This unit explores the biological foundations of behavior, covering the nervous system's structure and function, neuron communication, and the roles of neurotransmitters and hormones. Students should be prepared to differentiate between nervous system divisions, understand the steps of neural firing, and distinguish between various drug classifications and their effects.
Nervous System Divisions
This section outlines the structural and functional organization of the nervous system.
Nervous System
Body's communication network, coordinating actions and transmitting signals.
Central Nervous System (CNS)
Brain and Spinal cord; processes sensory info, initiates responses, stores memories.
Brain
Receives/processes sensory information, initiates responses, stores memories, generates thoughts/emotions.
Spinal cord
Conducts signals to/from brain, controls reflex activities.
Peripheral Nervous System (PNS)
Nerves extending from CNS to body; connects CNS to sensory organs, muscles, glands.
Sensory Neurons (Afferent Neurons)
Send signals from sensory receptors to the CNS (Approach).
Motor Neurons (Efferent Neurons)
Send signals from the CNS to muscles and glands (Exit).
Somatic Nervous System
Controls voluntary movements (skeletal muscles).
Autonomic Nervous System
Controls involuntary responses (heartbeat, digestion, breathing).
Sympathetic Division
'Fight or Flight': Prepares body for action (increased heart rate, dilated eyes, increased breathing).
Parasympathetic Division
'Rest or Digest': Conserves energy (relaxes muscles, slows heart rate, aids digestion).
Distinction between voluntary and involuntary control.
| Somatic Nervous System | Autonomic Nervous System | |
|---|---|---|
| Control | Controls voluntary movements | Controls involuntary responses |
| Targets | Skeletal muscles | Heart, smooth muscles, glands |
Opposing functions of the autonomic nervous system divisions.
| Sympathetic Division | Parasympathetic Division | |
|---|---|---|
| Function | Prepares body for stress/action | Calms body, conserves energy |
| Key Phrase | 'Fight or Flight' | 'Rest or Digest' |
| Heart Rate | Increases | Decreases |
| Digestion | Inhibits | Stimulates |
Neuron Structure and Types
This section details neuron components, types, and the reflex arc pathway.
Concept
Glial Cells (Sidekicks)
Provide structure, insulation, and waste transportation for neurons; they do not process information or transmit signals.
Concept
Neurons
The basic functional unit of the nervous system; process information using electrical impulses (Action potential) and chemical signals (Neurotransmitters).
Different types of neurons and their roles in the nervous system.
| Sensory Neuron | Interneuron | Motor Neuron | |
|---|---|---|---|
| Function | Transmit signals from sensory receptors to the CNS (Afferent) | Connect sensory and motor neurons within the CNS; facilitate communication | Transmit signals from the CNS to muscles/glands (Efferent) |
| Role | Detect stimuli | Process information, enable complex thought | Initiate responses |
- 1
Stimulus
An external or internal change detected by the body (e.g., touching a hot object).
- 2
Receptor
Specialized cells or organs that detect the stimulus and convert it into a neural signal.
- 3
Sensory Neuron
Transmits the signal from the receptor to the spinal cord (CNS).
- 4
Interneuron (Spinal Reflex Arc)
Within the spinal cord, the sensory neuron synapses directly with a motor neuron or an interneuron, bypassing the brain for rapid, involuntary response.
- 5
Motor Neuron
Transmits the signal from the spinal cord to the effector.
- 6
Effector
A muscle or gland that carries out the response (e.g., muscle contracts to withdraw hand).
- 7
Response
The action taken by the effector (e.g., hand withdrawal). Decision Making (conscious thought) occurs separately in the brain, often after the reflex.
Concept
Neuron Anatomy
The structural components of a neuron.
Concept
Soma (Cell body)
Contains the nucleus and other organelles; the main part of the neuron where genetic material (DNA) is located, providing information for cell function.
Concept
Dendrite
Branch-like extensions that receive information from other neurons and transmit electrical signals to the soma.
Concept
Axon
A long projection that carries signals away from the soma to the axon terminal.
Concept
Myelin Sheath
A fatty substance (formed by Schwann cells) that insulates the axon and speeds up the transmission of signals; gaps are called Nodes of Ranvier.
Concept
Axon Terminal (Terminal buttons)
The end of the axon that transmits signals to other neurons by emitting neurotransmitters (via vesicles).
Concept
Reuptake
The process by which excess neurotransmitters are reabsorbed by the presynaptic neuron from the synaptic gap (further detailed in Synaptic Transmission).
Neural Firing (Action Potential)
This section explains the electrical signal transmission through a neuron.
Concept
Action Potential
A rapid, short-lasting electrical signal that travels along the cell membrane of an excitable neuron, caused by the movement of ions (charged particles) into and out of the cell, changing the voltage across the membrane.
- 1
Resting Potential
The neuron is polarized with a baseline charge of -70mV. More K+ ions inside the cell and more Na+ ions outside. The Sodium-Potassium Pump actively maintains this by moving 3 Na+ out and 2 K+ in.
- 2
Threshold
A stimulus causes the membrane potential to reach -55mV. This is the 'All or Nothing' Threshold; if reached, the neuron fires completely.
- 3
Depolarization (Action Potential)
Once threshold is met, Na+ ion channels open, causing a rapid influx of Na+ into the cell. The neuron becomes less polarized, and the voltage quickly rises to +30mV.
- 4
Repolarization
Immediately after depolarization, K+ ion channels open, and K+ ions exit the cell, causing the membrane potential to drop back towards resting levels.
