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The Human Nervous System Lecture

01/24/01 and 01/25/01

The nervous system can be broken into two(2) parts, the central nervous system (CNS) and the peripheral nervous system (PNS). Further more, the brain and the spinal cord makes up the CNS, while the sensory nerves and the motor nerves makes up the PNS. The PNS is composed of the sense organs (e.g. the eye, the ear, touches nerve cells, taste buds, and olfactory nerve cells). The somatic nervous system and the autonomic nervous system are the parts of the motor nerves.

The nervous system acts like a telephone system. Information is transmitted from and to the brain, the brain receiving information from the sensory nerves, and from the motor nerves. Information about our environment is received by the sensory nerves, then sent to our brain. At the same time, information about our bodies (e.g. we are hungry) is received by the motor nerves, and it too is sent to the brain.

Our brain controls how our body acts. Voluntary movement is initiated by our brain, and sent to the somatic nervous system, which controls our biceps, triceps and other voluntary muscles. Involuntary movement, such the beating of our hearts, does not need our brain to work. But we when we run, our bodies need more oxygen, so our brain tells the autonomic nervous system that controls our heart to beat faster. One very easy difference to see between the two nervous system's is that the autonomic nervous system system act much faster.

The Sense Organs

Our sensory system is composed of many different types of nerve cells. Nerve receptors are the nerve cells that receive information about our environment (e.g.: pain, cold, heat, sweet taste, bitter taste, etc.). The following table defines important terms used to describe our sensory nerves.

  Sense Cell

Function

Mechanoreceptors

Nerve receptor cells that detect different senses of touch, and hearing.

Meissner's corpuscles

Nerve receptor cells that tells the brain the shape and feel of an object in the hand, or the touch of a kiss, always adjusting to a constantly changing environment, which is why the brain eventually ignores clothing that you are wearing.

Pacinian corpuscle

Nerve receptor cells that detects pressure, telling the brain when a limb has moved. After the brain has told a limb, such as an arm, to move into the correct position.

Free Nerve endings

Nerve receptor cells that inform the brain about pain, and are located over the entire body.

Thermoreceptors

Nerve receptor cells that detect changes in temperature.

Ruffini's end-organ

A type of nerve receptor cell that detects heat.

End bulb of Krause

A type of nerve receptor cell that detects cold.

Chemoreceptors

Nerve receptor cells that detect chemicals in food, drink, or in the air.

Odor

A collection of chemicals drifting in the air.

Olfactory bulb

A collection of nerve receptor cells that detect chemicals drifting in the air.

Olfactory tract

The nerve cells that send information about odor to the brain. They act like a phone line.

Taste buds

Nerve receptor cells that detect different types of flavors.

Photoreceptors

Nerve receptor cells that detect light, depth, and color.

Retina

A layer in the back of the eye that contains the eye's photoreceptors.

Rods

The photoreceptor that detects light the best. There are 20X more rods than cones.

Cones

The photoreceptor that detects color.

At the heart of the nervous system is the brain. The brain can be broken into five (5) parts, the Brain stem, the thalamus, the hypothalamus, the cerebellum, and the cerebrum. The following table defines the roles of each part of the brain.

Anatomy of the Brain

Section of Brain
Function
Brain stem The heart, and the most primitive part of the brain. It is here that the brain controls the body's most basic functions, such as our heart rate, respiration, blood pressure, swallowing, coughing, sneezing, and vomiting.
Thalamus Just like the phone jack in the back of your computer, this is the main information pathway between the brain and the spinal cord and cerebrum.
Hypothalamus This is the control center for our body's homeostasis. It is this part of the brain that controls our body temperature, appetite, thirst for water (keeping us hydrated), desire for salt (keeping our pH steady), and endocrine system (hormones).
Cerebellum This section of our brain keeps our movements smooth and coordinated, helping maintain our posture, muscle tone, and equilibrium (balance).
Cerebrum This is what gives humans intellect, the power of speech, and memory.

A nerve cell, or neuron acts exactly like a phone line or a computer cable. It conducts electrons, just like wires and electricity. For these electrons to be able to move from one cell to another, they must pass through across a gap or space lying between two nerve cells called a synapse. But electrons cannot move across a gap. So a neurotransmitter carries these electrons across the synapse, in the form of Na+ and K+. As the neurotransmitter move across the synapse, they are caught by a receptor protein on the next nerve cell.

Nerve cells never touch each other, or other cells. If they did, the nervous system would short out like an unprotected wire. So nerve cells are coated by a myelin sheath, which insulates it from other cells in our body, exactly like the plastic coating on an electrical wire, or computer cable. This myelin sheath prevents the nerve cells from shorten out, and keeps our bodies working properly.

The following diagram shows the parts of a neuron.