Sensory Receptors, Senses and the Nervous System: Structure and Function

Sensory Receptors

What is the mission of sensory receptors? Their function is to receive information and transmit it to the central nervous system, which processes it and produces a response. The response can be sensory or motor.

Types of Sensory Receptors

What types of sensory receptors do you know?

• Photoreceptors: detect light stimuli, located in the eyes.
• Mechanoreceptors: respond to mechanical stimuli such as pressure, contact, changes in position and sound waves. They are found in several groups: in the skin, in the muscles and joints, and in the hearing receptors.
• Chemoreceptors: capture information about chemical changes. The taste and smell receptors belong to this group.
• Thermoreceptors: respond to temperature changes, as some receptors in the skin do.

Sense Organs

How does the eye function?

The eye performs two processes:

• Regulation of light intensity: if the light is too intense it can damage the cells (photoreceptors), and if it is too weak the visual process cannot take place. The pupil enlarges or contracts to let in more or less light, through the contraction or relaxation of the iris muscles.
• Focusing: objects are focused on the retina. Focus is achieved by modifying the thickness of the lens: it bulges to focus close objects and flattens to focus distant ones. The image is formed inverted on the retina and the brain interprets it correctly.

Human stereoscopic vision is possible thanks to the images obtained by both eyes, which overlap in the same area and give the brain a spatial sensation.

How is sound produced and how is hearing?

Sound waves are produced by the vibration of molecules in the air or in any other object. When these waves reach the eardrum, it vibrates and transmits the vibration to the auditory ossicles (malleus, incus and stapes). The stapes transmits the vibration to the oval window. This vibration causes the perilymph of the inner ear to move, which in turn stimulates the hair cells of the organ of Corti. This generates a nerve impulse that travels along the auditory nerve to the brain, where it is interpreted.

How does balance work in the ear?

Balance depends on the position of the head and body. In the utricle and the saccule there are sensory cells covered with cilia embedded in a gelatinous mass that contains mineral particles. When the head changes position, these particles shift and bend the cilia, generating a nerve impulse that informs the brain about the position. Movement is detected by the semicircular canals: when we move, the endolymph moves relative to the canal walls, bending the cilia of the vestibular cells and generating a nerve impulse that is transmitted to the brain.

What is touch? What sensations does it include?

Touch is detected by sensitive receptors in the skin, which capture very different stimuli. Touch sensations include the shape and texture of objects, pressure, high temperatures, low temperatures and pain.

What is taste like? And smell?

Taste allows us to capture chemical information about foods. Smell consists in detecting gaseous molecules that reach the nose.

Effector Organs: The Locomotor System

What is the mission of the locomotor system?

Its mission is to move the body. It is made up of the skeletal system and the muscular system.

What is the mission of the muscular system?

Its mission is to cause movement by producing changes in the position of body parts.

What are muscles made of? How is muscle contraction produced?

Muscles are made up of bundles of elongated muscle cells called muscle fibers, which are covered by connective tissue. They are attached to bones by tendons. Muscle contraction consists of shortening. For it to occur, it requires: a nerve impulse from the central nervous system, the motor neuron and its motor endplate, which connects through synapses, and neurotransmitters that cause muscle contraction. Within muscle cells, energy is obtained through cellular respiration.

What is the mission of the skeletal system?

It allows muscles to anchor, supports and protects important organs of the body, stores calcium and produces blood cells.

What are bones like? What are their characteristics?

Bone is a tissue that forms the skeleton. It has two types: compact bone, which is hard and dense, and spongy bone, which has internal cavities. Bones are strong and also provide living tissue.

What are joints? What types do you know?

Joints are the points where bones meet. They can be classified as fixed (immovable), mobile and semi-mobile.

Common Diseases

• Conjunctivitis: inflammation of the conjunctiva of the eye, caused by allergy or infection. It produces itching, tearing and redness.
• Cataracts: the lens becomes cloudy and does not let light through. It frequently appears in older people.
• Blindness: the affected person cannot see, or cannot distinguish colors.
• Deafness: includes the lack of hearing caused by lesions of the inner ear or the auditory nerve, and difficulties in transmitting sound vibrations.
• Rheumatoid arthritis: chronic inflammation of the joints that causes swelling and heat. Joints lose mobility over time and can become deformed. It may be caused by infections or by unknown causes.
• Gout: a very painful joint inflammation accompanied by redness and swelling.
• Osteoporosis: a reduction in bone mass that makes bones more fragile and prone to fractures. It is usually due to calcium malabsorption, difficulties in intestinal absorption, or calcium deposits.
• Hernia and disc herniation: a structure comes out of its normal position. A disc herniation originates when the cartilage of a vertebra moves out of its usual position and compresses nerve roots of the spinal cord, causing pain.