Neurons, Neurotransmitters, and the Central Nervous System: Foundations of Biological Psychology

Module 2.1: Neurons and Neurotransmitters

The Neurons: Billions of Brain Cells

Neurons are the fundamental units of the brain and nervous system, responsible for receiving sensory input, sending motor commands, and relaying electrical signals throughout the body. The human brain contains billions of these specialized cells, each with a unique structure and function.

• Definition: Neurons are specialized cells that transmit nerve impulses.

• Parts of a Neuron:

  • Cell Body (Soma): Contains the nucleus and metabolic machinery.

  • Dendrites: Branch-like structures that receive signals from other neurons.

  • Axon: Long, slender projection that transmits electrical impulses away from the cell body.

  • Axon Terminals: Endings where the neuron communicates with other cells.

• Myelin Sheath: Fatty layer that insulates axons, increasing the speed of impulse transmission. Gaps in the sheath are called nodes of Ranvier.

Example: The Argus Retinal Prosthesis System is a neural implant that helps restore vision by converting visual information into electrical impulses, which are then transmitted to the brain via neurons.

Supporting the Neurons: Glial Cells

Glial cells are non-neuronal cells that provide support and protection for neurons. They outnumber neurons and perform various functions essential for brain health.

• Functions:

  • Provide structural support

  • Insulate neurons (e.g., oligodendrocytes form myelin in the CNS)

  • Maintain the chemical environment

  • Assist in repair and waste removal

Communication Between Neurons: The Synapse

Neurons communicate at specialized junctions called synapses. The synapse consists of the axon terminal of the sending neuron, the synaptic cleft (gap), and the dendrite or cell body of the receiving neuron.

• Electrical Impulse: Travels down the axon to the axon terminal.

• Neurotransmitter Release: Electrical impulse triggers the release of chemical messengers (neurotransmitters) into the synaptic cleft.

• Receptor Binding: Neurotransmitters bind to receptors on the receiving neuron, influencing whether it will generate its own electrical impulse.

Example: A single neuron may form thousands of synapses, allowing for complex communication networks in the brain.

Neurotransmitters: The Chemical Messengers of the Brain

Neurotransmitters are chemicals that transmit signals across synapses from one neuron to another, playing a crucial role in regulating mood, cognition, and bodily functions.

• Definition: Neurotransmitters are chemical messengers released by neurons to communicate with other neurons, muscles, or glands.

• Storage: Stored in synaptic vesicles at the axon terminal.

• Release: Released into the synaptic cleft in response to an action potential.

• Receptors: Bind to specific receptor sites on the postsynaptic neuron, like a key fitting into a lock.

The Variety of Neurotransmitters: Some Excite and Some Inhibit

Neurotransmitters can have excitatory or inhibitory effects on the receiving neuron, influencing whether an action potential will be generated.

• Excitatory Neurotransmitters: Increase the likelihood that the receiving neuron will fire (e.g., glutamate).

• Inhibitory Neurotransmitters: Decrease the likelihood of firing (e.g., GABA).

Major Neurotransmitters and Their Functions

Neurotransmitter	Main Function	Example/Application
Acetylcholine (ACh)	Excitatory at neuromuscular junctions; involved in muscle contraction, learning, and memory	Enables skeletal muscle movement; deficits linked to Alzheimer's disease
Dopamine	Involved in movement, motivation, reward, and emotion	Deficits linked to Parkinson's disease; excess linked to schizophrenia
Norepinephrine (Noradrenaline)	Regulates arousal, alertness, and mood	Implicated in stress response and mood disorders
Serotonin	Regulates mood, appetite, and sleep	Low levels associated with depression
GABA (gamma-aminobutyric acid)	Main inhibitory neurotransmitter in the brain	Helps regulate anxiety
Glutamate	Main excitatory neurotransmitter in the brain	Involved in learning and memory

Example: Acetylcholine causes skeletal muscles to contract, while dopamine is involved in reward pathways and movement control.

Module 2.2: The Central Nervous System

Overview of the Central Nervous System (CNS)

The CNS consists of the brain and spinal cord. It is responsible for integrating sensory information and responding accordingly.

• Brain: The control center for thoughts, emotions, and behaviors.

• Spinal Cord: Transmits signals between the brain and the rest of the body; also coordinates reflexes.

The Brainstem: The Most Primitive Part of the Brain

The brainstem controls basic life functions such as breathing, heart rate, and sleep. It connects the brain to the spinal cord.

