Chapter 14: The Somatic Nervous System

Somatic Nervous System Overview

• Definition: The somatic nervous system (SNS) allows voluntary movement and controls skeletal muscles throughout the body.
• Functionality: It transmits sensory information from special sensory organs to the central nervous system (CNS).

Sensory Receptors

• Role: Sensory receptors help us perceive the environment and our internal states.
• Process:
• Stimuli are received and converted into electrochemical signals.
• Change in cell membrane potential leads to an action potential in sensory neurons.
• Signals are relayed to the CNS for integration and conscious perception.
• Special Senses: Taste, smell, hearing, vision will be analyzed in this chapter.

Special Senses

Taste (Gustation)

• Chemical Stimuli: The sense of taste responds to chemicals dissolved in food.
• Five Recognized Tastes:
• Sweet
• Salty
• Sour
• Bitter
• Umami (savory)
• Anatomy of Taste:
• Papillae: Raised bumps on the tongue containing taste buds.
• Gustatory Receptor Cells: Located within taste buds; have microvilli (taste hairs) that project into taste pores.
• Transmission: Action potentials from gustatory cells activate sensory neurons in cranial nerves (facial, glossopharyngeal, vagus) to relay taste information to the brain.

Smell (Olfaction)

• Chemical Stimuli: Olfaction also responds to airborne odorant molecules.
• Anatomical Structures:
• Olfactory Receptor Cells: Located in the olfactory epithelium in the superior nasal cavity.
• Odorant molecules dissolve in the mucus, bind to olfactory receptors, and generate action potentials.
• Pathway:
• Axons extend through the cribriform plate into the brain.
• Signal travels to the primary olfactory cortex and limbic system for emotional memory association.

Hearing (Audition)

• Sound Waves: Transduction of sound waves into neural signals.
• Ear Anatomy:
• External Ear: Auricle directs sound to the auditory canal, ending at the tympanic membrane (eardrum).
• Middle Ear: Contains three auditory ossicles (malleus, incus, stapes) connected to the Eustachian tube.
• Inner Ear: Houses the cochlea (hearing) and vestibule (balance).

Cochlea Function

• Oval Window: First point of fluid wave initiation after ossicle vibrations.
• Scala Vestibuli and Scala Tympani: Fluid-filled tubes causing movement of the basilar membrane, triggering hair cells in the organ of Corti to generate neural signals based on sound frequency.
• Sound Frequency Encoding: Hair cells respond based on sound frequency; cochlea processes frequencies from 20 to 20,000 Hz.

Equilibrium and Balance

• Vestibular System: Senses head position and acceleration through the utricle and saccule.
• Utricle: Detects horizontal acceleration.
• Saccule: Detects gravity and linear acceleration.
• Maculae: Contain hair cells surrounded by otolithic membranes that move in response to head movements, sending signals to the brain.

Vision

• Transduction of Light: Vision involves the conversion of light stimuli into neural signals.
• Eye Structures:
• Tunics of the Eye:
  • Fibrous Tunic: Includes sclera (white part) and cornea (clear part).
  • Vascular Tunic: Comprises choroid, ciliary body, and iris; responsible for blood supply and light adjustment (accommodation).
  • Neural Tunic (Retina): Contains photoreceptors (rods and cones) for light absorption and processing.
• Photoreceptors:
• Rods: Sensitive to low levels of light (grayscale vision).
• Cones: Enable color vision, sensitive to specific light wavelengths (red, green, blue).

Visual Pathway

• Light signals from photoreceptors are transmitted to the brain via bipolar and ganglion cells, then through the optic nerve to the primary visual cortex for processing.
• Optic Chiasma: Each cerebral hemisphere processes information from different retina halves (lateral/medial).

Conclusion

• Integration: The somatic nervous system coordinates sensory information (special and general senses) and motor responses across various systems (taste, smell, hearing, vision).
• Importance: Understanding these systems is crucial for grasping how we interact with the world and respond to stimuli.