Sensations

The general purpose of sensation is to transmit information on external and internal conditions affecting the body to the brain. There are both general senses and special senses.

General Senses: the four general senses are

1. pain,
2. temperature,
3. touch/pressure/position, and
4. chemical detection.

Special Senses: the five special senses are

1. smell,
2. taste,
3. vision,
4. hearing and
5. balance.

Sensation is relayed via the sensory pathway which includes: receptors, sensory neurons, sensory tracts, and sensory areas (of the cortex).

Receptors are protein molecules that respond to chemical or physical stimuli and cause some form of cellular/tissue response, generally by stimulating action potentials in sensory neurons.

This process is called SENSORY TRANDUCTION. These action potentials travel through the peripheral nerves up the sensory tracts and into the sensory areas of the spinal cord or brain.

 

The Spine

To understand sensations properly you must understand the anatomy of the spine.

LINEAR ANATOMY: The spinal cord has 31 segments, each with a pair of ventral (anterior, MOTOR) and dorsal (posterior--SENSORY) spinal nerve roots.

NOTE: Do not confuse the dorsal spinal nerve roots with the dorsal horns (dorsal spinal horns). [SEE BELOW]

Immediately exiting the spine, the ventral and dorsal nerve roots combine on each side to form spinal nerves.

CROSS-SECTION ANATOMY: The ventral and dorsal spinal roots emerge from the spine as continuations of neurons within the spine. In cross-section, these collections of neurons are darker, hence called gray matter. The lie in a butterfly shape in cross-section, wiith two anterior wings (anterior horns) and two posterior wings (the posterior horns). The anterior and posterior spinal horns are also called the anterior and posterior spinal tracts. Axons exiting the gray matter are the white matter, surrounding the "butterfly" of gray matter. 

• DORSAL HORNS: carry sensation--the entry point of sensory information into the central nervous system. It doesn't just relay information, but works on the information ("modulation") to alter it, such as in suppressing pain sensations. 
• ANTERIOR HORNS: carry motor, the exit point for innervation of muscles.

SENSATION and INNERVATION:

• Fine touch, vibration, 2-poiunt discrimination, proprioception:

Skin and joints → afferent fibers in spinal nerves → dorsa nerve roots → dorsal spinal column → brain.

• Pain and deep pressure:

Skin and viscera → afferent fibers in spinal nerves → dorsal nerve roots → spinothalamic tract → brain.

• Volitional motor:

Brain → anterior spinal column → ventral nerve root → spinal nerve → muscle.

 

Hunger and Thirst

The sensations of hunger and thirst are based on inputs from visceral receptors and hypothalamic detection of salt/hormone/fluid status.

Visceral sensations along the vagus nerve originate via stretch receptors in the stomach and GI tract. A bigger stomach (stretched by food and liquid in the digestive system) activates these stretch receptors, and the resulting signals cause hormone changes (→ insulin) and reduce the feeling of hunger. These create a "fed" signal of satisfaction and fullness to the brain.

The hypothalamus constantly monitors the blood’s concentration of sodium and other substances, as well as changes in blood pressure and blood volume. When blood pressure or blood volume is low, the hypothalamus triggers a response to induce the feeling of thirst to restore the blood volume. This same response occurs when the concentration of salts in the bloodstream rises too high.

 

Smell

The pathway for the sense of smell involves the olfactory chemoreceptors and the olfactory cranial nerves.

The olfactory epithelium within the nasal passages is lined by olfactory receptors, which contain "Golf protein," which responds to specific odor molecules. Once stimulated, the Golf proteins stimulate the release of certain enzymes that initiate depolarization of the receptor cells. After depolarization, the impulse travels through the olfactory nerve (CN I) and is transferred by the primary olfactory cortex to the hypothalamus, the thalamus, and the orbitofrontal cortex.

 

Taste

The pathway for the sense of taste includes the taste buds, chemoreceptors, and the facial (VII), glossopharyngeal (IX), and vagus (X) nerves.

On the tongue are small bumps ("taste buds") called papillae that contain the receptors for taste. These receptors depolarize and release neurotransmitters in response to

• sweet,
• sour, and
• salty foods.

After depolarization, the sensory information travels through the facial nerve (CN VII) from the anterior two-thirds of the tongue, and then from the throat, palate and posterior tongue through the glossopharyngeal nerve (CN XI) and the vagus nerve (CN X) to the thalamus. The thalamus relays the information to the somatosensory cortex where the perception of taste is processed.

 

Pain

"And life without Pain is a long endless chain
Of errors repeated again and again" --Pose Ode

Pain sensation relies on sensors in the skin (nociceptors) that send their signals down the peripheral nerves to the spinal cord when tissue injury releases pain-mediating substances. At the spinal cord, they arrive at the dorsal root ganglia, then synapse with neurons of the spinothalamic tract, which send the signals up the spine to the thalamus.

Larger, myelinated fibers (fast) carry sharp pain and temperature sensation, while small, unmyelinated fibers (slow) carry dull pain and itching. Many hormone systems also modify the perception of pain, mainly by acting on the neurons of the spinal cord. When overwhelmed by glutamate (an excitatory neurotransmitter), inhibitory interneurons of the spine can convert into excitatory ("amplification") neurons, which leads to chronic pain and extreme responses to minor pain in some patients.

Referred pain is felt in a part of the body apart from its actual source. There is no definitive consensus on why this occurs, but the general idea is that the nervous system is unable to pinpoint a specific receptor causing the pain. If there are other receptors along the pathway of the pain signal, the brain can misinterpret the source of the signal as one of the other receptors on said pathway. Also, there is evidence that:

• a nociceptive signal arriving at a spinal segment's dorsal root ganglia tends to spread up and down the adjacent few segments and
• in the brain, excitatory pain perception can excite adjacent sensory real estate, making the parts of the body innervated by these neurons seem involved as well. 

 

Touch

The dorsal spinal column is a sensory pathway of the central nervous system that conveys sensations of fine touch, vibration, two-point discrimination, and proprioception (position--SEE NEXT) from the skin and joints.

Proprioception

Proprioception or "muscle sense" is the ability to sense the relative position of neighboring body parts and the strength of effort being applied during movement. It would be impossible, for instance, to drive a car without constantly looking at your hands if not for the sensation of proprioception.

Proprioception requires special organs found within the tendons of muscles. These organs are attached to the neurons of the peripheral nerves, which then run to the spinal cord and synapse with the dorsal spinal columns. The signals ultimately end in the thalamus, which distributes them to several areas of the cerebral cortex.