Nervous System Study Guide: Structure, Function, and Physiology

• Nervous System

Major Divisions of the Nervous System

The nervous system is divided into several major components, each with distinct functions and anatomical features.

• Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for processing and integrating information.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS; subdivided into the somatic and autonomic nervous systems.

• Somatic Nervous System: Controls voluntary movements via skeletal muscles.

• Autonomic Nervous System: Regulates involuntary functions such as heart rate, digestion, and respiratory rate.

Example: The CNS interprets sensory input and initiates motor output, while the PNS transmits signals between the CNS and the rest of the body.

Functional Anatomy and Organization

The nervous system includes various structures such as ganglia, nerves, tracts, and nuclei, each serving specific roles in signal transmission and processing.

• Ganglia: Clusters of neuron cell bodies in the PNS.

• Nerves: Bundles of axons in the PNS.

• Tracts: Bundles of axons in the CNS.

• Nuclei: Clusters of neuron cell bodies in the CNS.

Example: Sensory information travels via nerves to the CNS, where it is processed in nuclei.

Types of Neurons and Nerve Fibers

Neurons are classified based on their structure and function. Nerve fibers transmit electrical impulses throughout the nervous system.

• Multipolar Neurons: Have multiple dendrites and one axon; common in the CNS.

• Bipolar Neurons: Have one dendrite and one axon; found in sensory organs.

• Unipolar Neurons: Have a single process; primarily sensory neurons in the PNS.

• Myelinated Fibers: Conduct impulses rapidly due to the presence of myelin sheath.

• Unmyelinated Fibers: Conduct impulses more slowly.

Example: Motor neurons are typically multipolar, while sensory neurons may be unipolar or bipolar.

Membrane Potentials and Action Potentials

Neurons communicate via electrical signals known as membrane potentials and action potentials.

• Resting Membrane Potential: The voltage difference across the neuronal membrane at rest, typically around -70 mV.

• Action Potential: A rapid change in membrane potential that propagates along the axon.

• Depolarization: The membrane potential becomes less negative.

• Repolarization: The membrane potential returns to its resting state.

• Hyperpolarization: The membrane potential becomes more negative than the resting potential.

Equation:

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

Example: The influx of sodium ions during depolarization initiates the action potential.

Synaptic Transmission

Neurons communicate with each other at synapses, where neurotransmitters are released to transmit signals.

• Chemical Synapses: Use neurotransmitters to transmit signals across the synaptic cleft.

• Electrical Synapses: Allow direct passage of ions between neurons via gap junctions.

• Excitatory Postsynaptic Potential (EPSP): Depolarizes the postsynaptic membrane, increasing the likelihood of an action potential.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes the postsynaptic membrane, decreasing the likelihood of an action potential.

Example: Acetylcholine is a common excitatory neurotransmitter at neuromuscular junctions.

Neurotransmitters and Receptors

Neurotransmitters are chemicals that transmit signals across synapses. Receptors on the postsynaptic membrane determine the response.

• Common Neurotransmitters: Acetylcholine, dopamine, serotonin, norepinephrine, GABA.

• Receptor Types: Ionotropic (directly open ion channels) and metabotropic (activate second messenger systems).

Example: GABA is an inhibitory neurotransmitter that opens chloride channels, causing hyperpolarization.

Integration and Summation

Neurons integrate multiple synaptic inputs through summation, which determines whether an action potential will be generated.

• Spatial Summation: Multiple synapses at different locations combine their effects.

• Temporal Summation: Rapid, repeated stimulation at a single synapse increases the effect.

Example: Simultaneous EPSPs from several presynaptic neurons can trigger an action potential.

Autonomic Nervous System

Somatic vs. Autonomic Nervous System

The somatic and autonomic nervous systems differ in their control and function.

• Somatic Nervous System: Controls voluntary movements; one neuron pathway from CNS to effector.

• Autonomic Nervous System: Controls involuntary functions; two-neuron pathway (preganglionic and postganglionic).

Example: The somatic system activates skeletal muscles, while the autonomic system regulates heart rate.

Sympathetic vs. Parasympathetic Divisions

The autonomic nervous system is divided into sympathetic and parasympathetic divisions, each with distinct effects.

• Sympathetic Division: Prepares the body for 'fight or flight' responses; increases heart rate, dilates pupils.

• Parasympathetic Division: Promotes 'rest and digest' activities; decreases heart rate, stimulates digestion.

Example: Sympathetic activation increases blood flow to muscles, while parasympathetic activation promotes digestion.

Neurotransmitters and Receptors in the Autonomic Nervous System

Different neurotransmitters and receptors mediate the effects of the autonomic nervous system.

• Preganglionic Neurons: Release acetylcholine in both divisions.

• Postganglionic Neurons: Release norepinephrine (sympathetic) or acetylcholine (parasympathetic).

• Receptors: Cholinergic (bind acetylcholine) and adrenergic (bind norepinephrine/epinephrine).

Example: Beta-adrenergic receptors mediate increased heart rate in response to sympathetic stimulation.

Organization and Pathways

The autonomic nervous system uses a two-neuron pathway to reach target organs.

• Preganglionic Neuron: Originates in the CNS and synapses in an autonomic ganglion.

• Postganglionic Neuron: Extends from the ganglion to the effector organ.

Example: Parasympathetic preganglionic neurons originate in the brainstem or sacral spinal cord.

Table: Comparison of Somatic and Autonomic Nervous Systems

Feature	Somatic Nervous System	Autonomic Nervous System
Control	Voluntary	Involuntary
Effector Organs	Skeletal muscles	Cardiac muscle, smooth muscle, glands
Neural Pathway	One neuron from CNS to effector	Two neurons (preganglionic and postganglionic)
Neurotransmitters	Acetylcholine	Acetylcholine, norepinephrine

Additional info: These notes expand upon the study guide questions by providing definitions, examples, and a comparative table for clarity and completeness.