The Somatic Nervous System - Study Notes

The Somatic Nervous System

Chapter Overview

• Author: Betts et al. (2022).

• Source: Anatomy and Physiology 2e, OpenStax

Chapter Objectives

• Describe the components of the somatic nervous system.

• Name the modalities and submodalities of the sensory systems.

• Distinguish between general and special senses.

• Describe regions of the central nervous system that contribute to somatic functions.

• Explain the stimulus-response motor pathway.

Sensory Perception (14.1)

• Defined as a component of the somatic nervous system.

• Responsible for:

  • Conscious perception of the environment.

  • Voluntary responses to perceptions using skeletal muscles.

• Sensory receptors:

  • Detect environmental stimuli.

  • Relay information through sensory neurons to the brain for processing.

Receptor Classification By Structure

• Free nerve endings (dendrites):

  • Components: Axon, Dendrite, Cell body.

  • Function: Pain, temperature, mechanical deformation.

• Encapsulated nerve endings.

• Specialized receptor cells (e.g., Rods).

Receptor Classification By Location of Stimulus

• Exteroceptor:

  • Receives external stimuli from the environment.

• Interoceptor:

  • Receives internal stimuli from visceral organs.

• Proprioceptor:

  • Receives stimuli regarding body position and movement.

Receptor Classification By Function

• Chemoreceptor: Responds to chemical stimuli.

• Nociceptor: Detects pain.

• Mechanoreceptor: Responds to physical stimuli (pressure, vibration, body position).

• Thermoreceptor: Sensitive to temperature.

Gustation (Sense of Taste)

• Associated with the tongue: Provides information about foods and liquids consumed.

• Basic Taste Sensations: Sweet, salty, sour, bitter, umami, and fats/lipids.

• Taste Buds:

  • Located in the papillae of the tongue.

  • Composed of chemoreceptors.

• Chemicals must be dissolved in saliva to taste.

• Ageusia: Loss of the sense of smell, with a noted decrease in taste sensitivity with age.

Structure of the Tongue

• Lingual Papillae:

  • Types: Circumvallate, Fungiform, Filiform, Foliate.

• Components of Taste Buds:

  • Taste hairs, Taste pores, Basal cells, Transitional cells, Gustatory cells.

Gustatory Pathway

• Involves the following structures:

  • Facial Nerve (VII), Glossopharyngeal Nerve (IX), Vagus Nerve (X).

  • Pathway:

  • Taste information travels from taste buds to the Solitary nucleus in the medulla oblongata.

  • Ascends to the Thalamic nucleus (ventral posteromedial) and then to the Gustatory cortex in the insula.

Olfaction (Sense of Smell)

• Anatomy:

  • Components include Olfactory bulb, Olfactory tract, Olfactory epithelium, Nasal conchae.

• Path of Inhaled Air: Contains odorant molecules that interact with olfactory receptor neurons.

• Olfactory Pathway:

  • From olfactory tracts to olfactory cortex located in the temporal lobe of the cerebrum.

• Anosmia: Loss of the sense of smell.

Audition (Sense of Hearing)

• Anatomy of the Ear:

  • Composed of three parts: External ear, Middle ear, Inner ear.

  • Key components include the auricle (pinna), ear canal, tympanic membrane, ossicles (malleus, incus, stapes), cochlea, and the vestibular apparatus.

Auditory Pathway

• Sound Wave Transmission:

  • Sound waves cause the tympanic membrane to vibrate.

  • Vibrations are amplified across the ossicles and set up standing waves in the fluid of the cochlea.

• Frequency of Sound Waves:

  • Measured in Hz (cycles per second).

• Organ of Corti: Contains hair cells that transform vibrations into neural signals.

Equilibrium

• Static Equilibrium:

  • Involves maculae, which sense linear acceleration and head tilting.

  • Stereocilia of hair cells bend due to differences in inertia with otoliths during head movement.

• Dynamic Equilibrium:

  • Encoded by hair cells in the semicircular canals responding to rotational movements of the head.

  • Involves bending of stereocilia due to fluid movement in response to changes in head position.

Disorders of the Ear

• Include:

  • Otitis media, Otitis externa, Presbycusis, different forms of Deafness (Conduction & Sensorineural), Vertigo, Tinnitus.

Somatosensation (General Sense)

• Group of sensory modalities associated with:

  • Touch, proprioception, and interoception.

• Mechanoreceptors of Somatosensation:

  • Free nerve endings: Pain, temperature.

  • Merkel’s discs: Low frequency vibration (5–15 Hz).

  • Meissner’s corpuscle: Light touch, vibrations below 50 Hz.

  • Pacinian corpuscle: Deep pressure, high frequency vibration (around 250 Hz).

  • Muscle spindles: Muscle contraction and stretch.

Accessory Structures of the Eye

• Include:

  • Eyebrow, Eyelashes, Conjunctiva, Cornea.

• Lacrimal Apparatus:

  • Involves lacrimal glands and ducts essential for tear production and drainage.

Extrinsic Eye Muscles

• Control eye movement and include:

  • Superior oblique muscle, Superior rectus muscle, Lateral rectus muscle, Medial rectus muscle, Inferior rectus muscle, Inferior oblique muscle.

Structures of the Eye

• Layers of the Eye: Fibrous tunic, Vascular tunic, and Neural tunic.

• Neural Tunic (Retina):

  • Contains photoreceptors: Rods (dim light) and Cones (color vision).

  • Fovea centralis: Site of highest visual acuity with only cones.

Visual Pathway

• Pathway of visual information from the eye to the brain involves:

  • Optic nerves, optic chiasm, lateral geniculate nucleus of the thalamus, and visual cortex.

Refraction of Light

• The bending of light rays by the cornea and lens focuses images onto the retina, which are then inverted and processed by the brain.

Disorders of the Eye

• Include:

  • Myopia (nearsightedness), Hyperopia (farsightedness), Astigmatism, Cataracts, Glaucoma, Retinal detachment, Diplopia (double vision), Strabismus, Colorblindness, Nyctalopia (night blindness).

Central Processing (14.2)

• Involves sensory pathways carrying information from peripheral sensations to the brain, mapped to specific areas in the somatosensory cortex.