Lecture Two: The nervous system

1. Definition:

The nervous system is a very complex system in the body. It has many parts. It is a complex network of nerves and cells that carry messages to and from the brain and spinal cord to various parts of the body. The nervous system is divided into two main systems, the central nervous system (CNS) and the peripheral nervous system (PNS).

The Nervous system has three main functions:

-Sensation: it receives the information from the environment and from inside of the body.

-Integration: it interprets and processes the information (the impulse) it receives to determine the appropriate response.

-Reaction: It makes the body responds to the information. 

1.1. The Central nervous system:

The central nervous system is divided into two parts: the brain and the spinal cord. The average adult human brain weighs 1.3 to 1.4 kg (approximately 3 pounds). The brain contains about 100 billion nerve cells (neurons) and trillions of "support cells". The spinal cord is about 43 cm long in adult women and 45 cm long in adult men and weighs about 35-40 grams. The vertebral column, the collection of bones (back bone) that houses the spinal cord, is about 70 cm long. Therefore, the spinal cord is much shorter than the vertebral column.

1.2. The Peripheral nervous system:

The PNS is made up of nerve cells or neurons that are "wired" together throughout the body, somewhat like communication system. Neurons carry messages in the form of electrical impulses. The messages move from one neuron to another to keep the body functioning.

The Nervous System

2. The nervous System Parts:

            The nervous system contains three main parts: Neurons, Brain and spinal Cord.

2.1. The Neurons:

They are the nerve cells that are specialized for communication. They are able to communicate with neurons and other cell types through specialized junctions called Synapses, at which electrical and electrochemical signals can be transmitted from one call to another.

The neuron is composed of:

-the cell body (the nucleus): It is responsible of sending and receiving signals.

-the axons: they are the extension from the cell. They receive and send the nerve impulses from and to the cell body.

-Dendrites: short branches from the cell body. They receive nerve impulses and send them to the cell body or to the other neuron.

- Terminal Buttons: the terminal (ending) of the axon.

-Myelin Sheath: fatty layer of cells that covers the axons in the brain and the Spinal Cord.

            2.2. The spinal cord:

It is the main pathway for information connecting the brain and peripheral nervous system. It is protected by the bony spinal column shown to the left. The spinal column is made up of bones called vertebrae (backbone).

            It is a cylinder of nerve tissue as thick as finger and about 45 cm long. It begins from the base of the scull and extends throughout most of the backbone. It can handle some information coming from the senses and provide motor responses that do not come from the brain (this is called reflex action).

2.3. The Brain:

It is the control center of the entire body. It receives messages from and to all organs and tissues of the body. It controls both voluntary and involuntary activities. It gives the ability to learn, to reason and to feel. Sensory nerves gather information from the environment; send that info to the spinal cord, which then speed the message to the brain. The brain then makes sense of that message and fires off a response. Motor neurons deliver the instructions from the brain to the rest of your body.

            The brain is made up of three main Parts: 

-          The Cerebrum: The cerebrum or cortex is the largest part of the human brain, associated with higher brain function such as thought and action. The cerebral cortex is divided into four sections, called "lobes": the frontal lobe, parietal lobe, occipital lobe, and temporal lobe.

The Table bellow shows the function of each part of the cerebrum.

Lobes	Function
Frontal lobe	Speech and Movement
Temporal lobe	Hearing and smell
Parietal lobe	Taste and touch
Occipital lobe	Sight

       

-          The Cerebellum: or "little brain", is similar to the cerebrum in that it has two hemispheres and has a highly folded surface or cortex. This structure is associated with regulation and coordination of movement, posture, and balance. The Cerebrum is responsible for thought, memory, sensation and action. It also regulates our higher intellectual processes.

-          The brain stem: This structure is responsible for basic vital life functions such as breathing, heartbeat, and blood pressure. Scientists say that this is the "simplest" part of human brains because animals' entire brains, such as reptiles (who appear early on the evolutionary scale) resemble our brain stem. It acts as a pathway for messages travelling between some parts of the brain and spinal cords.

The brain is composed of two hemispheres:

    - Left hemisphere: controls the right side of the body. It is the center for mathematical skills, speech, writing, and logical thinking (Language, speech, writing, calculation, time sense, rhythm, ordering of complex movement).

   - Right hemisphere: controls the left side of the body. It is the center of creativity, artistic talent, and musical talent (non verbal, perceptual skills, visualization, recognition of patterns, faces, melodies, recognition and expression of emotion, spatial skills, and simple language comprehension.)

3. The levels of Neuroscience studies:

 The nervous system can be studied at four main levels:

   1- The Molecular level: neuroscientists are interested in the morphology and molecular identity and the physiological characteristics of neurons, and how they relate to different types of behavior. They also use biology and genetics to understand how neurons develop and die and how genetic changes affect biological changes. 

   2- The cellular level: they are interested in mechanisms of how neurons process signals which are physiologically and electrochemically studied. 

   3- The System level: they are interested in the circuits. How they are formed as functional circuits. They try to understand how the circuits are are formed and used automatically and physiologically to produce physiological functions (such as: reflexes, motor coordination, emotional responses (like crying and laughing  ...etc) learning, memory ...etc

   4- The cognitive level: they are interested in the ways neurons and their connections are modified by experience.

End of lecture Two.