Study Guide | Module 1.3 [The Neuron & Neural Firing] | AP Psych Unit 1

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• Neurons

  • Neurons are specialized cells responsible for transmitting information within the nervous system. They communicate through electrical impulses and chemical signals to coordinate bodily functions, thoughts, emotions, and behaviors. There are approximately 86 billion neurons in the human brain, each playing a vital role in neural communication.

    • Key Structure of Neurons:

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

      • Cell Body (Soma): Contains the nucleus and processes incoming signals.

      • Axon: A long, thin fiber that transmits electrical impulses away from the cell body.

      • Myelin Sheath: A fatty layer that insulates the axon, increasing the speed of neural transmission.

      • Axon Terminals: The endpoints of the axon, where neurotransmitters are released to communicate with other neurons.

    • Types of Neurons:

      • Sensory Neurons (Afferent): Transmit information from sensory organs to the central nervous system (CNS).

      • Motor Neurons (Efferent): Carry signals from the CNS to muscles and glands.

      • Interneurons: Connect sensory and motor neurons within the CNS and process information.

    • Neural Communication:
      Neurons communicate via:

      1. Electrical Transmission: An action potential travels down the axon.

      2. Chemical Transmission: Neurotransmitters are released at the synapse to pass the signal to the next neuron.

    • Example 1: Sensory neurons in the skin detect heat and send signals to the brain, which processes the information and triggers a motor neuron response to withdraw the hand.

    • Example 2: Interneurons in the spinal cord process reflex actions without direct input from the brain, such as the knee-jerk reflex.

    • Importance:

      • Neurons form the basis of all cognitive and physical functions, from simple reflexes to complex thought processes.

      • Damage or dysfunction of neurons is associated with neurological conditions such as multiple sclerosis, Parkinson's disease, and Alzheimer's disease.

      • Understanding neurons is essential for studying how the brain controls the body and processes information.

• Glial Cells

  • Glial cells (or glia) are the support cells of the nervous system, providing essential functions that help neurons operate efficiently. While neurons transmit signals, glial cells maintain the environment necessary for proper neural function.

    • Key Functions:

      • Provide structural support for neurons.

      • Nourish neurons by delivering nutrients.

      • Protect neurons by maintaining homeostasis and insulating axons.

    • Importance:

      • Essential for maintaining neural health and facilitating efficient communication between neurons.

      • Support brain function and protect the nervous system from damage.

• Reflex Arc

  • A reflex arc is the simplest neural pathway that controls an involuntary response to a stimulus, often bypassing the brain to ensure a quick reaction. Reflex arcs are essential for protecting the body from harm and maintaining homeostasis.

    • Components of a Reflex Arc:

      1. Sensory Receptor: Detects the stimulus (e.g., heat, pressure).

      2. Sensory Neuron: Transmits the signal to the spinal cord.

      3. Interneuron: Processes the information and connects sensory and motor neurons (in the spinal cord).

      4. Motor Neuron: Sends the signal from the spinal cord to an effector (muscle or gland).

      5. Effector: Executes the response (e.g., muscle contraction).

    • Key Aspects:

      • Does Not Require Conscious Thought: Reflexes occur automatically without input from the brain.

      • Purpose: Designed for protection and survival, minimizing injury by providing a rapid response.

    • Importance:

      • Reflex arcs allow for immediate reactions to potentially harmful stimuli, such as pulling your hand away from a hot surface.

      • They help maintain bodily functions, such as posture and balance, through stretch reflexes.

• Sensory Neurons

  • Sensory neurons, also known as afferent neurons, are specialized nerve cells that carry information from sensory receptors located throughout the body to the central nervous system (CNS), which includes the brain and spinal cord. These neurons allow the body to detect and respond to environmental stimuli.

    • Key Functions:

      • Transmit sensory information such as touch, pain, temperature, light, sound, and chemical stimuli.

      • Connect peripheral sensory organs (e.g., eyes, ears, skin) to the CNS for processing.

    • Pathway:

      1. Sensory Receptor: Detects a stimulus (e.g., heat, light, sound).

      2. Sensory Neuron: Transmits the signal as an electrical impulse to the CNS.

      3. CNS: Processes the information and determines the appropriate response.

    • Importance:

      • Sensory neurons are essential for perceiving the external environment and maintaining homeostasis by detecting internal changes in the body.

      • Damage to sensory neurons can result in sensory deficits or conditions like neuropathy, which affects sensation and reflexes.

• Motor Neurons

  • Motor neurons, also known as efferent neurons, are specialized nerve cells that carry signals from the central nervous system (CNS) to effectors, such as muscles or glands, to produce a response. These neurons play a critical role in voluntary and involuntary movements.

    • Key Functions:

      • Transmit motor commands from the brain and spinal cord to muscles.

      • Control both voluntary movements (e.g., walking) and involuntary responses (e.g., reflexes).

    • Pathway:

      1. CNS: Sends signals via motor neurons.

      2. Motor Neurons: Transmit electrical impulses to specific effectors.

      3. Effector: Executes the action (e.g., muscle contraction or gland secretion).

    • Types of Motor Neurons:

      • Somatic Motor Neurons: Control voluntary movements of skeletal muscles.

      • Autonomic Motor Neurons: Regulate involuntary activities of smooth muscles, cardiac muscles, and glands.

    • Importance:

      • Essential for all physical activities and reflexes.

      • Damage to motor neurons, such as in conditions like amyotrophic lateral sclerosis (ALS), can result in muscle weakness or paralysis.

• Interneurons

  • Interneurons are the "middlemen" of the nervous system, located entirely within the central nervous system (CNS). They process incoming sensory information and determine the appropriate response by relaying signals to motor neurons. Interneurons are critical for reflexes, decision-making, and complex thought processes.

