Nervous System: CNS, PNS, and Brain

Nervous System

• The nervous system executes motor responses and regulates body mechanisms.
• It consists of the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

Central Nervous System (CNS)

• Composed of the spinal cord and the brain.

Brain Components

• Hindbrain includes the Medulla (for breathing, blood pressure, heart rate), Pons (coordinates signals and regulates), and Cerebellum (coordinates balance and movement).
• Midbrain is involved in alertness, sleep/wake cycles, and motor activity.
• Forebrain includes the Cerebrum, which enables speech, thinking, reasoning, sensing, and emotions.

Peripheral Nervous System (PNS)

• PNS provides sensory information to the CNS for processing.
• Autonomic Nervous System (ANS) governs internal environment processes and autonomic reflexes.
• Somatic Nervous System (SNS) controls skeletal muscle motor functions, enabling voluntary actions and somatic reflexes.

Neurons and Glia

• Neurons transmit signals.
• Glia are supporting cells that help neurons connect.

Neuron Composition

• Cell body contains the nucleus and most organelles.
• Dendrites are branched structures that receive signals.
• Axon is the fiber that carries a signal away to another cell.
• Synapse is the area where a neuron communicates with another cell.

Glia Functions

• They maintain chemical balance in the spaces between cells.
• They maintain the blood-brain barrier, restricting substance entry into the brain.
• They produce myelin, forming a myelin sheath around axons to insulate and enhance signal transfer.
• They produce cerebrospinal fluid, which is essential for the brain and homeostasis.

Action Potential Process

• Neurons use action potentials to communicate signals.
• Resting potential occurs when a neuron is at rest without signal transmission and there is a higher concentration of Na+ outside the neuron and K+ inside the cell, resulting in a negative charge relative to its surroundings.
• An action potential starts when a neuron receives a signal, rapidly changing the charge around the axon.
• Na+ ions flood into the axon while K+ ions flow out during depolarization, propagating the signal.
• Repolarization restores the original state by reversing the ion flow.

Neurotransmitters

• Signals reach axon terminals (synapses) at the end of a neuron.
• Synapses emit neurotransmitters derived from amino acids, amino acid precursors, or gases.
• Neurotransmitters travel across the synaptic cleft (the space between the terminal and dendrite) to reach the dendrite.
• Neurotransmitters combine with specific receptors on the next neuron, and the dendrite initiates a new action potential.

Synapse Details

• The synapse is the meeting point between two neurons, which translates and converts signals.
• Synapses facilitate brain connections for learning and memory, but are also related to psychiatric disorders and addictions.
• Electrical signals are immediate.
• Chemical signals are controlled, slower, more abundant, precise, and selective.
• An action potential triggers the opening of voltage-gated calcium channels (Ca^2+) at the axon terminal, causing calcium to be released into the neuron's cytoplasm.
• The influx of positively charged calcium ions causes synaptic vesicles to fuse with the cell membrane and release neurotransmitters.

Types of Neurotransmitters

• Excitatory neurotransmitters cause graded potentials that bring the neuron closer to the threshold for an action potential and depolarize the axon by making its inside more positive.
• Inhibitory neurotransmitters generate graded potentials that push the neuron away from the threshold, making an action potential less likely and hyperpolarizing the axon by making it more negative.

Drugs and Synapses

• Drugs with structures similar to transmitter substances can affect postsynaptic protein receptors.
• Agonists stimulate the nervous system.
• Antagonists inhibit the nervous system.
• Dopamine is the master molecule of addiction, primarily in the brain's primitive areas.

Drug Effects on Dopamine

• Cocaine blocks dopamine absorption, causing dopamine to accumulate in the synapse.
• Amphetamines stimulate excess dopamine release, overwhelming reuptake and breakdown processes.
• Nicotine stimulates dopamine release, while compounds in cigarette smoke inhibit monoamine oxidase (MAO).

Withdrawal

• Withdrawal is a physiological response to the absence of drug effects.
• It occurs abruptly when a drug addict quits.
• Over time, the body returns to normal physiology, and cravings subside.

Drug Administration Methods

• Oral ingestion requires absorption via the digestive system.
• Inhaling delivers the drug faster through the bloodstream.
• Injecting directly introduces chemicals into the bloodstream for the quickest effect.

Reflex Arc

• The reflex arc is a specific neural pathway of a reflex action.
• Stimulus initiates change.
• Receptor detects stimulus.
• Sensory neuron carries signal to spinal cord.
• Relay neuron (interneuron) processes the signal in the spinal cord.
• Motor neuron sends response to effector.
• Effector (muscle or gland) carries out response.

Reflex Actions

• Reflex actions are fast and automatic.
• Protective in nature.
• Uses a reflex arc.
• Bypasses the brain in some cases for a quicker response.