Biological base of human behavior

Biological base of human behavior

2.1 Introduction

The scientific study of understanding the biology of behavior is called biopsychology. This term covers the broad scope of trying to understand how behavior and cognition are related to complex biological processes. Biopsychologists recognize that the brain does not exist by itself, but works together with aspects of our physical and social environment. Research in recent years has provided clarity to our understanding of how biological processes interact with environmental factors to determine how we think, feel, and behave.

Thus behavior patterns which have developed as a result of evolution are as much a part of the nature of the species as its anatomical structure. The presence of speciestypical behavior or instincts contributes to our understanding of the biology of behavior. Another aspect of the biology of behavior is the study of the relationship between behavioral and mental events – memory, learning, perception, motivation and speech – and processes in the nervous system, particularly the brain.

2.2 Species typical behavior patterns

A species typical behavior pattern is one which is displayed by all normal members of the species under certain circumstances. Species typical behaviors originate from the genetic heritage of the species as it evolved over time. These behaviors are part of the species’ nature. Species typical behavior is dependent on environmental factors when young animals are growing up. Environmental factors are more or less the same for all members of a species as they live in a similar habitat and since species typical behaviors are based on a common genetic heritage, it results in all members of the species displaying the behavior. Within the general framework provided by the species heritage, particular behaviors are learned.

2.3 Brain and behavior

The brain is an extraordinarily complex structure composed of billions of cells working together with one another to enable us to move, perceive, feel and think. A system of thousands of nerves extends through the body, carrying messages to and from the brain, enabling us to do anything from jumping hurdles to memorizing poems. The brain is ultimately responsible for all our actions and is what makes each of us unique.

2.3.1 The Neuron

The brain and nervous system essentially consist of neurons – cells that transmit information throughout the body as well as within the brain. All behavior is a result of neuronal activity. Every movement, thought, depends on what happens at the neuronal level. Each neuron is a tiny information processing system with thousands of connections for receiving and sending signals to other neurons.

 

Structure of neuron. No two neurons are alike, although they share three basic features: dendrites, soma, and an axon. Dendrites carry information toward the cell body, whereas the axons carry information away from it. In many neurons the axon is covered by a sheath of fatty material known as myelin. The myelin sheath has small gaps which play an important role in the neuron’s ability to transmit information. The myelin sheath is produced by glial cells found within the nervous system. The axon divides into small branches near its end called axon terminals which are very closely placed, but do not actually touch other cells. The region at which the axon terminals of a neuron closely approach other cells is known as the synapse.

Function of neuron. The basic function of the neuron is to communicate and pass information to other cells and neurons. How does information travel within the neuron?  When a neuron is at rest, it has a tiny electrical charge, due to the presence of ions in different concentrations inside and outside the cell. Thus, the neuron has a slightly negative charge called resting potential which it achieves by discharging the positively charged ions outside the cell body. This state of rest is changed when the neuron is stimulated, either directly or by messages from other neurons. If stimulation crosses the threshold of the neuron, complex biochemical changes occur in the cell membrane. When these changes occur, the neuron allows positive ions from outside to enter through ion channels, briefly gaining a positive charge. With the entry of the positive ions, the resting potential of the neuron is destroyed. After a brief period, the neuron actively sends the positive ions back outside, regaining the resting potential. Once again the neuron is ready to “fire”. This change in electrical charges from negative to positive and back again is termed action potential. The action potential is an all-or-none response – either it occurs at full strength or does not occur at all.

How does communication between neurons occur? The relaying of information from one neuron to another begins at the synapse. When the action potential reaches the axon terminals, a small amount of neurotransmitter is released into the synaptic gap, the space between the two cells which binds to receptor sites on the membrane of the receiving cell. If there is a sufficient change in electrical charge as a result of this binding, the receiving cell will fire an action potential. Neurotransmitters have either excitatory or inhibitory effects on their target cells – when they are excitatory, they cause the receiving cell to initiate an action  potential; when they are inhibitory, they do not cause an action potential to be fired in the receiving cell.

 

2.3.2 Neurotransmitters

            Neurotransmitters are special chemicals released from axon terminal buttons that cross the synaptic gap and bind to receptor sites on the membrane of another neuron. Neurotransmitters are released when the action potential of the cell is fired. Scientists have identified a number of the chemical substances that act as neurotransmitters at synapses such as serotonin, dopamine, etc. Neurotransmitters have been identified as playing a key role in the onset of mental disorders. For example, the dopamine hypothesis states that an overactivity of dopamine in the brain causes schizophrenia. Research has identified that depression is caused due to a depletion of norepinephrine and serotonin, while an excess of these neurotransmitters leads to mania.

