Chapter 8: The Nervous System – Anatomy & Physiology Study Notes

The Nervous System

Introduction to the Nervous System

The nervous system is a complex network responsible for coordinating body activities and processing sensory information. It consists of the central and peripheral nervous systems, each with specialized functions.

• Central Nervous System (CNS): Includes the brain and spinal cord; processes and integrates information.

• Peripheral Nervous System (PNS): Consists of nerves outside the CNS; transmits signals between the CNS and the rest of the body.

• Neural tissue: Contains two main cell types: neurons (signal transmission) and neuroglia (support and protection).

Organs of the Nervous System

The nervous system is organized into distinct organs that perform specialized functions.

• Brain and Spinal Cord: Make up the CNS and are the main processing centers.

• Sensory Receptors: Specialized cells that detect changes in the environment (e.g., eyes, ears).

• Peripheral Nerves: Connect the CNS to other body systems.

Neurons

Structure and Function of Neurons

Neurons are the fundamental units of the nervous system, specialized for communication and signal transmission.

• Cell body (soma): Contains the nucleus and organelles.

• Dendrites: Receive incoming signals.

• Axon: Transmits electrical impulses away from the cell body.

• Axon terminals: Communicate with other neurons or effector cells.

Functional Classifications of Neurons

Neurons are classified based on their function within the nervous system.

• Sensory Neurons: Transmit sensory impulses toward the brain and spinal cord.

• Motor Neurons: Carry impulses from the CNS to muscles, organs, and glands.

• Interneurons: Connect neurons within the CNS, integrating and evaluating information.

Types of Sensory Receptors

Sensory receptors detect specific types of stimuli and relay information to the CNS.

• Exteroceptors: Detect external stimuli (touch, temperature, pressure).

• Proprioceptors: Monitor body position and movement.

• Interoceptors: Sense internal conditions (digestive, respiratory, cardiovascular systems).

Neuroglia

Neuroglia in the CNS and PNS

Neuroglia are supportive cells that maintain the environment for neurons and assist in their function.

• Astrocytes: Maintain the blood-brain barrier and support neurons.

• Oligodendrocytes: Form myelin sheaths in the CNS.

• Microglia: Act as immune cells, removing debris.

• Ependymal cells: Line ventricles and produce cerebrospinal fluid.

The Membrane Potential

Resting Membrane Potential

The resting membrane potential is the electrical charge difference across the neuron's plasma membrane when the cell is not transmitting a signal.

• Typical value: About -70 mV in neurons.

• Maintained by: Selective permeability of the membrane and ion pumps.

Equation:

$V_m = V_{inside} - V_{outside}$

Factors Responsible for Membrane Potential

• Ion concentration gradients: Mainly sodium (Na+) and potassium (K+).

• Selective permeability: Membrane allows certain ions to pass more easily.

• Electrical gradients: Separation of positive and negative ions.

Sodium and Potassium Channels

• Leak channels: Always open, allow passive movement of ions.

• Gated channels: Open or close in response to stimuli.

Graded Potentials

Graded potentials are temporary changes in membrane potential due to stimulus.

• Can spread from the site of stimulation.

• Produce a graded potential.

Action Potential Generation

An action potential is a rapid, propagated change in membrane potential that travels along the axon.

• Initiated at the axon hillock.

• Propagates along the axon to axon terminals.

Equation:

$I = g(V_m - E)$

Propagation of Action Potentials

• Continuous propagation: Occurs in unmyelinated axons.

• Saltatory propagation: Occurs in myelinated axons, jumping between nodes of Ranvier.

Synapses

Chemical Synapses

Synapses are junctions where neurons communicate with other neurons or effector cells.

• Neurotransmitters: Chemicals such as acetylcholine (ACh) or norepinephrine (NE) transmit signals across the synaptic cleft.

• Events at a cholinergic synapse: Action potential arrives, calcium enters, ACh is released, binds to receptors, and is broken down by acetylcholinesterase.

Types of Synapses

• Excitatory synapses: Promote action potentials in the postsynaptic cell.

