Innate and Learned Behavior Pt. 2

Learned Behavior

Learned behavior involves the central nervous system and is more difficult to investigate than innate behavior. The Russian physiologist Ivan Pavlov carried out a series of experiments using dogs.

Pavlov inserted a tube through the cheek of his experimental animals that could be used to collect saliva from the  dogs mouth. He then was able to give stimuli and measure the amount of saliva created by the dogs. He found that the saliva was secreted in response to the sight or smell of food. These types of stimulus, to which all dogs respond without learning, are called unconditioned stimuli. After a while, Pavlov observed that the dogs were starting to secrete saliva even before they received the unconditioned stimulus. Something else had turned into a stimulus that allowed the dogs to anticipate the arrival of food. He found that the dogs could learn to use a variety of signals in this way, including the ringing of a bell, the flashing of a light, a metronome ticking or a musical box playing. These are a few examples of conditioned stimuli and the secretion of saliva that these stimuli elicit is the conditioned response.

Conditioned responses are used extensively in many animal’s behavior and can greatly increase survival chances. There are other types of learning as well, which also help animals to survive, reproduce, and pass on their genes.

Innate and Learned Behavior

Innate Behavior:

Some types of animal behavior develop whatever external influences an animal experiences – they develop independently of the environmental context. This is because they are genetically programmed and this is called innate behavior. An example of this would be the movement of Planaria flatworms towards food. Other types of behavior develop as a result of the animal’s experiences. This is called learned behavior. An example is the use by chimpanzees of narrow twigs to extract termites from termite mounds.

Stimulus and Response

The study of the nervous system of animals is called neurobiology. Nervous systems are used in responses to stimuli. A stimulus is a change in the environment, either internal or external, that is detected by a receptor and elicits a response. A response is a change in an organism, produced by a stimulus. Behavior is the pattern of responses in an animal. The simplest example of animal behavior is reflex. A reflex is a rapid unconscious response. An example of a reflex takes place when a young rabbit touches a stinging nettle plant for the first time with the tip of its nose. The rabbit pulls its nose away from the nettle in a fraction of a second. This is called reflex.

Drugs and the Nervous System

Psychoactive drugs

The brain has many synapses that vary in their organization and use a wide variety of neurotransmitters. There are over a hundred different brain neurotransmitters that are known today. “Psychoactive drugs affect the brain and personality by altering the functioning of some of these synapses. Some drugs are excitatory, because they increase post-synaptic transmission. Others are inhibitory because they decrease it.” One example of each will be described below.

Cocaine

“Cocaine acts at synapses that use dopamine as a neurotransmitter. It binds to dopamine reuptake transporters, which are membrane proteins that pump dopamine back into the pre-synaptic neuron. Because cocaine blocks these transporters, dopamine builds up in the synaptic cleft and the post-synaptic neuron is continuously excited. Cocaine is therefore an excitatory psychoactive drug. Synapses that use dopamine are part of what is known as the reward pathway that gives us pleasurable feelings during certain activities. Cocaine gives feelings of euphoria that are not related to any particular activity. Excitatory drugs can also work by mimicking excitatory neurotransmitters, for example, heroin mimics endorphins.”

Tetrahydrocannabinol (THC)

“THC binds to cannabinoid receptors in pre-synaptic membranes. Binding inhibits the release of neurotransmitters that cause excitation of post-synaptic neurons. THC is therefore an inhibitory psychoactive drug. Cannabinoid receptors are found in synapses in various parts of the brain, including the cerebellum, hippocampus and cerebral hemispheres. Several naturally occurring or endogenous cannabinoids have been discovered. They are an unusual example of retrograde signaling as they are released by the post-synaptic neuron and convey a message to the pre-synaptic neuron. People have reported a wide variety of feelings due to THC. The main effects are disruption of psychomotor behavior, short term memory impairment, intoxication and stimulation of appetite.”

Drug addiction and its causes

According to the American Psychiatric Association, addiction is:

“a chronically relapsing disorder that is characterized by three main elements:

• compulsion to seek and take the drug
• loss of control in limiting intake and
• emergence of a negative emotional state when access to the drug is prevented”

Only certain drugs can cause addiction and usually repeated use over a prolonged period of time is needed. If a few drugs are being used at once, addiction will develop more rapidly. Some people are more prone to addiction than others. The causes of addiction are clearly not simple and three areas need to be considered.

1. “Dopamine secretion is associated feelings of well-being and pleasure. Many addictive drugs, including opiates, cocaine, nicotine, and alcohol affect dopamine secreting synapses”
2. “Genetic predisposition is the increased chance of developing an addiction that some people have because of their genes. One example is the gene, DRD2, that codes for the dopamine receptor protein. There are multiple alleles of this gene and a recent study showed that people with one or more copies of the A1 allele consumed less alcohol than those homozygous for the A2 allele.”
3. “Social factors affect drug use and addiction. Cultural traditions are very important and help to explain why different drugs cause problems in different parts of the world. Peer pressure, poverty and social deprivation, traumatic life experiences and mental health problems may also contribute.”

