Brain and Behavior
Brain is the command center of human behavior, controlling everything from basic survival functions to complex thought processes. Understanding how neurons function as a biological computer, the structure of the nervous system, and the role of the cerebral cortex provides valuable insight into how the brain influences perception, decision-making, and actions.
By the end of this section, you should know about:
- Neurons—Building a “Biocomputer”
- What are the major parts of the nervous system?
- Research Methods — Charting the Brain’s Inner Realms
- The Cerebral Cortex — My, what a Wrinkled Brain You Have!
Let’s take a closer look at them.
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Neurons—Building a “Biocomputer”
The brain contains about 100 billion neurons (nerve cells), which create the foundation for all thoughts, emotions, and actions. Neurons carry sensory input to the brain and motor output from the brain to muscles and glands. Supporting these neurons are glial cells, which assist in neuron function. Neurons form intricate networks, linking with each other to enable consciousness, intelligence, and complex behaviors.
Parts of a Neuron
Neurons have four main components:
Dendrites: Tree-like structures that receive signals from other neurons.
Soma (Cell Body): Houses the nucleus and generates nerve impulses.
Axon: A long fiber that transmits the neuron’s signal to other cells; it can range from 0.1 mm to a meter in length.
Axon Terminals: Bulb-like endings that connect with other neurons, passing information through synaptic connections.
In a metaphor, imagine standing in a line of people passing a hand-squeeze signal. The left hand represents dendrites receiving a signal, the body as the soma decides to pass it, and the right hand (axon terminal) sends it to the next person.
The Nerve Impulse
Resting and Action Potential: Ions and Resting Potential: Neurons maintain a resting state with a charge difference, about -60 to -70 millivolts. Positive ions reside outside the neuron, and negative ions inside, creating a resting potential. Threshold and Action Potential: When a neuron’s charge increases to -50 millivolts, it hits the threshold, triggering an action potential (a nerve impulse), which travels down the axon at high speeds. Ion Channels and Propagation: As the action potential travels, ion channels in the axon open, allowing sodium ions to rush in, creating a domino-like wave of electric activity that moves down the axon. This is an “all-or-nothing” process; once it starts, it goes through the axon completely.
After the impulse, potassium ions exit the neuron, restoring its resting potential. The neuron then recharges to prepare for the next impulse.
Saltatory Conduction and Myelin
Myelin Sheath: Some axons are coated in a fatty layer called myelin, which insulates the axon and speeds up the nerve impulse by allowing it to “jump” from gap to gap in a process called saltatory conduction. This rapid conduction is essential for quick responses, such as braking a car. Myelin Damage: In conditions like multiple sclerosis, myelin damage leads to impaired nerve transmission, causing symptoms like numbness, weakness, or paralysis.
Synapses and Neurotransmitters
While the nerve impulse within a neuron is electrical, communication between neurons is chemical. Synapse: A small gap between neurons through which messages are passed. Neurotransmitters: When an action potential reaches the axon terminals, neurotransmitters are released into the synaptic gap, where they attach to receptor sites on the next neuron, altering its likelihood of firing.
Types and Functions of Neurotransmitters
Different neurotransmitters play various roles:
Excitatory vs. Inhibitory: Some neurotransmitters increase the likelihood of firing in the next neuron, while others decrease it. Examples: Dopamine: Associated with pleasure and reward. Serotonin: Linked to mood and emotional states; deficits may lead to depression. Acetylcholine: Essential for muscle activation and movement; its blockage leads to paralysis.
Certain drugs mimic or block neurotransmitters. For instance, cocaine increases dopamine, inducing a “high,” while long-term use disrupts the reward system, leading to addiction. Similarly, curare blocks acetylcholine, preventing muscle activation and causing paralysis.
Neural Regulators and Neuropeptides
Neuropeptides are chemicals that regulate neuron activity rather than directly transmitting signals.
Functions: They influence memory, pain, mood, hunger, and pleasure, among other processes. Endorphins and Enkephalins: These neuropeptides act as natural painkillers, reducing pain under stressful conditions, allowing individuals to tolerate pain better.
Painkilling Effects of Endorphins
Endorphins, a type of neuropeptide, are released in response to pain and stress, producing euphoria and pain relief similar to the effects of morphine. This phenomenon can explain: Placebo Effect: Fake treatments can raise endorphin levels, reducing pain. Runner’s High and Other High-Stress Activities: Activities like running, childbirth, acupuncture, and even extreme sports stimulate endorphin release, inducing feelings of pleasure or “high.” Masochistic Practices: Activities involving controlled pain, like hot saunas followed by cold showers, may lead to increased endorphin levels, explaining the pleasurable sensations afterward.
