Coordination and Response: The Human Nervous System and Reflexes

Organisms require proper coordination between various organs and systems to work together and sustain life.
This coordination allows organisms to sense changes in their environment and respond to them appropriately.
The nervous system is primary to the coordination and regulation of body functions.
Responsibilities of the nervous system include:
- Receiving information from the environment.
- Receiving information from within the body.
- Processing gathered information.
- Generating appropriate responses to maintain homeostasis, which is the stable internal environment of the body.

The nervous system consists of $\text{millions}$ of neurons, also known as nerve cells.
Information is transmitted through the system as electrical impulses that travel along neurons to facilitate quick communication.
A bundle of neurons is defined as a nerve.
Example of a nervous system response to heat regulation:
- When exposed to cold temperatures, neurons detect the drop in temperature.
- A signal is sent to the brain.
- The brain triggers a response to generate heat through shivering in order to maintain body temperature.

The mammalian nervous system is divided into two main components:
- Central Nervous System (CNS): Consists of the brain and the spinal cord (noted as being shown in yellow in diagrams).
- Peripheral Nervous System (PNS): Consists of all the nerves in the body located outside of the brain and the spinal cord (noted as being shown in pink in diagrams).
- Note: In visual materials, the spinal cord is also referenced as being shown in pink in separate instances.

There are three distinct types of neurons that must be identifiable in diagrams and images:

Sensory Neurons:
- Function: Carry impulses from the sense organs to the Central Nervous System (the brain or spinal cord).
- Structure: Long neurons characterized by having a cell body located in the middle of the axon.
Relay Neurons:
- Function: Found inside the Central Nervous System; they serve to connect sensory neurons and motor neurons.
- Structure: Short neurons with a small cell body at one end and many dendrites branching off from it.
Motor Neurons:
- Function: Carry impulses from the Central Nervous System to effectors, specifically muscles or glands.
- Structure: Long neurons with a large cell body at one end and long dendrites branching off that body.

Stimulus:
- Definition: Any change in the environment that can be detected by sensory neurons.
- Examples: Light, sound, touch, temperature, or chemicals (e.g., fire).
Receptor:
- Definition: A structure that can detect a stimulus.
- Function: Generates an electrical impulse upon detection, which is sent to the CNS.
- Example: Receptors in the skin of the hand detecting fire.
Effector:
- Definition: A structure that responds to a signal from the nervous system resulting in a specific action or behavior.
- Types: Often a muscle or a gland.
- Example: Muscles of the arm pulling the hand away from a fire.

Voluntary Response:
- Characteristic: A conscious action performed on purpose by choice.
- Origin: The response begins in the brain.
- Example: Deciding to raise your hand to answer a question in class.
Involuntary Response (Reflex):
- Characteristic: A quick and automatic reaction to a stimulus that occurs without conscious thought.
- Coordination: The brain is not required to coordinate this response.
- Awareness: The individual is often not aware the action has occurred until after it has happened.
- Importance: Vital for survival as they are significantly faster than voluntary responses.
- Example: Touching something hot and pulling the hand away immediately before realizing the object is hot.

A reflex arc refers to the specific pathway followed by a nerve impulse during an involuntary or reflex response.
The reflex action is a means of automatically and rapidly integrating and coordinating stimuli with effector responses (muscles and glands).
Sequential steps of the reflex arc (using the example of touching fire):
1. Stimulus: The fire itself.
2. Receptor: Receptor cells in the skin detect the stimulus and convert it into an electrical impulse.
3. Sensory Neuron: Carries the electrical impulse to the spinal cord (the coordinator).
4. Relay Neuron: In the spinal cord, the sensory neuron passes the impulse to the relay neuron.
5. Motor Neuron: The relay neuron connects to the motor neuron and passes the impulse along.
6. Effector: The motor neuron carries the impulse to the effector, which is the muscle in the arm.
7. Response: The muscle contracts and pulls the arm away from the fire.

Synapse Definition: A junction between two neurons.
Gap: Neurons do not touch each other; the gap between them is called the synaptic gap.
Structural Components of a Synapse:
- Vesicles containing neurotransmitter molecules.
- The synaptic gap.
- Receptor proteins.
Transmission Process at the Synapse:
- Neurons utilize chemical messengers called neurotransmitters.
- This is the only portion of the nervous system where the message is chemical rather than electrical.
- An impulse travels along the first neuron, $\text{Neuron A}$ .
- This impulse stimulates the release of neurotransmitter molecules from vesicles into the synaptic gap.
- The neurotransmitter molecules diffuse across the gap.
- The neurotransmitter molecules bind with receptor proteins on the membrane of the next neuron.
- This binding stimulates a new electrical impulse in the next neuron.
Function of Synapses: They ensure that nerve impulses travel in one direction only.

Interjection regarding habits: During the explanation, an aside was made regarding common struggles with personal habits.
Question: "Did you know there are $2$ basic reasons that people struggle to keep a Bible habit?"
Answer provided: They don't have time.