Nervous System and Sensory Organs: Structure, Function, and Disorders
Cells of the Nervous System
The nervous system is formed by very distinct cells. Neurons are the cells responsible for conducting nerve impulses. Neurons communicate by connecting the axon with the dendrites.
Types of Neurons
- Sensory or afferent neurons
- Interneurons or association neurons
- Motor neurons
Sensory Receptors
Receptors are a set of specialized cells that perceive stimuli. According to the stimulus, they are classified as:
- Thermoreceptors
- Chemoreceptors
- Mechanoreceptors
- Photoreceptors
Internal receptors detect changes within the body, while external receptors provide information about the environment.
Smell
The sense of smell resides in the nostrils and recognizes volatile substances.
Taste
Taste is located on the tongue. There are four types of receptors that give rise to the four basic tastes: salty, sweet, bitter, and sour. Food tastes are detected through various senses: smell, taste, and touch.
The Ear
External Ear
The external ear is made up of the pinna and ear canal, which terminate at the eardrum.
Middle Ear
The middle ear contains ossicles and the Eustachian tube.
Inner Ear
The inner ear is formed by the cochlea and the auditory nerve.
Mechanism of Hearing
Sound waves reach the eardrum, causing it to vibrate.
Vision
Vision is the ability to perceive light waves. The structures involved in vision are the eyeball and adnexa.
Adnexa
- Lacrimal glands: keep the eyeball moist.
- Eyelids: prevent foreign agents from entering the eye.
- Ocular muscles: move the eyeballs.
Eyeball
The eyeball has three layers: the sclera (posterior) and cornea, the iris, and the choroid. The iris determines the eye’s color and has a central hole called the pupil. The pupil regulates the amount of light that enters the eye. The lens is a biconvex structure that allows focusing on objects at different distances. The retina contains two types of photoreceptors: rods and cones.
Anatomy of the Nervous System
The nervous system is formed by the central nervous system and the peripheral nervous system.
Central Nervous System
The central nervous system is made up of the brain and spinal cord.
Brain
The brain consists of the medulla oblongata, cerebellum, thalamus, hypothalamus, and cerebrum.
Spinal Cord
The spinal cord is located within the backbone. Its functions include coordinating reflex actions and communicating with the brain.
Peripheral Nervous System
The peripheral nervous system includes the somatic and autonomic systems. The somatic system is responsible for conscious movement, while the autonomic system controls involuntary movements.
Reflex and Voluntary Acts
Reflex Acts
Reflex acts are automatic responses of the organism to a stimulus. The elements of a reflex act are: receptor stimulation, a sensory neuron that carries information to the spinal cord, an interneuron, a motor neuron that carries information to the effector organ, and muscle movement.
Voluntary Acts
Voluntary acts are performed consciously.
Eye Diseases
Eye diseases include alterations of the eyeball such as myopia, hyperopia, and astigmatism. Cataracts occur when the lens becomes opaque. Conjunctivitis is an eye infection.
Diseases Due to Trauma
These occur when a bone is broken.
Degenerative Diseases
- Alzheimer’s: neurons lose their functionality.
- Parkinson’s: neurons die.
Mental Illnesses
- Hypochondria: experiencing symptoms of a disease that does not exist.
- Schizophrenia: characterized by hallucinations.
- Bipolar disorder: characterized by periods of euphoria.
Locomotor System
The locomotor system is formed by bones and muscles.
Bones
Bones support the body, protect internal organs, store calcium and phosphorus, and contain bone marrow. Bones are classified as:
- Long bones: tibia and fibula.
- Flat bones: bones of the cranium and clavicle.
- Short bones: vertebrae.
Bones are connected by articulations, which are classified as:
- Immovable: areas of the skull.
- Semi-movable: vertebrae.
- Movable: knee and elbow.
Muscles
Muscle fibers are made up of smooth, cardiac, and striated muscles. Smooth muscles are found in the walls of internal organs. Cardiac muscle is found in the heart. Striated muscles include the biceps.
Locomotor System Diseases
- Sprain: ligament tear.
- Fissure: total or partial fracture of bones.
- Muscle cramps: involuntary contraction.