PNS

Peripheral Nervous System Overview

  • Focus on the Peripheral Nervous System (PNS) after discussing the Central Nervous System (CNS).

  • PNS links the CNS to the external environment.

Definition and Scope of the Peripheral Nervous System

  • Includes structures outside the brain and spinal cord:

    • Sensory receptors

    • Peripheral nerves and ganglia

    • Efferent motor endings

  • Importance of PNS discussed using the analogy of sensory deprivation tanks. Participants experience lack of external stimulation which can lead to hallucinations after a while.

Neurological Structure Review

  • Overview of nervous system divisions:

    • CNS (brain and spinal cord)

    • PNS (covered in Chapter 13)

    • Autonomic Nervous System (autonomic functions classified separately in Chapter 14)

  • Survival relies on awareness and interpretation of internal and external stimuli.

Two Key Processes for Sensation

  1. Sensation: Awareness of changes, e.g., pressure or temperature.

  2. Perception: Interpretation of these changes, contributing to behavioral responses.

    • Example: Recognizing discomfort to adjust behavior.

Types of Sensory Input in PNS

  • Types of Sensory Receptors:

    • Exteroceptors: Receive external stimuli (e.g., touch, temperature).

    • Interoceptors: Monitor internal bodily conditions (e.g., visceral organs).

    • Proprioceptors: Inform body movement and position (found in muscles, tendons).

Levels of Neural Integration in Sensory Processing

  • Receptor Level: Types of sensory receptors activate in response to stimuli.

  • Circuit Level: Pathways transmit impulses to higher brain centers.

  • Perceptual Level: In the cerebral cortex, information is processed and integrated, distinguishing between various qualities and features of stimuli.

Sensory Receptor Classification

By Type of Stimulus Detected

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

  • Thermoreceptors: Detect changes in temperature.

  • Photoreceptors: Respond to light energy (found in eyes).

  • Chemoreceptors: Detect chemicals in solution (taste, smell).

  • Nociceptors: Sensitive to painful stimuli (extreme temperature, pressure).

By Location of Stimulus

  • Exteroceptors: React to external stimuli, typically at the body surface.

  • Interoceptors: Monitor internal stimuli from viscera and blood.

  • Proprioceptors: Located in specific spots related to body movement (muscles, tendons).

By Structure

  • Simple Receptors: Most common, associated with general senses (e.g., touch, pain).

  • Complex Receptors: Usually tied to special senses (e.g., vision, hearing).

Detailed Types of Simple Receptors

  • Free Nerve Endings: Non-encapsulated receptors; respond to temperature, pain, and itch. Examples:

    • Cold receptors: Sensitive between 10°C and 40°C.

    • Heat receptors: Sensitive between 32°C and 48°C. Nociceptors activate outside this range.

  • Tactile Discs (Merkel Discs): Light touch receptors.

  • Hair Follicle Receptors: Sense bending of hair.

Complex Receptors: Encapsulated Nerve Endings

  • All mechanoreceptors; examples include:

    • Tactile Corpuscles (Meissner’s Corpuscles): Light touch; located in papillary dermis of hairless skin.

    • Lamellar Corpuscles (Pacinian Corpuscles): Detect deep pressure and vibration.

    • Bulbous Corpuscles (Ruffini’s Endings): Detect sustained deep pressure.

    • Muscle Spindles: Detect muscle stretch; initiate reflexes.

    • Golgi Tendon Organs: Monitor tension in tendons.

    • Joint Kinesthetic Receptors: Monitor stretch in joint tissues.

Cranial Nerves in Peripheral Nervous System

  • 12 pairs of cranial nerves associated with the brain.

  • Most are connected to the brainstem (10 pairs) and two arise from the forebrain.

  • Primarily serve head and neck functions, with the exception of one that extends further.

Cranial Nerves Functional Overview

Each nerve has a Roman numeral, name, function, and testing method.

  • I. Olfactory Nerve: Sensory nerve for smell.

    • Testing: Identify aromatic substances.

    • Damage: Potential anosmia (loss of smell).

  • II. Optic Nerve: Sensory nerve for vision.

    • Testing: Visual acuity using Snellen chart.

    • Damage: May lead to blindness in the affected eye.

  • III. Oculomotor Nerve: Controls four extrinsic eye muscles and pupil constriction.

    • Damage: Impairs eye movement or results in double vision.

  • IV. Trochlear Nerve: Innervates superior oblique muscle (eye mover).

    • Testing: Follow a moving object to assess movement.

  • V. Trigeminal Nerve: Major sensory nerve for the face; has three branches (ophthalmic, maxillary, mandibular).

    • Testing: Sensation checks (touch, pain, temperature) and jaw movement.

  • VI. Abducens Nerve: Controls lateral rectus muscle (eye abduction).

    • Damage: Resulting eye deviation medially at rest.