- 5
Hyperpolarization (Refractory Period)
K+ channels close slowly, causing the membrane potential to briefly overshoot the resting potential, becoming more negative (e.g., -90mV). During this brief period, the neuron cannot fire again.
- 6
Return to Resting Potential
The Sodium-Potassium Pump restores the ion balance, bringing the membrane back to its -70mV resting state, ready for a new signal.
Distribution and movement of key ions during neural firing.
| K+ (Potassium Ions) | Na+ (Sodium Ions) | |
|---|---|---|
| Resting State | Higher concentration inside the cell | Higher concentration outside the cell |
| Depolarization | Moves out of the cell (later stage) | Rushes into the cell |
| Pump Action | Pumped into the cell (2 ions) | Pumped out of the cell (3 ions) |
All or Nothing Threshold
A neuron either fires completely when the -55mV threshold is reached, or it doesn't fire at all. The intensity of the stimulus does not change the strength of the action potential, only the frequency of firing.
Concept
Ion Channels
Protein channels in the cell membrane that open and close to allow specific ions (like Na+ and K+) to pass through, driven by the electrochemical gradient.
Concept
Sodium-Potassium Pump
An active transport protein that uses energy to pump 3 Na+ ions out of the cell and 2 K+ ions into the cell, maintaining the resting potential.
Synaptic Transmission
This section describes how signals transmit between neurons across the synapse.
Concept
Synapse
A small pocket of space (the synaptic gap or synaptic cleft) between the axon terminal of a presynaptic neuron and the dendrites or cell body of a postsynaptic neuron, where signals are transmitted.
Key differences between electrical and chemical synaptic transmission.
| Electrical Synapse | Chemical Synapse | |
|---|---|---|
| Signal Type | Electrical signals (ions) | Chemical signals (neurotransmitters) |
| Transmission | Immediate and direct | Slower but more flexible |
| Mechanism | Direct flow of ions through gap junctions | Release and binding of neurotransmitters |
| Components | Gap junctions | Synaptic vesicles, neurotransmitters, receptors |
- 1
Action Potential Arrives
An action potential travels down the axon and reaches the presynaptic terminal (axon terminal).
- 2
Neurotransmitter Release
Ion channels in the presynaptic terminal open, triggering synaptic vesicles (containing neurotransmitters) to fuse with the membrane and release neurotransmitters into the synaptic gap (chemical messages).
- 3
Binding to Receptors
Neurotransmitters diffuse across the synaptic gap and bind to specific neurotransmitter receptors (receptor sites) on the postsynaptic neuron's dendrites or cell body.
- 4
Postsynaptic Effect
Binding causes ion channels on the postsynaptic neuron to open, leading to either excitation or inhibition of the postsynaptic neuron.
- 5
Neurotransmitter Inactivation
Neurotransmitters unbind from receptors; some are broken down by enzymes (degradation), while others are reabsorbed by the presynaptic neuron via reuptake transporters (reuptake).
Drugs and Psychoactive Substances
This section explores drug classifications, mechanisms, and effects.
How drugs interact with neurotransmitter systems.
| Agonist Drugs | Antagonist Drugs | |
|---|---|---|
| Effect on NT | Increase the effectiveness of a neurotransmitter | Decrease the effectiveness of a neurotransmitter |
| Mechanism 1 | Act like a neurotransmitter by binding to receptors (e.g., Opioids mimic Endorphins; Xanax enhances GABA) | Block the release of neurotransmitters from the presynaptic axon terminal |
| Mechanism 2 | Block the reuptake process (e.g., SSRIs like Prozac for Serotonin in Depression) | Attach to receptors and block the intended neurotransmitter from binding (e.g., Narcan/Naloxone for opioid overdose) |
Opposing effects of stimulants and depressants on the nervous system.
| Stimulants | Depressants | |
|---|---|---|
| Neural Activity | Excite and promote neural activity | Reduce neural activity |
| Physiological Effects | Increased energy, reduced appetite, irritability, increased alertness/wakefulness | Drowsiness, muscle relaxation, lowered breathing, initial high followed by relaxation |
| Examples | Caffeine, cocaine, nicotine, Methamphetamine, Ecstasy (MDMA) | Alcohol, sleeping aids |
Concept
Opioids
A special class of depressants known for pain relief; highly addictive. Examples include Morphine, Heroine, oxycodone.
Concept
Hallucinogens
Cause an individual to sense things that are not actually there (altered perceptions). Examples include Marijuana, peyote, LSD, Ecstasy (MDMA).
Concept
Tolerance
The need to ingest or inject more dosage of a drug to achieve the same effect over time.
Concept
Withdrawal symptoms
Unpleasant physical and psychological effects that occur when a person stops or reduces the use of an addictive substance.
Hormones (Endocrine System)
This section introduces the endocrine system and key hormone functions.
Concept
Endocrine System
A slow-moving communication system that sends hormones through the bloodstream to target larger areas of the body.
Concept
Hormones
Chemical messengers produced by endocrine glands that travel through the bloodstream to regulate various bodily functions.
Opposing roles of Leptin and Ghrelin in hunger regulation.
| Leptin | Ghrelin | |
|---|---|---|
| Primary Effect | Inhibiting Hunger (signals satiety) | Stimulating Hunger (signals emptiness) |
| Source | Fat cells | Stomach |
Concept
Melatonin
A hormone that regulates sleep and Circadian rhythms (the body's natural sleep-wake cycle).
Concept
Oxytocin
A hormone associated with social bonding, trust, and love; also plays a role in childbirth and lactation.