The Biological Perspective: Neurons, Systems, and Brain Structures

Overview of the Nervous System and Neuroscience

• The nervous system is defined as an extensive network of specialized cells that carry information to and from all parts of the body.

• Neuroscience is the field of study that deals with the structure and function of neurons, nerves, and nervous tissue. It specifically examines the relationship between these biological structures and behavior and learning.

The Structure of the Neuron

• The neuron is the basic cell that makes up the entire nervous system. Its primary role is to receive and send messages within that system.

• Parts of a neuron:

  • Dendrites: These are branch-like structures that receive incoming messages from other neurons.

  • Soma: This is the cell body of the neuron. It is responsible for maintaining the life and vital functions of the cell.

  • Axon: A long, tube-like structure that carries the neural message away from the soma to other cells.

  • Axon terminals: Rounded areas at the end of the branches at the end of the axon. These are responsible for communicating with other nerve cells.

• Other types of brain cells:

  • Glial cells: These are grey fatty cells that provide support for neurons to grow on and around. They deliver nutrients to neurons, produce myelin to coat axons, and clean up waste products and dead neurons.

  • Myelin: A fatty substance produced by certain glial cells that coats the axons of neurons. It serves to insulate, protect, and speed up the neural impulse.

The Neural Impulse: Generating the Message

• The neural impulse involves the movement of ions, which are charged particles.

  • Inside the neuron: Negatively charged.

  • Outside the neuron: Positively charged.

• Potential states:

  • Resting potential: The state of a neuron when it is not firing a neural impulse.

  • Action potential: The release of the neural impulse, consisting of a reversal of the electrical charge within the axon. This process allows positive sodium ions to enter the cell.

• Mechanism of firing:

  • Threshold: The cell reaches a specific threshold to trigger an action potential.

  • All-or-none: A neuron either fires completely or it does not fire at all.

  • Timing: Voltage readings at a given place on the neuron occur over a period of approximately $20$ or $30\,ms$ (thousandths of a second).

  • Hyperpolarization: After firing, the cell undergoes a brief hyperpolarization period before returning to its resting potential.

Communication Between Neurons

• Neurons communicate across a microscopic fluid-filled space called the synapse or synaptic gap. This space exists between the axon terminals of one cell and the dendrites or surface of the next cell.

• Key components of communication:

  • Synaptic vesicles: Sack-like structures found inside the axon terminal that contain chemicals.

  • Neurotransmitters: Chemicals found in the synaptic vesicles which, when released, have a specific effect on the next cell.

  • Receptor sites: Holes in the surface of the dendrites (or certain cells of the muscles and glands) shaped to fit only certain neurotransmitters.

• Types of neurotransmitters:

  • Excitatory: Causes the receiving cell to fire.

  • Inhibitory: Causes the receiving cell to stop firing.

• Chemical interactions:

  • Agonists: Mimic or enhance the effects of a neurotransmitter on the receptor sites, thereby increasing or decreasing the activity of that cell.

  • Antagonists: Block or reduce a cell's response to the action of other chemicals or neurotransmitters.

• Cleaning up the synapse:

  • Reuptake: The process by which neurotransmitters are taken back into the synaptic vesicles. For example, dopamine is removed from the synapse by reuptake sites.

  • Cocaine impact: Cocaine blocks dopamine reuptake sites, allowing dopamine to remain active in the synapse for a longer duration.

  • Enzymes: Complex proteins manufactured by cells. One specific enzyme breaks up acetylcholine because muscle activity must happen rapidly; the reuptake process would be too slow for this function.

The Central and Peripheral Nervous Systems

• The Central Nervous System (CNS) consists of the brain and the spinal cord.

  • Spinal cord: A long bundle of neurons that carries messages to and from the body to the brain. It is responsible for very fast, life-saving reflexes.

• The Reflex Arc involves three types of neurons:

  • Sensory neuron (Afferent neuron): Carries information from the senses to the CNS.

  • Motor neuron (Efferent neuron): Carries messages from the CNS to the muscles of the body.

  • Interneuron: Found in the center of the spinal cord; it receives information from sensory neurons and sends commands to muscles through motor neurons. Interneurons make up the bulk of the neurons in the brain.

• The Peripheral Nervous System (PNS) includes all nerves and neurons not contained in the brain or spinal cord. It is divided into two systems:

  • Somatic Nervous System: Consists of nerves carrying information from the senses to the CNS and from the CNS to the voluntary muscles of the body.

  • Sensory pathway: Nerves from sensory organs to the CNS (sensory neurons).

  • Motor pathway: Nerves from the CNS to voluntary muscles (motor neurons).

  • Autonomic Nervous System (ANS): Controls all involuntary muscles, organs, and glands. It has two divisions:

  • Sympathetic division: The "fight-or-flight" system; reacts to stressful events and handles bodily arousal.

  • Parasympathetic division: Restores the body to normal functioning after arousal and manages day-to-day functions of organs and glands.

• Neuroplasticity is the ability of the brain to constantly change both the structure and function of cells in response to experience or trauma.

The Endocrine Glands and Hormones

• Endocrine glands secrete chemicals called hormones directly into the bloodstream.

• Major endocrine glands:

  • Pituitary gland: Located in the brain, known as the "master gland." It secretes human growth hormone and influences all other hormone-secreting glands.