These insights into endorphin release offer clues for understanding various psychological and neurological issues, including depression, schizophrenia, and addiction.
Brain and Behavior: Neural Networks
Understanding Neural Networks: Neural networks are interconnected groups of neurons that process information in complex ways. Each neuron receives multiple signals from others, which are either excitatory (stimulating firing) or inhibitory (preventing firing). Whether a neuron fires an action potential depends on the balance of these inputs:
Example: A neuron receiving mostly excitatory signals, like a person receiving encouragement to buy something, is more likely to “fire” (send a message). Complexity of Networks: Each neuron may be influenced by hundreds or thousands of other neurons. After firing, neurons reassess new inputs to decide whether to fire again, forming a continuous and dynamic processing system.
Brain Capacity as a “Biocomputer”: Although a single neuron operates slower than a computer, the human brain’s 100 billion neurons and 100 trillion synapses form a highly powerful processing network. Together, these neurons create a computing system far more capable than any machine, enabling advanced cognition, memory, and learning.
Neuroplasticity
The Brain’s Adaptability: Neuroplasticity is the brain’s ability to reorganize itself by forming new connections in response to learning, experience, or injury. For example:
Environmental Influence on Brain Structure: Rats raised in stimulating environments develop more synapses and longer dendrites than those in simpler settings. Adaptation to Injury: People who have lost parts of their brain can often recover function as their brain forms new pathways to compensate.
Brain and Behavior: Neuroplasticity in Adulthood
While adult brains are less adaptable than those of children, they still exhibit neuroplasticity with continued practice and persistence. This capacity for change, even in later life, enables learning, skill development, and recovery from injury.
What are the major parts of the nervous system?
Playing catch may look simply, but it requires complex coordination within the nervous system. This system is divided into two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS).
Brain and Behavior: Central Nervous System (CNS)
The CNS includes:
Brain: Acts as the primary “computer,” processing and interpreting information. For example, Harry uses his brain to estimate when and where the football will arrive. Spinal Cord: Connects the brain to the rest of the body. Messages from the brain travel down the spinal cord and then through nerves to reach different body parts.
The spinal cord not only relays messages but also handles some simple reflexes. For example, if Maya steps on a thorn, a reflex arc within the spinal cord will automatically cause her foot to withdraw before she even consciously feels the pain.
Brain and Behavior: Peripheral Nervous System (PNS)
The PNS connects the CNS to the rest of the body. It includes large bundles of neuron axons, called nerves, which carry sensory and motor messages between the CNS and various body parts. The PNS consists of:
Somatic Nervous System (SNS): Controls voluntary actions, like catching or throwing a ball. Autonomic Nervous System (ANS): Manages involuntary actions, such as heartbeat, digestion, and perspiration.
The ANS has two branches:
Sympathetic Nervous System: Activates the “fight or flight” response during times of danger or high emotion. It increases heart rate, blood pressure, and energy levels. Parasympathetic Nervous System: Calms the body after stress, helping to maintain balanced heart rate and digestion.
Nerve Damage and Regeneration
Unlike neurons within the brain or spinal cord, nerves in the PNS can regenerate if damaged. The neurilemma, a thin layer of cells around PNS axons, forms a “tunnel” that helps guide repair, allowing some recovery in damaged limbs.
Reflexes and the Role of the Spinal Cord
The spinal cord can independently control simple reflexes without involving the brain. When Maya steps on a thorn:
Sensory Neurons in her foot detect pain and send a message to the spinal cord. Connector Neurons relay the signal to Motor Neurons. Effector Cells in her muscles contract, pulling her foot away.
The brain is informed of the pain after the fact, which allows for a rapid protective response.
Advances in Nervous System Repair
Research is making strides in repairing damaged CNS neurons: Cellular Bridges and Nerve Grafts: These methods help severed nerve fibers reconnect. Stem Cells: Injection of stem cells into damaged areas can lead to new neuron growth and repair.
These techniques hold promise for conditions like spinal cord injuries. While still experimental, they offer hope for future treatments that may improve mobility for individuals with CNS injuries.