 

2.4 The Nervous System

If neurons are considered the building blocks, then the nervous system is the structure which they build. The nervous system regulates our internal bodily functions and enables us to react to the external world in various ways. The nervous system is often studied as two major parts – the central nervous system and the peripheral nervous system.

 

2.4.1 The Peripheral Nervous System

The peripheral nervous system (PNS) includes all nerves going to and from the brain and spinal cord. The PNS has two major subdivisions – the somatic nervous system and the autonomic nervous system. The PNS works jointly with the central nervous system and the endocrine system in carrying out their functions. The PNS is also responsible for involuntary tasks such as heart rate, digestion, and breathing.

 

2.4.1.1 Somatic Nervous System

The somatic nervous system consists of all the nerves that carry incoming sensory information and outgoing motor information. Incoming information comes to the brain or spinal cord from sense organs and muscles through the afferent nerves. They carry information about external stimulation and the position of the skeletal muscles and limbs. Outgoing information travels through the efferent nerves from the brain or spinal cord in the form of neural impulses. These impulses give instructions for skeletal muscles to contract or relax. The somatic nervous system responds to external stimuli and regulates voluntary actions.

 

2.4.1.2 Autonomic Nervous System

The primary function of the autonomic nervous system is to maintain homeostasis, the body’s steady state of normal functioning. This is done by regulating the endocrine glands, the heart muscle, and the smooth muscles of the blood vessels and internal organs. The autonomic nervous system is divided into two branches: the parasympathetic and the sympathetic. These tend to work in opposition to each other in order to regulate the functioning of target organs like heart, intestines, and the lungs.

 

  • The parasympathetic nervous system is normally dominant when a person is relaxed, and is physically and mentally non-stressed. The main function of the parasympathetic is to slow heart rate, lower blood pressure, and increase digestive and eliminative processes. In other words, it looks after basic housekeeping, and bodily maintenance.
  • When a person is under some stress, either physical or mental, the sympathetic nervous system takes over. The sympathetic nervous system stops digestive and eliminative processes, increases respiration, heart rate, and blood pressure, and causes several hormones to be released into the bloodstream. The result of sympathetic activation is to get more oxygenated blood to the skeletal muscles, thereby, enabling the person to deal better with the source of stress. The sympathetic nervous system is also referred to as the fight-flight system – it prepares the body to fight or flee from the stressful situation.

2.4.2 The Central Nervous System

The central nervous system (CNS) consists of the brain and the spinal cord. The brain is the control centre for all voluntary behavior (driving) and some involuntary behavior (breathing). The spinal cord contains the structures responsible for reflex actions and the nerve fibres that link the brain and other parts of the body.

 

2.4.2.1 Spinal Cord

Extending from the base of the brain down the back, the spinal cord is surrounded and protected by vertebrae. The spinal cord is involved in all voluntary and reflex responses of the body below the neck. It is the communications link between the brain and the body, and relays incoming sensory information to the brain and sends messages from the brain to the muscles. The spinal cord is made up of two major components: gray matter and white matter. The gray matter mostly contains cell bodies, and information is processed here. The white matter contains myelinated axons which transmit information to and from the brain.

 

2.4.2.2 Cerebellum

The cerebellum is located at the base of the brain behind the brain stem, and is a very old structure. The cerebellum is responsible for maintaining smooth movement and coordinating motor activity. The motor control area of the frontal lobe is actually involved in initiating voluntary movements, but it is the cerebellum that makes these movements smooth, coordinated, synchronized, and on target. For example, as we type, it is the cerebellum that enables us to hit the correct keys in order. The cerebellum also controls the automatic adjustments of posture that allow us to stay upright when we walk and that keep us from falling out of our chairs. To know what postural adjustments to make, the cerebellum receives input from all areas of the brain, including the cortex, the subcortex, and the brain stem.

 

2.4.2.3 Brain Stem

The brain stem lies below the subcortical brain areas and in front of the cerebellum. The three major structures of the brain stem are the pons, the medulla, and the reticular activating system. The pons is located in the upper portion of the brain stem, below the subcortex. It is in front of the cerebellum. The pons contains several types of fibres some of which connect the two halves of the cerebellum, whereas others carry visual and auditory information either to the brain or to the cerebellum. Other fibres are associated with respiration, movement, facial expression, and sleep, including the initiating of rapid eye movements (REM) of dream sleep.

The medulla is found below the pons at the bottom of the brain stem, just above the spinal cord. Its functions are similar to those of the pons. As it is an extension of the spinal cord, the medulla has many nerve fibres passing through it which carry information to and from the brain. The medulla also contains nerve fibres that control automatic bodily functions such as respiration.

