Chapter 12: Nervous System and Nervous Tissue – Structured Study Notes

Nervous System and Nervous Tissue

Overview and Functions of the Nervous System

The nervous system is responsible for monitoring internal and external changes, processing information, and responding to stimuli. It is essential for sensory perception, integration, and motor output.

• Sensory Input: Detection of changes (stimuli) via sensory receptors.

• Integration: Processing and interpretation of sensory input in the central nervous system (CNS).

• Motor Output: Response to stimuli by activating effectors such as muscles and glands.

• Afferent: Signals traveling toward the CNS.

• Efferent: Signals traveling away from the CNS.

Structural Divisions of the Nervous System

The nervous system is divided into two main anatomical divisions:

• Central Nervous System (CNS): Consists of the brain and spinal cord, located in the cranium and vertebral canal.

• Peripheral Nervous System (PNS): Includes cranial nerves, spinal nerves, and ganglia. Cranial nerves carry signals to and from the brain; spinal nerves carry signals to and from the spinal cord; ganglia are clusters of neuronal cell bodies.

Functional Divisions in the PNS

• Sensory (Afferent) Division: Transmits sensory information to the CNS.

• Motor (Efferent) Division: Transmits motor commands from the CNS to effectors.

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

• Autonomic Nervous System: Controls involuntary functions (cardiac/smooth muscle, glands). Includes sympathetic (fight or flight) and parasympathetic (rest and digest) divisions.

Cells of Nervous Tissue

Nervous tissue consists of two main cell types:

• Neurons: Excitable cells that transmit electrical signals.

• Neuroglia (Glial Cells): Non-excitable supportive cells that maintain the environment for neurons.

Neurons: Structure and Function

Neurons are the fundamental units of function in the CNS, with unique properties and specialized structures.

• Cell Body (Soma): Contains the nucleus, nucleolus, and organelles; biosynthetic center of the neuron.

• Dendrites: Branching processes that receive signals and conduct them toward the cell body.

• Axon: Single, long process that conducts impulses away from the cell body; may be myelinated.

• Synaptic Terminals: Endings of the axon that contain synaptic vesicles filled with neurotransmitters.

Neuron Cell Body (Soma)

• Diameter: 5–140 μm

• Single nucleus with large nucleolus

• Contains Nissl bodies (clusters of rough ER), mitochondria, lysosomes, Golgi apparatus

• Renews cell membranes and cytosolic proteins

Dendrites

• Numerous branching processes

• Receptive sites for incoming signals

• Increase surface area for signal reception

• Conduct electrical signals toward the cell body

Axon

• One per neuron, uniform diameter

• Ends in multiple terminals

• May have collateral branches

• Axon terminals contain synaptic vesicles

• Impulse generation at initial segment; conduction away from cell body

• Often myelinated

Synapses: Structure and Function

Synapses are specialized junctions where neurons communicate with other neurons or effectors. Most synapses are chemical, involving neurotransmitter release.

• Presynaptic Neuron: Releases neurotransmitter into the synaptic cleft.

• Synaptic Cleft: Space between presynaptic and postsynaptic membranes.

• Postsynaptic Neuron: Contains receptors for neurotransmitters.

• Neurotransmitters can be excitatory or inhibitory.

Classification of Neurons

Structural Classification

Neurons are classified based on the number and arrangement of their processes:

• Multipolar: Multiple dendrites, one axon; most abundant (>99% of all neurons).

• Bipolar: One dendrite, one axon; rare, found in special sensory organs.

• Unipolar: Single process; most sensory neurons.

Functional Classification

• Sensory Neurons: Transmit sensory information toward the CNS.

• Motor Neurons: Transmit motor commands from the CNS to effectors.

• Interneurons: Confined to the CNS; integrate and process information.

Neuroglial Cells (Glia)

Glial cells support and maintain the neural environment. They are mitotically active and can give rise to brain tumors.

• Astrocytes: Most numerous; maintain ionic homeostasis, support blood-brain barrier, promote neuronal growth.

• Oligodendrocytes: Myelinate axons in the CNS; one cell can myelinate several axons.

• Schwann Cells: Myelinate axons in the PNS; one cell myelinates one axon.

• Microglia: Macrophages of the CNS; remove debris and act as immune cells.

• Ependymal Cells: Form epithelial lining of brain ventricles; produce and circulate cerebrospinal fluid (CSF).

Myelin Sheath

Myelin sheaths are concentric layers of plasma membrane produced by glial cells, wrapping around axons.

• Protects axons and increases speed of impulse transmission.

• Electrically insulates nerve fibers from one another.

• Schwann Cells: Form myelin sheaths in the PNS (one cell per axon).

• Oligodendrocytes: Form myelin sheaths in the CNS (one cell for several axons).

• Nodes of Ranvier: Gaps in the myelin sheath where impulse conduction is facilitated.

Gray Matter vs. White Matter in the CNS

• Gray Matter: Contains neuronal cell bodies, dendrites, and axon terminals; site of synapses. In the spinal cord, it forms an H-shaped internal region; in the brain, it is found in the cortex and deep areas.

• White Matter: Composed of myelinated axons; organized in tracts that connect different regions.

Nerves: Structure and Components

Nerves are organs of the PNS, consisting of bundles of nerve fibers (axons), Schwann cells, blood vessels, and connective tissue layers.

• Endoneurium: Surrounds a single axon.

• Perineurium: Surrounds a bundle of axons (fascicle).

• Epineurium: Surrounds the whole nerve.

• Most nerves contain both sensory and motor fibers, which can be myelinated or unmyelinated.

Summary Table: Types of Sensory and Motor Fibers in the PNS

Type	Sensory Components	Motor Components
Somatic	Touch, pain, pressure, vibration, temperature, proprioception (skin, body wall, limbs)	Motor innervation to skeletal muscles
Visceral	Stretch, pain, temperature, chemical changes, irritation (internal organs)	Motor innervation to smooth muscle, cardiac muscle, glands
Special	Hearing, equilibrium, vision, taste, smell	---

Key Equations and Concepts

• Resting Membrane Potential: $V_{m} = V_{inside} - V_{outside}$

• Action Potential Propagation: $\text{Depolarization} \rightarrow \text{Repolarization} \rightarrow \text{Hyperpolarization}$

Additional info:

• Interneurons constitute >99% of all neurons in the CNS and are essential for complex processing and integration.

• Loss of myelination (e.g., in multiple sclerosis) severely impairs nervous system function.