Nervous System: CNS & PNS

• The nervous system is the primary internal communication network of the body
• It is composed of the brain, spinal cord, and nerves

Functions of the Nervous System

• Sensory input involves detecting internal and external stimuli through sensory receptors
• Integration involves processing sensory information and making decisions
• Motor output involves initiating responses via effectors, such as muscles or glands

Divisions of the Nervous System

Central Nervous System (CNS)

• Consists of the brain and spinal cord
• Processes and coordinates sensory data
• Relays motor commands to effectors

Peripheral Nervous System (PNS)

• Includes all neural tissue outside the CNS
• Connects the CNS to the limbs and organs
• Delivers sensory information to the CNS
• Carries motor commands to peripheral tissues and systems

Divisions of the PNS

Somatic Nervous System (SNS)

• Controls skeletal muscle contractions
• Enables voluntary and involuntary muscle control via reflexes
• Is a component of the PNS

Autonomic Nervous System (ANS)

• Regulates smooth muscle, cardiac muscle, and glands
• Facilitates involuntary control
• Is a component of the PNS

Divisions of the ANS

Sympathetic Division

• Is a part of the Autonomic Nervous System
• Exerts a stimulating effect, often known as "fight or flight"
• Increases heart rate, dilates pupils, and inhibits digestion

Parasympathetic Division

• Is a part of the Autonomic Nervous System
• Produces a relaxing effect, often known as "rest and digest"
• Slows heart rate, constricts pupils, and stimulates digestion

Cells of the Nervous System

Neurons

• Transfer information in the nervous system
• Transmit electrical signals called action potentials
• High metabolic rate
• Have extreme longevity
• Typically non-mitotic

Neuroglia (Glial Cells)

• Support and maintain neurons
• Are mitotic

Parts of a Neuron

Cell Body (Soma)

• Contains the nucleus and other organelles
• The control center of the neuron

Dendrites

• Numerous, short, highly branched processes extend from the cell body
• Receive signals from other neurons

Axon

• A single, long process extending from the cell body
• Transmits signals to other neurons or effectors

Axon Hillock

• The cone-shaped region where the axon joins the cell body
• Where the action potential is initiated

Myelin Sheath

• A fatty insulating layer around the axon
• Formed by glial cells: oligodendrocytes in the CNS and Schwann cells in the PNS
• Increases the speed of signal transmission

Nodes of Ranvier

• Gaps in the myelin sheath where the axon is exposed
• Allow for saltatory conduction (jumping) of the action potential

Axon Terminals (Synaptic Terminals)

• Branched endings of the axon that form connections with other neurons or effectors
• Release neurotransmitters to transmit the signal

Types of Neurons

Sensory Neurons (Afferent Neurons)

• Transmit sensory information from receptors to the CNS
• Detect stimuli such as touch, temperature, light, and chemicals

Motor Neurons (Efferent Neurons)

• Transmit motor commands from the CNS to effectors, such as muscles or glands
• Cause muscle contraction or gland secretion

Interneurons (Association Neurons)

• Located within the CNS
• Integrate sensory information and coordinate motor output
• Involved in higher functions such as learning and memory

Neuron Communication

Resting Membrane Potential

• The electrical potential difference across the plasma membrane of a neuron when it is not actively transmitting a signal
• Typically around -70mV (inside negative relative to outside)
• Maintained by ion channels and pumps, such as the Na+/K+ pump

Ion Channels

• Proteins in the plasma membrane that allow specific ions to pass through
• Leak channels are always open
• Gated channels open or close in response to a stimulus (voltage, ligand, mechanical)

Action Potential

• A rapid, short-lasting change in the membrane potential of a neuron
• Used to transmit signals over long distances

Steps of an Action Potential

• Depolarization occurs as the membrane potential becomes less negative (more positive) due to influx of Na+
• Repolarization occurs as the membrane potential returns to its resting value due to efflux of K+
• Hyperpolarization occurs when the membrane potential becomes more negative than the resting value due to continued efflux of K+

Threshold

• The depolarization level required to trigger an action potential
• Typically around -55mV

All-or-None Principle

• An action potential either occurs fully or does not occur at all
• The strength of the stimulus does not affect the size of the action potential but can affect the frequency of action potentials

Propagation of Action Potential

• The action potential travels down the axon from the axon hillock to the axon terminals
• In myelinated axons, the action potential jumps from one node of Ranvier to the next (saltatory conduction), increasing the speed of transmission

Synaptic Transmission

• The process of transmitting a signal from one neuron to another across a synapse

Synapse

• The junction between two neurons or between a neuron and an effector cell
• Consists of the presynaptic neuron, the synaptic cleft, and the postsynaptic neuron

