The Nervous System: Neurons and Nerve Cells

• The nervous system has three main functions: receiving information, interpreting it, and enabling the body to respond.

Nerve Cells (Neurons)

• Nerve cells, also known as neurons, are akin to electrical wires distributed throughout the house.
• Millions of neurons are scattered throughout the whole body.
• Neurons convert stimuli into impulses, which are electric signals.
• A stimulus is an environmental change that initiates an impulse within a neuron.

Parts of a Neuron

• Cell Body: Contains the nucleus and other cell organelles.
• Dendrites: Short fibers extending from the cell body; they are receiving parts of the neuron.
• Axon: Transmits impulses to other neurons; it's a long fiber with a myelin sheath for insulation and protection.
• Myelin Sheath: A protective layer around axons that helps insulate neurons and increase the number of transferred electrical signals.
• Schwann Cells: Glial cells that form the myelin sheath around nerve fibers in the peripheral nervous system
• Nodes of Ranvier: Microscopic gaps in myelinated axons that speed up action potential propagation through saltatory conduction.

Types of Neurons

• Sensory Neurons: Detect and respond to external signals, relaying sensory information to electrical impulses via their receptors in the PNS.
• Motor Neurons: Carry impulses from the CNS to effectors like muscles and glands and are also known as efferent neurons.
• Relay Neurons or Interneurons: Process sensory information and facilitate responses through motor neurons and are also called association neurons

Neuron vs. Neuroglia/Glial Cells

• Neuroglia or glial cells are located in the CNS and connect nervous tissues together.
• Glial cells cannot transmit nerve impulses but replace neurons after injuries or diseases and can cause glioma, a malignant brain tumor.
• Oligodendrocytes: Produce myelin, an insulating sheath on axons of nerve fibers that is found in the central nervous system of invertebrates and vertebrates.
• Astrocytes: Are the most abundant glial cells in the CNS. Together with oligodendrocytes, they form macroglia, which originate within the neural plate.
• Microglia: A type of neuronal support cell (neuroglia) occurring in the central nervous system of invertebrates and vertebrates that functions primarily as an immune cell.

Central Nervous System (CNS)

• The CNS is the body's command center, comprising the brain and spinal cord, integrating sensory input, and reacting to environmental changes.
• The brain, encased in the skull for protection, is an important organ, wrapped to nourish and bathed in cerebrospinal fluid (CSF) for cushioning.
• The brain’s gray matter is composed of cell bodies and appears gray, while the white matter underneath is composed of myelinated axons.
• Cerebrum: Constitutes 85% of the human brain and is identified by a cerebral cortex with wrinkled grooves and is the area for learning, intelligence, and judgement, and is divided into hemispheres.

Lobes of the Brain

• Frontal Lobe: Manages reasoning, motor control, emotion, and language and contains the motor cortex for movement planning/coordination, the prefrontal cortex for cognitive function, and Broca's area for language production.
• Parietal Lobe: Processes information from the body’s senses; located behind the frontal lobe + has the somatosensory cortex for sensory information such as touch, temperature, and pain.
• Temporal Lobe: Located on the side of the head (near the temples) and associated with hearing, memory, emotion, and some language aspects.
• The left hemisphere of the brain is responsible for language, mathematics, logical reasoning, and problem-solving.
• The right hemisphere enhances talent in art, music, dancing, drawing, and other creative expressions.
• Cerebellum: Coordinates muscle actions for smooth, steady movements and is located behind the brain stem, and oversees balance, posture, and equilibrium.
• Brain Stem: Connects the spinal cord to the brain to coordinate survival functions like breathing, heart rate, sleep, and wakefulness
• Medulla Oblongata: Controls involuntary actions
• Pons: Help control respiratory functions.
• Midbrain: Processes visual and auditory reflexes.
• Spinal cord: Relays nerve impulses to and from the brain through a tubelike structure containing neurons and blood vessels.
• PNS Impulses: PNS impulses are transmitted to the brain via the Spinal Cord.

Peripheral Nervous System (PNS)

• The PNS connects the CNS to the rest of the body
• The PNS is composed “nerves”: A network of bundled ropelike structures.
• The PNS gathers and delivers information to and from the CNS.
• The PNS consists of 43 pairs of nerves extending from the brain and spinal cord to organs in the body.

Somatic and Autonomic Systems

• Somatic Nervous System: Facilitates willful movement by using skeletal muscles, and is responsible for functions we are aware of, such as arm and leg movement.
• Autonomic Nervous System: Regulates involuntary body functions like heartbeat, blood flow, breathing, and digestion.
• Reflex Arcs: Consist of sensory nerves transmitting signals to the spinal cord and immediately relaying them via motor neurons to trigger reflexive muscle responses.
• Reflexes: Reflexes occur involuntarily through direct nerve pathways in the spine.
• Examples of reflexes include jerking a hand back after touching a hot pan and involuntary knee jerks.
• Parasympathetic Nervous System: Maintains normal bodily functions, conserves resources by controlling the bladder, slowing the heart rate, and constricting eye pupils.
• Sympathetic Nervous System: Regulates fight-or-flight responses, relaxes the bladder, speeds up heart rate, and dilates eye pupils.
• Enteric Nervous System: Controls the gastrointestinal tract and digestion.

Explore the nervous system's functions: receiving, interpreting, and responding to information. Learn about neurons, the body's electrical wires, and their parts, including the cell body, dendrites, and axon. Discover how myelin sheath and Schwann cells contribute to nerve insulation and signal transmission.