A stimulus is a change in the environment of an organism. Animals respond to both internal and external stimuli through their central nervous system (CNS). The response to the stimulus helps to maintain the homeostasis or a constant internal environment within them. The CNS is composed of the brain and the spinal cord. Various types of receptors in the body respond to stimuli and generate nerve impulses that are transmitted to the brain and spinal cord through sensory neurons. Brain and the spinal cord process the nerve impulses and the corresponding information is transmitted to the effector organs through motor neurons.
Key Areas Covered
1. What is the Function of Central Nervous System
– Definition, Brian, Spinal Cord
2. How Does the Brain Receive the Information from the Receptor
– Receptors, Sensory Neurons
Key Terms: Brain, Central Nervous System (CNS), Receptors, Sensory Neurons, Spinal Cord, Stimulus
What is the Function of Central Nervous System
The central nervous system is the processing unit of the nervous system. It includes the brain and the spinal cord. It receives nerve impulses from the peripheral nervous system and sends information to the peripheral nervous system in the form of nerve impulses. The brain processes the sensory information and sends the information to the spinal cord. The anatomy of the nervous system is shown in figure 1.
The brain is the control center of the central nervous system. The wrinkled appearance of the brain forms gyri and sulci. The medial longitudinal fissure divides the brain into two hemispheres. The three divisions of the brain are forebrain, brainstem, and hindbrain. The largest part of the forebrain is the cerebrum. The processing of most of the sensory impulses occurs in the cerebral cortex. Midbrain and hindbrain form the brainstem.
The spinal cord is a cylindrical bundle of nerves that are connected to the brain. It extends from the neck to the lower back. The nerves of the spinal cord transmit sensory nerve impulses of both internal and external stimuli to the brain and transmit the information from the brain back to the corresponding effector organs of the body. The ascending nerve tracts are responsible for the transmission of sensory impulses to the brain while the descending nerve tracts transmit motor impulses to the effector organs.
How Does the Brain Receive the Information from the Receptor
Various types of receptors that respond to both internal and external stimuli are found in the body. Most of these receptors are found in the skin, responding to the external stimuli such as temperature, touch, pressure, and pain. In addition to the skin, complex organs also serve as receptors. Some of these receptors are;
- Light receptors in the retina of the eye
- Sound receptors in the ear
- Position receptors in the ear
- Chemical receptors in the nose and tongue
- Secreting cells in the glands
- Muscle cells
- Different organs
Various organs that produce internal stimuli (interoception) are shown in figure 2.
These receptors send sensory information in the form of nerve impulses to the brain through sensory neurons.
- The free nerve endings and corpuscles are the two types of neurons found on the skin. Free nerve endings are embedded in the dermis. They detect mechanical stimuli such as touch, pressure, and stretch. They also detect temperature and danger (nociception) as well. The sensory pathway of the receptors in the skin is shown in figure 3.
- Rod cells and cone cells in the retina are sensitive to the light. They activate nerves known as retinal ganglia. The nerve impulses generated in the retinal ganglia are transferred to the brain through the optical nerve, sensing the sight.
- The odor of a molecule dissolves in the mucous membrane and attaches to the microvilli of the epithelium in the hood of the nose. Dendrites of the olfactory nerves are found in the microvilli. The contact of odor molecules with the dendrites stimulates the sensory neurons to send the impulses to the brain, sensing the smell.
- The tastes buds are the terminals of the sensory neurons which are present on the tongue. The cranial nerve 7 (2/3 of the tongue) and glossopharyngeal N (1/3 of the tongue) nerve transmit sensation for the taste to the brain.
- The inner hair cells in the ear stimulate the afferent audio nerve and signals send to the brain, allowing an organism to sense different sounds.
Sensory neurons assemble to form sensory nerves. These sensory nerves reach the brain through the spinal cord.
Both internal and external stimuli are recognized by various types of receptors in the skin and organs. This information is transmitted to the brain through sensory neurons. Sensory neurons assemble to form sensory nerves that reach the brain through the spinal cord. The brain processes the sensory impulses and sends information to the corresponding effector organs through motor nerves.
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2. “GCSE Bitesize: Responding to a stimulus.” BBC, .
3. “Sensory Coding: Getting Messages from Receptors to Your Brain.” Study.com, .
1. “1201 Overview of Nervous System” By OpenStax – via
2. “Interoception and the body” By Schappelle – Own work via
3. “1212 Sensory Neuron Test Water” By – via