Nervous System and Neuron Structure

• The nervous system is composed of nerves, which are bundles of individual nerve cells called neurons.
• Neurons transmit messages known as nerve impulses, which are electrochemical in nature and travel very quickly.

Neuron Structure

• A neuron consists of a cell body (containing the nucleus and organelles), dendrites, and axons.
• Dendrites receive nerve impulses from other cells.
• Axons transmit nerve impulses to other cells.
• Axons are surrounded by Schwann cells, which are composed of myelin, a fatty substance that insulates the axon, speeding up nerve signal transmission.
• Nodes of Ranvier are gaps between Schwann cells where ions move in and out of the axon, facilitating impulse transmission.
• A single neuron can have thousands of dendrites and multiple axons, enabling communication with many other cells.

Nerve Impulse Transmission: Resting State

• Nerve impulses are electrical, resulting from a difference in electrical charge across the neuron's plasma membrane.
• When a neuron is not actively transmitting a nerve impulse, it is in a resting state.
• During the resting state, the sodium-potassium pump maintains a charge difference by using ATP to pump positive sodium ions (Na+) out of the cell and potassium ions (K+) into the cell.
• The inside of the neuron is negatively charged compared to the extracellular fluid, creating a resting potential due to more positively charged ions outside the cell.

Nerve Impulse Transmission: Action Potential

• A nerve impulse is a sudden reversal of the electrical charge across the membrane of a resting neuron, known as an action potential.
• The action potential begins when a neuron receives a chemical signal, causing sodium ion channels to open and allowing positive sodium ions to flow into the cell.
• As a result, the inside of the cell becomes positively charged compared to the outside.
• This reversal of charge rapidly ripples down the axon.
• In myelinated neurons, ions flow across the membrane only at the nodes between myelin sections, causing the action potential to jump from node to node, increasing transmission speed.

The Synapse

• A synapse is the junction where the axon of one neuron meets the dendrite of another, separated by a tiny gap.
• Synapses are also found between neurons and muscle cells, known as the neuromuscular junction.
• When a nerve impulse reaches the axon tip, neurotransmitters are released, which travel across the synaptic gap and bind to the dendrite of the next neuron, triggering a nerve impulse in the receiving neuron.
• Examples of neurotransmitters include serotonin, dopamine, and acetylcholine.

Types of Neurons

• Neurons are classified based on their role in the body, including sensory neurons and motor neurons.
• Sensory neurons carry nerve impulses from sense organs and internal organs to the central nervous system.
• Interneurons (relay neurons) process signals within the central nervous system.
• Motor neurons carry nerve impulses from the central nervous system to internal organs, glands, and muscles.
• Sensory neurons carry information to the central nervous system, which processes the information and sends messages through motor neurons for the body to respond.
• Reflex arcs, requiring only 2 or 3 neurons, produce the simplest processing and actions.

Central Nervous System

• The central nervous system (CNS) comprises the brain and the spinal cord.
• The brain is protected by the skull, and the spinal cord is protected by the vertebrae.
• The entire CNS is surrounded by protective coverings called meninges.

The Brain

• The brain is the control center of the nervous system, containing 100 billion neurons with synapses connecting each neuron to thousands of others.
• It consumes nearly a quarter of the body's total energy.
• The brain enables interpretation of sensory information, learning, thinking, memory, language use, and controls internal body processes and external movements.
• The brain consists of three main parts: the cerebrum, cerebellum, and brain stem.

Cerebrum

• The cerebrum is the largest part of the brain, located on top of the brainstem.
• It controls functions such as thinking, feeling, problem-solving, and speech, and serves as the storage place for most memories.
• It also controls all voluntary movements.

Cerebellum

• The cerebellum is the second largest part of the brain, located under the cerebrum and behind the brain stem.
• It controls body position, coordination, and balance.

Brain Stem

• The brain stem is the smallest of the three main parts of the brain.
• It consists of the midbrain, pons, and medulla oblongata.
• The midbrain is associated with vision, hearing, motor control, sleep/wake, arousal, and temperature regulation.
• The pons helps control breathing, sleep, and serves as a relay station between the cortex and the cerebellum.
• The medulla oblongata regulates breathing, heart and blood vessel function, digestion, sneezing, and swallowing.

Cerebral Hemispheres and Lobes

• The cerebrum is divided into right and left hemispheres connected by the corpus callosum, a thick bundle of axons.
• The corpus callosum carries messages between the two hemispheres.
• The right hemisphere controls the left side of the body, and the left hemisphere controls the right side of the body.
• Each hemisphere is divided into four lobes: frontal, parietal, temporal, and occipital.
• The frontal lobe is responsible for reasoning, planning, parts of speech, movement, emotions, and problem-solving.
• The parietal lobe is responsible for feeling, orientation, recognition, perception of stimuli.
• The temporal lobe is responsible for perception and recognition of auditory stimuli, memory, speech, language.
• The occipital lobe is responsible for sight and visual processing.

Peripheral Nervous System

• The peripheral nervous system (PNS) includes all nerves outside the central nervous system.
• This network includes nerves of the hands, arms, feet, legs, trunk, scalp, neck, and face, as well as nerves supplying internal organs and glands.
• The sensory division of the PNS carries messages from sense organs and internal organs to the central nervous system.
• The motor division of the PNS carries messages from the central nervous system to internal organs and muscles.