Neurotransmitters

• Chemical messengers released from the presynaptic neuron that bind to receptors on the postsynaptic neuron
• Examples include acetylcholine, norepinephrine, dopamine, serotonin, and GABA

Steps of Synaptic Transmission

• The action potential arrives at the axon terminal of the presynaptic neuron
• Voltage-gated Ca2+ channels open, allowing Ca2+ to enter the axon terminal
• Ca2+ influx triggers the release of neurotransmitter from synaptic vesicles into the synaptic cleft
• Neurotransmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic neuron
• Binding of neurotransmitter to receptors causes a change in the membrane potential of the postsynaptic neuron, resulting in either depolarization or hyperpolarization
• Neurotransmitter is removed from the synaptic cleft by degradation, reuptake, or diffusion

Postsynaptic Potentials

Excitatory Postsynaptic Potential (EPSP)

• A depolarization of the postsynaptic membrane
• Increases the likelihood of the postsynaptic neuron firing an action potential

Inhibitory Postsynaptic Potential (IPSP)

• A hyperpolarization of the postsynaptic membrane
• Decreases the likelihood of the postsynaptic neuron firing an action potential

Summation

• The integration of multiple postsynaptic potentials at the axon hillock
• If the sum of EPSPs and IPSPs reaches the threshold, an action potential is triggered

Neuroglia (Glial Cells) of the CNS

Astrocytes

• Are the most abundant glial cell in the CNS
• Form the blood-brain barrier, which regulates the passage of substances from the blood into the brain
• Provide structural support and regulate the chemical environment around neurons

Oligodendrocytes

• Form the myelin sheath around axons in the CNS
• Insulate axons and increase the speed of signal transmission

Microglia

• Immune cells of the CNS
• Phagocytize pathogens and cellular debris

Ependymal Cells

• Line the ventricles of the brain and the central canal of the spinal cord
• Produce and circulate cerebrospinal fluid (CSF)

Neuroglia (Glial Cells) of the PNS

Schwann Cells

• Form the myelin sheath around axons in the PNS
• Insulate axons and increase the speed of signal transmission
• Can only myelinate one segment of one axon

Satellite Cells

• Surround neuron cell bodies in ganglia
• Provide support and regulate the chemical environment around neurons

The Brain

Cerebrum

• Largest part of the brain
• Responsible for higher-level functions such as thought, language, memory, and voluntary movement
• Divided into two hemispheres (left and right) connected by the corpus callosum

Cerebral Cortex

• The outer layer of the cerebrum
• Composed of gray matter
• Highly folded to increase surface area
• Contains sensory areas, motor areas, and association areas

Lobes of the Cerebrum

• Frontal lobe: controls motor functions, planning, decision-making, and personality
• Parietal lobe: processes sensory information, spatial awareness, and language
• Temporal lobe: processes auditory information, memory, and object recognition
• Occipital lobe: processes visual information

Diencephalon

• Located between the cerebrum and the brainstem
• Includes the thalamus, hypothalamus, and epithalamus

Thalamus

• Relays sensory information to the cerebral cortex

Hypothalamus

• Regulates body temperature, hunger, thirst, sleep-wake cycles, and hormone release
• Controls the autonomic nervous system

Brainstem

• Connects the brain to the spinal cord
• Includes the midbrain, pons, and medulla oblongata

Midbrain

• Involved in visual and auditory reflexes and motor control

Pons

• Relays information between the cerebrum and the cerebellum
• Involved in breathing and sleep

Medulla Oblongata

• Controls vital functions such as heart rate, blood pressure, and breathing

Cerebellum

• Located posterior to the brainstem
• Coordinates voluntary movements and maintains balance and posture

Spinal Cord

• Extends from the brainstem to the lower back
• Transmits sensory information from the body to the brain
• Transmits motor commands from the brain to the body
• Controls reflexes

Protection of the CNS

Meninges

• Three layers of membranes that surround the brain and spinal cord
• Dura mater: the outermost, tough layer
• Arachnoid mater: the middle layer, contains cerebrospinal fluid
• Pia mater: the innermost, delicate layer that adheres to the surface of the brain and spinal cord

Cerebrospinal Fluid (CSF)

• Clear fluid that surrounds the brain and spinal cord
• Provides cushioning and protection
• Transports nutrients and removes waste products

Blood-Brain Barrier (BBB)

• A selective barrier that regulates the passage of substances from the blood into the brain
• Protects the brain from harmful substances and pathogens
• Formed by tight junctions between endothelial cells in brain capillaries and astrocytes

The nervous system is the primary internal communication network, composed of the brain, spinal cord, and nerves. It has three main functions: sensory input, integration, and motor output. The nervous system is divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).