Understanding Neurons, Synapses, and Sensory Systems
Relations Function
The ability to capture pair changes or stimuli in the environment and respond if necessary.
Neurons
Santiago Ramon y Cajal concluded that the nervous system comprised independent cells, or “neurons.” His work earned him the Nobel Prize in 1906 and led to the neuron theory. Neurons are special cells with a common pattern:
- A central, enlarged part (soma or cell body). This contains the core and most cellular organelles.
- A unique, often enormously long prolongation, the “axon.” It has a bump at the presynaptic button, where many secretory vesicles are located. Axons are wrapped in an insulating sleeve: the myelin sheath. This is a lipid-rich, white envelope formed by extensions of other cells flattened and rolled around the axon.
- Many thin, highly branched extensions called dendrites, which are not covered by myelin, unlike axons.
Glia
Glia include astrocytes (provide nutrients to neurons and form structures), oligodendrocytes (form the myelin sheath), and microglia (eliminate damaged cells).
The Synapse
Each neuron communicates with others through nerve synapses. The two cells involved come into contact. There is always a space between them called the synaptic cleft. Vesicles in the presynaptic button are loaded with a chemical substance, neurotransmitters, which can induce changes in the adjacent neuron’s membrane.
Nerve Impulses
The neuronal membrane is excitable and can transmit nerve impulses. The nerve impulse always exits neurons via the axon and enters through the dendrites in the “polarization of the nerve impulse.”
Nerve Fibers
Each axon is a nerve fiber, which can be:
- Sensory: Carrying nerve impulses from receptors (sense organs) to the central nervous system.
- Motor: Carrying impulses from the central nervous system to effector organs (muscles and glands).
Axons join to form nerves. Throughout the nerve, there may be bumps called ganglia, which contain the cell bodies.
Sensory Receptors
Sensory receptors capture stimuli that are transmitted to the central nervous system. A certain energy level is required for a sensory receptor to capture a stimulus; this is the minimum threshold. If the intensity exceeds a certain level, it can damage the organ (maximum threshold). Sometimes, simple sensory receptors can excite isolated cells, as with diffuse senses. We have mechanoreceptors, thermoreceptors, photoreceptors, and chemoreceptors (quimiorreceptores).
The Cutaneous Senses
Touch is due to the stimulation of mechanoreceptors located immediately beneath the skin: the Merkel and Meissner corpuscles. Deeper are the Pacinian and Ruffini corpuscles. Heat and cold are detected by thermoreceptors, specifically the Krause corpuscles.
The Senses of Hearing and Balance
The ear is divided into three parts:
- External ear: Auricle/pinna (collects waves) & ear canal (with hair and wax glands).
- Middle ear: Tympanic membrane, ossicles (hammer, anvil, stirrup), Eustachian tube (connects to the pharynx).
- Inner ear: Oval window, round window, bony labyrinth (perilymph), membranous labyrinth (endolymph), vestibule (utricle & saccule), semicircular canals (position), cochlea (sounds / CORTI).
Smell
The olfactory epithelium is yellow and located at the top of the nostrils. It is formed by nerve cells (neurons) that capture odors and supporting cells interspersed among others. Only gas and soluble or partially soluble substances are sensed.