regulation

• Up-Regulation: Target cells form more receptors in response to hormone.

• Down-Regulation: Target cells lose receptors in response to high hormone levels.

Endocrine Glands and Hormones

Major endocrine glands include the pituitary, thyroid, parathyroid, adrenal, pancreas, and gonads. Each secretes specific hormones with distinct functions.

Hypophyseal Portal System

A network of blood vessels connecting the hypothalamus to the anterior pituitary, allowing hormones to regulate pituitary function.

Tropic Hormones

Hormones that regulate the activity of other endocrine glands (e.g., TSH, ACTH, FSH, LH).

Disorders of Hormone Regulation

• Conditions can result from hypo- or hypersecretion of hormones (e.g., diabetes mellitus, gigantism, hypothyroidism).

Example: Pancreas and Blood Glucose Regulation

• Insulin: Secreted by beta cells; increases glucose uptake by cells, lowering blood glucose.

• Target Tissues: Liver, skeletal muscle, adipose tissue.

Additional info: For a more comprehensive review, students should consult their textbook and lecture notes for detailed mechanisms, clinical correlations, and additional examples.