Chapter 18: The Nervous System - General and Special Senses

Introduction to Sensation and Perception

• Sensation: This occurs when the afferent division of the nervous system detects a stimulus and sends a neural signal to the Central Nervous System (CNS).
• Perception: This is the conscious awareness of a sensation. It occurs only if the neural signal reaches the cerebral cortex.
• The Olfaction Example:
  • Sensation: Chemicals in the air are detected by olfactory receptors in the nose, which transmit a signal to the olfactory cortex.
  • Perception: The olfactory cortex processes this signal to create a "smell" that the brain recognizes.
  • Note: If a sensation fails to reach the cerebral cortex, no perception is created.

Categorization of Senses

• General Senses (Somatic Senses):
  • These are sensations that can be felt throughout the entire body.
  • Examples include: temperature, pain, touch, pressure, vibration, and proprioception (body position).
  • Processing Location: These sensations are processed in the primary somatosensory cortex located in the parietal lobe.
• Special Senses:
  • These are sensations created by highly specialized sense organs.
  • Examples include: olfaction (smell), gustation (taste), equilibrium (balance), hearing, and vision.
  • Processing Location: Each special sense is processed in a distinct, dedicated part of the cerebral cortex.

Sensory Receptors

• Definition: Specialized cells capable of sensing different types of stimuli.
• Complexity: Receptors range from simple structures to highly complex cells.
  • Free Nerve Endings: The simplest type of sensory receptor; these are neurons that utilize their dendrites to detect stimuli.
  • Specialized Receptors: Cells with unique structures built for only one type of stimulus, such as photoreceptors in the eye.

Classification of Sensory Receptors by Stimulus

• Nociceptors: Detect tissue damage; commonly referred to as "pain receptors."
• Thermoreceptors: Detect changes in temperature.
• Mechanoreceptors: Detect physical stimuli that distort the cell membrane (stretching, compression, twisting).
• Chemoreceptors: Detect chemical signals and changes in chemical concentrations.
• Photoreceptors: Detect light.

Classification of Sensory Receptors by Location

• Exteroceptors: Provide information regarding the external environment (e.g., skin receptors for touch).
• Interoceptors: Provide information regarding the body’s internal environment (e.g., blood pressure receptors in blood vessels).
• Proprioceptors: A sub-type of interoceptor that specifically senses body position; typically located in joints and skeletal muscles.

Detailed General Senses

Nociceptors

• Function: Interoceptors that detect damage to the body.
• Structure: All nociceptors are free nerve endings.
• Stimuli: Responds to extreme temperatures, physical damage to cells, and noxious chemicals.
• Distribution: Heavily concentrated in superficial areas like the skin; deeper areas, such as the organs in the abdominopelvic cavity, possess very few nociceptors.

Thermoreceptors

• Function: Interoceptors that detect temperature changes.
• Structure: Free nerve endings.
• Types: Two distinct types exist—one for decreasing temperatures (cold) and one for increasing temperatures (warm).
• Distribution: Found in the dermis, skeletal muscles, liver, and hypothalamus.
• Adaptation: These are strongly activated by changes but tend to shut off when temperature remains constant.

Mechanoreceptors

• Tactile Receptors: Exteroceptors detecting touch, pressure, and vibration.
• Baroreceptors: Interoceptors that monitor pressure in internal organs by measuring the degree of stretch. Located in blood vessels, lungs, digestive tract, and urinary tract.
• Proprioceptors: Interoceptors monitoring joint position, tendon/ligament tension, and muscle contraction.
  • Muscle Spindles: Measure skeletal muscle stretch.
  • Golgi Tendon Organs: Measure tension in tendons.
  • Joint Capsule Free Nerve Endings: Monitor joint movement.

Tactile Receptor Sub-Types

• Unencapsulated Tactile Receptors:
  • Free Nerve Endings: Sensitive to light contact.
  • Root Hair Plexuses: Specifically detect movement of the hair.
  • Merkel Cells: Specialized for detecting light touch; they synapse with sensory neurons to send signals to the CNS.
• Encapsulated Tactile Receptors: Dendrites covered in a connective tissue capsule.
  • Tactile (Meissner) Corpuscles.
  • Bulbous (Ruffini) Corpuscles.
  • Lamellar (Pacinian) Corpuscles: Surrounded by multiple layers of collagen fibers and fluid; they are only activated by strong vibrations and deep pressure.

Chemoreceptors

• Function: Specialized neurons detecting changes in chemical concentrations.
• Visceral Chemoreceptors: Monitor internal changes in chemicals such as $O_2$, $CO_2$, ions, and glucose.

Olfaction (Smell)

• Olfactory Organs: A pair of organs located in the nasal cavities.
• Composition: Olfactory sensory neurons, supporting cells, basal epithelial cells (regenerative), and olfactory glands.
• The Mucous Layer: Olfactory glands produce a thick mucus. Olfactory sensory neurons (chemoreceptors) extend dendrites with receptor proteins into this mucus.
• Olfactory Activation: Inhaled chemicals dissolve in the mucus and bind to receptor proteins.
• Sensitivity: Extremely high; as few as $4$ molecules can activate a neuron.
• Neural Pathway:
  1. Olfactory sensory neurons.
  2. Second-order neurons in the olfactory bulb.
  3. Olfactory tract.
  4. Olfactory cortex.
• Unique Feature: Olfaction is the only special sense that does not synapse in the thalamus.
• Limbic Connection: Signals also go to the hypothalamus and limbic system, explaining why smells trigger strong emotions and memories.
• Regeneration: Neurons are replaced every $4 – 6 \text{ weeks}$; however, the replacement rate declines with age.

