Study guide: Human Sensory Systems

📋 Course Outline

1. Nervous System Divisions
2. Sensory Receptors and Sensation
3. Somatic Senses
4. Taste and Smell
5. Eye Structure and Accommodation
6. Retina and Photoreceptors
7. Visual Abnormalities
8. Ear Structure and Hearing
9. Equilibrium and Inner Ear

📖 1. Nervous System Divisions

★ Must-know

• The central nervous system (CNS) consists of the brain and spinal cord, while the peripheral nervous system (PNS) includes cranial and spinal nerves.

• Sensory (afferent) nerves carry information into the brain and spinal cord, whereas motor (efferent) nerves carry information from the CNS to effectors.

• Somatic motor nerves signal skeletal muscles for voluntary action, whereas autonomic motor nerves signal smooth muscle, cardiac muscle, and glands involuntarily.

Further detail

• Somatic sensory nerves carry signals from the skin, muscles, joints, and special senses, whereas visceral sensory nerves carry signals from body organs.

📌 The parasympathetic division is associated with “rest and digest,” whereas the sympathetic division is associated with “fight or flight.”

💡 Memory Hook

Afferent arrives at the CNS; efferent exits toward effectors.

📖 2. Sensory Receptors and Sensation

🔑 Key Concepts & Definitions

• Sensory receptor: A dendrite specialized to detect a certain type of stimulus.
• Sensory adaptation: A decrease in response to a repetitive stimulus, such as an odor.

📝 Essential Points

📌 Exteroceptors detect stimuli from outside the body, such as taste, hearing, and vision, whereas interoceptors receive stimuli from inside the body, such as changes in blood pressure.

• The five sensory receptor types are:

  • Chemoreceptors, which respond to nearby chemicals
  • Nociceptors, which respond to chemicals released by damaged tissue
  • Photoreceptors, which respond to light energy
  • Mechanoreceptors, which respond to mechanical forces such as pressure
  • Thermoreceptors, which respond to temperature changes

• 🔄 Sensation proceeds through:

  1. Sensory receptors respond to environmental stimuli
  2. Nerve impulses travel to the cerebral cortex
  3. The stimuli are consciously perceived

💡 Memory Hook

Stimulus → receptor → sensory impulse → cerebral cortex → conscious sensation.

📖 3. Somatic Senses

🔑 Key Concepts & Definitions

• Proprioceptors: Mechanoreceptors involved in reflex actions that maintain muscle tone and thereby support body equilibrium and posture.
• Cutaneous receptors: Receptors in the dermis that make the skin sensitive to touch, pressure, pain, and temperature.

📝 Essential Points

• Skin receptors and their reported sensations include:
  • Free nerve endings for pain, heat, and cold
  • Merkel disks for touch
  • Krause end bulbs for touch
  • Root hair plexuses for touch
  • Meissner corpuscles for touch
  • Ruffini endings for pressure
  • Pacinian corpuscles for pressure

📖 4. Taste and Smell

★ Must-know

• The five main taste types are:

  • Sweet
  • Sour
  • Salty
  • Bitter
  • Umami (savory)

• About 80–90% of what people perceive as taste is actually due to smell.

Further detail

• Adult humans have about 4,000 taste buds, located primarily on the tongue.

• Smell depends on 10–20 million olfactory cells, which are modified neurons located in the roof of the nasal cavity.

📖 5. Eye Structure and Accommodation

🔑 Key Concepts & Definitions

• Visual accommodation: Occurs when the lens changes shape to focus light on the retina and form an image.

★ Must-know

• The eye has three coats:
  • The mostly white, fibrous sclera
  • The darkly pigmented, vascular choroid
  • The inner retina, which contains photoreceptors

📌 The anterior compartment lies between the cornea and lens and contains aqueous humor, whereas the posterior compartment lies behind the lens and contains vitreous humor.

Further detail

• The cornea is the transparent portion of the sclera and refracts light, while the pupil is an opening that allows light into the eyeball.

• The iris regulates pupil size, and the ciliary body contains a muscle that controls lens shape.

• For distant focus, the ciliary muscle is relaxed, the lens is flattened, and the suspensory ligament is taut; for near focus, the ciliary muscle contracts, the lens rounds, and the ligament relaxes.

📖 6. Retina and Photoreceptors

★ Must-know

• The retina contains rods and cones: rods are sensitive to light, while cones require bright light and respond to wavelengths associated with color.

• Rods contain the visual pigment rhodopsin, support peripheral and night vision, and require vitamin A for proper functioning.

• Cones are located mostly in the fovea, enable fine detail and color detection, and occur in three kinds containing red, green, and blue pigments.

Further detail

• The fovea centralis is densely packed with cones and is the retinal area where images are focused.

• Retinal sensory receptors form the optic nerve, which carries impulses to the brain, and the blind spot is where the optic nerve attaches and lacks photoreceptors.

💡 Memory Hook

Rods serve dim-light and peripheral vision; cones provide color and fine detail.

📖 7. Visual Abnormalities

★ Must-know

📌 A cataract is clouding of the lens, whereas glaucoma involves a buildup of fluid pressure in the eye.

📌 Nearsightedness results from an eyeball that is too long and makes distant objects difficult to see, whereas farsightedness results from an eyeball that is too short and makes near objects difficult to see.

Further detail

• Color blindness is a genetic abnormality, usually involving difficulty distinguishing red and green, and is most common in males.

• Astigmatism occurs when the cornea or lens is uneven, producing a fuzzy image.

💡 Memory Hook

Myopia blurs distant objects; hyperopia blurs nearby objects.

📖 8. Ear Structure and Hearing

★ Must-know

• The outer and middle ear are air-filled and function in hearing, while the fluid-filled inner ear functions in both hearing and balance.

• The tympanic membrane vibrates in response to sound, and the malleus, incus, and stapes are three ossicles that amplify sound waves.

• 🔄 Hearing signals proceed through:

  1. The stapes vibrates against the oval-window membrane
  2. Fluid waves form in the cochlea
  3. Embedded hairs in the spiral organ bend and initiate nerve impulses
  4. The cochlear nerve carries impulses to the brain

Further detail

• The outer ear includes the pinna, which catches sound waves, and the auditory canal, which directs them to the tympanic membrane; the canal has fine hairs and modified sweat glands that secrete ear wax.

• The auditory (Eustachian) tube connects the throat to the middle ear and equalizes pressure to prevent the eardrum from bursting.

📌 Pitch is determined by sound-wave frequency detected at different parts of the spiral organ, whereas volume is determined by sound-wave amplitude.

💡 Memory Hook

Sound travels from pinna and canal to eardrum, ossicles, oval window, cochlea, and brain.

📖 9. Equilibrium and Inner Ear

★ Must-know

• The vestibule functions in gravitational equilibrium, whereas the semicircular canals function in rotational equilibrium.

Further detail

• Hair cells in the ampulla at the base of each semicircular canal detect angular movement associated with rotational equilibrium.

• Hair cells in the utricle and saccule detect head movement in vertical and horizontal planes associated with gravitational equilibrium.

💡 Memory Hook

Semicircular canals sense rotation; utricle and saccule sense gravity and linear head movement.

📊 Synthesis Tables

Receptors for Sensation and Balance

Receptor or structure	Stimulus or role	Location or pathway
Exteroceptors	Stimuli from outside the body	Examples include taste, hearing, and vision
Interoceptors	Stimuli from inside the body	Examples include changes in blood pressure
Semicircular canal hair cells	Angular movement and rotational equilibrium	Ampullae
Utricle and saccule hair cells	Head movement in vertical and horizontal planes	Vestibule