Quick Answer
The organ with the most nerve endings is the skin. The skin is the human body’s largest organ and is richly supplied with nerves that allow us to perceive touch, temperature, and pain. On average, the skin contains around 640,000 sensory nerve endings per square inch. This high concentration of nerves makes the skin exquisitely sensitive to external stimuli.
Nerve Endings in Skin
The skin contains a variety of specialized nerve endings that allow us to sense our environment. Here are some of the main types:
- Mechanoreceptors – Detect touch, pressure, vibration
- Nociceptors – Detect pain
- Thermoreceptors – Detect temperature (warmth and cold)
Mechanoreceptors, which detect touch and pressure, are the most abundant nerve endings in the skin. There are four main types:
- Merkel cells – Detect light touch
- Meissner’s corpuscles – Detect touch and low-frequency vibration
- Ruffini corpuscles – Detect skin stretch
- Pacinian corpuscles – Detect deep touch and high-frequency vibration
These specialized receptors are found in different concentrations on different parts of the body. For example, mechanoreceptors are densely packed in the fingertips, lips, and tongue, allowing excellent tactile sensitivity in these areas.
The skin’s rich nerve supply allows us to make fine tactile discriminations and protects us by prompting a rapid withdrawal response from dangerous stimuli.
Distribution of Nerve Endings
While the skin as a whole contains a very high density of nerve endings, they are not evenly distributed over the entire skin surface. Some areas have a far greater concentration of nerve endings than others.
The areas of thickest skin, such as the soles of the feet and the palms of the hands, have the highest number of nerve endings overall. However, the density (number per square inch) is actually higher in the thinner, more sensitive skin covering the fingers, lips, genitals, and face.
Here is a breakdown of the estimated density of nerve endings in different skin regions:
| Area of skin | Nerve endings per square inch |
|---|---|
| Fingertip | 2,500 |
| Lip | 1,000 |
| Face | 900 |
| Genitals | 800 |
| Hands | 630 |
| Feet | 200 |
| Forearm | 140 |
| Lower leg | 75 |
| Back | 45 |
As you can see, the density ranges widely from 2,500 nerve endings per square inch in the fingertips to just 45 per square inch in the skin of the back. This helps explain why our fingertips and lips are so exquisitely sensitive to the slightest touch.
Other Organs with Many Nerve Endings
While the skin wins in terms of total number and density of nerve endings, other organs also have very rich nerve supplies. Here are some of the other most innervated organs:
- Tongue – Like the fingertips, the tongue has an extremely high density of nerve endings (around 2,000 per square inch for taste buds). This allows for refined sensations of taste and texture when eating.
- Cornea – The transparent outer layer of the eye is one of the most sensitive tissues in the body, with around 7,000 nerve endings per square inch. This allows any speck of dust or other irritant to be felt immediately.
- Lips – With around 1,000 nerve endings per square inch, the lips are highly sensitive to touch and temperature. This helps with functions like kissing and sampling food temperatures.
- Clitoris – The clitoris has around 8,000 nerve endings concentrated in a small area, making it extremely sensitive to sexual stimulation.
- Hands – With over 17,000 nerve endings in each hand, our hands are capable of amazingly delicate sensation and dexterity.
So while the skin as a whole tops the list, in terms of specific organs, the tongue and cornea could be considered to have the highest densities of nerve endings.
Why Skin Has so Many Nerve Endings
The fact that the skin, our largest organ, contains such a high concentration of nerve endings is not surprising when you consider its vital sensory functions:
- Acts as the body’s first line of defense – Nerve endings detect potential sources of harm/damage and prompt withdrawal reflexes.
- Allows nuanced tactile discrimination – Sensory impulses to the brain allow us to respond appropriately to different types of touch (light, soft, pain, vibration, etc).
- Provides information about the environment – Nerve endings send signals about surface texture, shape, temperature and more.
- Allows social interaction through touch – Nerve endings make touch and skin-to-skin contact pleasurable and communicate emotion.
Having such a dense network of specialized nerve endings allows the skin to provide a detailed and nuanced sensory “map” of our environment at every moment. This constant sensory input is sent to the brain and allows appropriate motor responses in our limbs and body.
In evolutionary terms, the development of an exquisitely sensitive skin conveyed major advantages in dealing with potential threats, navigating complex environments, and connecting with other members of the species.
Humans likely have the highest number of cutaneous sensory receptors among mammals, which fits with the highly advanced tactile and manipulative abilities of our species.
Development of Skin Nerves
The system of sensory nerves in the skin begins to develop early in embryonic growth, arising from neural crest cells.
By week 7 of gestation, primitive free nerve endings can be detected in the developing epidermis. By week 13, specialized nerve corpuscles like Meissner’s and Pacinian are visible.
The density of cutaneous nerves continues to increase until late gestation, when the nerves reach the layers of the skin where their endings will reside.
Interestingly, the number and types of receptors are genetically determined and form independent of stimulus from the external environment. Once born, the newborn’s skin immediately provides sensory information to the brain. The total number and density of cutaneous receptors declines with age beginning around puberty.
