Chapter 13: The Peripheral Nervous System and Reflex Activity – Sensory Receptors, Cranial and Spinal Nerves, and Reflexes

Major Organ Systems of the Body

Overview

• The human body is organized into major organ systems, each with specialized functions.

• This chapter focuses on the nervous system, particularly the peripheral nervous system (PNS), sensory receptors, cranial and spinal nerves, and reflexes.

Functional Overview of the Nervous System

Central and Peripheral Nervous Systems

• Central Nervous System (CNS): Consists of the brain and spinal cord; integrates, processes, and coordinates sensory data and motor commands.

• Peripheral Nervous System (PNS): All neural tissue outside the CNS; delivers sensory information to the CNS and carries motor commands to peripheral tissues.

Divisions of the PNS

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

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

• Somatic Nervous System (SNS): Controls voluntary skeletal muscle contractions.

• Autonomic Nervous System (ANS): Regulates involuntary functions (smooth muscle, cardiac muscle, glands, adipose tissue).

Sensory Receptors – Function and Classification

Interpretation of Sensory Information

• Sensation: The conscious or subconscious awareness of changes in the external or internal environment.

• Perception: The conscious interpretation of sensations, mainly by the cerebral cortex. Only about 1% of sensory input reaches conscious awareness.

Pathway of Sensory Information

• Stimulation of a receptor → Transduction (conversion to electrical signal) → Generation of nerve impulses → Integration and projection in the CNS.

Sensory Receptors

• Specialized cells or nerve endings that monitor specific conditions in the body or external environment.

• Connect internal and external environments with the nervous system.

• When stimulated, transmit information to the CNS as action potentials along sensory neurons.

Receptor Adaptation

• Adaptation: Decreased sensitivity to a continuous stimulus.

• Phasic receptors: Rapidly adapting; active only briefly (e.g., touch, vibration, smell).

• Tonic receptors: Slowly adapting; always active (e.g., nociceptors for pain, proprioceptors for body position).

Sensory Receptor Classification

• By Distribution:

  • General: Somatic (skin, muscle, joints); Visceral (walls of viscera, blood vessels)

  • Special: Smell, taste, vision, hearing, equilibrium

• By Location (Origin of Stimulus):

  • Exteroceptors: External environment

  • Interoceptors: Within the body

  • Proprioceptors: Body movements

• By Modality (Type of Stimulus):

  • Chemoreceptors: Chemicals, pH

  • Thermoreceptors: Temperature

  • Photoreceptors: Light, color

  • Mechanoreceptors: Touch, pressure, vibration

  • Nociceptors: Pain

General and Special Senses

General Senses

• Simple in structure, widely distributed (skin and internal organs).

• Include: Temperature, Pain, Touch, Pressure, Vibration, Proprioception.

Special Senses

• Complex in structure, located in sense organs in the head.

• Include: Olfaction (smell), Gustation (taste), Vision (sight), Hearing (auditory), Equilibrium (balance).

Receptors for Somatic Sensations

• Tactile Receptors: Detect touch, pressure, vibration.

• Thermoreceptors: Detect temperature changes.

• Pain Receptors (Nociceptors): Detect tissue damage.

• Proprioceptors: Detect body position and movement.

Tactile Receptors – Mechanoreceptors

• Most abundant sensory receptors.

• Located in skin and mucous membranes.

• Endings may be encapsulated or unencapsulated.

Nociceptors (Pain Receptors)

• Free nerve endings (branching dendrites).

• Two types of pain:

  • Fast pain: Acute, sharp, pricking; perceived within 1 second.

  • Slow pain: Chronic, burning, aching, throbbing; perceived after 1 second or more.

• Superficial somatic pain: From skin receptors.

• Deep somatic pain: From skeletal muscles, joints, tendons, fascia.

Referred Pain

• Pain perceived in a region of the body that is not the source of the stimulus.

• Organ pain is often referred to the skin (e.g., heart attack pain felt in the left arm).

• Occurs due to convergence of somatic and visceral sensory neurons in the spinal cord.

Common Sites of Referred Pain

• Heart: pectoral region, medial arm

• Kidney/ureter: lower abdomen

• Gallbladder: right shoulder

• Appendix: right lower quadrant

• Bladder: perineum, buttocks

Proprioceptors – Mechanoreceptors

• Located in muscles and tendons; recognize position of body parts.

• Muscle spindles: Monitor muscle length; involved in stretch reflexes.

• Tendon organs: Located at junction of tendon and muscle; protect from overstretching.

• Joint kinesthetic receptors: In and around synovial joints; respond to pressure, acceleration, deceleration.

