Neurons and the Nervous System

• Behavior and actions are controlled by different parts of the brain
• Neurons facilitate behavior

Learning Outcomes

• Describes the structure and function of neurons and the nervous system
• Explains the roles of heredity and environment on human behavior
• Elaborates on the socio-cultural and ecological factors underlying human behavior

Structure and Function of Neurons

• Neurons are the most basic unit of the brain
• The human brain contains around 150 billion neurons, also known as nerve cells
• Neurons connect to many other neurons, making uncountable connections
• Connections between neurons are called synapses
• A neuron is a specialized cell in the nervous system that receives and sends messages
• Neurons act as messengers in the body and possess a special structure
• Dendrites (branches) receive messages from other cells
• Dendrites attach to the soma (cell body) that contains the nucleus, keeping the cell alive and functional
• Axon, a fiber-like structure attached to the soma, carries messages to other cells
• Neurons make up only 10% of the cells in the brain
• Glial cells compose the other 90% of the brain and creates a sort of structure on which the neurons develop and work, and also as nutrients and oxygen supplier to the neurons
• Schwann cells produce myelin to coat axons and clean up waste products and dead neurons
• Axons may or may not have a myelin sheath which is interrupted at the Nodes of Ranvier
• A neuron with a myelin sheath conducts messages faster
• Bundles of myelin-coated axons travel in cable-like structures
• Axons are coated with a thin membrane called Schwann's membrane which serves as a tunnel through which damaged nerve fibers can repair themselves
• Axons in the brain and spinal cord usually do not have this coating and are therefore more likely to be damaged on a permanent basis

Types of Neurons

• Neurons are divided into three categories based on the number of processes extending directly from the cell body
• Unipolar neurons comprise nerves conducting excitation from sense organs to the brain and spinal cord
• Bipolar neurons have a single axon and dendrite, believed to be the primitive type
• Multipolar neurons possess several short dendrites and a single axon, connecting them to numerous neurons

Functions of Neurons

• Neurons function on the basis of the "All or None" law
• Depending upon stimulus intensity, a neuron will fire with total strength if a critical value is reached, if below it will not fire
• Nerve impulses are electrical events of short duration that move along the axon
• The inside of the membrane has a negative electrical charge when a neuron is not conducting any impulse (resting)
• When the cell membrane is excited by the stimulus, the cell becomes less negative, changing membrane characteristics and opening channels allowing sodium to enter
• Sodium ions are positively charged
• The inside of the cell becomes momentarily positive, resulting in a nerve impulse
• Potassium ions move out to neutralize charges, restoring the potential
• In an axon, the active portion triggers a spike in the next region and so on down the axon
• Once the potential is restored and the stimulation is ready to carry another impulse
• The tip of the axon makes a functional connection with dendrites/cell bodies of the following neurons, called Synapse
• The gap between the axon terminal and dendrite is known as the Synaptic Cleft
• Small packets (vesicles) at the terminal position of the axon contain neurotransmitters
• Neurotransmitters can either excite or inhibit transmission of impulse in the receiving cell

Role of Nervous System and Endocrine System in Human Behavior

• Signals from the nervous system and hormonal release from the endocrine system are internal sources for behavior undertaken as action or reaction
• Nervous System can be categorized based on location and function

Central Nervous System(CNS)

• Comprises the brain and spinal cord within the skull and spine

Peripheral Nervous System (PNS)

• Parts of the nervous system outside the skull and spine that comprises the autonomic and somatic nervous system

Central Nervous System

• Comprised of the brain that continues as the spinal cord in the form of a long stalk
• Integrates sensory organ inputs, analyzes them, and carries out the necessary motor activities
• Relies on the functioning of the spinal cord

Spinal Cord

• It is encased in the spinal column
• There are 31 segments of the spinal cord
• Nerves following the dorsal pathway contain sensory fibers
• Nerves that follow the ventral pathway contain the motor fibers
• The cross-sectional image of the spinal cord appears like a butterfly
• It is mainly comprised of neuron's cell bodies appear grey, and hence known as grey matter
• Outer neuron part, mainly composed of axons and nerves, look white
• The outer section is responsible for carrying to and fro information
• Maintains the reflex action
• Three types of neurons involved include sensory, motor, and interneurons that link the sensory and motor neurons and form the inside of the spinal cord and the brain itself