Synapses & Neurotransmitters

Synaptic transmission is when an action potential in the pre-synaptic neuron causes release of neurotransmitter into the synaptic cleft, and an action potential is stimulated in the post-synaptic neuron when the neurotransmitter binds to receptors in its membrane. There are two additional features of synapses:

• “More than one pre-synaptic neuron can form a synapse with the same post-synaptic neuron. Sometimes there are hundreds of synaptic neurons! Usually a single release of neurotransmitter from one of the presynaptic neuron is insufficient to trigger an action potential. Either one pre-synaptic neuron is insufficient to trigger an action potential. Either one pre-synaptic neuron must repeatedly release neurotransmitter, or several different pre-synaptic neurons must release neurotransmitter together. The addictive effect from multiple releases of neurotransmitter called summation”
• “Neurotransmitters that stimulate action potentials in the post-synaptic neuron are called excitatory. Some neurotransmitters have a different effect- they inhibit action potentials. Neurons only release one type of neurotransmitter. Pre-synaptic neurons therefore either excite or inhibit post-synaptic transmission- they cannot do both.”

There can be interaction between the activities of the excitatory and inhibitory neurons where there are many sunapses between pre-synaptic neurons and a post-synaptic neuron. “Whether a post-synaptic action potential is excited or not is a summation of the effects of all of these neurons. This is the basis of decision-making processes in the central nervous system.”

Perception: Hearing

Neurobiology and behavior

Neurobiology is the study of the nervous system of animals. Nervous systems are used in responses to stimuli. A stimulus is a change in the environment, either internal or external, that is detected by a receptor and elicits a response. A response is known as a change in an organism, produced by a stimulus. The pattern of responses in an animal is known as behavior. Reflex is the simplest example of animal behavior. A reflex is a rapid unconscious response. 

Perception of stimuli

“Stimuli are detected by receptors. Some receptors are nerve endings of sensory neurons, e.g. pain receptors. Others are special cells located in a sense organ. Animals can detect a wide variety of stimuli, using different types of receptor. Humans have the following types of receptor.”

• Mechanoreceptors perceive movements, for example sound vibrations or movements due to pressure or gravity.
• Chemoreceptors perceive chemical substances, either dissolved in water or as vapors diffusing through the air.
• Thermoreceptors perceive heat and temperature
• Photoreceptors perceive electromagnetic radiation, usually light.

Perception: Vision

Visual Perception:

“Light entering the eye is focused to form an image on the retina.” The rod and cone cells detect the image and convert it into nerve impulses. Rods are very sensitive to light, so they work well in dim light conditions. “In bright light the pigment in them is temporarily bleached, and they do not work. Rod cells absorb all the visible wavelengths of light and do not distinguish between them so they give us black and white vision.”

“Groups of rod cells send impulses to the brain by a single neuron, so the brain cannot distinguish which rod absorbed the light. Therefore, there is less detail in images transmitted to the brain by rods than by the cones, each of which sends impulses to the brain by an individual neuron. There are three types of cones, which absorb different ranges of wavelength of light. They are named according to the colour that they absorb most: red, blue or green. By analysing the amount that each of the three cone types are stimulated, the colour of light can be very precisely determined. Cones are only stimulated by bright light and therefore the colour vision that they give us does not work well in dim light.”

Processing of visual stimuli:

“The retina begins the task of processing the visual stimuli that are collected by the rods and cones and passed by the bipolar cells to the ganglion cells. Each ganglion cell is stimulated when light falls on a small circular area of the retina called the receptive field. There are two types of ganglion cells. Both types are more stimulated if the edge of light/ dark areas is within the receptive field. This is called the edge enhancement and it is the explanation of an optical illusion called the Hermann grid.”

“Contra-lateral processing of visual stimuli means that some impulses from the left eye pass to the right side of the brain, and vice versa. This exchange happens in the optic chiasma. The right side of the brain processes visual stimuli from the left side of the visual field, and vice versa.”

 

 

Homeostasis: Glucose Regulation

Blood Glucose

Blood glucose levels are monitored by cells in the pancreas. If the concentration deviates from the set point of about 5 mmol dm-3, then homeostatic mechanisms involving the pancreatic hormones insulin and glucagon are initiated. Diabetes is a condition where an individual has consistently elevated levels of sugar in the blood. Type 1 diabetes is caused by an inability to produce sufficient quantities of insulin. Due to this, Type 1 diabetes is often referred to as insulin dependent diabetes. Type 2 diabetes is characterized by an inability to process or respond to insulin due to a deficiency of insulin receptors on target cells.

Homeostasis: Thermoregulation

Homeostasis

The human body maintains many variables within defined limits, such as body temperature, blood pH, carbon dioxide concentration, and blood glucose concentration. The nervous system and the endocrine system play a role in homeostasis by monitoring levels of variables and correcting changes in levels by negative feedback mechanisms. Negative feedback involves the product of a process inhibiting the process that creates the product.

An example of homeostatic mechanism would be thermoregulation. Cells in the hypothalamus, which is a part of the brain, detect changes in the core body temperature, and effect different responses depending on whether the core body temperature is above or below the set point (37degrees Celsius).

Neurons & Synapses

Synapses

A synapse is the junction between two neurons, where there is a fluid filled gap that separates the plasma membranes of the two neurons.

1. The nerve impulse causes Ca2+ ions to diffuse through channels in the membrane.
2. The calcium influx causes vesicles containing neurotransmitters to fuse with the pre-synaptic membrane.
3. Neurotransmitter is released into the synaptic cleft. The fusion of vesicles leading to the release of materials outside the cell is called exocytosis.
4. The neurotransmitter diffuses across the synaptic cleft and binds to receptors on the post-synaptic membrane.
5. The binding of the neurotransmitter to the receptors causes adjacent ion channels to open, and ions diffuse down their concentration gradient.
6. The local depolarization leads to an impulse being generated in the post-synaptic neuron.
7. The neurotransmitter is either recycles, degraded or diffuses out of the synapse.