9.1 Introduction to the Nervous System

The nervous system is the body's communication and control network, coordinating everything from breathing to complex thought. For pharmacology, understanding this system is essential because so many drugs work by altering nervous system signaling, whether that's blocking a receptor, mimicking a neurotransmitter, or changing how signals travel along a nerve.

This unit covers the basic architecture of the nervous system, how neurons communicate, and how the system divides into central and peripheral components.

Overview of the Nervous System

Structure and Function of the Nervous System

The nervous system is a vast network of specialized cells and tissues that transmits signals throughout the body. It's composed of the brain, spinal cord, and an extensive web of nerves reaching every organ and tissue.

The system carries out three core functions:

• Sensory input: Receives and interprets information from both internal and external stimuli (touch, sight, sound, organ stretch, blood chemistry changes)
• Integration: The brain and spinal cord process and analyze that sensory information, essentially "deciding" what to do with it
• Motor output: Sends signals to muscles, glands, and organs to trigger appropriate responses (muscle contraction, hormone release, changes in heart rate)

Nervous System Cells

Two broad categories of cells make up the nervous system: neurons and glial cells.

Neurons are the specialized cells that actually transmit electrical and chemical signals. Each neuron has three main parts:

• Cell body (soma): Contains the nucleus and organelles; the metabolic center of the cell
• Dendrites: Branched extensions that receive incoming signals from other neurons
• Axon: A long, thin extension that carries signals away from the cell body toward other neurons or effector cells like muscles and glands

Glial cells are non-neuronal support cells. They don't transmit signals themselves, but neurons can't function without them:

• Astrocytes provide structural support, help regulate neurotransmitter levels, and maintain the blood-brain barrier
• Oligodendrocytes (in the CNS) and Schwann cells (in the PNS) wrap around axons to form the myelin sheath, an insulating layer that dramatically speeds up signal transmission
• Microglia act as the immune cells of the CNS, protecting against infection and clearing cellular debris

Nervous System Communication for Homeostasis

Neurons communicate through two types of signaling: electrical and chemical. Understanding both is critical for pharmacology, since drugs can target either process.

Electrical signaling involves changes in the membrane potential of neurons:

• Resting potential: When a neuron isn't actively firing, it maintains a stable, negative charge inside the cell (around $-70 \text{ mV}$). This "ready state" is maintained by ion pumps and channels.
• Action potential: When a stimulus is strong enough to reach threshold, a rapid, all-or-nothing change in membrane potential fires and propagates down the axon. This is the electrical impulse that carries the signal.

Chemical signaling happens at synapses, the tiny gaps between neurons:

1. An action potential reaches the end of the presynaptic neuron's axon
2. The presynaptic neuron releases neurotransmitters (chemical messengers like acetylcholine, dopamine, or serotonin) into the synaptic cleft
3. These neurotransmitters cross the gap and bind to receptors on the postsynaptic cell
4. Depending on the neurotransmitter and receptor type, the postsynaptic cell is either excited (more likely to fire) or inhibited (less likely to fire)

This process of synaptic transmission is where a huge number of drugs exert their effects, by enhancing, blocking, or mimicking neurotransmitter activity.

Homeostasis and Feedback

The nervous system constantly monitors and adjusts bodily functions to maintain internal stability:

• Negative feedback detects a deviation from a set point and initiates a corrective response to bring the system back to normal. Examples include thermoregulation (you shiver when cold, sweat when hot) and blood pressure regulation.
• Positive feedback amplifies a change to drive a process to completion. A classic example is oxytocin release during childbirth, where contractions trigger more oxytocin, which triggers stronger contractions.

The autonomic nervous system is the primary driver of involuntary homeostatic regulation, split into two divisions:

• Sympathetic division: Activates "fight or flight" responses by increasing heart rate, blood pressure, and glucose release
• Parasympathetic division: Promotes "rest and digest" functions by slowing heart rate, increasing digestive activity, and promoting relaxation

These two divisions often oppose each other, and their balance determines the body's overall state at any given moment. Many drugs you'll encounter in pharmacology target one division or the other.

Nervous System Adaptability and Protection

• Neuroplasticity is the brain's ability to reorganize itself and form new neural connections throughout life. This is what allows learning, memory formation, and recovery after brain injuries. The brain isn't "fixed" after development; it continues to adapt.
• The blood-brain barrier (BBB) is a selective barrier formed partly by astrocytes that protects the brain from potentially harmful substances circulating in the bloodstream. It tightly regulates which molecules can pass from the blood into the CNS. This has major pharmacological implications: many drugs cannot cross the BBB, which limits treatment options for brain disorders and is a key consideration in drug design.

Divisions of the Nervous System

Central vs. Peripheral Nervous Systems

The nervous system divides into two main components: the central nervous system (CNS) and the peripheral nervous system (PNS).

Central Nervous System (CNS)

The CNS consists of the brain and spinal cord. It's the integration and processing center, where sensory information is analyzed and decisions about responses are made.

The brain has three major regions:

• Cerebrum: The largest part, responsible for higher cognitive functions (thinking, language, planning), sensory processing, and voluntary movement
• Cerebellum: Coordinates balance, posture, and fine motor control
• Brainstem: Regulates vital involuntary functions like respiration, heart rate, and consciousness

The spinal cord is a bundle of nerves extending from the brainstem down through the vertebral column. It serves as the main conduit for signals traveling between the brain and the rest of the body:

• Ascending tracts carry sensory information up from the body to the brain
• Descending tracts carry motor commands down from the brain to the body
• Spinal reflexes are rapid, involuntary responses (like the knee-jerk reflex) that occur at the spinal cord level without needing input from the brain, allowing for faster protective reactions

Peripheral Nervous System (PNS)

The PNS consists of all the nerves and ganglia (clusters of nerve cell bodies) located outside the brain and spinal cord. Its role is to connect the CNS to the rest of the body.

The PNS has two functional divisions:

Sensory (afferent) division carries information toward the CNS:

• Somatic sensory: Detects touch, pressure, temperature, pain, and proprioception (body position)
• Visceral sensory: Monitors conditions in internal organs and glands

Motor (efferent) division carries commands away from the CNS:

• Somatic nervous system: Controls voluntary movements of skeletal muscles
• Autonomic nervous system: Regulates involuntary functions of smooth muscle, cardiac muscle, and glands, further divided into the sympathetic and parasympathetic divisions described above