Nervous System, Sensory Receptors, and Human Senses
Coordination Systems
Coordination involves the nervous system and the endocrine system.
Neurons: Structure and Function
The nervous system uses independent cells called neurons. Key parts include:
- Soma: The cell body.
- Axon: A projection often surrounded by a myelin sheath.
- Dendrites: Numerous extensions from the soma.
Sensory Receptors and Stimuli Transformation
Sensory receptors detect stimuli and transform them into electrical signals (nerve impulses).
Receptor Classification
- Mechanoreceptors: Stimulated by mechanical action (touch, pressure, vibration, sound).
- Thermoreceptors: Stimulated by temperature changes.
- Photoreceptors: Stimulated by light.
- Chemoreceptors: Stimulated by chemical substances (smell, taste).
Cutaneous Senses
These senses originate in the skin:
- Touch: Results from stimulating mechanoreceptors just below the skin, including free nerve endings, Meissner’s corpuscles, and Merkel discs.
- Pressure: Stimulates receptors in deeper layers, such as Ruffini endings and Pacinian corpuscles.
- Vibration: Detected primarily by Pacinian corpuscles.
- Heat and Cold: Detected by thermoreceptors, which are often free nerve endings (e.g., Krause end bulbs for cold).
The Human Ear: Structure and Function
Outer Ear Anatomy
Extends from the auricle (pinna) through the ear canal. It contains protective elements like hair and wax-producing glands and ends at the eardrum (tympanic membrane).
Middle Ear Anatomy
The eardrum is connected to a chain of small bones called ossicles: the malleus (hammer), incus (anvil), and stapes (stirrup). The stapes rests on the oval window. The oval window and the round window are entrances to the inner ear.
Inner Ear Anatomy
Consists of two main parts:
- The bony labyrinth: Contains a fluid called perilymph.
- The membranous labyrinth: Suspended within the bony labyrinth, contains a fluid called endolymph.
Semicircular Canals and Balance
These three ducts are filled with endolymph. They contain sensory hair cells covered by a gelatinous structure (cupula). When the head moves, the endolymph shifts, stimulating the hair cells. This provides information about body position and changes in acceleration, contributing to our sense of balance.
The Cochlea and Hearing
The cochlea (‘snail’) is a spirally coiled tube. Its interior is divided longitudinally into ducts. The upper duct (scala vestibuli) connects to the middle ear via the oval window. Below it lies the cochlear duct containing endolymph and the Organ of Corti, which houses the auditory hair cells situated on the basilar membrane. The lower duct (scala tympani) ends at the round window. The scala vestibuli and scala tympani contain perilymph.
How Hearing Works
When sound waves reach the ear, they cause the eardrum to vibrate. This vibration is transmitted through the ossicles (malleus, incus, stapes) to the oval window. The movement of the oval window creates pressure waves in the perilymph of the cochlea. These waves travel through the cochlear fluids (perilymph and endolymph), causing the basilar membrane and the Organ of Corti to vibrate. The hair cells within the Organ of Corti transform these mechanical vibrations into electrical impulses. These impulses are then transmitted via the auditory nerve to the brain, where they are interpreted as sound.
Chemoreceptors: The Sense of Smell (Olfaction)
The sense of smell relies on chemoreceptors located in the olfactory mucosa, a specialized region within the nasal cavity. This area contains olfactory receptor neurons with numerous cilia. For a substance to be detected by smell, it must be volatile (able to become airborne), allowing its molecules to reach the olfactory mucosa.
Chemoreceptors: The Sense of Taste (Gustation)
The perception of flavors (taste) primarily occurs in the mouth via chemoreceptors. Sensitive cells that detect dissolved substances are located within taste buds. Taste buds are often grouped together on small bumps on the tongue called papillae. Each taste bud contains supporting cells and sensory hair cells (gustatory cells) that project microvilli (often called ‘hairs’ or cilia) through an opening called the taste pore.
Taste Types and Perception
Taste buds detect several basic tastes:
- Sweet: Typically associated with sugars. Detected across the tongue, often perceived strongly at the tip.
- Sour: Associated with acids (e.g., vinegar, lemon juice). Detected mainly along the sides of the tongue.
- Bitter: Often associated with alkaloids (e.g., quinine, caffeine, nicotine) and potential toxins. Detected mainly at the back of the tongue.
- Salty: Associated with ionic salts (e.g., sodium chloride). Detected across the tongue, particularly on the sides.
- (Note: Umami, associated with glutamate, is also widely recognized as a basic taste.)