Introduction to Psychology/Biological basis of behavior

The physical structure of the body plays an important role in the behavior of an individual. The most important physical structure for psychologists is the nervous system. The nervous system carries orders from the brain and spinal cord to various glands and muscles, it also carries signals from stimuli receptors to the spinal cord and brain. If you wanted to blink your eye a signal would be created in the brain, then it would be transported by neurons to the muscle controlling the eyelid.

Neurons

The base of the nervous system is the neuron. Neurons are cells that are specialized for communicating information. They are the basic tissue and element of the nervous system.

Neurons have a basic structure of:

• One cell body
• One axon
• One or more dendrites

The cell body (or soma) is the bulbous end of a neuron, containing the cell nucleus. The soma makes use of nutrients to supply energy for neuronal activity.

Axons are organelles that carry information away from the cell body. Axons may be as small as several microns or as long as several meters in giraffes and whales. The axons main job is to send a signal to the dendrites of another neuron, but some say that they may also receive signals in certain situations. Each neuron has only one axon, but the axon may have branches with what are called terminal buttons at its end.

Dendrites are organelles that sense the neurotransmitter secreted by the axon of another neuron. Most neurons have more than one dendrite. Dendrites and axons do not directly touch each other; there is a gap, called a synapse.

The Transmission of the Signal

The transmission of the signal is basically the same in all cells, the signal is sent across the synapse by the axon and the dendrite of the next cell picks up the signal.

Synapse

The synapse is a gap between two cells. Synapse are one way junctions between neurons and other cells. The neurotransmitter is emitted from the axon of one cell and usually goes to the dendrite of the next cell. Sometimes the signal goes to the soma or the axon of the next cell instead of the dendrite (Arnold Wittig 2001).

Neurotransmitter

The terminal button at the end of the axon holds the synaptic vesicles. When the signal reaches the end of the axon the vesicles discharge a chemical called a neurotransmitter. Neurotransmitters are chemicals that are used to relay, amplify and modulate electrical signals between a neuron and another cell. There are approximately 40 to 60 different chemicals that are used as neurotransmitters. The neurotransmitters from the axon fit into receptors of the dendrite on the next neuron. They will then either excite the cell and make it fire or inhibit it and stop it from doing so. The sum of the excitation and inhibition of the neuron is called the graded potential. If the graded potential is greater than that cell's threshold then the cell fires, sending the message to the next cell. Goto Here to see a list of some neurotransmitters.

Resting Potential

When the cell hasn't fired for a certain amount of time it is considered at its resting potential. The resting potential of a neuron is approx. -70 mV because the membrane surrounding the cell lets in positive potassium ions (K+) and negative chloride ions (Cl-) and keeps out positive sodium ions (Na+). It is easier to fire a cell that is at its resting potential than one that is in the refractory phase.

Action potential

When the graded potential passes the neurons threshold, an action potential takes place. The action potential sends the signal the entire length of the cell and never dies within the cell, which can be referred to as the all-or-none-principle. During firing the inside of the cell becomes positive, which is sometimes incorrectly called Depolarization and should be called the raising phase of the action potential. After the action potential hits its peak the cell starts the refractory phase.

Refractory Phase

After the action potential changes the neuron from negative to positive there is a refractory phase where it changes back to negative. At the beginning of this period it is impossible for another signal to be transmitted, this is called absolute refractory phase. After the absolute refractory phase is the relative refractory phase where it is possible to send another signal but more excitation than normal is needed.

Signal Strength

For the signal to be passed from one neuron to the next it must have enough energy to break a point called the threshold. Once the threshold is broken the signal is transmitted. The neuron fires at the same strength every time. The strength of a signal is decided by how many different neurons are being fired and at what frequency they are being fired.

Glial cells

The amount of glial cells to every neuron in the nervous system is disputed. Glial cells function as support for the neurons; they produce the myelin sheath which surrounds some neurons and also form part of the blood-brain barrier. The blood-brain barrier is a structure that prevents certain substances in the bloodstream from reaching the brain. Many axons are sheathed with tubes of myelin, which is a fatty material. Myelin is produced by the glial cells. The myelin sheaths on axons have gaps, which are called the nodes of Ranvier. Myelinated sheaths help transmit information quickly and efficiently.

Organization of the nervous system

The neurons can all be placed in one of two systems, the central nervous system or the peripheral nervous system.

The Central nervous system

The central nervous system has a fundamental role in the control of behavior. It contains the brain and the spinal cord which are both encased in bone which shows their importance. Both the brain and spinal cord receive signals from the afferent neurons and send signals to muscles and glands through efferent neurons.

The peripheral nervous system

Any part of the nervous system that is not part of the central nervous system is part of the peripheral nervous system. The nerves in the peripheral nervous system are split up into the autonomic and somatic. The somatic connect the central nervous system to sensory organs (such as the eye and ear) and muscles, while the autonomic connect other organs of the body, blood vessels and glands.

The glandular systems

The body has two types of glandular systems: the endocrine, which generally secrete hormones through the bloodstream, and the exocrine which secrete fluids to the outer surfaces of the body, such as sweating.

Exocrine glands

Exocrine glands release their secretions into ducts which in turn release them onto the surface of organs. Examples of exocrine glands are sweat glands, salivary glands, mammary glands, etc. The pancreas is both an exocrine as well as an endocrine gland hence, one should keep that in mind. It secretes digestive enzymes that are released into the digestive system while it also contains the Islets of Langerhans which secrete insulin into the blood.

Endocrine glands

• Pituitary Gland

• Adrenal Cortex
• Adrenal Medula
• Thyroid Gland
• Parathyroid Gland
• Islets of Langerhans
• Gonads
• Placenta

Structure and function of the brain

The brain is split up into three major layers, the hindbrain is the first, the second is the midbrain, and the forebrain is last.

Hindbrain

A diagram showing the parts of the brainstem

The hindbrain is a well protected central core of the brain and includes the cerebellum, reticular formation, and the brain stem. The cerebellum plays an important role in the integration of sensory perception and motor output. It utilizes constant feedback on body position to fine-tune motor movements. The brain stem contains the pons, and the medulla oblongata. The pons relays sensory information between the cerebellum and cerebrum. The medulla oblongata is the lower portion of the brainstem. It controls autonomic functions such as breathing and vomiting, and relays nerve signals between the brain and spinal cord. The reticular formation is a part of the brain which is involved in stereotypical actions, such as walking, sleeping, and lying down.

Midbrain

This part of the brain is located between the hindbrain and the forebrain making up part of the brain stem. All sensory and motor information going to and from the fore brain and the spinal cord must pass through the midbrain. It can also be referred to as the relay station.

Forebrain

The anteriormost division of the developing vertebrate brain that contains the most complex neural network in the CNS. The forebrain has two major divisions, the lower diencephalon, which contains the thalamus and the hypothalmus, and the upper telencephalon, which contains the cerebrum.

Methods for observing or evaluating brain activity

In the past only two methods of observation were available. The first was observing individuals who have received brain damage and assume that the part of the brain that was damaged controlled the behavior or sense that had changed. The second was connecting electrodes to the outside of someone's head and recording the readings.

Newer methods include computed tomography (CT scan), positron emission tomography (PET scan), magnetic resonance imaging (MRI), and superconductive quantum interference devices (SQUID).

Footnotes

(1) Voltage is a measure of the potential energy that moves electrons in an electric current: the electron motive force (force, in physics, is the first derivative of energy) The flow of electric current is from a relatively positive potential to a relatively negative potential. Voltages are not absolute, but only exist as differences in potential between any two given points.

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