Premed · Premed · Introductory Psychology
Lecture 3: Biological Bases of Behavior: The Neuron and Nervous System
Introductory Psychology
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the structure and function of a neuron and its key components
- Explain the process of neural transmission including the action potential and synaptic transmission
- Identify the major neurotransmitters and their behavioral effects
- Outline the organization of the nervous system into its major divisions
- Describe the role of the endocrine system in behavior
Lecture Content
I. The Neuron: Building Block of the Nervous System
The nervous system contains approximately 86 billion neurons and an even greater number of glial cells. A neuron is a specialized cell designed to transmit electrochemical signals, and although neurons come in many shapes and sizes, they share a common basic structure.
Dendrites are the branching extensions that receive signals from other neurons; they are covered in receptor sites for neurotransmitters. The cell body, or soma, contains the nucleus and organelles and serves as the integration center, combining all incoming signals. Extending from the cell body is the axon, a long fiber that carries electrical impulses away from the soma toward other neurons. Axons can range from microscopic lengths to over a meter in the case of motor neurons running through the spinal cord. Many axons are wrapped in a myelin sheath, a fatty insulating layer produced by Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system. Myelination dramatically speeds up signal transmission through a process called saltatory conduction, in which the electrical signal "jumps" between the Nodes of Ranvier — small gaps in the myelin sheath where the action potential is regenerated. Demyelination, the loss of this insulating layer, causes disorders such as multiple sclerosis. At the end of the axon are the axon terminals (or terminal buttons), which release neurotransmitters into the synapse.
Neurons come in three functional types. Sensory (afferent) neurons carry information from sensory receptors to the central nervous system. Motor (efferent) neurons carry commands from the central nervous system to muscles and glands. Interneurons, the most numerous type, connect neurons within the central nervous system itself.
Glial cells, or neuroglia, outnumber neurons and play essential supporting roles. Astrocytes help maintain the blood-brain barrier and regulate neurotransmitter levels. Microglia serve as the immune defense system within the central nervous system. Oligodendrocytes and Schwann cells produce the myelin sheath in the central and peripheral nervous systems, respectively.
<image>A detailed labeled diagram of a neuron. Panel A: Full neuron structure showing dendrites with receptor sites, the cell body with nucleus, axon hillock, a long axon wrapped in myelin sheath segments with Nodes of Ranvier between them, and axon terminals at the end. Arrows indicate the direction of signal flow from dendrites to axon terminals. Panel B: An enlarged cross-section of the myelin sheath showing the lipid bilayer wrapping around the axon. Panel C: A comparison of myelinated vs. unmyelinated signal conduction speed, with arrows showing saltatory conduction jumping between nodes.</image>
II. The Action Potential
Neurons communicate through electrochemical signals. At rest, a neuron has a resting potential of approximately -70 millivolts, meaning the inside of the cell is negatively charged relative to the outside. This electrical gradient is maintained by the sodium-potassium pump, which continuously moves three sodium ions out of the cell for every two potassium ions it brings in, and by a selectively permeable membrane that keeps large negative ions trapped inside.
When a neuron receives sufficient excitatory input, it reaches a threshold of about -55 mV, triggering an action potential. The action potential obeys the all-or-none law: the neuron either fires completely or not at all, much like pulling a trigger. During depolarization, sodium channels open and sodium ions rush into the cell, driving the internal charge up to approximately +40 mV. Potassium channels then open, and potassium ions flow out, restoring the negative charge in a phase called repolarization. A brief period of hyperpolarization follows, during which the membrane potential dips below -70 mV before returning to the resting state. After firing, the neuron enters a refractory period during which it cannot fire again. In the absolute refractory period, no stimulus can trigger another action potential; in the relative refractory period, a stronger-than-normal stimulus is needed.
The nervous system encodes the intensity of a stimulus not by changing the size of the action potential, but through two mechanisms: the firing rate (more intense stimuli cause neurons to fire more rapidly) and the number of neurons activated.
<image>A graph of the action potential over time. The x-axis shows time in milliseconds, the y-axis shows membrane potential in millivolts. The graph shows: resting potential at -70 mV (labeled), a stimulus arrow, threshold at -55 mV (dashed line), rapid depolarization rising to +40 mV as Na+ channels open (labeled), repolarization falling as K+ channels open (labeled), hyperpolarization dipping below -70 mV, and return to resting potential. Small inset diagrams at each phase show the state of Na+ and K+ ion channels (open, closed, or inactivated) in the cell membrane.</image>
III. Synaptic Transmission
The synapse is the junction between two neurons, comprising the presynaptic terminal, the synaptic cleft, and the postsynaptic membrane. When an action potential arrives at the axon terminal, voltage-gated calcium channels open and calcium ions flood into the terminal. This calcium influx triggers synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft via exocytosis. These neurotransmitters cross the cleft and bind to receptors on the postsynaptic membrane, causing ion channels to open or close and producing postsynaptic potentials. The signal is terminated through reuptake (neurotransmitter is reabsorbed by the presynaptic neuron), enzymatic degradation (enzymes break down the neurotransmitter), or diffusion away from the synapse.
