A&P chapter 11 shudy Guide

Chapter: Sensory System

1. Introduction to the Senses & Sensory Receptors

Key Concepts:

• A stimulus is defined as an environmental change that initiates a nerve impulse.

• A sensation occurs when the cerebral cortex interprets that impulse.

• Sensory receptors are parts of the nervous system that detect a stimulus.

1.1 What is a Sensory Receptor?

• The sensory receptor is the structure that detects a stimulus. It can be:

  • A free dendrite of a sensory neuron (e.g., those detecting pain and temperature).

  • A modified ending on a dendrite (e.g., those responsible for touch).

  • A specialized cell associated with a sensory neuron (e.g., rods and cones found in the retina).

1.2 Categories of Sensory Receptors (by stimulus type):

• Chemoreceptors: Detect chemicals in solution such as those responsible for taste and smell.

• Photoreceptors: Respond to light, specifically found in the retina of the eye.

• Thermoreceptors: Detect temperature changes.

• Mechanoreceptors: Respond to movement such as pressure, vibration, and stretch. This includes receptors involved in hearing, equilibrium, and body position.

1.3 Special vs. General Senses:

• Special Senses: Localized in specific sense organs. These include:

  • Vision

  • Hearing

  • Equilibrium

  • Taste

  • Smell

• General Senses: Receptors are widely distributed throughout the body. These include:

  • Pressure

  • Temperature

  • Pain

  • Touch

  • Sense of position (found in muscles, tendons, and joints).

2. Sensory Adaptation

• Sensory Adaptation: This is the process through which sensory receptors adjust to a continuous, unimportant stimulus, resulting in the sensation becoming less acute.

  • Examples:

  • Noticing a watch when first worn but then becoming accustomed to it.

  • Adapting to warm water after a period of immersion.

• Variation:

  • Receptors for warmth, cold, and light pressure adapt rapidly.

  • Pain receptors do not adapt.

3. The Eye and Vision

Key Concepts:

• Vision is considered the dominant special sense.

• The eye is protected by bones, eyelids, eyelashes, tears, and the conjunctiva.

3.1 Protective Structures & Eye Muscles

• Protective Structures:

  • Eye orbit: The bony socket housing the eye.

  • Eyelids: Protect the eye by closing.

  • Eyelashes: Help to keep debris out of the eye.

  • Eyebrows: Protect from sweat running into the eyes.

  • Lacrimal apparatus: Produces tears for protection and lubrication of the eye.

  • Conjunctiva: Thin membrane lining the eyelids and covering the sclera to protect and lubricate.

• Extrinsic Eye Muscles: Six muscles responsible for moving the eyeball. They enable convergence (the process of both eyes focusing on a single visual field) for clear, three-dimensional vision.

3.2 Nerve Supply to the Eye

• Sensory Nerves:

  • Optic Nerve (CN II): Carries visual impulses from the retina to the brain.

  • Ophthalmic branch of the Trigeminal Nerve (CN V): Carries sensations of pain, touch, and temperature to the brain.

• Motor Nerves:

  • Oculomotor (CN III): Supplies most eye muscles, allowing for movement and focusing.

  • Trochlear (CN IV): Supplies the superior oblique muscle, aiding in eye movement.

  • Abducens (CN VI): Supplies the lateral rectus muscle, responsible for outward eye movement.

3.3 Structure of the Eyeball (Three Tunics)

1. Fibrous Tunic (Outer):

   • Sclera: The white, tough connective tissue of the eye.

   • Cornea: The transparent, anterior portion of the eye, which is the main refracting structure.

2. Vascular Tunic (Middle):

   • Ciliary Body: Contains the ciliary muscle which controls the shape of the lens for focusing.

   • Iris: The colored part of the eye controlling the size of the pupil.

3. Nervous Tunic (Inner):

   • Retina: Contains photoreceptors (rods and cones) responsible for detecting light.

3.4 Pathway of Light & Refraction

• Refraction: This is described as the bending of light rays as they pass through substances of varying densities.

