Chapter 11: Introduction to the Nervous System and Nervous Tissue – Study Notes

Overview of the Nervous System

General Functions

The nervous system is the primary control center for perception, voluntary movement, consciousness, learning, memory, and homeostasis. It works closely with the endocrine system to regulate vital functions such as respiratory rate, blood pressure, body temperature, sleep/wake cycles, and blood pH.

• Controls voluntary and involuntary actions

• Seat of consciousness and cognition

• Regulates homeostasis

Anatomical Divisions

• Central Nervous System (CNS): Consists of the brain and spinal cord. The brain contains billions of neurons and is protected by the skull, while the spinal cord extends from the foramen magnum to the lumbar vertebrae, enabling communication between the brain and the body below the head and neck.

• Peripheral Nervous System (PNS): Includes all nerves outside the CNS. Nerves are bundles of axons, blood vessels, and connective tissue. The PNS is divided into cranial nerves (12 pairs) and spinal nerves (31 pairs).

Functional Divisions

• Sensory (Afferent) Division: Gathers information from internal and external environments and transmits it to the CNS.

• Integrative Functions: Analyze and interpret sensory information to determine appropriate responses. Most sensory input is subconsciously disregarded.

• Motor (Efferent) Division: Carries out responses via motor neurons to effectors (muscles and glands). Subdivided into:

  • Somatic Motor Division: Voluntary control of skeletal muscles.

  • Autonomic Nervous System (ANS): Involuntary control of smooth muscle, cardiac muscle, and glands.

Nervous Tissue

Neurons: Structure and Function

Neurons are excitable cells responsible for transmitting electrical signals (action potentials). They have three main parts:

• Cell Body (Soma): Contains the nucleus, organelles, and is the site of most metabolic activity. Includes Nissl bodies (rough ER), Golgi apparatus, nucleoli, and mitochondria.

• Dendrites: Short, branched processes that receive input from other neurons and transmit it toward the cell body.

• Axon: A single, long process that conducts action potentials away from the cell body. Key regions include the axon hillock, axon collaterals, telodendria, and axon terminals (synaptic bulbs).

Axonal Transport

• Slow Axonal Transport: Moves cytoskeletal proteins at 1–3 mm/day.

• Fast Axonal Transport: Uses motor proteins and ATP to move vesicles and organelles at up to 400 mm/day (anterograde and retrograde).

Functional Regions of Neurons

• Receptive Region: Dendrites and cell body

• Conducting Region: Axon

• Secretory Region: Axon terminals

Neuron Classification

• Structural:

  • Multipolar: One axon, multiple dendrites (most common)

  • Bipolar: One axon, one dendrite (eye, olfactory epithelium)

  • Pseudounipolar: Single fused axon that splits (sensory neurons)

• Functional:

  • Sensory (Afferent): Carry information to CNS

  • Interneurons: Relay information within CNS

  • Motor (Efferent): Carry information from CNS to effectors

Structural Class	Multipolar Neurons	Bipolar Neurons	Pseudounipolar Neurons
Features	One axon, many dendrites	One axon, one dendrite	Single short process splits into two axons
Location	CNS, motor neurons in PNS	Special sensory organs	Sensory neurons in PNS

Neuron Groupings

• CNS: Nuclei (cell bodies), tracts (axons)

• PNS: Ganglia (cell bodies), nerves (axons)

Neuroglia (Neuroglial Cells)

Types and Functions

Neuroglia provide support, protection, and maintenance for neurons. They can divide and fill spaces left by dead neurons.

• CNS Neuroglia:

  • Astrocytes: Anchor neurons, regulate environment, form blood-brain barrier, repair tissue

  • Oligodendrocytes: Myelinate axons in CNS

  • Microglia: Phagocytic cells, remove debris

  • Ependymal Cells: Line CNS cavities, produce and circulate cerebrospinal fluid

• PNS Neuroglia:

  • Schwann Cells: Myelinate axons in PNS

  • Satellite Cells: Support cell bodies in PNS

The Myelin Sheath

Structure and Function

The myelin sheath is formed by layers of plasma membrane from Schwann cells (PNS) or oligodendrocytes (CNS). It insulates axons, increasing the speed of action potential conduction.

• Myelinated axons: Conduct action potentials 15–20 times faster than unmyelinated axons.

• Internodes: Segments covered by myelin

• Node of Ranvier: Gaps between myelinated segments

• White matter: Myelinated axons

• Gray matter: Cell bodies, unmyelinated dendrites and axons

Regeneration of Nervous Tissue

Regeneration in PNS

Regeneration is limited in the PNS and nearly nonexistent in the CNS. It can occur only if the cell body remains intact.

