Understanding the Nervous System

• The nervous system consists of components that work together to facilitate communication and control throughout the body.

Central Nervous System (CNS)

• Brain: Orchestrates thoughts, beliefs, and emotions, processes sensory information, controls motor function, and is crucial for higher functions like reasoning, problem-solving, and decision-making.
• Spinal Cord: Serves as the main information highway between the brain and body and enables reflex actions independent of the brain.

Peripheral Nervous System (PNS)

• Sensory: Gathers and transmits sensory data from the environment, such as touch, sight, sound, taste, and smell, to help interact with the world and react to changes and dangers.
• Somatic Motor: Controls all voluntary movements by sending signals from the brain to skeletal muscles, offering conscious control over body movements.

Autonomic Nervous System (ANS)

• Parasympathetic Branch: Responsible for "rest and digest" functions, enhancing digestion, lowering heart rate, and conserving energy to promote a state of calm and balance after stress.
• Sympathetic Branch: Prepares the body for "fight or flight" responses during stress by increasing heart rate, boosting energy supplies, and redirecting blood flow to essential muscles.

Motor Neuron Structure

• Dendrites: Branched extensions receive signals from other neurons.
• Cell Body (Soma): Contains the nucleus and organelles and integrates signals from dendrites.
• Axon: A long projection that transmits signals away from the cell body.
• Myelin Sheath: A fatty insulating layer that increases signal transmission speed.
• Nodes of Ranvier: Gaps in the myelin sheath that facilitate saltatory conduction.
• Axon Terminal: Branched end of the axon where neurotransmitters are released.

Sensory Neuron Structure

• Dendrite: Receives signals from other neurons or sensory receptors.
• Cell Body (Soma): Integrates signals and initiates the nerve impulse.
• Axon: Transmits nerve impulses away from the cell body.
• Myelin Sheath: Insulates the axon to increase the speed of the nerve impulse transmission.
• Axon Terminal: Releases neurotransmitters to communicate with other neurons or effector cells.
• Satellite Cell: Supports and protects the neuron.

Reflex Arc

• The reflex arc is a rapid, involuntary response to a stimulus.
• A sensory receptor detects a stimulus and sends an impulse along the sensory neuron to the spinal cord.
• In the spinal cord, the impulse passes through a relay neuron (interneuron).
• The motor neuron carries the impulse to an effector, like a muscle, causing a contraction.
• This process bypasses the brain, allowing for faster reaction times.

Neuron Potentials

• Resting Potential (-70 mV): Neuron at rest, Na+ and K+ gates are closed, and the inside is negative.
• Threshold Potential (-40 mV to -55 mV): Minimum level needed to trigger depolarization.
• Depolarization (rising phase, up to +40 mV): Na+ channels open, Na+ floods into the cell, making the inside positive.
• Action Potential (peak at +40 mV): Membrane is fully depolarized.
• Repolarization (falling phase): Na+ channels close, K+ channels open, K+ floods out, making the inside negative again.
• Hyperpolarization (dip to -80 mV): Too much K+ leaves, and the membrane becomes temporarily more negative than resting.
• Refractory Period: Neuron cannot fire another action potential yet.
• Return to Resting Potential (-70 mV): Normal ion balance is restored using sodium-potassium pumps.

Threshold Level and All-or-Nothing Response

• The threshold level ensures only strong stimuli trigger an action potential, filtering out weak stimuli.
• Neurons have an all-or-nothing response, meaning the impulse is the same size when the threshold is reached, preventing constant stimulation and fatigue.

Refractory Period

• Refractory Period: Allows neuron time to recover after an action potential and ensures impulses travel only in one direction.
• Absolute Refractory Period: No new action potential can be generated.
• Relative Refractory Period: A stronger-than-normal stimulus is required to trigger another impulse.

Myelin Sheath

• The myelin sheath is a fatty layer wrapping around the axon, insulating it and preventing interference between nerve impulses.
• It speeds up conduction by enabling impulses to jump between gaps (nodes of Ranvier).
• Impulses travel faster along myelinated neurons than unmyelinated ones.

Cholinergic Synapse

• Synaptic Vesicles: Contain acetylcholine (ACh) neurotransmitter.
• Pre-synaptic membrane: Releases ACh into the synaptic cleft when an action potential arrives.
• Calcium Ion Channels: Open when the action potential arrives, allowing Ca2+ to enter and trigger vesicle release.
• Synaptic Cleft: Gap between two neurons where ACh diffuses across.
• Post-synaptic Membrane: Contains receptors for acetylcholine (nicotinic or muscarinic).
• ACh Receptors: Bind acetylcholine to generate a new action potential in the post-synaptic cell.
• Enzyme (Acetylcholinesterase): Breaks down ACh to stop the signal and allow reset.

Synapse Function

• The synapse is a junction between two neurons where electrical impulses are converted to chemical signals.
• It ensures impulses travel in one direction and can filter out weak stimuli or amplify strong ones.
• Controlled transmission is vital for accurate and efficient nervous system function.

Impulse Transmission Across Synaptic Cleft

• When an action potential reaches the end of a neuron, neurotransmitters are released from synaptic vesicles into the synaptic cleft.
• They diffuse across the cleft and bind to receptors on the dendrites of the postsynaptic neuron.
• This stimulates the postsynaptic neuron, opening ion channels and generating a new action potential if the signal is strong enough.
• The impulse is effectively transmitted from one neuron to another, facilitating communication within the nervous system.