 

2.4.2.4 Reticular Formation

In the centre of the brain stem, running from the medulla to the midbrain, is a dense network of interconnected neurons. This is called the reticular formation and is involved in the activation of the cerebral cortex (layer of neurons covering the cerebrum of the forebrain). The reticular activating system (RAS) serves as a filter for incoming sensory information. After receiving input from most of the sensory systems, the RAS filters it and rejects unimportant sensory input. As the RAS serves as a sensory filter, it is also important for attention and arousal. The RAS is the part of the brain that plays a key role in sleep and arousal.

2.5 Association Cortex, Behavior and Experience

Most of the cerebral cortex lies outside the primary sensory areas and the principal motor area. These regions in the frontal, parietal, temporal and occipital lobes are called association areas. Association areas in the cortex have no specific motor or sensory functions but involve mental operations such as perception, emotion, memory, language, and thinking. We use these areas when solving a math problem, planning for a trip or sculpting. Association areas organize and integrate sensory information received from other brain areas to enable the person to perform specific functions.

2.5.1 Frontal Lobe Association Cortex

The association area of the frontal lobe is called the prefrontal cortex. The prefrontal cortex monitors emotional behavior and is interconnected with the visual, auditory, and somatosensory cortical areas, with other association cortex in the parietal and temporal lobes, with the thalamus, and a number of other noncortical structures of the brain.

Damage to the prefrontal cortex can severely affect the emotionality of the individual. Lack of restraint, impulsiveness, immaturity in social relationships, and vulgar behavior characterize the personality of the individual after prefrontal damage. Apathy, indifference to others, loss of initiative, decrease in spontaneous talking, and reduced emotional expression are other features which are observed in people with prefrontal cortex damage. Planning, or changing a course of action are severely limited intellectual capacities in a person with damage to the prefrontal cortex.

2.5.2 Parietal Lobe Association Cortex

The parietal lobe association cortex lies behind the primary somatosensory cortex. The association areas of the parietal lobe are involved with integrating information from the environment and integrating tactile information with visual and auditory information. The abilities of using memory to orient oneself in space and identifying objects within that space are controlled by these association areas. For instance, when something touches you on the shoulder, you are able to identify whether that touch is a friend’s tap or something falling on you. This requires integration of the visual and auditory areas of the brain also.

 

When the right parietal association cortex is damaged, patients ignore the left side of space and the left side of the body. This is known as contralateral neglect. Distortions of spatial perception occur. Problems of writing, reading, and doing simple arithmetic problems often show up after damage to the left parietal lobe association cortex. There can also be difficulty with short term memory.

 

2.5.3 Temporal Lobe Association Cortex

The major functions of the temporal lobe located on the sides of the brain are auditory perception, language, memory and some emotional control. The temporal lobes are also important in the formation of new concepts and memories. The hippocampus and the amygdala though not association cortex, seem to work in a similar manner. Auditory and visual agnosia, and impaired attention may occur due to damage to the parietal lobe association cortex. There is difficulty in selecting and focusing on auditory and visual inputs. Damage to Wernicke’s area, which is responsible for thinking and interpreting aspect of language, impairs the understanding of speech and written language.

 

2.6 Hemisphere Function

It is now an established fact that the right and left hemispheres of the brain have different functions. The connecting link between the two hemispheres is the corpus callosum which enables communication between the two halves. The left hemisphere is specialized for language functions – speaking, reading, writing, and understanding language – and is better equipped to think with language symbols, and for analytical functions such as mathematics. The right hemisphere is specialized for nonverbal abilities such as musical abilities, perceptual and “spatio-manipulative” skills like maneuvering through space, drawing or building geometric designs, working puzzles, painting pictures and recognizing faces. However, none of these abilities are located exclusively in the hemispheres; the specializations are only a matter of degree.

 

2.7 Summary

Neurons are the cells responsible for transmitting information throughout the body. Information from one neuron to another is transferred at the synapse via chemicals called neurotransmitters.

The nervous system is divided into the peripheral nervous system consisting of the somatic and autonomic systems and the central nervous system composed of the brain and the spinal cord.

The spinal cord is the communication link between the brain and the rest of the body.

The cerebellum, located at the base of the brain, is responsible for coordinated motor activity.

The brain stem lies in front of the cerebellum and is divided into pons, medulla, and reticular formation.

Studies on left and right brain hemispheres prove that they have different functions. Damage to the association cortex in the lobes can bring about a change in the personality and behavior of an individual.