Protecting the Nervous System
To minimize CNS injury risks: Use seat belts, helmets, and protective gear. Avoid high-risk activities that could damage the head or spinal cord.
With advancements in neuroscience, the potential for repairing brain and spinal cord injuries is becoming more realistic, providing cautious optimism for the future.
Research Methods — Charting the Brain’s Inner Realms
Biopsychology investigates how different parts of the brain contribute to behavior and mental processes. By identifying and understanding specific brain regions, biopsychologists map out how functions are localized within the brain.
Mapping Brain Structure
Dissection: Anatomists study brain anatomy by dissecting human and animal brains, identifying distinct brain areas. CT Scans: Computed Tomography (CT) uses X-rays from various angles to create cross-sectional images of the brain, revealing details such as injuries, tumors, and strokes. MRI Scans: Magnetic Resonance Imaging (MRI) uses magnetic fields to generate detailed 3D images of the brain’s structure. Unlike CT scans, MRIs do not use radiation and allow for a clearer view of brain tissue.
Exploring Brain Function
Determining which parts of the brain control certain functions requires linking behavior or mental abilities to brain structures through several methods:
Clinical Case Studies: Observing changes in behavior or perception following brain injury can indicate which areas control specific functions. For example, personality changes after damage to the frontal lobe can localize emotion regulation to that area. Neurological Soft Signs: Subtle indicators of brain issues, such as poor coordination or clumsiness, help in diagnosing neurological disorders when obvious symptoms are not present. Electrical Stimulation of the Brain (ESB): In ESB, a mild electric current stimulates specific brain areas, allowing researchers to observe corresponding behaviors and sensations, such as feelings or muscle movements. Ablation and Deep Lesioning: In cases where specific brain areas are surgically removed (ablation) or destroyed (deep lesioning), observing changes in behavior helps researchers understand the roles of affected areas.
Measuring Brain Activity Without Invasion
Several methods allow researchers to study brain function non-invasively, providing valuable insights into which parts of the brain are active during different tasks.
Electroencephalography (EEG): EEGs measure brain wave activity using electrodes on the scalp, detecting brain activity associated with different mental states (e.g., sleep, alertness) and identifying irregularities like epilepsy. PET Scans: Positron Emission Tomography (PET) detects energy use in the brain by tracking weakly radioactive glucose, allowing researchers to observe which areas are active during tasks like speaking or thinking. More efficient brain activity often correlates with higher intelligence. fMRI: Functional MRI (fMRI) measures brain activity by detecting changes in blood flow, identifying which areas are involved in tasks. For example, fMRI has shown that the front of the brain is more active when a person lies compared to when they tell the truth.
The Future of Brain Mapping
Advances in brain imaging technologies, including digital 3D brain maps or “atlases,” are making it easier for researchers and clinicians to understand the structure-function relationship within the brain. These tools promise to enhance both medical treatments and our knowledge of complex psychological functions.
These methods are revolutionizing our understanding of the brain, bringing biopsychologists closer to uncovering the “wiring” behind thought, emotion, and behavior.
The Cerebral Cortex
The cerebral cortex plays a fundamental role in what distinguishes human intelligence, enabling complex skills like language, reasoning, and social behaviors. This cortex, covering the upper brain in two halves or hemispheres, has specialized regions called lobes that each serve unique functions and contribute to our overall brain activity.
Hemispheric Differences
The left and right hemispheres of the brain are interconnected by the corpus callosum and are specialized in different cognitive functions. Generally, the left hemisphere is more analytical, responsible for tasks involving language, math, sequential processing, and logical reasoning. For instance, most people use their left brain for understanding and producing language, handling rhythm, and managing complex movements.
In contrast, the right hemisphere excels at holistic processing, such as recognizing faces, patterns, spatial orientation, and interpreting emotional cues. It is also better at recognizing overall context, contributing to humor, irony, and detecting subtle language nuances. However, despite these specialties, both hemispheres usually work in tandem, sharing tasks depending on the complexity of the activity.
Split-Brain Research
Roger Sperry’s pioneering work with “split-brain” patients, who had their corpus callosum severed to treat epilepsy, highlighted the hemispheres’ independence and specialization. In split-brain patients, each hemisphere can function almost independently. For instance, if information is only processed by one hemisphere, the other hemisphere might not be aware, which has led to situations where one hand might act in opposition to the other—a rare phenomenon because normally both hemispheres coordinate through the corpus callosum.
