Human Nervous System & Sensory Organs: Structure and Health

The Sense of Hearing and Balance

The sense of hearing is located inside the ears. Sounds are vibrations of the air, received by the eardrum. The eardrum transfers these vibrations to the cochlea, where mechanoreceptors transform them into nerve impulses. These impulses then travel to the brain through the auditory nerve. The sense of balance is also located inside the ear and is closely related to the sense of hearing.

The Organization of the Nervous System

The nervous system is composed of two main divisions: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

Central Nervous System (CNS)

• The Central Nervous System (CNS) comprises the brain and the spinal cord. This is where all stimuli are evaluated, and responses are generated.

  • The Brain

    Protected by the skull, the brain has three primary parts:

    • Cerebrum: Controls voluntary actions, emotional processes, and intellectual functions.
    • Cerebellum: Manages posture, balance, and movement coordination.
    • Brain Stem: Connects the brain to the spinal cord and regulates involuntary actions like heartbeat and body temperature.

  • The Spinal Cord

    Protected by the spinal column, the spinal cord connects the PNS with the brain and controls reflex actions.

Peripheral Nervous System (PNS)

• The Peripheral Nervous System (PNS) consists of all the nerves that extend into and out of the CNS.

Health of the Nervous System and Sense Organs

Common Nervous System Conditions

• Meningitis

  An inflammation (due to infection) of the layers protecting the brain and spinal cord (the meninges), which can lead to death.

• Multiple Sclerosis

  Damage to the axons of neurons reduces the transmission of nerve impulses, often leading to paralysis.

• Alzheimer’s Disease

  A type of dementia caused by a loss of neurons and connections in the brain.

• Parkinson’s Disease

  Neurons controlling movement and posture die, causing the patient to lose muscle control.

Mental Health Disorders

• Mental Disorders

  These conditions produce changes in thought, emotions, and behavior:

  • Phobias

    Irrational fears that cause significant anxiety.

  • Dementia

    A decline in brain function, affecting memory, thinking, and reasoning.

  • Depression

    A mood disorder that interferes with daily life, often caused by changes in brain chemistry. It is not merely sadness. Symptoms include persistent mood changes, frustration, insomnia, lack of energy, loss of pleasure in activities, hopelessness, and social withdrawal.

Eye Anomalies

• Myopia (Nearsightedness)

  The eye becomes too long, preventing images from focusing correctly on the retina. This results in difficulty seeing distant objects clearly. It is corrected with specific lenses or glasses.

• Hypermetropia (Farsightedness)

  The opposite of myopia, where the eye is too short. Patients have difficulty seeing close objects clearly. It is also corrected with specific lenses or glasses, different from those used for myopia.