The sensory system

Learning Objectives

• Identify and assess the anatomy of the human special senses: sight, sound, taste, smell, and balance
• Examine the functionality of the senses
• Analyse the importance of pain
• Investigate pain gate theory in caring for patients

Introduction to the Senses

In addition to vision, hearing, smell, taste, and touch, there are other senses vital for survival:https://www.youtube.com/embed/0abA8gh3eZ8&t=1s

• Interoception sense of the internal state of the body, including:
  • Hunger
  • Thirst
  • Heartbeat
  • Breathing
  • Nausea
  • Fullness
  • Internal pain (e.g., stomach ache)
  • Temperature (core/internal)
• Equilibrioception (Vestibular sense) sense of balance and spatial orientation, governed by the inner ear.
• Proprioception – awareness of body position and movement (e.g., knowing where your limbs are without looking).
• Nociception – sense of pain:
  • External pain (e.g., injury to skin)
  • Internal/visceral pain (e.g., cramps)
• Chronoception – perception of time (less understood, but likely involves brain integration rather than a single sense organ) (Wittmann, 2009)
• Magnetoreception – very weak in humans (if present (Wang et al., 2019)), this would be the ability to sense magnetic fields (strong in birds and some animals)
• Chemoreception – detection of chemical stimuli internally (e.g., CO₂ levels in blood)
• Stretch receptors – in organs like the lungs or stomach, to detect expansion (e.g., feeling full)

Watch Video 1 Pain Physiology | Nociception, Dr Matt & Dr Mike follow this link https://www.youtube.com/embed/QidHfIj5rYQ

The senses help detect and monitor changes in the environment by sending information to the brain through sensory (afferent) neurons. The brain then processes this information and sends out the appropriate response via motor (efferent) neurons.

Sight / Vision

Rods in the eye detect dim light and provide black-and-white images. Cones detect color in bright light. Each retina has about 3 million cones and 100 million rods.

Rods are more sensitive in the dark and are located around the edge of the retina, explaining why stars seem clearer when viewed indirectly.

Figure 1.1 Anatomy of the human eye (Purves et al., 2001)

How Can Humans See in the Dark?

Human vision in low light is possible due to rods, the eye’s photoreceptors specialized for dim environments. Rods produce black-and-white images and function well in darkness, while cones, which are active in bright light, allow us to perceive color.

Each retina contains approximately 100 million rods and 3 million cones, enabling us to see across various lighting conditions. Remarkably, the human eye can detect around 500 shades of grey and even spot a candle flame from over a mile away (Krisciunas and Carona, 2015).

Why Do Stars Seem to Disappear When You Look Directly at Them?

This phenomenon is due to the uneven distribution of rods and cones in the retina. The central retina (fovea) is densely packed with cones, which are less effective in low light. The peripheral retina contains more rods, which are better at detecting faint light.

As a result, when you look directly at a dim object like a star, it may fade or vanish because the central vision relies on cones. Looking slightly to the side allows rods to pick up the light, making the star more visible (Ben McAllister, 2019).

How to Find Your Dominant Eye

Most people have one eye that is naturally dominant—meaning it provides slightly more reliable input to the brain for tasks requiring visual alignment, such as aiming or focusing on distant objects. To identify which of your eyes is dominant, follow these steps:

1. Choose a small, distant object to focus on (e.g., a clock on the wall).
2. Extend both arms fully in front of you, keeping both eyes open.
3. Form a small triangular opening by overlapping your hands so that the space between your thumbs and index fingers creates a window.
4. Center the distant object within that triangular opening.
5. Close one eye at a time, while keeping your hands and head still.
6. The eye that continues to see the object through the opening is your dominant eye. If the object shifts out of view when one eye is closed, that eye is non-dominant (Porac and Coren, 1976).

Note: While many right-handed individuals have a right-eye dominance, this is not always the case. Eye dominance and handedness do not always align.

Hearing / Sound

The ear is responsible for two vital functions:

• Hearing (auditory perception)
• Balance (equilibrium)

Humans perceive sound by detecting vibrations, typically transmitted through air. These vibrations are converted into nerve impulses by the structures of the ear and transmitted to the brain via the eighth cranial nerve (also known as the vestibulocochlear nerve).

Anatomically, the ear is divided into three main sections:

1. External Ear – Captures sound waves and directs them inward.
2. Middle Ear – Amplifies and transmits sound from the eardrum to the inner ear via ossicles.
3. Inner Ear (Labyrinth) – Contains the cochlea for hearing and the vestibular system for balance (Tortora and Derrickson, 2009).

Watch Video 2 Hearing & Balance: Crash Course Anatomy & Physiology #17, CrashCourse follow this link https://www.youtube.com/embed/Ie2j7GpC4JU

Sound waves travel through the outer ear, vibrate the eardrum and ossicles, and are transferred through the cochlea. Vibrations stimulate nerve cells which send signals to the brain.

Sound travels better through solids or liquids than air, hence underwater communication among whales or tapping in mining accidents.

