Chapter 14: Somatic Nervous System and Special Senses (Part 1)

Sensation: Awareness of external or internal stimuli, can be conscious or subconscious. It involves the detection of a stimulus by a receptor.
Perception: The interpretation of sensations, which is a conscious process. People interpret sensations differently due to various factors.
- Example: Different individuals may perceive the same color (e.g., a shirt appearing black or blue).
Personal Example: A temperature drop of 20 degrees experienced when exiting Walmart.
- Individual interpretation of cold varies; for instance, some may consider 50°F cold, while others may not.

Modalities: Unique characteristics that distinguish one sensation from another, such as pain, touch, hearing, etc.
- Each sensory neuron carries only one type of message (e.g., pain, touch, hearing).
- General Senses: Includes somatic (external) and visceral (internal) senses.
- Somatic senses: Pain, thermal sensations, proprioceptive sensations.
- Visceral senses: Related to internal organs (e.g., stretch, nausea, hunger, pressure).
- Special Senses: Localized senses including smell, vision, hearing, equilibrium, and taste.

Four Requirements for Sensation:
1. Stimulation of the sensory receptor (must be close enough to detect changes).
- Example: Receptors detected cold when leaving Walmart due to the proximity of temperature change.
1. Transduction of stimulus into a graded potential (varying amplitude based on stimulus strength).
- Important note: Graded potentials are not action potentials.
1. Generation of a nerve impulse to the brain or spinal cord (when graded potentials summate to a threshold).
2. Integration of sensory input by a specific brain region to produce conscious sensation.

Most sensory receptors are adaptable, meaning their sensitivity decreases with prolonged exposure to a stimulus.
- Example: Initial cold sensation from outdoor temperature adapts over time, resulting in diminished perception of cold.
Fast-adapting receptors include touch and smell. No longer able to sense clothing or strong colognes after time elapses.
Some receptors adapt more slowly or do not adapt at all: e.g., pain receptors remain sensitive to alert about potential harm.

Proprioceptors: Critical for maintaining body awareness and control.
- Found within muscles, joints, tendons, and inner ear, they provide information on body position and movement, influencing systems like blood pressure regulation.
Visceral receptors: Monitor internal conditions (e.g., baroreceptors for blood pressure).

Receptor Modes:
- Mechanoreceptors: Detect stretching and pressure.
- Thermoreceptors: Detect temperature changes.
- Nociceptors: Detect painful stimuli.
- Photoreceptors: Activated by light.
- Chemoreceptors: Detect chemicals.
- Osmoreceptors: Detect osmotic pressure.

Cutaneous sensations arise from receptors in skin, subcutaneous tissue, and mucous membranes.
Sensitivity varies by body region (e.g., tongue and fingertips have more receptors compared to forearm).

Impulses from cutaneous receptors travel through somatic afferent neurons to the thalamus, then to the somatosensory area of the parietal lobe for perception.

Nociceptors: Free nerve endings responsible for pain, present in most body tissues.
- Highly significant for survival since they alert the brain of potential harm.
Types of Pain:
- Acute Pain: Quick and sharp, carried by myelinated neurons (faster).
- Chronic Pain: Slow, throbbing pain, transmitted by unmyelinated neurons (slower).
Superficial vs. Deep Somatic Pain:
- Superficial pain: Occurs at the skin level.
- Deep somatic pain: Occurs in muscles, tendons, and joints.
Referred Pain: Pain felt at the surface over internal organs due to shared nerve pathways (e.g., pain during a heart attack felt in the left arm).
Phantom Pain: Sensation following a limb amputation, generated by nerve impulses from residual nerves, often resulting in pain, itching, or pressure where the limb was removed.

Anesthesia: Blocks pain sensations from reaching the brain.
- General anesthesia: Induces unconsciousness and loss of all sensations.
- Spinal anesthesia: Injected into the subarachnoid space, blocking sensations below it.
Analgesics: Reduce pain perception and can include medications like aspirin which block pain signal transmission.

Special Senses: Smell, taste, vision, hearing, and equilibrium are distinct receptor types, predominantly localized in the head.
Olfaction (smell) and Gustation (taste) work closely together, transmitting chemical impulses to the brain.
- Emotional responses triggered in the limbic system associated with olfactory cues, impacting memories (e.g., smell of apple pie).

Five Primary Tastes: Sweet, sour, bitter, salty, umami (savory).
Approximately 10,000 taste buds located on the tongue, soft palate, pharynx, and larynx, diminishing with age.
Structure of Taste Buds:
- Composed of gustatory receptor cells, supporting cells, and basal cells for continual renewal.
Tongue Papillae Types:
- Circumvallate Papillae: Large, at back of the tongue (100-300 taste buds each).
- Fungiform Papillae: Scattered, with about 5 taste buds each.
- Foliate Papillae: Located in trenches on tongue margins (most degenerate in childhood).
- Filiform Papillae: Lacking taste buds; provide traction for food.
For tasting, substances must dissolve in saliva; receptor potential triggers nerve impulses transmitted to cranial nerves (VII, IX, and X) to the brain.

Olfactory receptors are located in the nasal epithelium, responding to odorants.
Basal cells renew receptors monthly; olfactory glands produce mucus essential for odorant detection.
Neural Pathway: Olfactory signals travel to the olfactory bulbs, then to limbic and cerebral cortex regions for perception.

Regions of the ear include:
- External Ear: Collects sound waves (pinna, auditory canal).
- Middle Ear: Amplifies sound via ossicles (malleus, incus, stapes).
- Inner Ear: Contains structures (cochlea, semicircular canals) critical for hearing and equilibrium.
Sound vibrations travel through the tympanic membrane into the ossicles and eventually to the cochlea, where auditory signals are transformed into nerve impulses sent to the cerebral cortex for perception.
Balance: Maintained through vestibular apparatus detecting head position and body movement (static and dynamic equilibrium).

Part one concludes with a detailed overview of somatic senses and introductory concepts of special senses (smell and taste, hearing).
Further Discussion: Vision will be covered in part two.