Brain

• The spinal cord progresses in the upward direction into the brain stem near the junction of the skull
• The brain can be categorized on the basis of the major functions it carries out:
  • Survival
  • Motivation and emotion
  • Higher mental processing
• The brain stem carries out survival functions
• The brain stem covers the medulla, pons, cerebellum, and reticular activating system
• The medulla is responsible for heart rate and breathing
• The pons facilitates the passing of sensory and motor information and is responsible for sleep-arousal, muscle tone, and cardiac reflexes
• The cerebellum transmits information to the higher parts of the brain and controls movements
• Reticular Activating System activates and arouses the central cortex and impacts sleep-arousal, regulation of muscle tone and cardiac reflexes
• Motivation and emotion are controlled by the hypothalamus which also plays a role in controlling the autonomic nervous system
• The thalamus is near the hypothalamus and receives sensory inputs that it transmits to the cerebral hemispheres and other parts of the brain
• Emotional reactions and behaviors are regulated by the limbic system that is comprised of a set of small structures in the deep layers of brain
• The hippocampus is the largest structure which is responsible for making memories
• The amygdala is responsible for people's reactions to dangerous situations and controls fear responses and memory of fear
• The cerebral cortex manages higher Mental Processes and covers the entire surface of the brain
• Though both hemispheres seem identical, they are responsible for different functions
• The left hemisphere is responsible for language aspects
• The right hemisphere is responsible for image formation, spatial relationships, and pattern recognition
• The hemispheres are interconnected with myelinated fibers, Corpus Callosum, for transmitting and receiving information
• On the basis of functionality, the cerebral cortex is divided into four lobes:
  • Frontal
  • Parietal
  • Temporal
  • Occipital
• The frontal lobe is responsible for cognitive functioning and for inhibitory effects
• The parietal lobe is responsible for processing information from touch
• The temporal lobe is responsible for memory and meaning of symbolic sounds
• The occipital lobe is responsible for the visual information
• Functions in the brain are interlinked and well-coordinated

Peripheral Nervous System (PNS)

• It consists of the nerve fibers or axons which carry nerve inputs from the sensory receptors to the body inward to the CNS
• PNS carry nerve impulses for the movement of muscles and excitation of certain glands outward from the CNS
• It comprises of Somatic Nervous System (SNS) and Autonomic Nervous System (ANS)
• SNS takes charge of activating the striped muscles through motor fibers and carries information from the sensing organs through sensory fibers
• All the involuntary muscles, organs, and glands are controlled by ANS
• ANS motor fibers activate smooth muscles, activate secretion from certain glands such as the salivary glands and regulate activities in the heart
• ANS sensory fibers carry information from the internal organs that is perceived as pain, warmth, cold, or pressure

Autonomic Nervous System (ANS)

• It is further categorized as Sympathetic and Parasympathetic Nervous System
• The Sympathetic division is known as the "fight or freezing" system
• The Parasympathetic division (PSD) is known as the “eat-drink- & rest" system and restores the normal functioning after a situation ends

Endocrine Glands

• The nervous system controls the endocrine glands and the secretion of hormones into the bloodstream

• The functioning of the nervous system and endocrine glands is known as Neuro-endocrine System

• Endocrine glands are ductless glands which secrete hormones into the bloodstream to impact a desired organ

• Hormones help elicit a desired behavior

• Hormones are responsible for the functioning of the heart, pancreas, and sex organs and influence our emotional reactions Different types of endocrine glands are:

  • Pituitary
  • Pineal
  • Thyroid
  • Pancreas
  • Gonads
  • Adrenal

• The pituitary gland is known as the master gland and influences other hormone secreting glands and releases hormones for human growth

• The pineal gland regulates the sleep-wake cycle

• The thyroid gland regulates metabolism

• The pancreas controls the level of blood sugar

• The gonads control sexual behavior and reproduction

• The adrenal glands regulate sympathetic arousal, physical stress, psychological stress, salt intake, and the functioning of the nervous system

Heredity and Environment in Human Development

• Genetics is the study of inheritance of physical and psychological makeup among offspring from parents
• Genes are found in chromosomes and have 23 pairs of chromosomes
• The 23rd pair of chromosomes determines the sex of the child
• Genes are the codes or commands responsible for the physical and psychological development of the offspring and do not work within their own system

Nature vs. Nurture

• A child's development is governed by unique genetic makeup and/or the environment
• Both nature and nurture play an important role in a child's development
• Psychologists study twins to understand the role of heredity and environment

Socio-cultural and Ecological Factors Underlying Human Behavior

• An individual's behavior is determined by both genetics and environment
• The term environment encompasses the physical, psychological, and socio-cultural dimensions
• Bronfenbrenner's Ecological Systems Theory focuses on interconnection between the individual and the environment at various levels
• There are five levels at which this interconnection can be understood:
  • Microsystem
  • Mesosystem
  • Exosystem
  • Macrosystem
  • Chronosystem
• The microsystem is the environment that a child comes in immediate contact with
• The mesosystem is the relationship between microsystem components
• The exosystem is the environmental factors that have an indirect effect
• The macrosystem is the culture in which an individual is brought up
• The chronosystem includes changes in the environment that occur over time

Major Social Agents That Guide Human Behavior

• The way individuals perceive, attend, analyze, and reciprocate to the external environment is affected by social agents like:
  • Parents
  • Siblings
  • Peers
  • School
  • Media
• Parents are the first social contact
• Siblings create influence
• School offers the first exposure to a structured social system
• Peers become a major influencing group
• Media are a scientific development that has been identified as a significant contributing agent of socializing
• Human behavior is complex and largely determined by hereditary, environmental, social, and cultural val