Organization of the Nervous System, Neurons, and Neurotransmitters

Organization of the Nervous System

Central and Peripheral Nervous Systems

The nervous system is divided into the Central Nervous System (CNS) and Peripheral Nervous System (PNS). Each plays a distinct role in processing and transmitting information throughout the body.

• CNS: Consists of the brain and spinal cord; responsible for integrating sensory information and responding accordingly.

• PNS: Includes all neural tissue outside the CNS; subdivided into spinal nerves and cranial nerves.

• PNS Divisions:

  • Afferent (sensory) Division: Transmits sensory information to the CNS.

  • Efferent (motor) Division: Carries motor commands from the CNS to effectors.

    • Autonomic Nervous System (ANS): Controls involuntary functions; includes Sympathetic and Parasympathetic divisions.

    • Somatic Nervous System: Controls voluntary movements.

Neurons and Neuroglia

Neuron Structure and Function

Neurons are the functional units of the nervous system, specialized for communication.

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

• Dendrites: Receive incoming signals.

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

• Axon hillock: Initiates action potentials.

• Axon terminal (synaptic terminal): Releases neurotransmitters to communicate with other cells.

• Myelin sheath: Insulates axons, increasing signal speed.

• Synapse: Junction between neurons where neurotransmitters are released.

Types of Neurons:

• Sensory neurons: Transmit sensory information.

• Motor neurons: Carry commands to muscles and glands.

• Interneurons: Connect neurons within the CNS.

Neuroglia (Glial Cells)

Support and protect neurons. Types include:

• Astrocytes

• Microglia

• Ependymal cells

• Oligodendrocytes

• Schwann cells

• Satellite cells

Membrane Potentials and Action Potentials

Resting and Action Potentials

Neurons communicate via electrical signals called action potentials, which are changes in membrane potential.

• Resting potential: The baseline electrical charge across the membrane, typically -70 mV.

• Graded potential: Local changes in membrane potential that may lead to an action potential.

• Action potential: Rapid, all-or-none electrical impulse.

• Voltage-gated channels: Proteins that open/close in response to changes in membrane potential.

• Depolarization: Membrane potential becomes less negative.

• Hyperpolarization: Membrane potential becomes more negative.

Propagation:

• Continuous: Occurs in unmyelinated axons.

• Saltatory: Occurs in myelinated axons; action potential jumps between nodes of Ranvier.

Refractory Periods:

• Absolute refractory period: No new action potential can be initiated.

• Relative refractory period: A stronger stimulus can initiate another action potential.

Key Equation:

• Resting membrane potential is determined by the Nernst equation: $E = \frac{RT}{zF} \ln \frac{[\text{ion outside}]}{[\text{ion inside}]}$

Synaptic Transmission and Neurotransmitters

Synapse Types and Function

Synapses are specialized junctions for neuron-to-neuron communication.

• Chemical synapse: Uses neurotransmitters to transmit signals.

• Electrical synapse: Direct flow of ions between cells.

Synaptic Events:

• Presynaptic neuron: Releases neurotransmitter.

• Postsynaptic neuron: Receives neurotransmitter.

• Reuptake: Neurotransmitter is taken back into the presynaptic cell.

• Enzymatic degradation: Neurotransmitter is broken down by enzymes.

Postsynaptic Potentials:

• EPSP (Excitatory Postsynaptic Potential): Depolarizes the postsynaptic membrane.

• IPSP (Inhibitory Postsynaptic Potential): Hyperpolarizes the postsynaptic membrane.

Summation:

• Spatial summation: Multiple synapses activate simultaneously.

• Temporal summation: Rapid, repeated activation of a single synapse.

Major Neurotransmitters

• Acetylcholine

• Dopamine

• Serotonin

• Norepinephrine

• GABA

• Glutamate

• Nitric oxide

Each neurotransmitter has specific functions and mechanisms for signal termination (reuptake, degradation).

Brain and Spinal Cord Organization

Major Brain Regions and Structures

The brain is organized into several regions, each with specialized functions.

• Cerebrum: Includes gray and white matter, gyri, sulci, and fissures.

• Basal nuclei: Involved in movement regulation.

• Cerebral cortex: Contains sensory and motor areas, association areas.

• Diencephalon: Includes thalamus, hypothalamus, limbic center.

• Brainstem: Composed of midbrain, pons, medulla oblongata.

• Cerebellum: Coordinates movement and balance.

• Ventricles: Contain cerebrospinal fluid (CSF).

• Meninges: Protective layers (dura mater, arachnoid, pia mater).

Blood-brain barrier: Protects the brain from harmful substances.

Spinal cord: Contains ascending and descending tracts, gray and white matter.

Functional Areas and Disorders

• Primary motor area: Controls voluntary movement.

• Primary somatic sensory area: Processes sensory input.

• Association areas: Integrate information.

• Broca's area: Speech production.

• Wernicke's area: Language comprehension.

• Common disorders: CVA (stroke), Alzheimer's, Parkinson's, Huntington's, cerebral edema, concussion.

Reflexes and Spinal Cord Function

Reflex Arc

A reflex arc is the basic unit of reflex action, involving sensory input, integration, and motor output.

• Components: Receptor, sensory neuron, integration center, motor neuron, effector.

• Examples: Knee-jerk reflex, withdrawal reflex.

Peripheral Nervous System (PNS)

PNS Vocabulary and Structure

The PNS consists of nerves and ganglia outside the CNS, responsible for transmitting signals to and from the CNS.

• Mixed nerves: Contain both sensory and motor fibers.

• Sensory (afferent) nerves: Carry information to the CNS.

• Motor (efferent) nerves: Carry commands from the CNS.

• Cranial nerves: 12 pairs, each with specific functions.

• Spinal nerves: 31 pairs, organized into plexuses.

Spinal Nerve Plexuses

Plexus	Main Nerves
Cervical	Phrenic nerve
Brachial	Axillary, Musculocutaneous, Radial, Median, Ulnar
Lumbar	Femoral, Obturator
Sacral	Sciatic (Tibial, Common fibular), Superior and inferior gluteal

Mnemonic: "UM_ARM" for brachial plexus nerves.

PNS Concepts

• Cranial nerves: Identified by name, Roman numeral, type (sensory, motor, both), and function.

• Plexuses: Clinical significance in nerve injury and disease.

Autonomic Nervous System (ANS)

ANS Structure and Function

The ANS regulates involuntary functions and is divided into sympathetic and parasympathetic divisions.

• Sympathetic division: Prepares the body for 'fight or flight'.

• Parasympathetic division: Promotes 'rest and digest' activities.

• Craniosacral division: Refers to the origin of parasympathetic fibers.

• Preganglionic neuron: First neuron in the ANS pathway.

• Postganglionic neuron: Second neuron, innervates the target organ.

• Cholinergic fibers: Release acetylcholine.

• Adrenergic fibers: Release norepinephrine.

Major ANS Neurotransmitters

• Acetylcholine: Used by all preganglionic neurons and parasympathetic postganglionic neurons.

• Norepinephrine: Used by most sympathetic postganglionic neurons.

Additional info:

• Ion gradients, channel proteins, and pumps are essential for generating and propagating action potentials.

• Disorders/injuries of the brain and spinal cord can affect sensory, motor, and autonomic functions.