Synapses, Neurotransmitters, and the Nervous System: Study Notes

Synapses and Neurotransmission

Introduction to Synapses

Synapses are specialized junctions where neurons communicate with each other or with other target cells. This communication is essential for all nervous system functions, including sensation, movement, and cognition.

• Synapse: The point of contact between a neuron and its target cell (another neuron, muscle cell, or gland).

• Pre-synaptic neuron: The neuron sending the signal.

• Post-synaptic neuron: The neuron receiving the signal.

• Neurotransmission: The process of chemical communication between neurons at the synapse.

Example: When you touch a hot surface, sensory neurons transmit signals to the spinal cord and brain via synapses, leading to a rapid withdrawal response.

Electrical and Chemical Signaling

Neurons communicate using both electrical and chemical signals. Electrical signals (action potentials) travel along the axon, while chemical signals (neurotransmitters) are released at the synapse.

• Action potential: An electrical impulse that travels down the axon.

• Neurotransmitter: Chemical messengers released from the pre-synaptic neuron into the synaptic cleft.

• Receptor: Protein on the post-synaptic cell that binds neurotransmitters, triggering a response.

Post-Synaptic Potentials: EPSPs and IPSPs

Excitatory and Inhibitory Potentials

Neurotransmitters can cause either excitatory or inhibitory effects on the post-synaptic neuron, influencing whether it will fire an action potential.

• EPSP (Excitatory Post-Synaptic Potential): Depolarizes the post-synaptic membrane, increasing the likelihood of an action potential. Example: Membrane potential changes from -70mV to -67mV.

• IPSP (Inhibitory Post-Synaptic Potential): Hyperpolarizes the post-synaptic membrane, decreasing the likelihood of an action potential. Example: Membrane potential changes from -70mV to -73mV.

Key Principle: The sum of all EPSPs and IPSPs determines whether the neuron reaches the threshold to fire an action potential.

• If the sum of EPSPs and IPSPs is greater than the threshold, an action potential occurs.

• If the neuron is in a refractory state or hyperpolarized, it will not fire.

Equation:

$\text{Action Potential Occurs if:} \quad \sum \text{EPSPs} + \sum \text{IPSPs} > \text{Threshold}$

Major Neurotransmitters and Their Functions

Overview of Neurotransmitters

Neurotransmitters are specific molecules that transmit signals across synapses. Each neurotransmitter has distinct functions and is often found in specific brain pathways.

• Lock and key mechanism: Neurotransmitters bind to specific receptors, triggering unique cellular responses.

Table: Major Neurotransmitters

Neurotransmitter	Main Function(s)	Example/Application
Glutamate	Major excitatory neurotransmitter; involved in learning, memory, sensory relay	MSG in food enhances flavor; critical for synaptic plasticity
GABA (Gamma-Amino Butyric Acid)	Major inhibitory neurotransmitter; reduces anxiety, stress, promotes sleep	Anti-anxiety drugs (e.g., benzodiazepines) enhance GABA activity
Acetylcholine	Controls voluntary muscle movement (PNS); regulates attention, learning, memory, sleep (CNS)	Loss of acetylcholine linked to Alzheimer's disease
Dopamine	Regulates motivation, pleasure, emotional arousal, motor behavior	Parkinson's disease involves dopamine deficiency
Serotonin	Regulates sleep, wakefulness, mood	SSRIs (antidepressants) increase serotonin levels
Norepinephrine	Involved in vigilance, heightened awareness, stress response	Fight-or-flight response
Endorphins	Reduce pain, elevate mood	Released during exercise ('runner's high')

Drugs and Synaptic Transmission

Impact of Drugs on Neurotransmission

Drugs can alter synaptic transmission by affecting neurotransmitter release, receptor binding, or neurotransmitter concentration in the synaptic cleft.

• Agonists: Drugs that enhance neurotransmitter action (e.g., increase release or mimic neurotransmitter).

• Antagonists: Drugs that inhibit neurotransmitter action (e.g., block receptors or decrease release).

• Reuptake inhibitors: Prevent neurotransmitter reabsorption, increasing its availability (e.g., SSRIs for serotonin).

Example: Cocaine blocks dopamine reuptake, increasing dopamine levels and producing euphoria.

The Nervous System: Structure and Divisions

Overview of the Nervous System

The vertebrate nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS), each with distinct roles in processing and transmitting information.

• CNS (Central Nervous System): Composed of the brain and spinal cord; responsible for processing information and generating consciousness.

• PNS (Peripheral Nervous System): Connects the CNS to the rest of the body; controls interactions with the environment and maintains homeostasis.

Divisions of the Peripheral Nervous System

• Somatic Nervous System (SNS): Interacts with the external environment; controls voluntary movements via afferent (sensory) and efferent (motor) nerves.

• Autonomic Nervous System (ANS): Regulates internal environment; controls involuntary functions via afferent and efferent nerves.

Table: Divisions of the Nervous System

Division	Main Components	Function
CNS	Brain, Spinal Cord	Information processing, consciousness, control of behavior
PNS	SNS, ANS	Connects CNS to body; controls voluntary and involuntary actions
SNS	Afferent and Efferent Nerves	Sensory input from environment; motor output to muscles
ANS	Sympathetic and Parasympathetic Nerves	Regulates internal organs; maintains homeostasis

Autonomic Nervous System: Sympathetic vs. Parasympathetic

• Sympathetic Nervous System: Prepares the body for 'fight or flight' responses; increases heart rate, dilates pupils, inhibits digestion.

• Parasympathetic Nervous System: Promotes 'rest and digest' activities; conserves energy, slows heart rate, stimulates digestion.

Example: During stress, the sympathetic system activates; after eating, the parasympathetic system dominates.

Summary Table: Key Terms and Concepts

Term	Definition
Synapse	Junction between two neurons or a neuron and another cell
Neurotransmitter	Chemical messenger released at synapse
EPSP	Excitatory post-synaptic potential; increases likelihood of action potential
IPSP	Inhibitory post-synaptic potential; decreases likelihood of action potential
Agonist	Drug that enhances neurotransmitter action
Antagonist	Drug that inhibits neurotransmitter action
CNS	Central nervous system; brain and spinal cord
PNS	Peripheral nervous system; connects CNS to body
SNS	Somatic nervous system; controls voluntary movement
ANS	Autonomic nervous system; controls involuntary functions
Sympathetic	Prepares body for action
Parasympathetic	Conserves energy, promotes rest

Additional info: Some content was inferred and expanded for clarity and completeness, including definitions, examples, and tables summarizing neurotransmitter functions and nervous system divisions.