  • VII. Facial Nerve: Mixed nerve controlling facial expressions; taste from the anterior tongue; parasympathetic fibers to glands.

    • Testing: Facial symmetry and expressions.

  • VIII. Vestibulocochlear Nerve: Mainly sensory; responsible for hearing and balance.

    • Testing: Tuning fork and balance tests.

  • IX. Glossopharyngeal Nerve: Mixed nerve; taste from posterior tongue; swallowing and salivary gland function.

    • Testing: Gag reflex.

  • X. Vagus Nerve: Only cranial nerve that wanders beyond the head and neck; regulates heart, lungs, and abdominal viscera.

    • Damage affects autonomic nervous functions.

  • XI. Accessory Nerve: Primarily motor; controls neck and shoulder muscles.

    • Damage affects head turns and shoulder elevations.

  • XII. Hypoglossal Nerve: Controls tongue movement.

    • Damage affects speech and swallowing.

Reflex Mechanisms

  • Types of reflexes:

    1. Inborn (Intrinsic) Reflexes: Predetermined, involuntary responses (e.g., knee jerk).

    2. Learned Reflexes: From repetition, can be called conditioned responses.

  • Reflexes classified as:

    • Somatic Reflexes: Involve skeletal muscles.

    • Autonomic Reflexes: Involve smooth or cardiac muscle or glands.

  • Importance of reflex testing for diagnosing nervous system conditions.

Stretch and Tendon Reflexes

  • Stretch Reflex: Initiated by muscle stretch; regulated by muscle spindles.

    • Helps maintain muscle tone and posture.

  • Tendon Reflex: Involve Golgi tendon organs to maintain tendon tension and prevent injury.

    • Patellar Reflex: Illustrates stretch reflex mechanics—monosynaptic and ipsilateral response involving reciprocal inhibition of antagonistic muscles.

Flexor and Crossed Extensor Reflexes

  • Flexor Reflex (Withdrawal Reflex): Protective reflex; initiated by painful stimulus.

    • Ipsilateral and polysynaptic.

    • Can be overridden by brain input.

  • Crossed Extensor Reflex: Accompanies flexor reflex to maintain balance; involves contralateral response.

Superficial Reflexes

  • Initiated by gentle skin stimulation:

    • Plantar Reflex: Tests spinal cord integrity (L4-S2); normal response is toe flexion.

    • Abdominal Reflex: Tests spinal cord integrity from T8-T12.

Introduction to Special Senses

  • Special senses include vision, hearing, and equilibrium.

Vision Overview

  • Dominant human sense; 70% of all sensory receptors in the eyes.

  • Visual processing occupies significant brain cortex (nearly 50%).

  • Eye structure: hollow sphere made from three layers, functioning to focus light.

Eye Structural Layers

  • Fibrous Layer: Outer layer with sclera (white part) and cornea.

  • Vascular Layer: Middle layer with choroid, ciliary body, and iris, responsible for nourishing and regulating light.

  • Inner Layer (Retina): Contains photoreceptors (rods and cones), which convert light into signals.

Photoreceptors

  • Rods: For dim light; peripheral vision, greater sensitivity; no color detection.

  • Cones: For bright light; detect color; three types corresponding to RGB color vision.

  • Macula lutea: Region for sharp vision; fovea centralis contains only cones.

Importance of Aqueous Humor

  • Filled with nutrients for lens and cornea; crucial in maintaining intraocular pressure (related to glaucoma if drainage is blocked).

Lens Functionality

  • Changes shape for focusing: thicker for near vision and flatter for far vision.

    • Myopia and Hyperopia explained with corrective measures for vision impairment.

Taste and Smell Connection

  • Taste is mainly a combination of five sensed modalities:

    • Sweet, sour, salty, bitter, umami (the most recent discovery).

  • Smell involves olfactory receptors sensitive to airborne chemicals.

Hearing and Equilibrium Mechanics

  • Ear structure divided into external, middle, and internal areas.

  • Sound waves cause tympanic membrane vibrations leading to auditory ossicles.

  • Internal ear contains both structures for hearing and balance (cochlea and vestibular apparatus).

  • Workplace of sound waves amplification and neural signal processing.

Types of Auditory Waves

  • Frequency and amplitude impact sound perception:

    • Frequency correlates to pitch (Hertz - Hz).

    • Amplitude relates to loudness (decibels - dB).

Equilibrium and Balance Mechanisms

  • Responds to head movements; relies on visual input and stretch receptors.

  • Semicircular canals monitor dynamic equilibrium while the macula detects static position.

  • Nystagmus: eye movement adjusting for rotational motion;

Conclusion

  • PNS plays critical roles in sensory intake and reflex mechanisms essential for survival.

  • Understanding the connections and processes offers insight into human physiology related to perception and reaction to stimuli through diverse techniques and structures in the body.