Nervous System Cells: Neurons and Glia
The nervous system consists of two distinct cell types: neurons and glial cells. These cells differ morphologically and functionally.
Neurons
Neurons are responsible for conducting nerve impulses. They are highly specialized and cannot regenerate. Neurons transmit information to the dendrites of other neurons via their axons. Communication between neurons occurs at junctions called synapses. At the synapse, the axon terminals of one neuron release substances called neurotransmitters, which interact with the receiving neuron.
Types of Neurons
- Sensory (afferent) neurons: Carry information from receptors to the central nervous system.
- Interneurons (association neurons): Transmit information between neurons.
- Motor (efferent) neurons: Carry responses from the central nervous system to effectors.
Glial Cells
Glial cells are capable of dividing and are classified into two main types:
- Macroglia: Provide support to neurons, playing structural, reconstructive (replacing lost neurons), and nutritional (providing glucose) roles.
- Microglia: Perform a defensive function, eliminating remnants of dead cells and debris through phagocytosis.
Receptors
Receptors are specialized cells that perceive stimuli. For a receptor to be activated, the stimulus must reach a certain intensity, called the sensation threshold. This threshold varies among individuals. Receptors are classified as follows:
According to the Stimulus
- Thermoreceptors
- Chemoreceptors
- Mechanoreceptors
- Photoreceptors
According to their Location
- Internal receptors
- External receptors
Senses
Touch
The sense of touch resides in the skin, which detects different sensations through thermoreceptors and mechanoreceptors. A very strong stimulus to these receptors causes a painful sensation.
Smell
Smell resides in the upper part of the nostrils, where neurons connect with the olfactory nerve. Humans can detect a wide variety of smells. This sense is one of the earliest to develop, and some animals have a highly developed sense of smell.
Taste
Taste detects chemicals in food through receptors on the tongue. There are several types of chemoreceptors on the tongue that respond to chemical substances, giving rise to four basic tastes: acid, bitter, salty, and sweet. The stimulation of these receptors is very important for digestion.
Hearing
The ear consists of the following parts:
- Outer ear: Includes the pinna (ear) and the ear canal, which ends at the eardrum, a membrane that vibrates when sound waves reach it.
- Middle ear: Consists of a chain of ossicles and the Eustachian tube, which connects to the pharynx and balances pressure.
- Inner ear: Connects to the middle ear through the oval window. It has a complex structure, also called the labyrinth, formed by a coiled tube, the cochlea.
Vision
Vision is the ability to perceive light waves. Several structures are involved in vision:
Adnexa (Accessory Structures)
- Lacrimal glands: Produce a watery liquid that keeps the eyeball moist.
- Eyelids: Prevent the entry of foreign agents, such as dust or sweat, and distribute tears across the eye.
- Eye muscles: Responsible for moving the eyeballs.
Eyeballs
The eyeballs consist of three layers:
- Sclera: Protects the rest of the eyeball; the front is transparent and is called the cornea.
- Choroid: A black layer containing the iris, a membrane with a central hole called the pupil, where light passes through.