Figure 1.2 (NIH, 2022)

Sound perception is a complex coordinated process involving the outer, middle, and inner ear. Here’s how sound travels through the auditory system:

1. Sound Wave EntrySound waves enter the outer ear and travel down a narrow passage called the ear canal, which directs them toward the eardrum (tympanic membrane).
2. Eardrum VibrationThe eardrum vibrates in response to the sound waves. These vibrations are transmitted to the ossicles—three tiny bones in the middle ear known as the:
   • Malleus (hammer)
   • Incus (anvil)
   • Stapes (stirrup)
3. Amplification and TransmissionThese bones amplify the vibrations and send them into the cochlea, a fluid-filled, snail-shaped structure in the inner ear.
4. The Cochlea and Basilar MembraneInside the cochlea, an elastic structure called the basilar membrane runs along its length, dividing it into upper and lower chambers. This membrane serves as the base for hair cells, the key sensory receptors for hearing.
5. Fluid Waves and Hair Cell ActivationThe vibrations cause the cochlear fluid to ripple, creating a traveling wave along the basilar membrane.
   • High-pitched sounds (e.g., an infant’s cry) activate hair cells near the base of the cochlea.
   • Low-pitched sounds (e.g., a dog barking) stimulate hair cells near the apex (center).
6. Stereocilia and Electrical SignalsAs hair cells move, their tiny projections—called stereocilia—bend against an overlying structure. This bending opens ion channels at the tips of the stereocilia, allowing chemicals to enter the cells and generate an electrical signal.
7. Signal Transmission to the BrainThe auditory nerve (part of the eighth cranial nerve) carries these electrical signals to the brain, where they are interpreted as recognizable sounds (Yost, 2013).

Watch Video 3 Journey of Sound to the Brain, National Institutes of Health (NIH) follow this link https://youtu.be/eQEaiZ2j9oc?si=JIGpGirUoni0RquX

Common Misconception About Sound Transmission

Many people mistakenly believe that sound travels best through air, simply because air is the medium we live in and experience sound most often.

However, this is not scientifically accurate. Sound travels through a medium by causing its molecules to vibrate, and this process—known as molecular collision—occurs more efficiently when the molecules are closely packed together.

• In solids, molecules are tightly packed, allowing vibrations to transfer quickly.
• In liquids, molecules are less tightly packed than in solids but still closer together than in gases.
• In gases (like air), molecules are much farther apart, making sound travel slower and less efficiently.

Real-World Examples

• Whales can communicate across vast distances—up to two miles underwater—because water transmits sound much more effectively than air. Source: TED-Ed (2016). “Why Do Whales Sing?”

Watch Video 4 Why do whales sing? – Stephanie Sardelis, TED-Ed follow this link https://www.youtube.com/watch?v=7Xr9BYhlceA

• Miners trapped underground often tap on solid surfaces (like rock or metal) rather than shout, because sound travels more efficiently through the solid walls than through the air in the tunnels.

Balance

The semicircular canals in the inner ear detect movement through fluid displacement. Sensory receptors send signals to the brain to help maintain equilibrium.

Taste / Flavour

Taste buds are located on the tongue, lips, and throat. They send signals to the brain through three cranial nerves. Humans detect five tastes: sweet, salty, sour, bitter, and umami.

Smell / Scent

Smell is processed in the olfactory region of the nasal cavity. It’s closely related to taste and memory. Sniffing helps move odors to olfactory receptors.

Touch / Feel

Touch receptors (tactile corpuscles) are concentrated in the skin, especially fingertips and the tongue. Separate temperature sensors detect heat and cold. Adaptation changes how we perceive ongoing temperature stimuli.

Two different systems:

• Discriminatory: Tells you where and what you are touching. So that we don’t have to rely on visual cues.

• Protective: Reacts to unexpected touch and alerts us to danger.

Neurons, the fundamental components of the nervous system, are categorized into three types:

• sensory neurons, 

• motor neurons,

• and interneurons.

Sensory neurons detect signals from sensory organs that track changes both inside and outside the body. Based on their specific roles, these neurons transmit information—such as temperature, light, pressure, muscle tension, and smell—to higher regions of the nervous system for interpretation.

Importance of Pain

Pain acts as a warning system. Pain receptors are found throughout the body and do not adapt. Types include:

• Referred pain: felt in a different location than the origin (e.g., liver pain in the shoulder)
• Phantom pain: felt in amputated body parts

Watch Video 5 Excretory System and the Nephron, Amoeba Sisters follow this link https://www.youtube.com/watch?v=q5qaGHfdmYM

Watch Video 6 Kidney Disease and Dialysis | Health | Biology | FuseSchool, FuseSchool – Global Education follow this link https://youtu.be/9KZHowze7lg

Watch Video 7 Digestive System, Amoeba Sisters follow this link https://youtu.be/1UvuBYUbFk0

Watch Video 8 Taste & Smell: Crash Course Anatomy & Physiology #16, CrashCourse follow this link https://youtu.be/mFm3yA1nslE

Watch Video 9 Vision: Crash Course Anatomy & Physiology #18, CrashCourse follow this link https://youtu.be/o0DYP-u1rNM