Chapter 13: The Nervous System – Neural Tissue (Anatomy & Physiology Study Notes)

Introduction to the Nervous System

Overview and Functional Role

The nervous system and endocrine system are the primary control and regulatory systems of the body. Both systems use chemical communication to influence the activities of other organ systems, but differ in speed and duration of their effects.

• Nervous system: Produces swift, brief responses to stimuli.

• Endocrine system: Produces slower responses that are often longer-lasting.

An Overview of the Nervous System

Anatomical Subdivisions

The nervous system consists of all neural tissue in the body and is divided into two main anatomical subdivisions:

• Central Nervous System (CNS):

  • Composed of the brain and spinal cord.

  • Responsible for integrating, processing, and coordinating sensory data and motor commands.

  • Functions include intelligence, memory, learning, and emotion.

• Peripheral Nervous System (PNS):

  • All neural tissue outside the CNS.

  • Provides sensory information to the CNS and carries motor commands to peripheral tissues.

Functional Divisions of the PNS

• Afferent Division: Brings sensory information from receptors to the CNS.

• Efferent Division: Carries motor commands from the CNS to muscles and glands.

• The efferent division is further subdivided into:

  • Somatic Nervous System (SNS): Controls voluntary and involuntary skeletal muscle contractions.

  • Autonomic Nervous System (ANS): Regulates involuntary activity of smooth muscle, cardiac muscle, and glands.

Types of Sensory Receptors

• Somatic sensory receptors: Detect stimuli in skeletal muscles, joints, and skin.

• Visceral sensory receptors: Monitor conditions in smooth muscle, cardiac muscle, and glands.

• Special sense organs: Include the eye, nose, tongue, and ear.

Cellular Organization in Neural Tissue

Neural Tissue Cell Types

Neural tissue is composed of two main cell types:

• Neurons: Specialized for the transfer and processing of information.

• Neuroglia (glial cells): Support, protect, and isolate neurons.

Neuron Structure

Neurons have a unique structure that enables their function:

• Dendrites: Receive stimuli from the environment or other cells.

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

• Axon: Conducts nerve impulses (action potentials) toward synaptic terminals.

• Terminal boutons: Affect another neuron or effector organ (muscle or gland).

Neuroglia Functions

• Provide structural framework for neural tissue.

• Maintain the intercellular environment.

• Act as phagocytes (removing debris and pathogens).

• Neuroglia outnumber neurons by approximately five to one (about 100 billion glial cells).

Classification of Neuroglia

Location	Type	Main Functions
Peripheral Nervous System	Satellite cells	Surround neuron cell bodies in ganglia; regulate O2, CO2, nutrients, and neurotransmitter levels
Peripheral Nervous System	Schwann cells	Surround axons; responsible for myelination and repair after injury
Central Nervous System	Oligodendrocytes	Myelinate CNS axons; provide structural framework
Central Nervous System	Astrocytes	Maintain blood-brain barrier; provide structural support; regulate ion, nutrient, and dissolved gas concentrations; absorb and recycle neurotransmitters; form scar tissue after injury
Central Nervous System	Microglia	Remove cell debris, wastes, and pathogens by phagocytosis
Central Nervous System	Ependymal cells	Line ventricles and central canal; assist in producing, circulating, and monitoring cerebrospinal fluid

Astrocytes

• Largest and most numerous glial cells in the CNS.

• Functions include:

  • Controlling the interstitial environment.

  • Maintaining the blood-brain barrier.

  • Creating a three-dimensional framework for the CNS.

  • Repairing damaged neural tissue.

  • Guiding neuron development.

Structural and Functional Classification of Neurons

Structural Types

Type	Structure	Location/Function
Anaxonic neuron	No distinct axon	Found in brain and special sense organs
Bipolar neuron	One dendrite, one axon	Special sensory organs (e.g., retina)
Pseudounipolar neuron	Single process splits into two branches	Most sensory neurons of PNS
Multipolar neuron	Multiple dendrites, one axon	Most common type; motor neurons and interneurons

Functional Types

• Sensory neurons: Most are pseudounipolar; transmit sensory information from receptors to CNS.

• Motor neurons: Most are multipolar; carry instructions from CNS to effectors (muscles/glands).

• Interneurons (association neurons): Most are multipolar; connect sensory and motor neurons within CNS.

Types of Receptors

• Exteroceptors: Monitor external environment (touch, temperature, pressure, special senses).

• Proprioceptors: Monitor position and movement of skeletal muscles and joints.

• Interoceptors: Monitor internal environment (digestive, respiratory, cardiovascular, urinary, reproductive systems; deep pressure and pain).

Neural Regeneration

Regeneration after Injury

Peripheral nerves have a limited ability to regenerate after injury, primarily due to the activity of Schwann cells. CNS neurons have a much lower capacity for regeneration.

• Schwann cells form a regeneration tube that guides the regrowth of axons.

• Regeneration is more successful in the PNS than in the CNS.

The Nerve Impulse

Action Potentials and Conduction

Excitability is the ability of a neuron's plasma membrane to conduct electrical impulses. When stimulated to threshold, an action potential is generated and travels along the axon as a nerve impulse.

• The rate of impulse conduction depends on:

  • Presence or absence of a myelin sheath

  • Axon diameter

Formula:

$\text{Conduction Velocity} \propto \text{Axon Diameter} \times \text{Degree of Myelination}$

Synaptic Communication

Types and Structure of Synapses

Synapses are specialized junctions where neurons communicate with other neurons, muscle cells, or gland cells.

• Synapses with other neurons: Dendrites or cell bodies of another neuron.

• Neuromuscular synapses: Skeletal muscle fibers.

• Neuroglandular synapses: Gland cells.

Synaptic transmission involves the release of neurotransmitters from the presynaptic neuron, which bind to receptors on the postsynaptic cell, leading to a response.

Neuron Organization and Processing

Neural Circuits

Neurons are organized into circuits that process information in specific ways:

• Divergence: One neuron sends signals to multiple neurons.

• Convergence: Multiple neurons send signals to a single neuron.

• Serial processing: Neurons are arranged in a linear sequence.

• Parallel processing: Neurons process the same information simultaneously in different pathways.

• Reverberation: Feedback loops maintain activity within the circuit.

Anatomical Organization of the Nervous System

Major Divisions and Glossary

Division	Description
Central Nervous System (CNS)	Brain and spinal cord; integrates and coordinates sensory and motor information
Peripheral Nervous System (PNS)	All neural tissue outside CNS; connects CNS to limbs and organs
Somatic Nervous System (SNS)	Controls voluntary movements of skeletal muscles
Autonomic Nervous System (ANS)	Regulates involuntary functions (smooth muscle, cardiac muscle, glands)

Additional info: The anatomical organization includes gray matter (neuron cell bodies), white matter (myelinated axons), tracts, nerves, ganglia, and nuclei, each with specific roles in neural processing and communication.