Human Sensory Receptors, Neurons, and Nervous System Structure

Sensory Receptors and Human Senses

Types of Sensory Receptors

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

The Five Sense Organs

• Sense of Sight (Eyes): Detected by the eyes. Each eye consists of the eyeball and accessory structures.
• Sense of Touch (Skin): The skin has two layers: the epidermis (the outer layer) and the dermis (the inner layer).
• Sense of Smell (Nose): Enables the body to detect chemical molecules in the air.
• Sense of Taste (Tongue): Enables the body to detect flavors.
• Sense of Hearing (Ears): Receptors are located in a cavity of the temporal bone; they are sensitive to sound waves.

Components of the Nervous System

Neurons: Structure and Function

Neurons generate and transmit nerve impulses. They consist of dendrites, a cell body, and an axon.

Types of Neurons

• Sensory Neurons: Send information from the receptors to the CNS (Central Nervous System), where it is interpreted and responses are made.
• Motor Neurons: Send responses from the CNS to the effectors, causing muscle contractions or gland secretions.
• Relay Neurons (Interneurons): Connect sensory and motor neurons.

Glial Cells (Neuroglia)

• Astrocytes: Star-shaped cells that transport nutrients to the neurons.
• Microglia: Remove cellular waste and protect against microorganisms like bacteria.
• Oligodendrocytes: Produce the myelin cover, or sheath, found on some axons. This sheath helps to transmit nerve impulses efficiently.

The Synapse and Nerve Impulse Transmission

Neurons transmit information as electrical signals. These impulses travel along the neuron, always in the same direction, and exit through the axon terminal.

Structure of the Nervous System

The Central Nervous System (CNS)

The CNS consists of the brain and the spinal cord.

Components of the Brain

• Cerebrum: Divided into left and right hemispheres separated by a large fissure. It interprets the information sent by the sense organs.
• Cerebellum: Located under the cerebrum, controls posture and balance.
• Brain Stem: Connects the brain to the spinal cord. It regulates vital functions such as heartbeat, breathing, body temperature, and sleep regulation.

The Spinal Cord

The spinal cord is protected by the spinal column and connects the brain to the PNS (Peripheral Nervous System).

A cross-section of the spinal cord shows:

• White Matter: Composed of myelin-covered axons.
• Grey Matter: Composed of cell bodies, axons without myelin, and dendrites.

The Peripheral Nervous System (PNS)

• Somatic Nervous System: Involved in voluntary muscle movements and involuntary reflex arcs.
• Autonomic Nervous System: Controls unconscious, automatic actions. It is divided into:
  • Sympathetic Nervous System
  • Parasympathetic Nervous System