Sensory Functions of the Skin
The wide array of cutaneous sensory receptors allows the skin to provide detailed information about various types of stimuli:
Touch and Pressure
The skin’s mechanoreceptors allow sensations of light touch, pressure, vibration, stretching, flutter, etc. The four main types detect stimuli differently:
- Merkel discs – light touch
- Meissner corpuscles – light touch, low-vibration (flutter)
- Ruffini endings – skin stretch
- Pacinian corpuscles – deep pressure, high-vibration
Pain
Nociceptors in the skin sense potentially damaging stimuli. They are activated by extremes of temperature, mechanical forces, or chemical irritants. Signals sent to the brain elicit sensations of pain and prompt defensive reactions.
Temperature
Thermoreceptors detect ambient temperature – some sense warmth while others sense cold. This provides important information about the external environment and potential tissue damage from extreme heat or cold.
Itch
Itch-specific nerve fibers detect histamine and other chemical irritants. The sensation of itch prompts scratching and removal of the irritant.
In addition to external stimuli, receptors in the skin can also be activated by internal factors like inflammation, providing warning signals of tissue damage or disease.
How Nerve Impulses Reach the Brain
When receptors are stimulated in the skin, they generate electrical nerve impulses that must travel to the brain to be perceived consciously.
Route
Receptors connect to sensory nerve fibers that gather into nerves. These peripheral nerves converge into the spinal nerves which enter the spinal cord. Ascending tracts in the spinal cord carry the signals up to the brain.
Steps
- Stimulation of a receptor triggers opening of sodium channels, creating a generator potential.
- If this depolarization reaches threshold, an action potential fires down the nerve fiber.
- The spinal nerve transmits the impulse into the spinal cord.
- Ascending pathways relay the signal up to the thalamus and sensory cortex.
Multiple synaptic relays occur to transmit the information to the cerebral cortex where conscious perception occurs.
Speed
Despite multiple synapses, sensory signals can travel to the brain extremely rapidly due to myelination of nerve fibers.
- 150 mph in myelinated nerves
- 15 mph in unmyelinated nerves
So a touch on the fingertips is perceived within milliseconds!
Mental Processing of Skin Sensations
Once impulses from cutaneous receptors reach the cerebral cortex, further processing elucidates spatial details and meaning.
Sensory Homunculus
The arrangement of the primary somatosensory cortex maps skin sensations from different body areas onto specific places. This sensory homunculus displays relative differences in touch sensitivity. For example, huge areas devoted to the sensitive lips and hands versus tiny areas for the less sensitive back.
Object Recognition
The brain integrates sensory details about surfaces, textures, shape, and dimensionality to identify objects through touch.
Two-point Discrimination
The minimum distance between two points that can be distinguished as separate is used to measure tactile acuity. It varies widely based on receptor density in different skin regions.
Subjective Perception
Higher order processing elicits the subjective aspects of sensation – the pleasantness versus unpleasantness, affective qualities, and emotional associations with certain types of touch.
Clinical Conditions Affecting Skin Sensation
Because the skin contains such a high proportion of the body’s sensory receptors, many neurologic and dermatologic conditions produce aberrations of cutaneous sensation. These include:
Peripheral Neuropathies
Nerve damage from diabetes, toxin exposure, autoimmune disease, infection, or trauma can cause numbness, tingling, reduced sensation, or chronic pain signals due to peripheral sensorimotor neuropathy. Common in legs and feet.
Complex Regional Pain Syndrome
Nerve malfunction results in burning pain, hypersensitivity, changes in skin temperature and swelling, often after injury. Most common in hands and feet.
Brachioradial Pruritus
Chronic itching, burning and tenderness on outer arm, possibly from nerve compression. More common in middle-aged females.
Postherpetic Neuralgia
Debilitating skin pain persisting after shingles rash clears, due to damage to sensory ganglia. Most often affects torso and face along dermatome distribution.
Trigeminal Neuralgia
Light touch triggers excruciating facial pain due to damage to trigeminal nerve. Also called tic douloureux.
Assessment of tactile discrimination and two-point testing can help diagnose conditions affecting large fiber sensory tracts. Changes in sensitivity to vibration, pain and temperature may indicate small fiber involvem
Conclusion
The skin contains an extraordinarily high number and density of sensory receptors, far surpassing most other organs and tissues. This allows the skin to provide detailed sensory information to the brain and react to the finest changes within our environment.
While the specific distribution varies, the average density of around 640,000 nerve endings per square inch makes the skin the most abundantly innervated organ. No other single organ contains the sheer number and diversity of sensory endings.
The skin’s prodigious sensory capabilities provide vital functions – warning of threats, allowing nuanced touch and manipulation, connecting us emotionally through contact. The loss of these capabilities through disease or injury severely impairs quality of life and normal function.
So the next time you marvel at the delight of a caress, the pain of a pinprick, or the ability to handle delicate objects, thank the millions of sensory receptors in your skin making these experiences possible!
