Study Guide: Nervous System Structure and Function (Chapter 12.1–12.8)

Introduction to the Nervous System

The nervous system is a complex network responsible for coordinating the body's activities. It is divided into several structural and functional components, each with specialized roles in maintaining homeostasis and enabling communication throughout the body.

Key Structures and Divisions

• Central Nervous System (CNS): Consists of the brain and spinal cord. It processes information and is the control center for the body.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS, such as cranial and spinal nerves. It connects the CNS to limbs and organs.

• Cells of the Nervous System: The two main types are neurons (nerve cells) and neuroglia (supporting cells).

12-1 – Divisions of the Nervous System

Central Nervous System (CNS)

• Components: Brain and spinal cord.

• Functions: Integrates sensory information, coordinates voluntary and involuntary responses.

• Other Tissues: Besides nervous tissue, the CNS contains blood vessels and connective tissue.

Peripheral Nervous System (PNS)

• Components: Cranial nerves (arise from the brain) and spinal nerves (arise from the spinal cord).

• Functions: Relays information between the CNS and the rest of the body.

Functional Divisions of the PNS

• Sensory (Afferent) Division: Carries sensory information from receptors to the CNS. Receptors detect changes in the environment.

• Motor (Efferent) Division: Carries motor commands from the CNS to effectors (muscles and glands). Effectors are target organs that respond to neural commands.

Subdivisions of the Motor Division

• Somatic Nervous System (SNS): Controls voluntary movements of skeletal muscles.

• Autonomic Nervous System (ANS): Regulates involuntary functions (e.g., heart rate, digestion).

  • Sympathetic Division: "Fight or flight" responses.

  • Parasympathetic Division: "Rest and digest" activities.

  • Enteric Nervous System: Controls gastrointestinal functions ("brain of the gut").

12-2 – Neurons

Neurons are the functional units of the nervous system, specialized for communication via electrical and chemical signals.

Structure of a Neuron

• Cell Body (Soma): Contains the nucleus and organelles.

  • Perikaryon: Cytoplasm surrounding the nucleus.

  • Nissl Bodies: Clusters of rough endoplasmic reticulum and ribosomes for protein synthesis.

  • Cytoskeleton Elements: Neurofilaments and neurotubules provide structural support.

  • Axon Hillock: Cone-shaped region where the axon originates.

• Cell Processes:

  • Dendrites: Receive incoming signals.

  • Axon: Conducts electrical impulses away from the cell body. Key regions include axoplasm, axolemma, initial segment, collaterals, telodendria, and axon terminals.

Classification of Neurons

• Structural:

  1. Anaxonic neurons

  2. Unipolar neurons

  3. Bipolar neurons

  4. Multipolar neurons

• Functional:

  1. Sensory (Afferent) neurons: Carry information to the CNS.

  2. Interneurons: Connect neurons within the CNS.

  3. Motor (Efferent) neurons: Carry commands from the CNS to effectors.

12-3 – Neuroglia

Neuroglia (glial cells) support, protect, and nourish neurons. They are essential for the maintenance of the nervous system.

Types of Neuroglia in the CNS

• Astrocytes: Maintain the blood-brain barrier, provide structural support, and regulate ion concentrations.

• Ependymal Cells: Produce, monitor, and circulate cerebrospinal fluid (CSF).

• Microglia: Act as phagocytes, removing debris and pathogens.

• Oligodendrocytes: Myelinate CNS axons, increasing the speed of electrical transmission.

Types of Neuroglia in the PNS

• Satellite Cells: Surround neuron cell bodies in ganglia, regulating the environment.

• Schwann Cells: Myelinate PNS axons and assist in their repair.

Myelination and Related Structures

• Myelin Sheath: Insulating layer that increases the speed of nerve impulse conduction.

• Neurilemma: Outer surface of a Schwann cell.

• Nodes of Ranvier: Gaps in the myelin sheath where action potentials are regenerated.

• Wallerian Degeneration: Process of axon degeneration and repair after injury.

12-4 – Membrane Potential

Membrane potential is the difference in electrical charge across the cell membrane. It is essential for the generation and propagation of electrical signals in neurons.

• Potential Difference: The inside of the cell is negatively charged relative to the outside.

• Measured in: Millivolts (mV).

Factors Contributing to Membrane Potential

• Ion concentration differences between extracellular fluid (ECF) and intracellular fluid (cytosol).

• Selective permeability of the membrane to different ions (mainly Na+ and K+).

• Presence of negatively charged proteins inside the cell.

Passive and Active Forces

• Passive Forces:

  • Chemical gradients (diffusion of ions)

  • Electrical gradients (attraction of opposite charges)

• Active Forces: Sodium-potassium exchange pump (Na+/K+ ATPase)

Resting Membrane Potential

• Typically about $-70\ \text{mV}$ in neurons.