• Medulla: Regulates vital functions like heartbeat and respiration.

• Pons: Involved in sleep and arousal.

• Midbrain: Coordinates sensory and motor information.

The Cerebellum: Coordination and Balance

The cerebellum is essential for motor control, balance, and coordination. It allows for smooth, precise movements.

• Function: Fine-tunes voluntary movements and maintains posture.

• Damage: Can result in loss of coordination (ataxia).

The Thalamus: The Relay Station

The thalamus acts as a relay station, directing sensory information to appropriate areas of the cerebral cortex for processing.

• Function: Processes and transmits sensory information (except smell).

The Hypothalamus: Master Regulator

The hypothalamus regulates vital bodily functions, including hunger, thirst, temperature, and the endocrine system via the pituitary gland.

• Function: Maintains homeostasis and controls the autonomic nervous system.

The Limbic System: Emotion and Memory

The limbic system is involved in emotional responses, motivation, and memory formation.

• Amygdala: Processes emotions such as fear and aggression.

• Hippocampus: Essential for forming new memories.

The Cerebral Cortex: Higher Mental Functions

The cerebral cortex is the outer layer of the brain, responsible for complex thought, perception, and voluntary movement. It is divided into four lobes:

• Frontal Lobes: Involved in movement, speech, reasoning, and problem-solving.

• Parietal Lobes: Processes sensory information related to touch, temperature, and pain.

• Occipital Lobes: Responsible for visual processing.

• Temporal Lobes: Involved in hearing, language, and memory.

Hemispheric Specialization

• Left Hemisphere: Dominant for language, logic, and analytical tasks.

• Right Hemisphere: Specializes in visual-spatial tasks, facial recognition, and processing emotions.

• Corpus Callosum: Bundle of nerve fibers connecting the two hemispheres, allowing communication between them.

Example: "Split-brain" patients (after corpus callosum surgery) demonstrate the specialized functions of each hemisphere.

Neuroplasticity: The Brain's Ability to Change

Neuroplasticity refers to the brain's capacity to reorganize itself by forming new neural connections throughout life. This ability is crucial for learning, memory, and recovery from brain injury.

• Example: After a stroke, undamaged parts of the brain may take over functions lost due to injury.

Studying the Brain: Methods and Techniques

• Electroencephalogram (EEG): Measures electrical activity in the brain using electrodes placed on the scalp.

• CT Scan (Computed Tomography): Uses X-rays to create detailed images of brain structures.

• MRI (Magnetic Resonance Imaging): Uses magnetic fields and radio waves to produce high-resolution images of the brain.

• PET Scan (Positron Emission Tomography): Measures brain activity by detecting radioactive tracers.

• fMRI (Functional MRI): Measures changes in blood flow to assess brain activity during tasks.

Module 2.3: The Peripheral Nervous System and Endocrine System

The Peripheral Nervous System (PNS)

The PNS connects the CNS to the rest of the body and is divided into the somatic and autonomic nervous systems.

• Somatic Nervous System: Controls voluntary movements of skeletal muscles.

• Autonomic Nervous System: Regulates involuntary functions (e.g., heart rate, digestion).

• Autonomic Subdivisions:

  • Sympathetic: Prepares the body for "fight or flight" responses.

  • Parasympathetic: Promotes "rest and digest" activities.

The Endocrine System: Hormonal Regulation

The endocrine system is a network of glands that secrete hormones into the bloodstream, regulating various bodily functions.

Gland	Main Hormone(s)	Function
Pituitary	Growth hormone, others	"Master gland"; regulates other endocrine glands
Thyroid	Thyroxine	Regulates metabolism
Adrenal	Adrenaline, cortisol	Stress response, metabolism
Pancreas	Insulin, glucagon	Regulates blood sugar
Sex Glands (Ovaries/Testes)	Estrogen, testosterone	Regulate sexual development and reproduction

Handedness

Handedness refers to the preference for using one hand over the other, often linked to brain hemisphere dominance. Most people are right-handed, but the reasons for handedness are not fully understood.

Key Equations and Concepts

• Resting Membrane Potential: The difference in electrical charge across the neuron's membrane at rest, typically about $-70\ \text{mV}$.

• Action Potential: A rapid change in membrane potential that travels along the axon. The process can be summarized as: $\text{Depolarization} \rightarrow \text{Repolarization} \rightarrow \text{Hyperpolarization}$

• All-or-None Law: A neuron either fires completely or not at all; the strength of the action potential is always the same.

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard academic sources in biological psychology.