Types of Signals- Sensory Receptors and Classification

Introduction:

Whenever we touch a hot object, our sensory receptors detect the change, the brain processes this information, and an immediate response of withdrawing our hand is observed.

Every organism must be aware of its surroundings and perform the actions required for survival. There are a lot of actions going inside our body of which we are unaware. All such actions must be properly timed and coordinated.

Our nervous system is responsible for processing this information and creating a response.

Explanation:

Nervous system:

The nervous system in our body keeps us informed about the outside world with the help of sense organs. It helps us remember things, think, and reason out. It controls all our voluntary actions like running, reading a book, etc. It also regulates involuntary actions like breathing, beating of the heart, etc.

Our nervous system consists of the brain, spinal cord, sensory receptors, and many nerves. The brain and the spinal cord are made up of specialized cells called neurons or nerve cells.

Neurons:

Neurons are specialized cells that make up the nervous system. These cells are responsible for receiving and transmitting signals to different parts of the body.

The Three Main Parts of the Neuron are:

1. Cell body
2. Dendrites
3. Axon

The cell body contains a well-defined nucleus that is surrounded by granular cytoplasm. It has all the organelles like a regular cell except centrosome because nerve cells cannot divide. The dendrites are branched cytoplasmic projections of the cell body. They conduct nerve impulses to the cell body. The axon is a tube-like projection that shows electrical impulses from the cell body to the axon terminals and passes the impulses to another neuron.

Synapse is the chemical junction between the terminus of one neuron and the dendrites of another neuron.

There are Three Types of Neurons:

The sensory neuron carries the signal from the (sense organs) to the main nervous system, i.e., the brain and the spinal cord. The motor neurons carry the impulse from the main nervous system to the effector muscles or glands. The interneurons are situated in the brain and the spinal cord and interconnect the sensory and motor neurons.

Nerves:

Nerves are thread-like white structures that emerge from the brain and spinal cord and branch out to almost all parts of the body.

There are three types of nerves. The sensory nerves contain only sensory fibers and bring information from the receptors (sense organs) to the brain and the spinal cord. For example, the optic nerve that emerges from the eyes and ends in the brain.

Whereas motor nerves contain only motor fibers and bring information from the brain and the spinal cord to effector organs like muscles or glands to bring them into action. For example, a nerve that arises from the brain to the muscles of the eyeball in order to rotate the eye. Mixed nerves contain sensory and motor fibers—for example, a spinal nerve.

Sensory Receptors:

Sensory receptors are the dendritic ends of sensory neurons, which are specialized for receiving particular types of stimuli. Three methods classify sensory receptors:

1. By complexity of the receptor
2. By location
3. By type of stimulus detected

Classification by Complexity of the Receptor:

1. Free nerve endings: Dendrites with terminal ends have little or no physical specialization.
2. Encapsulated nerve endings: Dendrites whose terminal ends are covered in a capsule of connective tissue.
3. Sense organs: These consist of sensory neurons with receptors for special senses (vision, hearing, smell, taste, and equilibrium) and connective, epithelial, or other tissues.

Classification by Location:

1. Exteroceptors are found near the skin’s surface and are sensitive to stimuli detected outside or on the body’s surface. Tactile receptors that detect sensations like touch, pain, and temperature, as well as vision, hearing, smell, and taste receptors, are included in this group.
2. Interceptors (visceroceptors) respond to stimuli occurring in the body from visceral organs and blood vessels.
3. Proprioceptors respond to skeletal muscles, tendons, ligaments, and joint stimuli. These receptors acquire information about body position and the physical conditions of these sites.

Classification by Type of Stimulus/Signals Detected:

1. Mechanoreceptors: They provide information about mechanical changes in the environment and respond to physical force such as pressure (touch or blood pressure), movement, and stretch.

They are divided into three main categories:

1. 1. Tactile receptors: Provides information regarding touch and vibrations
   2. Baroreceptors: Detects changes in pressure
   3. Proprioceptors: Detects the position of joints
2. Photoreceptors: They respond to light. In the human eye, photoreceptor cells are located in the retina. There are two types of photoreceptor cells: cones and rods.
3. Thermoreceptors: They respond to temperature changes. They are accessible nerve endings situated in the skin, liver, and skeletal muscles. Cold thermoreceptors are 3.5 times more common than heat receptors.
4. Chemoreceptors: They respond to dissolved chemicals during sensations of taste and smell. They can also detect changes in internal body chemistry, such as blood variationsand PH. Taste buds are an example of chemoreceptors.
5. Nociceptors: They respond to various stimuli associated with tissue damage. The brain interprets pain.

Types of Signals:

We have learned that different sensory receptors respond to other stimuli/signals. As a result, changes are happening around us that are detected by these receptors. These changes are signals that activate these receptors. These signals are classified into various categories depending on the type of receptor that they activate.

For example, mechanical signals, chemical signals, electromagnetic signals, etc.

• Mechanical Signals: When a signal is triggered by a mechanical force such as shear stress or a pull/push, it is called mechanical signaling.
  • Examples of mechanical signals:

1. 1. Your friend pushes you suddenly.
   2. You twist your ankle while walking.
   3. An object falls on your hand, etc.

• Chemical Signals: when a chemical change in the body triggers a signal, it is called chemical signaling. These signaling substances are proteins or other molecules which are secreted by the receptor cells. The receptor cells release These molecules inside our body in the extracellular space.

Examples include smelling or eating food.

• Electromagnetic Signals: Visible light is a form of electromagnetic radiation, as are radio waves, infrared radiation, ultraviolet radiation, X-rays, and microwaves.

Visible light is an electromagnetic signal detected by rods and cones, i.e., the receptor cells in our eyes.