Gustation (Taste)

• Receptors: Gustatory epithelial cells located in taste buds.
• Anatomy of the Tongue: Taste buds are situated on small bumps called lingual papillae.
• Taste Bud Structure: Contains $40 – 100$ gustatory epithelial cells that extend microvilli through taste pores.
• Activation: Chemicals from food/drink dissolve in saliva and bind to receptor proteins on the microvilli.
• Regeneration: Basal cells produce new gustatory epithelial cells, which are replaced every $10 – 12 \text{ days}$.

Anatomy of the Ear

• External Ear: Visible portion; contains the auricle (elastic cartilage) and the external acoustic meatus (passageway).
• Middle Ear: Located in the temporal bone; contains the air-filled tympanic cavity.
• Inner Ear: Contains sensory receptors for hearing (cochlea) and equilibrium (vestibular complex).

The Middle Ear and Sound Transfer

• Tympanic Membrane (Eardrum): Connective tissue separating external and middle ear; vibrates in response to sound.
• Auditory tube (Eustachian tube): Connects tympanic cavity to the nasopharynx to equalize air pressure.
• Auditory Ossicles: Three bones that transfer and amplify vibrations:
  • Malleus: Attached to the tympanic membrane.
  • Incus: Between the malleus and stapes.
  • Stapes: Transmits vibrations to the cochlea via the oval window.
• Protective Muscles: The tensor tympani and stapedius muscles pull on ossicles to reduce vibrations from loud noises.

Equilibrium and the Inner Ear

• Labyrinths: The fluid-filled membranous labyrinth is enclosed by the hard bony labyrinth.
• Vestibular Complex:
  • Semicircular Canals: Three canals (anterior, lateral, posterior) for detecting head rotation. Each canal has an enlarged area called an ampulla.
  • Ampulla Mechanism: Contains hair cells with a large kinocilium and multiple stereocilia embedded in a gel-like ampullary cupula. Fluid movement pushes the cupula, bending the hair cells.
• Vestibule:
  • Contains the utricle (horizontal forces) and saccule (vertical forces/gravity).
  • Otolithic Membrane: Gel-like matrix containing stereocilia.
  • Otoliths: Calcium carbonate crystals (surface "ear stones") that move with gravity or acceleration, bending the hair cells.

Hearing and the Cochlea

• Cochlea Structure: Snails-shell shape spiraling around a central hub called the modiolus.
• Cochlear Ducts:
  • Scala Vestibuli: Receives vibrations from the oval window via the stapes.
  • Scala Tympani: Ends at the round window.
  • Cochlear Duct: Sits between the two; contains the sensory receptors.
• Spiral Organ (Organ of Corti): Sits on the basilar membrane. Vibrations move the basilar membrane upward, pushing hair cell stereocilia against the tectorial membrane, activating the sense of hearing.

Vision and the Eye

Accessory Structures

• Lacrimal Apparatus: Produces tears via lacrimal glands to keep the eye moist and clean.
• Palpebrae (Eyelids): Spread tears across the eye.
• Lacrimal Lake: Area in the medial angle where excess tears collect before draining into the nasal cavity.
• Orbits and Orbital Fat: Protects, cushions, and insulates the eye.

Layers of the Eye Wall

1. Fibrous Layer (Outer):
   • Sclera: The "white of the eye"; dense fibrous connective tissue.
   • Cornea: Transparent layer of collagen fibers where light enters.
   • Corneoscleral Junction: Where the sclera and cornea merge.
2. Vascular Layer (Uvea/Middle):
   • Iris: Contains pigment cells (eye color) and smooth muscle to regulate the pupil width.
   • Ciliary Body: Extends from the corneoscleral junction to the ora serrata. Controls lens shape via ciliary zonules (suspensory ligaments).
   • Choroid: Vascular tissue separating the sclera and retina.
3. Inner Layer (Retina):
   • Neural Layer: Contains photoreceptors and neurons.
   • Pigmented Layer: Deep to the neural layer; absorbs light that passes through to prevent reflection.

Cavities and Fluids

• Anterior Cavity: Between the cornea and lens; filled with aqueous humor (similar to CSF). Failure to drain causes glaucoma.
• Posterior Cavity: Posterior to the lens; filled with thick, viscous vitreous humor to maintain eye shape and press the retina against the wall.

Retina and Photoreceptors

• Rods: Highly light-sensitive; provide black-and-white vision. Denser at the outer edges of the retina.
• Cones: Require more light; provide detailed color vision (Red, Green, and Blue types). Denser in the center.
• Retinal Landmarks:
  • Macula: Center of the retina where light is focused; contains only cones.
  • Fovea Centralis: Center of the macula; highest concentration of cones; provides sharpest vision.
  • Optic Disc: Where the optic nerve exits; lacks photoreceptors, creating a "blind spot."

Neural Organization of the Retina

• Bipolar Cells: Receive signals from photoreceptors.
• Horizontal Cells: Modify communication between photoreceptors and bipolar cells.
• Ganglion Cells: Receive signals from bipolar cells; axons form the optic nerve.
• Amacrine Cells: Modify communication between bipolar and ganglion cells.

The Visual Pathway

1. Signal travels from photoreceptors to bipolar cells, then to ganglion cells.
2. Axons leave via the optic nerve ($II$).
3. Optic nerves meet at the optic chiasm.
4. Some axons cross over; the right visual cortex receives signals from the left visual field, and vice versa.
5. Signal synapses in the lateral geniculate nuclei of the thalamus.
6. Signal reaches the visual cortex.
7. Depth Perception: Created by the visual cortex integrating slightly different signals from each eye.