• Wallerian degeneration: Axon and myelin sheath degenerate distal to injury

• Growth processes form from the proximal end

• Schwann cells and basal lamina form a regeneration tube

• New axon grows toward target cell and reconnects

Electrophysiology of Neurons

Resting Membrane Potential

Neurons maintain a resting membrane potential (RMP) of approximately –70 mV, with a thin layer of negative ions inside and positive ions outside the membrane.

• Voltage: Electrical gradient across the membrane

• Membrane potential: Source of potential energy for the cell

Ion Channels and Gradients

• Leak Channels: Always open, allow ions to flow down gradients

• Gated Channels: Open in response to specific stimuli

  • Ligand-gated: Open when a chemical binds

  • Voltage-gated: Open with voltage changes

  • Mechanically-gated: Open with mechanical stimulation

Changes in Membrane Potential

• Depolarization: Membrane potential becomes less negative (e.g., –70 mV to –60 mV)

• Repolarization: Return to resting potential

• Hyperpolarization: Membrane potential becomes more negative (e.g., –70 mV to –80 mV)

Local Potentials

• Small, graded changes in membrane potential

• Reversible and decremental (decrease over distance)

• Trigger action potentials if threshold is reached

Action Potentials

Phases and Mechanisms

Action potentials are rapid, uniform changes in membrane potential that travel along axons. They involve voltage-gated sodium and potassium channels.

• Depolarization: Sodium channels open, Na+ enters

• Repolarization: Sodium channels inactivate, potassium channels open, K+ exits

• Hyperpolarization: Potassium channels remain open briefly, membrane potential becomes more negative than RMP

Refractory Periods

• Absolute Refractory Period: No new action potential can be generated

• Relative Refractory Period: Only a strong stimulus can trigger another action potential

Propagation of Action Potentials

• Action potentials are self-propagating and travel in one direction (from trigger zone to axon terminals)

• Saltatory Conduction: In myelinated axons, action potentials jump between nodes of Ranvier, increasing speed

• Continuous Conduction: In unmyelinated axons, action potentials propagate along every segment, slowing conduction

Neuronal Synapses

Types of Synapses

• Electrical Synapses: Direct flow of ions via gap junctions; bidirectional and nearly instantaneous; found in brain regions for automatic behaviors and in cardiac/smooth muscle

• Chemical Synapses: Use neurotransmitters to transmit signals across a synaptic cleft; unidirectional and allow for variable signal strength

Postsynaptic Potentials

• Excitatory Postsynaptic Potential (EPSP): Small depolarization, brings membrane closer to threshold (Na+ or Ca2+ influx)

• Inhibitory Postsynaptic Potential (IPSP): Small hyperpolarization, moves membrane away from threshold (K+ efflux or Cl– influx)

Neural Integration and Summation

• Neural Integration: Postsynaptic neuron integrates all incoming EPSPs and IPSPs

• Summation: Temporal (rapid, repeated signals from one neuron) and spatial (simultaneous signals from multiple neurons)

• Action potential is generated only if threshold is reached at the trigger zone

Termination of Synaptic Transmission

• Neurotransmitters are removed by diffusion, enzymatic degradation, or reuptake into the presynaptic neuron

Neurotransmitters

General Features

• Made in the cell body or axon terminal, packaged into vesicles

• Released into the synaptic cleft, bind to receptors on the postsynaptic membrane

• Effects are rapidly terminated by removal or degradation

Example: Acetylcholine (ACh)

• Widely used in the nervous system (neuromuscular junction, brain, spinal cord, autonomic nervous system)

• Mostly excitatory, but can be inhibitory in the PNS

• Degraded by acetylcholinesterase (AChE)

Functional Groups of Neurons

Neuronal Pools and Circuits

• Neuronal Pools: Groups of interneurons that process specific types of information

• Diverging Circuits: One neuron communicates with multiple targets (e.g., sensory information distribution)

• Converging Circuits: Multiple neurons converge on a single postsynaptic neuron (e.g., motor control)

• Inhibitory circuits and synaptic fatigue help stabilize neural activity

Additional info: This guide covers the foundational concepts of nervous tissue, neuron structure and function, neuroglia, myelination, electrophysiology, synaptic transmission, neurotransmitters, and neural circuits, as outlined in a typical Anatomy & Physiology curriculum.