    • Key Functions:

      • Act as a link between sensory neurons (input) and motor neurons (output).

      • Integrate and process information received from sensory neurons.

      • Play a role in reflex arcs and higher-level functions like memory and reasoning.

    • Pathway Example in a Reflex Arc:

      1. A sensory neuron detects a stimulus and sends the signal to the spinal cord.

      2. An interneuron in the spinal cord processes the signal and determines the response.

      3. The interneuron transmits the signal to a motor neuron, which activates a muscle or gland.

    • Importance:

      • Interneurons allow for rapid reflexive responses to stimuli, bypassing the brain for efficiency.

      • They enable higher-order functions, such as learning and memory, by facilitating complex neural circuits.

      • They are the most abundant type of neuron in the human brain, forming networks for information processing.

• Neural Transmission

  • Neural transmission is the process that allows neurons to communicate and transmit information throughout the nervous system. It involves electrical signals (action potentials) traveling along the axon of a neuron and chemical messengers (neurotransmitters) facilitating communication across synapses.

    • Key Steps in Neural Transmission:

      1. Action Potential: An electrical impulse is generated in the neuron's axon when it reaches the threshold.

      2. Propagation: The action potential travels down the axon to the axon terminal.

      3. Synaptic Transmission: At the synapse, neurotransmitters are released from vesicles in the axon terminal into the synaptic cleft.

      4. Receptor Binding: Neurotransmitters bind to receptors on the postsynaptic neuron, triggering a response (excitatory or inhibitory).

      5. Reuptake/Degradation: Neurotransmitters are reabsorbed into the presynaptic neuron or broken down.

    • Importance of Neural Transmission:

      • Essential for all bodily functions, from basic reflexes to complex behaviors and thoughts.

      • Enables the nervous system to process sensory inputs, coordinate movements, and regulate internal states like emotions and homeostasis.

    • Key Features:

      • Transmission speed varies based on axon characteristics (e.g., myelinated axons are faster).

      • Imbalances in neurotransmitter activity can lead to neurological or psychological disorders (e.g., dopamine in Parkinson's or serotonin in depression).

• Action Potential

  • An action potential is a rapid electrical signal that travels along the axon of a neuron, enabling the transmission of information. This process is crucial for communication within the nervous system and occurs when a neuron is sufficiently stimulated to exceed its threshold.

    • Key Steps in the Process:

      1. Resting Potential: The neuron is at rest, with a negative charge inside the cell relative to the outside.

      2. Depolarization: Sodium ions (Na⁺) flow into the cell after stimulation, causing the charge to become more positive.

      3. Repolarization: Potassium ions (K⁺) exit the cell, restoring the negative internal charge.

      4. Refractory Period: The neuron briefly resets before it can fire again.

    • Key Aspects:

      • Requires a stimulus strong enough to surpass the neuron's threshold.

      • Moves in one direction along the axon due to the refractory period.

      • Insulated axons with myelin sheaths allow for faster conduction (saltatory conduction).

    • Importance:

      • Critical for transmitting sensory inputs, motor outputs, and other neural messages.

      • Disruptions in this process can lead to neurological issues, such as epilepsy.

• All-or-Nothing Principle

  • The all-or-nothing principle refers to the phenomenon where a neuron either generates a full action potential or does not fire at all, depending on whether the stimulus exceeds the threshold level. Once the threshold is reached, the response is always the same in terms of intensity; the neuron does not fire a stronger or weaker signal.

    • Key Features:

      • The strength of the stimulus does not affect the action potential's intensity.

      • A stimulus must reach the threshold potential to trigger the response; anything less is insufficient to cause firing.

      • This principle ensures that signals transmitted along neurons are consistent in magnitude, allowing for clear communication between cells.

    • Importance:

      • Helps maintain the reliability and speed of neural communication across the nervous system.

      • Assures that all messages sent by neurons are uniform in strength, avoiding confusion in interpreting signals.

• Depolarization

  • Depolarization occurs when a neuron’s membrane potential becomes less negative (more positive), which is a key step in the generation of an action potential. This process happens when ion channels open and positively charged ions, like sodium (Na⁺), flow into the cell.

    • Key Process:

      1. Resting State: The neuron is polarized, with a negative charge inside the cell relative to the outside.

      2. Triggering Depolarization: A stimulus causes sodium channels to open, allowing Na⁺ ions to enter the neuron.

      3. Positive Shift: The influx of sodium ions leads to a decrease in the difference between the inside and outside of the neuron, making the inside more positive.

    • Importance:

      • Depolarization is essential for the transmission of signals between neurons.

      • If depolarization reaches the threshold, it triggers an action potential, enabling communication across the nervous system.

    • Key Aspect:

      • This phase is followed by repolarization, where potassium (K⁺) ions exit the cell to restore the negative internal charge.

• Refractory Period

  • The refractory period is the brief time after a neuron has fired an action potential during which it is less responsive to additional stimuli. This period ensures that each action potential is discrete and that the signal moves in one direction along the axon.

    • Key Phases:

      1. Absolute Refractory Period: The neuron cannot fire another action potential, no matter the strength of the stimulus. This occurs because the sodium channels are inactivated.

      2. Relative Refractory Period: The neuron can fire again, but only if the stimulus is stronger than usual, as the neuron is still in the process of resetting.

    • Importance:

      • Prevents the neuron from firing multiple times in rapid succession, ensuring the signal is clear and directed.

      • Helps maintain the unidirectional flow of action potentials along the axon.

    • Key Aspect:

      • The length of the refractory period varies depending on the neuron’s type and state, but it is essential for proper neural function.