The nervous system is the body's command center, orchestrating our thoughts, movements, and responses to the world around us. It's divided into two main parts: the central nervous system (brain and spinal cord) and the peripheral nervous system (nerves connecting to the rest of the body).

This complex network of neurons and glial cells works together to process information, make decisions, and control bodily functions. From voluntary movements to automatic processes like breathing, the nervous system keeps us alive and helps us interact with our environment.

Nervous System Divisions and Components

Central and Peripheral Nervous Systems

• The nervous system is divided into two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS)
  • The CNS consists of the brain and spinal cord, which are protected by the skull and vertebral column, respectively
  • The PNS consists of all the nerves that extend from the CNS to the rest of the body, including sensory receptors (touch, pain, temperature), ganglia, and nerve plexuses (brachial, lumbar)

Brain Regions and Functions

• The brain is divided into several regions, each with specific functions:
  • The cerebrum is the largest part of the brain and is responsible for higher cognitive functions, such as perception, memory, and decision-making. It is divided into two hemispheres connected by the corpus callosum
  • The cerebellum is located at the back of the brain and is involved in motor coordination (balance, posture, smooth movements)
  • The brainstem connects the brain to the spinal cord and regulates vital functions, such as breathing, heart rate, and sleep cycles
  • The thalamus acts as a relay center for sensory information, while the hypothalamus regulates homeostasis (body temperature, hunger, thirst)

Spinal Cord and Neurons

• The spinal cord is a long, thin bundle of nervous tissue that extends from the brainstem to the lower back. It serves as a conduit for sensory and motor information between the brain and the rest of the body
  • The spinal cord contains neural circuits for simple reflexes (knee-jerk reflex) that can occur without input from the brain
• Neurons are the basic functional units of the nervous system, responsible for transmitting electrical and chemical signals. They consist of a cell body, dendrites (receive signals), and an axon (transmits signals)
• Glial cells are non-neuronal cells that provide support, protection, and maintenance for neurons. Types of glial cells include astrocytes (support and nutrient exchange), oligodendrocytes (myelination), and microglia (immune defense)

Functions of the Central and Peripheral Nervous Systems

Central Nervous System (CNS) Functions

• The central nervous system (CNS) is responsible for integrating and processing sensory information, making decisions, and coordinating motor responses
  • The brain is the primary control center of the nervous system, responsible for higher cognitive functions, such as perception, memory, emotion, and decision-making
  • The spinal cord serves as a relay center for sensory and motor information between the brain and the rest of the body. It also contains neural circuits for simple reflexes (withdrawal reflex)

Peripheral Nervous System (PNS) Functions

• The peripheral nervous system (PNS) connects the CNS to the rest of the body, allowing for communication between the brain, spinal cord, and peripheral organs and tissues
  • Sensory neurons in the PNS detect stimuli from the internal and external environment (pain, temperature, pressure) and transmit this information to the CNS for processing
  • Motor neurons in the PNS carry signals from the CNS to effector organs, such as muscles (skeletal, cardiac, smooth) and glands (sweat, salivary), to initiate responses
• The PNS is further divided into the somatic nervous system and the autonomic nervous system, each with specific functions

Somatic vs Autonomic Nervous Systems

Somatic Nervous System (SNS)

• The somatic nervous system (SNS) is responsible for voluntary, conscious control of skeletal muscles
  • Sensory neurons in the SNS detect stimuli from the skin, muscles, and joints, providing information about touch, pressure, temperature, and proprioception (body position and movement)
  • Motor neurons in the SNS innervate skeletal muscles, allowing for conscious control of movement (walking, talking, writing)

Autonomic Nervous System (ANS)

• The autonomic nervous system (ANS) regulates involuntary, unconscious functions of internal organs and glands
  • The sympathetic division of the ANS is responsible for the "fight or flight" response, preparing the body for action in stressful situations by increasing heart rate, blood pressure, and blood glucose levels (adrenaline release)
  • The parasympathetic division of the ANS is responsible for the "rest and digest" response, promoting relaxation and conservation of energy by slowing heart rate, increasing digestion (peristalsis), and promoting urination and defecation
  • The enteric nervous system is a subset of the ANS that specifically controls the gastrointestinal tract, regulating motility, secretion (digestive enzymes), and blood flow

Maintaining Homeostasis

• The SNS and ANS work together to maintain homeostasis, ensuring that the body's internal environment remains stable despite changes in the external environment
  • For example, the SNS increases heart rate and breathing during exercise, while the ANS regulates body temperature (sweating, shivering) and blood glucose levels (insulin, glucagon secretion)

Blood-Brain Barrier: Protection for the Brain

Structure and Function

• The blood-brain barrier (BBB) is a highly selective, semipermeable barrier that separates the circulating blood from the brain and extracellular fluid in the central nervous system (CNS)
• The BBB is formed by tight junctions between endothelial cells lining the capillaries in the brain, as well as astrocytes and pericytes that surround the endothelial cells
• The primary function of the BBB is to protect the brain from potentially harmful substances circulating in the blood, such as toxins (heavy metals), pathogens (bacteria, viruses), and inflammatory molecules (cytokines)

Selective Permeability

• The tight junctions between endothelial cells prevent the passage of large or hydrophilic molecules, while allowing the diffusion of small, lipid-soluble molecules, such as oxygen and carbon dioxide
• Specific transport proteins in the endothelial cells facilitate the selective uptake of essential nutrients, such as glucose and amino acids, while efflux transporters (P-glycoprotein) remove metabolic waste products and xenobiotics (drugs, pollutants) from the brain
• The BBB also helps to maintain a stable environment for proper neuronal function by regulating the ionic composition of the extracellular fluid in the brain

BBB Disruption and Neurological Disorders

• Disruption of the BBB can occur in various neurological disorders, such as stroke (ischemia), traumatic brain injury, and multiple sclerosis (autoimmune attack), leading to the infiltration of harmful substances and immune cells into the brain, which can exacerbate neuroinflammation and neurodegeneration
• Understanding the structure and function of the BBB is crucial for developing targeted therapies that can effectively deliver drugs to the brain while minimizing systemic side effects (nanoparticles, receptor-mediated transport)
• Research on the BBB also aims to identify biomarkers (proteins, microRNAs) that can be used to diagnose and monitor the progression of neurological disorders