Human Body Systems: Nerves, Senses, and Hormones

Posted on May 21, 2025 in Biology

The Nervous System and Environmental Interaction

The nervous system interacts with both internal and external environments. This interaction allows the body to detect changes and provides appropriate responses to them. Sensory receptors are specialized cells that detect these changes. A detectable change is known as a stimulus.

The Path of a Nerve Impulse

The journey of a nerve impulse from stimulus to response follows a specific sequence:

1. A stimulus occurs.
2. Sensory receptors detect the stimulus.
3. Sensory neurons receive the information and transmit it to the Central Nervous System (CNS).
4. The CNS receives the information and a response is decided.
5. Motor neurons transmit the response to the effectors.
6. Effectors carry out the response. An effector can be a muscle or a gland.
7. The response is given. There are two main types:
   • Motor responses: Carried out by muscles (e.g., movement).
   • Secretory responses: Carried out by glands (e.g., hormone release).

Coordination Systems of the Body

The human body utilizes two primary systems for coordination and communication:

• Nervous System:
  • Receives and transmits information rapidly.
  • Information is transmitted by electrical impulses through sensory neurons and processed in the CNS.
  • Responses are typically short-lived.
• Endocrine System:
  • Composed of glands that secrete hormones.
  • Hormones are carried in the blood to the cells of the target tissue or organ.
  • Responses are generally slow and long-lasting.

Sensory Receptors: Detecting the World

Sensory receptors capture information from both external and internal environments. When a receptor detects a stimulus, it converts the stimulus into an electrical impulse. Nerves then carry these impulses from the receptors to the CNS, where they are received as sensations.

Types of Sensory Receptors (by Location)

• Interoceptors: Located within the body, in muscles and organs. They are sensitive to general body conditions and to sensations like thirst, the need to urinate, etc.
• Exteroreceptors: Located in the sense organs at or near body surfaces. They are sensitive to external stimuli.

Types of Sensory Receptors (by Stimulus)

• Chemoreceptors: Found in the nose and tongue, sensitive to chemical substances.
• Mechanoreceptors: Located in the skin and ears, sensitive to physical stimuli such as touch, pressure, sound waves, and gravity.
• Photoreceptors: Found in the retina, sensitive to light.
• Thermoreceptors: Located in the skin, sensitive to temperature changes.
• Nociceptors: Distributed throughout the body, sensitive to stimuli that cause pain.

The Five Senses

The five traditional human senses are: sight, touch, hearing, smell, and taste.

The Sense of Sight

Eyeball Structure

The eyeball is a spherical organ composed of three main layers: the sclera, choroid, and retina.

Accessory Structures of the Eye

These structures protect and support the eye:

• Eyebrows: Protect eyes by directing sweat away.
• Eyelids: Cover the eyes and spread tears.
• Eyelashes: Protect the eye from dust and insects.
• Lacrimal Glands: Secrete tears which moisten, lubricate, and clean the surface of the eye. Tears also have antibacterial properties.
• Lacrimal Sac: Collects excess tears and directs them into the nasal cavity.

The Sense of Touch

The skin, our largest organ, is responsible for the sense of touch and has two primary layers:

• Epidermis: The outer layer, made up of several layers of cells.
• Dermis: The inner layer, which contains the touch receptors.

Types of Touch Receptors

Tactile corpuscles are nerve endings protected by connective tissue, perceiving diverse sensations:

1. Ruffini Corpuscles: Sensitive to heat.
2. Meissner Corpuscles: Sensitive to light touch or contact.
3. Krause Corpuscles: Sensitive to cold.
4. Vater-Pacini Corpuscles: Sensitive to pressure and vibration.
5. Free Nerve Endings: Sensitive to a wide variety of intense stimuli, including pain, cold, heat, and pressure.