Neural Tissue: Structure and Function in the Nervous System

Neural Tissue

An Introduction to the Nervous System

The nervous system is a complex network responsible for coordinating the body's activities by transmitting signals to and from different parts. Neural tissue is specialized for communication and consists of two main cell types: neurons and neuroglia.

• Neurons: Cells that transmit and receive signals.

• Neuroglia (glial cells): Cells that support, protect, and nourish neurons.

Major organs: Brain, spinal cord, sensory organs, and nerves.

Functions: Sensory reception, integration of information, and motor output.

Divisions of the Nervous System

Anatomical Divisions

• Central Nervous System (CNS): Consists of the brain and spinal cord. Integrates, processes, and coordinates sensory data and motor commands.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS. Delivers sensory information to the CNS and carries motor commands to peripheral tissues and systems.

Components of the PNS

• Nerves: Bundles of axons with connective tissue and blood vessels.

• Cranial nerves: Connect to the brain.

• Spinal nerves: Attach to the spinal cord.

Functional Divisions of the PNS

• Afferent division: Carries sensory information from receptors to the CNS.

• Efferent division: Carries motor commands from the CNS to muscles and glands (effectors).

Receptors and Effectors

• Receptors: Detect changes or respond to stimuli (e.g., sensory organs).

• Effectors: Target organs that respond to motor signals (e.g., muscles, glands).

Subdivisions of the Efferent Division

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

• Autonomic Nervous System (ANS): Controls subconscious actions, contractions of smooth and cardiac muscle, and glandular secretions.

  • Sympathetic division: Stimulating effect ("fight or flight").

  • Parasympathetic division: Relaxing effect ("rest and digest").

Neurons

Structure and Function

Neurons are the basic functional units of the nervous system, specialized for the transmission of electrical impulses.

• Cell body (soma): Contains nucleus, nucleolus, mitochondria, and organelles. Site of most metabolic activity.

• Dendrites: Highly branched processes that receive signals from other neurons. Account for 80–90% of neuron surface area.

• Axon: Long process that carries electrical signals (action potentials) to target cells. Structure is critical for function.

Axon Structures

• Axoplasm: Cytoplasm of the axon, containing neurofibrils, enzymes, and organelles.

• Axolemma: Specialized plasma membrane covering the axon.

• Axon hillock: Thickened region where the axon attaches to the cell body.

• Initial segment: First part of the axon, attached to the axon hillock.

• Collaterals: Branches of a single axon.

• Telodendria: Fine extensions of distal axon, ending in synaptic terminals.

The Synapse

The synapse is the site where a neuron communicates with another cell. It consists of:

• Presynaptic cell: Sends the message (usually a neuron).

• Postsynaptic cell: Receives the message (can be a neuron, muscle, or gland cell).

• Synaptic cleft: Small gap separating the presynaptic and postsynaptic membranes.

• Synaptic terminal: Contains synaptic vesicles filled with neurotransmitters.

Neurotransmitters

• Chemical messengers released at the presynaptic membrane.

• Affect receptors on the postsynaptic membrane.

• Broken down by enzymes and reassembled at the synaptic terminal.

Axoplasmic Transport

• Movement of materials between the cell body and axon terminals.

• Uses molecular motors (e.g., kinesin, dynein) and is powered by mitochondria.

Types of Synapses

• Neuromuscular junction: Synapse between neuron and muscle.

• Neuroglandular junction: Synapse between neuron and gland.

Structural Classification of Neurons

Type	Structure	Location/Function
Anaxonic	Small, all processes look alike	Brain and special sense organs
Bipolar	One dendrite, one axon	Special sensory organs (sight, smell, hearing)
Unipolar	Single process with cell body to one side	Most sensory neurons of PNS
Multipolar	Multiple dendrites, one axon	Most common in CNS; all motor neurons to skeletal muscles

Functional Classification of Neurons

• Sensory neurons (afferent): Carry information from sensory receptors to CNS.

• Motor neurons (efferent): Carry instructions from CNS to effectors (muscles/glands).

• Interneurons: Connect sensory and motor neurons; involved in higher functions (memory, learning).

Types of Sensory Receptors

• Interoceptors: Monitor internal systems (digestive, respiratory, cardiovascular, etc.) and internal senses (taste, pain).

• Exteroceptors: Monitor external environment (touch, temperature, pressure, sight, smell, hearing).

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

Motor Neurons

• Carry instructions from CNS to peripheral effectors via efferent fibers.

• Two main types:

  • Somatic motor neurons: Innervate skeletal muscles.

  • Visceral motor neurons: Innervate smooth muscle, cardiac muscle, glands, and adipose tissue.

• Two groups of efferent axons:

  • Preganglionic fibers: From CNS to autonomic ganglia.

  • Postganglionic fibers: From ganglia to effectors.

Neuroglia

Overview

Neuroglia, or glial cells, make up about half the volume of the nervous system and provide support, protection, and nourishment for neurons. There are several types in the CNS and PNS.

Types of Neuroglia in the CNS

• Ependymal cells: Form epithelium called ependyma, line central canal of spinal cord and ventricles of brain, secrete and monitor cerebrospinal fluid (CSF), and may have cilia or microvilli.

• Astrocytes: Maintain the blood-brain barrier, create a structural framework, repair damaged neural tissue, and regulate the neural environment.

• Oligodendrocytes: Myelinate CNS axons, provide structural framework.

• Microglia: Remove cell debris, waste, and pathogens by phagocytosis.

Types of Neuroglia in the PNS

• Satellite cells: Surround neuron cell bodies in ganglia, regulate environment around neurons.

• Schwann cells: Form myelin sheath around peripheral axons, aid in repair after injury.

Summary Table: Types of Neuroglia

Neuroglia Type	Location	Main Function
Ependymal cells	CNS	Produce, monitor, and circulate CSF
Astrocytes	CNS	Maintain blood-brain barrier, support, repair
Oligodendrocytes	CNS	Myelinate CNS axons
Microglia	CNS	Phagocytosis of debris and pathogens
Satellite cells	PNS	Regulate environment in ganglia
Schwann cells	PNS	Myelinate PNS axons, repair

Key Terms and Concepts

• Action potential: Electrical signal transmitted along the axon.

• Neurotransmitter: Chemical messenger released at synapses.

• Myelin sheath: Insulating layer around axons, increases speed of signal transmission.

• Ganglion: Cluster of neuron cell bodies in the PNS.

• Nucleus (in CNS): Cluster of neuron cell bodies in the CNS.

Additional info:

• Some diagrams and images referenced in the slides (e.g., neuron structure, synapse, neuroglia) are not included in this text but are important for visual understanding.

• For a deeper understanding, students should review the mechanisms of action potential generation and propagation, as well as the molecular details of synaptic transmission.