• Inhibitory synapses: Suppress action potentials.

Neurotransmitters and Neuromodulators

Neurotransmitters are chemicals that transmit signals across synapses. Neuromodulators modify the response of neurons to neurotransmitters.

• Examples: Norepinephrine (NE), dopamine, serotonin, gamma-aminobutyric acid (GABA).

The Meninges

The Three Meningeal Layers

The brain and spinal cord are protected by three layers of connective tissue called meninges.

• Dura mater: Tough, outermost layer.

• Arachnoid mater: Middle, web-like layer.

• Pia mater: Innermost, delicate layer adhering to the CNS surface.

Gross Anatomy of the Spinal Cord

Spinal Cord Structure

The spinal cord is a long, cylindrical structure that transmits information between the brain and the body.

• Enlargements: Cervical and lumbar regions for limb innervation.

• Segments: 31 segments, each giving rise to a pair of spinal nerves.

Sectional Anatomy

• White matter: Contains myelinated and unmyelinated axons.

• Gray matter: Contains neuron cell bodies, neuroglia, and unmyelinated axons.

The Brain

Principal Structures of the Brain

The brain consists of several major regions, each with specialized functions.

• Cerebrum: Largest part, responsible for higher mental functions.

• Diencephalon: Contains thalamus and hypothalamus.

• Brain stem: Includes midbrain, pons, and medulla oblongata.

• Cerebellum: Coordinates movement and balance.

Cerebrum

• Divided into hemispheres: Left and right, separated by the longitudinal fissure.

• Surface layer: Gray matter (cerebral cortex).

Diencephalon

• Thalamus: Relay station for sensory information.

• Hypothalamus: Regulates homeostasis and endocrine functions.

Brain Stem

• Midbrain: Processes sight, sound, and reflexes.

• Pons: Connects cerebellum to brain stem.

• Medulla oblongata: Controls vital autonomic functions.

Cerebellum

• Coordinates repetitive body movements.

• Influences balance.

• Covered with cerebellar cortex.

Cerebrospinal Fluid (CSF)

• Produced in ventricles: Cushions and supports the brain.

• Circulates through subarachnoid space.

The Cerebral Cortex

Motor and Sensory Areas

• Primary motor cortex: Controls voluntary movements.

• Primary sensory cortex: Receives somatic sensory information.

Special Sensory Cortices

• Visual cortex: Processes visual information.

• Auditory cortex: Processes sound.

• Olfactory cortex: Processes smell.

• Gustatory cortex: Processes taste.

Association Areas

• Somatic sensory association area: Interprets sensory input.

• Somatic motor association area: Coordinates learned movements.

Other Integrative Areas

• General interpretive area: Integrates sensory information.

• Speech center (Broca's area): Produces speech.

• Prefrontal cortex: Involved in complex behaviors and decision-making.

Hemispheric Lateralization

• Left hemisphere: Language, math, logic.

• Right hemisphere: Spatial awareness, emotion, creativity.

Monitoring Brain Activity

• Electroencephalogram (EEG): Measures electrical activity (brain waves).

• Types of brain waves: Alpha, beta, theta, delta.

The Limbic System

Functions of the Limbic System

The limbic system is involved in emotion, memory, and motivation.

• Establishes emotional states.

• Links conscious and unconscious functions.

• Facilitates memory storage and retrieval.

HTML Table: Principal Neuroglia of the CNS

Neuroglia Type	Main Function
Astrocytes	Maintain blood-brain barrier, support neurons
Oligodendrocytes	Form myelin sheaths in CNS
Microglia	Phagocytosis, immune defense
Ependymal cells	Produce and circulate cerebrospinal fluid

HTML Table: Major Brain Regions and Functions

Region	Main Function
Cerebrum	Higher mental functions, voluntary movement
Diencephalon	Sensory relay, homeostasis
Brain Stem	Autonomic functions, relay between brain and spinal cord
Cerebellum	Coordination, balance

Additional info: Some explanations and definitions have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.