• No proprioceptors in visceral organs of thoracic and abdominopelvic cavities.

Cranial Nerves

Overview

• Part of the PNS; connect to the brain.

• 12 pairs (I to XII, Roman numerals).

• 1 pair connects to cerebrum; 11 pairs to diencephalon or brainstem.

• All serve head and neck except the vagus nerve (extends into thoracic and abdominal cavities).

Classifications of Cranial Nerves

• Primary sensory nerves: Touch, pressure, vibration, temperature, pain.

• Special sensory nerves: Smell, sight, hearing, balance.

• Motor nerves: Somatic motor neuron axons.

• Mixed nerves: Both motor and sensory fibers.

List of Cranial Nerves and Functions

Number	Name	Type	Main Function	Innervation
I	Olfactory	Special sensory	Smell	Olfactory epithelium
II	Optic	Special sensory	Vision	Retina of eye
III	Oculomotor	Motor	Eye movement, pupil constriction	4/6 extrinsic eye muscles, levator palpebrae superioris, intrinsic eye muscles
IV	Trochlear	Motor	Eye movement (down, to side)	Superior oblique muscle
V	Trigeminal	Mixed	Sensations from face, chewing	Ophthalmic, maxillary, mandibular branches
VI	Abducens	Motor	Lateral eye movement	Lateral rectus muscle
VII	Facial	Mixed	Taste, facial expression	Taste receptors on tongue, facial muscles
VIII	Vestibulocochlear	Sensory	Hearing, equilibrium	Cochlea, vestibule
IX	Glossopharyngeal	Mixed	Taste, swallowing, BP regulation	Tongue, pharynx, palate, BP/pH/CO2/O2 receptors
X	Vagus	Mixed	Visceral sensory/motor, taste, heart rate, GI tract	Pharynx, diaphragm, thoracic/abdominal viscera
XI	Accessory	Motor	Swallowing, head/neck/shoulder movement	Pharynx, palate, larynx (with vagus)
XII	Hypoglossal	Motor	Tongue movement	Tongue muscles

Spinal Nerves and Plexuses

Overview

• 31 pairs of spinal nerves connect the spinal cord to the periphery (muscles, skin, organs).

• Each nerve is surrounded by three connective tissue layers and contains blood vessels.

• Most spinal nerves exit through the intervertebral foramen.

• Spinal nerves are mixed nerves (carry both sensory and motor fibers).

Roots and Branches of Spinal Nerves

• Ventral (anterior) root: Motor neurons; carry commands from CNS to effectors.

• Dorsal (posterior) root: Sensory neurons; bring information to CNS.

• Dorsal root ganglion: Contains cell bodies of sensory neurons.

• Ventral and dorsal roots merge to form a spinal nerve.

• Branches (rami) distribute sensory and motor information to the body.

Dermatomes

• Specific bilateral regions of skin monitored by a single pair of spinal nerves.

• Clinically important for diagnosing nerve or dorsal root damage.

Spinal Nerve Plexuses

• Ventral rami (except thoracic nerves) form networks called plexuses before innervating target regions.

• Four major plexuses:

  1. Cervical plexus (neck, diaphragm; major nerve: phrenic nerve)

  2. Brachial plexus (shoulder, arm, hand; major nerves: radial, ulnar, median)

  3. Lumbar plexus (abdominal wall, thigh; major nerves: femoral, obturator)

  4. Sacral plexus (pelvis, leg; major nerves: sciatic, pudendal)

Reflexes and Information Processing

Reflexes

• Rapid, automatic responses to specific stimuli; help maintain homeostasis.

• Spinal reflexes: Occur in the spinal cord (e.g., flexor reflex).

• Cranial reflexes: Occur in the brain; often involve special senses.

Reflex Arc Components

• Sensory receptor → Sensory neuron → Integration center → Motor neuron → Effector

Types of Reflex Arcs

• Ipsilateral: Stimulus and response on the same side of the body.

• Contralateral: Stimulus and response on opposite sides.

Examples of Reflexes

• Stretch reflex (monosynaptic): Involves muscle spindles; e.g., patellar reflex.

• Crossed extensor reflex (polysynaptic): Involves withdrawal of injured limb and extension of opposite limb for balance (e.g., stepping on a tack).

Clinical Reflexes

• Plantar reflex: Normal response is toe flexion (negative Babinski sign).

• Babinski sign: Toe extension; normal in infants, pathological in adults (indicates CNS damage).

Additional info: This summary expands on the provided lecture slides and notes with definitions, examples, and clinical relevance for exam preparation.