• Maintained by leak channels and the sodium-potassium pump.

Ion Channels

• Leak Channels: Always open, contribute to resting membrane potential.

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

  • Chemically gated

  • Mechanically gated

  • Voltage-gated

Graded Potentials

• Localized changes in membrane potential.

• Can lead to depolarization (membrane potential becomes less negative) or hyperpolarization (more negative).

12-5 – Action Potential

An action potential is a rapid, large change in membrane potential that travels along the axon, enabling communication between neurons and effectors.

Steps in an Action Potential

1. Graded depolarization brings the axon hillock to threshold.

2. Voltage-gated sodium channels open, Na+ enters the cell (depolarization).

3. At $+30\ \text{mV}$, sodium channels close and voltage-gated potassium channels open, K+ leaves the cell (repolarization).

4. Potassium channels close slowly, causing a brief hyperpolarization.

Refractory Periods

• Absolute Refractory Period: No new action potential can be initiated.

• Relative Refractory Period: A stronger stimulus is required to initiate another action potential.

Propagation of Action Potentials

• Continuous Propagation: Occurs in unmyelinated axons; action potential moves along every segment.

• Saltatory Propagation: Occurs in myelinated axons; action potential "jumps" from node to node, increasing speed.

Axon Diameter and Conduction Velocity

• Larger diameter axons conduct impulses faster due to lower resistance.

Types of Axons (Fibers)

• Type A: Largest, fastest, carry important information (e.g., motor to skeletal muscle).

• Type B: Medium diameter, moderate speed.

• Type C: Smallest, slowest.

12-6 – Synapses

A synapse is the junction between two neurons or between a neuron and another cell (e.g., muscle). It is essential for neural communication.

Elements of a Synapse

• Presynaptic neuron and presynaptic membrane

• Synaptic vesicles containing neurotransmitters

• Synaptic cleft

• Postsynaptic cell and postsynaptic membrane

Chemical vs. Electrical Synapses

• Chemical Synapse: Uses neurotransmitters to transmit signals.

• Electrical Synapse: Direct electrical connection via gap junctions.

Neuromuscular and Neuroglandular Junctions

• Neuromuscular Junction: Synapse between a neuron and a muscle fiber.

• Neuroglandular Junction: Synapse between a neuron and a gland cell.

Synaptic Fatigue

• Occurs when neurotransmitter stores are depleted after prolonged activity.

Steps at a Chemical Synapse

1. Action potential arrives at the axon terminal.

2. Calcium channels open, allowing Ca2+ to enter the terminal.

3. Calcium triggers exocytosis of synaptic vesicles, releasing neurotransmitter.

4. Neurotransmitter binds to receptors on the postsynaptic membrane.

5. Neurotransmitter is broken down or recycled.

12-7 – Neurotransmitters

Neurotransmitters are chemicals that transmit signals across a synapse. They can be excitatory or inhibitory, depending on the receptor and target cell.

Major Classes of Neurotransmitters

• Biogenic Amines:

  • Norepinephrine (NE): Used in adrenergic synapses.

  • Dopamine: Involved in reward and motor pathways.

  • Serotonin: Affects mood and sleep; SSRIs increase serotonin levels.

• Amino Acids:

  • Gamma Aminobutyric Acid (GABA): Main inhibitory neurotransmitter.

• Neuropeptides: Modulate the activity of other neurotransmitters (e.g., opioids).

• Dissolved Gases: Nitric oxide (NO) and carbon monoxide (CO) act as neurotransmitters.

• Acetylcholine (ACh): Used in cholinergic synapses; important for muscle contraction.

12-8 – Information Processing

Neural information processing involves the integration of excitatory and inhibitory signals at the postsynaptic membrane, determining whether an action potential will be generated.

Postsynaptic Potentials

• Excitatory Postsynaptic Potential (EPSP): Graded depolarization that increases the likelihood of an action potential.

• Inhibitory Postsynaptic Potential (IPSP): Graded hyperpolarization that decreases the likelihood of an action potential.

Summation

• Temporal Summation: Rapid, repeated stimuli at one synapse.

• Spatial Summation: Simultaneous stimuli at multiple synapses.

Facilitation and Inhibition

• Facilitation: Increases the likelihood of an action potential.

• Inhibition: Decreases the likelihood of an action potential.

Net Effect

• The sum of all EPSPs and IPSPs at the axon hillock determines whether the neuron will fire an action potential.

Additional info: Neuromodulators and hormones can alter the sensitivity of neurons to neurotransmitters, shifting the balance between excitation and inhibition.