Human Body's Communication Systems: Nerves and Hormones

The Nervous System: Communication Network

Neurons: The Building Blocks of the Nervous System

• Neurons: Specialized cells that conduct electrical impulses throughout the body. They are elongated cells with a cell body and a thin axon.
• Dendrites: Thin projections that extend from the cell body and connect with other neurons to allow electrical impulses to pass through.
• Myelin Sheath: An insulating lipid layer that surrounds some axons, speeding up impulse transmission.
• Nerve: A bundle of nerve fibers enclosed within a protective sheath.
• Nerve Fibers: Long axons of neurons together with any associated tissue.

Types of Neurons

The nervous system is comprised of three specialized types of neurons:

• Sensory Neurons: Transmit messages from receptors (e.g., eyes, nose, ears) to the brain or spinal cord.
• Motor Neurons: Transmit impulses from the brain or spinal cord to effectors (e.g., muscles or glands), causing a reaction. Impulses reach muscles through the axon of motor neurons.
• Relay Neurons: Pass nerve impulses from the spinal cord to the brain and to an effector.

The Reflex Arc: An Involuntary Response Pathway

The reflex arc is a neural pathway that mediates a reflex action. It typically follows this sequence:

Receptor → Sensory Neuron → Relay Neuron → Motor Neuron → Effector

Process:

1. A receptor generates a nerve impulse in response to a stimulus.
2. The impulse is sent to the spinal cord along the sensory neuron's axon.
3. Within the spinal cord, the sensory neuron sends the impulse to a relay neuron.
4. The relay neuron then sends the impulse to the brain for processing and simultaneously to an effector along the motor neuron.

Key Characteristics of a Reflex Arc:

• Involuntary: You do not consciously think about the response.
• Very Fast: Responses occur rapidly.
• Brain Independence: The immediate response does not rely on conscious brain processing.
• Effectors:
  • Muscles: Produce mechanical responses (e.g., contraction).
  • Glands: Produce chemical responses (e.g., secretion).

Divisions of the Nervous System: CNS and PNS

The nervous system is broadly divided into two main parts:

• Peripheral Nervous System (PNS):
  • Composed of sensory and motor neurons.
  • Gathers and sends information between the central nervous system and the rest of the body (e.g., muscles and glands).
  • Has some capacity for repair.
• Central Nervous System (CNS):
  • Composed of the brain and spinal cord.
  • Contains neurons that coordinate and process information.
  • Sends appropriate responses along appropriate nerve fibers.
  • Has very limited capacity for repair.

The Endocrine System: Hormonal Regulation

Hormones and Glands: Chemical Messengers

The endocrine system is a chemical communication system that uses hormones to control and coordinate body functions.

• Comprises: Secretory cells located in glands throughout the body.
• How Information is Transmitted: Chemical messengers called hormones are transported in the blood.

Hormone Action Pathway:

1. Hormones are released from glands.
2. Hormones travel in the bloodstream.
3. Hormones are detected by specific target organs or cells.
4. A response is triggered in the target organ or cell.

The endocrine system primarily controls hormonal responses. Hormones are chemical messengers released from glands that travel in the blood and are detected by other organs, initiating specific physiological effects.

Comparison: Nervous System vs. Endocrine System

Example: Adrenaline Release (Fight or Flight Response)

A classic example of endocrine system action is the release of adrenaline in a "fight, flight, or freeze" situation. Adrenaline causes an increase in heart rate and breathing, which in turn increases respiration to provide more energy for immediate action.