• Pathway & Refractive Structures:

1. Cornea

2. Aqueous Humor (watery fluid in the anterior chamber)

3. Lens (adjusts thickness for focus)

4. Vitreous Body (jelly-like substance in the posterior chamber)

3.5 Accommodation & Iris Function

• Accommodation: Process of adjusting lens thickness to focus on near or far objects.

  • Distant Vision:

  • Ciliary muscle relaxes → suspensory ligaments are taut → lens is flattened.

  • Close Vision:

  • Ciliary muscle contracts → suspensory ligaments relax → lens becomes rounder/thicker.

• Iris Function: Regulates the amount of light entering the eye through the pupil.

  • In Bright Light: Circular muscles contract and pupil constricts.

  • In Dim Light: Radial muscles contract and pupil dilates.

3.6 Function of the Retina

• The retina contains photoreceptors that are essential for vision.

4. The Ear: Hearing and Equilibrium

Key Concepts:

• The ear is divided into three main sections for its functions.

4.1 Divisions of the Ear

1. Outer Ear:

   • Pinna (Auricle): The external part of the ear.

   • External Auditory Canal: Channel leading to the tympanic membrane (eardrum).

2. Middle Ear:

   • Aair space containing three ossicles (small bones):

   • Malleus (Hammer)

   • Incus (Anvil)

   • Stapes (Stirrup)

   • Auditory (Eustachian) Tube: Connects to the pharynx to equalize pressure in the ear.

3. Inner Ear (Labyrinth):

   • Bony Labyrinth: The outer shell filled with perilymph fluid.

   • Membranous Labyrinth: Inside the bony shell, filled with endolymph fluid.

   • Three Parts:

   • Vestibule: Contains receptors for equilibrium (utricle and saccule).

   • Semicircular Canals: Contain receptors for equilibrium (cristae).

   • Cochlea: Contains receptors for hearing (spiral organ of Corti).

4.2 The Mechanics of Hearing

1. Sound waves enter the external auditory canal.

2. Waves hit the tympanic membrane, causing vibrations.

3. Ossicles amplify and transmit vibrations to the oval window.

4. Fluid (perilymph) in the vestibular duct moves, creating waves in the cochlear duct.

5. Waves cause membranes to vibrate, bending the organ of corti which is embedded in the tectorial membrane.

6. Movement of cilia generates nerve impulses.

7. Impulses travel via the vestibulocochlear nerve (CN VIII) to the auditory cortex of the brain.

4.3 Types of Equilibrium

• Static Equilibrium:

  • Utricle and saccule respond to the position of the head relative to gravity.

• Dynamic Equilibrium:

  • Semicircular canals respond to head rotation and movement.

  • Receptors are located in the crista, responding to movement of the cupula.

4.4 Disorders of the Ear

• Otitis Media: Infection of the middle ear.

• Otitis Externa: Known as "Swimmer's ear," it is an infection of the external auditory canal.

• Hearing Loss:

  • Conductive: Obstruction or damage in the outer or middle ear (e.g., cerumen, otosclerosis).

  • Sensorineural: Damage to the inner ear, cochlear nerve, or brain (e.g., noise exposure, presbycusis which is age-related hearing loss).

• Equilibrium Disturbances:

  • Vertigo: Sensation of spinning.

5. Other Special Senses: Taste and Smell

• Both senses are categorized as chemoreceptors as they respond to chemical stimuli.

5.1 Sense of Taste (Gustation)

• Receptors: Taste buds located within papillae on the tongue.

• Five Primary Tastes:

  • Sweet

  • Salty

  • Sour

  • Bitter

  • Umami (savory)

• Nerves:

  • Facial Nerve (CN VII)

  • Glossopharyngeal Nerve (CN IX)

5.2 Sense of Smell (Olfaction)

• Receptors: Olfactory receptor cells located in the upper nasal cavity.

• Nerve: The Olfactory Nerve (CN I) carries impulses directly to the temporal lobe and limbic system, closely linking the sense of smell with taste.

6. The General Senses

• General Senses are receptors scattered throughout the body. These include:

  • Touch, Pressure, Temperature, Pain, Position (proprioception).

• Touch Receptors: Located in the skin, particularly concentrated in areas like fingertips and lips, where sensitivity is highest.