Lobes of the Cerebral Cortex
Each hemisphere has several lobes with distinct roles:
Frontal Lobes: Located at the front of the brain, these lobes are crucial for higher mental functions like planning, self-awareness, and decision-making. They contain the primary motor cortex, which directs voluntary muscle movements, especially those requiring fine motor skills, such as hand movements. The frontal lobes also include mirror neurons, which are involved in mimicking and understanding others’ actions, underpinning social cognition.
Parietal Lobes: Positioned behind the frontal lobes, the parietal lobes integrate sensory information, especially touch, temperature, and pain. This region enables spatial awareness and helps coordinate movement in response to sensory stimuli.
Occipital Lobes: At the back of the brain, the occipital lobes are dedicated to vision. They process visual information and help interpret shapes, colors, and motion, making them essential for recognizing objects and navigating our surroundings.
Temporal Lobes: Located on the sides of the brain, near the temples, the temporal lobes play a key role in processing auditory information and language comprehension. They also support memory formation and emotional responses, contributing significantly to our ability to connect sensory input with memories and emotions.
The cerebral cortex is a thin, wrinkled layer that covers the brain and is essential for human intelligence. It is only 3 millimeters thick but contains 70% of the neurons in the central nervous system, allowing humans to think, use language, and interact in complex ways. Its unique structure, with folds and wrinkles (called corticalization), sets humans apart from most animals, which have smooth and smaller cortices.
Hemispheres of the Brain
The cortex is divided into two hemispheres, left and right, connected by the corpus callosum, a thick bundle of nerve fibers. Each hemisphere controls the opposite side of the body; for example, the left hemisphere controls the right side of the body and vice versa. Damage to a hemisphere can lead to loss of function on the opposite side, as seen in conditions like strokes.
Hemispheric Specialization
Each hemisphere has specialized functions:
Left Hemisphere: Manages language (speech, writing, understanding), math, time, rhythm, and complex movements. Right Hemisphere: Handles perceptual tasks like recognizing patterns, faces, melodies, and interpreting emotions and context. Though limited in language, it helps understand humor, sarcasm, and the “big picture” of conversations.
Split-Brain Studies
Studies on split-brain patients, who have had their corpus callosum cut to treat epilepsy, reveal that the two hemispheres can operate independently. For example, one hemisphere may control one hand while the other controls the opposite hand, leading to situations where one hand might act in conflict with the other. These patients also demonstrate how each hemisphere can process different information simultaneously.
Lobes of the Cerebral Cortex
Each hemisphere is divided into four lobes: frontal, parietal, temporal, and occipital. Each lobe has unique functions crucial for various types of perception, movement, and cognition.
Frontal Lobes
Location: Front of the brain.
Functions: Involved in higher mental abilities, personality, and self-awareness.
Primary Motor Cortex: Controls voluntary muscle movements. Larger areas are devoted to body parts needing fine control, like hands and face.
Broca’s Area: Responsible for speech production, located in the left frontal lobe for most people. Damage here causes expressive aphasia, where speech is slow and labored.
Prefrontal Cortex: Supports reasoning, planning, and complex behaviors. Damage can alter personality, reduce self-control, and impair decision-making. The prefrontal cortex also plays a role in managing impulses and is linked to intelligence.
Parietal Lobes
Location: Top of the brain.
Functions: Processes bodily sensations like touch, temperature, and pain.
Somatosensory Cortex: A specific area here registers touch sensitivity. Areas like hands and lips have larger representations, reflecting their greater sensitivity.
Temporal Lobes
Location: Each side of the brain.
Functions: Key to processing auditory information (hearing) and understanding language.
Primary Auditory Area: First receives sound signals.
Wernicke’s Area: Located in the left temporal lobe, essential for language comprehension. Damage causes receptive aphasia, where speech is fluent but lacks meaning.
Occipital Lobes
Location: Back of the brain.
Functions: Primary visual processing area.
Primary Visual Area: First to receive and interpret visual signals.
Association Areas: Damage here can lead to visual agnosia, where a person can see but not recognize objects (e.g., someone may see a candle but not identify it without touching it). Facial agnosia, or difficulty recognizing faces, is also associated with damage here.
Integration of Functions
While each hemisphere and lobe has specialized functions, both hemispheres work together for most activities. For example, playing a musical instrument involves timing and rhythm from the left hemisphere, while the right hemisphere helps interpret and enjoy melodies.