Postsynaptic potentials come in two varieties. An excitatory postsynaptic potential (EPSP) depolarizes the postsynaptic neuron, making it more likely to fire. An inhibitory postsynaptic potential (IPSP) hyperpolarizes the neuron, making it less likely to fire. The cell body integrates all incoming EPSPs and IPSPs through summation. Temporal summation occurs when rapid successive signals arrive from a single neuron, while spatial summation occurs when simultaneous signals arrive from multiple neurons.
IV. Major Neurotransmitters
Acetylcholine (ACh) plays roles in muscle contraction, attention, memory, and learning. Deficits in acetylcholine are linked to Alzheimer's disease. The drug curare blocks ACh receptors and causes paralysis, while nicotine boosts ACh activity.
Dopamine is involved in movement, motivation, reward, and pleasure. Excess dopamine activity is associated with schizophrenia, while dopamine deficits are linked to Parkinson's disease. Antipsychotic medications work by blocking dopamine receptors, and drugs of abuse such as cocaine and amphetamines increase dopamine levels.
Serotonin regulates mood, sleep, appetite, and pain. Low serotonin is associated with depression, anxiety, and obsessive-compulsive disorder, and it is the target of SSRIs (selective serotonin reuptake inhibitors) such as fluoxetine (Prozac). Norepinephrine contributes to alertness, arousal, and the fight-or-flight response, and is implicated in mood disorders and attention.
GABA (gamma-aminobutyric acid) is the brain's major inhibitory neurotransmitter, reducing neural activity. Deficits in GABA are linked to anxiety and seizures, and its activity is enhanced by benzodiazepines, alcohol, and barbiturates. Glutamate, the brain's major excitatory neurotransmitter, is critical for learning and memory through a process called long-term potentiation. Excessive glutamate can cause excitotoxicity, leading to cell damage and death.
Endorphins are the body's natural opioids, involved in pain reduction and pleasure. They are released during exercise (producing the "runner's high"), stress, and injury, and are mimicked by drugs such as morphine and heroin.
<image>A detailed diagram of a synapse. Panel A: A magnified view of the synaptic junction showing the presynaptic axon terminal with synaptic vesicles containing neurotransmitter molecules, the synaptic cleft (labeled as approximately 20 nanometers wide), and the postsynaptic membrane with receptor proteins. Arrows show the sequence: vesicle fusion, neurotransmitter release, receptor binding, and reuptake pumps. Panel B: A table listing major neurotransmitters (ACh, Dopamine, Serotonin, NE, GABA, Glutamate, Endorphins) with columns for their primary functions and associated disorders when levels are abnormal.</image>
V. The Nervous System: Organization
The nervous system is divided into two major branches. The central nervous system (CNS) consists of the brain and spinal cord. The spinal cord connects the brain to the peripheral nervous system and mediates reflexes through spinal reflex arcs, in which a sensory neuron communicates with an interneuron that then activates a motor neuron, bypassing the brain entirely for the sake of speed.
The peripheral nervous system (PNS) encompasses all the nerves outside the brain and spinal cord, and it is further divided into two components. The somatic nervous system controls voluntary skeletal muscle movements and carries sensory information from the body to the CNS. The autonomic nervous system (ANS) controls involuntary functions such as heart rate, digestion, and breathing. It has two divisions that work in opposition to maintain homeostasis. The sympathetic division activates the body for emergencies — the "fight or flight" response — by increasing heart rate, dilating pupils, inhibiting digestion, and releasing glucose. The parasympathetic division calms the body and conserves energy — the "rest and digest" response — by slowing heart rate, constricting pupils, and stimulating digestion.
VI. The Endocrine System
The endocrine system is a network of glands that produce hormones, chemical messengers released into the bloodstream. Compared to neurotransmitters, hormones act more slowly but their effects last longer.
The hypothalamus serves as the critical link between the nervous system and the endocrine system, controlling the pituitary gland, often called the "master gland" because it releases hormones that regulate other glands. The pituitary produces growth hormone, oxytocin, and vasopressin, among others. The adrenal glands sit atop the kidneys; their inner portion (the adrenal medulla) releases epinephrine and norepinephrine during stress, while their outer layer (the adrenal cortex) releases cortisol, the body's primary stress hormone. The thyroid gland regulates metabolism through thyroxine. The gonads — ovaries in females and testes in males — produce sex hormones (estrogen, progesterone, and testosterone) that influence sexual development, reproductive behavior, and certain aspects of aggression. Finally, the pineal gland secretes melatonin, which helps regulate circadian rhythms.
