The Nervous System, Ear, Hearing and Balance — Page 60, Lesson 93

I want to walk you through the nervous system, because before we can properly understand how the ear and the eye work, we need to understand the network that carries their signals. Our knowledge of the outside world comes through our sense organs — our eyes and ears provide the majority of that information, but there are other sources too. Your senses of taste and smell both send information to your brain. Special nerve cells in your skin tell you about touch sensations, temperature changes, and also detect pain. Inside your muscles, there are pressure-sensitive cells that help you appreciate the relative position of your limbs and detect the effects of gravity. Now, the nervous system itself is the most complex system in the human body. It is responsible for sending, receiving, and processing nerve impulses. It serves as the body's control centre and its electrical-chemical communications network. It integrates countless pieces of information and generates reactions by sending these electrochemical impulses through the nerves to trigger organs such as muscles or glands. Every muscle and organ in your body relies on these nervous impulses to function. The nerve cells themselves are called neurons. The connection between two neurons is called the synapse. When a nerve impulse — which is electrical — travels across a neuron to the synapse, it causes a release of chemicals that carry the signal to the next neuron. So messages are sent through the nervous system by both electrical and chemical means — we call that electrochemical transmission. Three systems work together to carry out the mission of the nervous system. Let me take them one at a time. First is the Central Nervous System, or CNS. It is encased in bone — that means the brain is protected by the skull and the spinal cord is protected by the vertebral column. The CNS is responsible for issuing nerve impulses and analysing sensory data. It consists of the brain and the spinal cord. The brain weighs an average of 1.4 kilograms and comprises 97% of the entire nervous system. Nerve pathways extend from the brain to virtually every tissue and structure of the body. Second is the Peripheral Nervous System, or PNS. The PNS is made up of a series of specialised cells that do two things. First, they pass information received from the body organs and muscles back to the CNS through sensory nerves. Second, they pass information from the CNS directly to the organs and muscles themselves through motor nerves. This means it is not always necessary for the brain to be involved in the reaction to a stimulus. For example, reflex actions — such as quickly withdrawing your hand from a painful stimulus — only involve a loop between the hand and the spinal cord. Because fewer neurons are involved, reflexes are rapid. Third is the Autonomic Nervous System, also called the Vegetative Nervous System, or ANS. This is a special autonomic — meaning independent — nervous system that manages the glands of the body and the involuntary muscles of the internal organs and blood vessels. Although the autonomic nerves have connections with the CNS, we are not aware of the autonomic system working and we have no conscious control over it. It runs automatically, without our thinking about it. So to summarise: the CNS is the command centre, the PNS is the communication network that carries signals to and from the CNS, and the ANS handles the automatic functions we don't consciously control. That's the foundation we need before we move on to the ear, hearing, and balance.