  • Pineal gland: Located near the base of the cerebrum; it secretes melatonin.

  • Thyroid gland: Located in the neck; it regulates metabolism.

  • Pancreas: Controls the levels of sugar in the blood.

  • Gonads: The sex glands (ovaries in females, testes in males) that regulate sexual development, behavior, and reproduction.

  • Adrenal glands: Located on top of each kidney; they secrete over $30$ different hormones to deal with stress and regulate salt intake. They also provide a secondary source of sex hormones for adolescent sexual changes.

Research Methods and Brain Imaging

• Clinical Studies:

  • Deep lesioning: Inserting a thin, insulated wire into the brain and sending an electrical current to destroy brain cells at the tip of the wire.

  • Electrical stimulation of the brain (ESB): Using a milder electrical current to cause neurons to react as if they received a message.

  • Transcranial magnetic stimulation (TMS): Applying magnetic pulses to the cortex using copper wire coils positioned over the head.

  • Repetitive TMS (rTMS) and Transcranial direct current stimulation (tDCS).

  • Analysis of human brain damage.

• Mapping Structure:

  • Computed tomography (CT): Using computer-controlled X-rays of the brain.

  • Magnetic resonance imaging (MRI): Using radio waves and magnetic fields to produce detailed images of the brain.

• Mapping Function:

  • Electroencephalogram (EEG): Recording electrical activity of the brain below specific areas of the skull.

  • Magnetoencephalography (MEG).

  • Positron emission tomography (PET): Injecting radioactive sugar into the subject to create a color-coded image of brain activity. Lighter colors indicate higher levels of activity.

  • Single photon emission computed tomography (SPECT): Uses radioactive tracers similar to PET.

  • Functional MRI (fMRI): A computer tracks changes in brain activity over time, essentially creating a "movie" of brain function.

Structures of the Hindbrain and Limbic System

• The Hindbrain:

  • Medulla: Located at the top of the spinal cord; responsible for life-sustaining functions such as breathing, swallowing, and heart rate.

  • Pons: Located above the medulla; connects the top and bottom of the brain. It plays a role in sleep, dreaming, left-right body coordination, and arousal.

  • Reticular formation (RF): A network of neurons running through the middle of the medulla and pons; responsible for selective attention.

  • Cerebellum: Located behind the pons; controls and coordinates involuntary, rapid, and fine motor movements.

• The Limbic System (involved in learning, emotion, memory, and motivation):

  • Thalamus: Located in the center of the brain; relays sensory information from the lower brain to the appropriate areas of the cortex and processes some information before relaying it.

  • Hypothalamus: Located below the thalamus and above the pituitary gland; responsible for motivational behaviors like sleep, hunger, thirst, and sex.

  • Hippocampus: Located within each temporal lobe; responsible for forming long-term memories and storing memory for the location of objects.

  • Amygdala: Located near the hippocampus; responsible for fear responses and the memory of fear.

  • Cingulate cortex: The limbic structure found in the cortex; involved in cognitive and emotional processing.

The Cerebral Cortex and Lobes

• The cortex is the outermost covering of the brain consisting of densely packed neurons. It is responsible for higher thought processes and sensory interpretation.

  • Corticalization: The wrinkling of the cortex, which allows a larger area of cortical cells to fit into the small space of the skull.

  • Cerebral hemispheres: The two sections (left and right) of the cortex.

  • Corpus callosum: A thick band of neurons that connects the left and right hemispheres.

• The Four Lobes:

  • Occipital Lobe: Located at the rear and bottom; contains the visual centers. The primary visual cortex processes information, while the visual association cortex makes sense of it.

  • Parietal Lobe: Located at the top and back; contains centers for touch, taste, and temperature. The somatosensory cortex at the front of this lobe processes information from the skin and internal body receptors.

  • Temporal Lobe: Located behind the temples; contains neurons for hearing and meaningful speech. Includes the primary auditory cortex and auditory association cortex.

  • Frontal Lobe: Located at the front and top; responsible for higher mental processes, decision-making, and fluent speech. The motor cortex at the back sends commands to the somatic nervous system.

Association Areas and Brain Lateralization

• Association areas are sections within each lobe responsible for coordinating and interpreting information and higher mental processing.

• Language Disorders (Aphasias):

  • Broca's aphasia: Caused by damage to Broca’s area (usually in the left frontal lobe). It results in an inability to speak fluently, mispronunciation, and halting speech.

  • Wernicke's aphasia: Caused by damage to Wernicke’s area (usually in the left temporal lobe). It results in an inability to understand or produce meaningful language.

• Spatial neglect: A condition where damage to the association areas of the right hemisphere leads to an inability to recognize objects or body parts in the left visual field.

• Left vs. Right Brain Functions:

  • Cerebrum: The upper part of the brain including both hemispheres.

  • Left side: Typically controls language, writing, logical thought, analysis, and mathematical abilities. It processes information sequentially and manages speech.

  • Right side: Typically controls emotional expression, spatial perception, and recognition of faces, patterns, melodies, and emotions. It processes information globally and does not influence speech.

Attention-Deficit/Hyperactivity Disorder (ADHD)

• Causes of ADHD are complex, likely involving more than one brain route.

• Current research areas include:

  • Environmental factors (e.g., low-level lead exposure).

  • Genetic influences.

  • The role of heredity and familial factors.

  • Personality factors.