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

11.1 Overview of the Nervous System

Nervous System Functions

The nervous system is essential for controlling perception, voluntary movement, and higher cognitive functions. It works closely with the endocrine system to maintain homeostasis of key physiological variables.

• Controls perception and experience of the world

• Directs voluntary movement

• Seat of consciousness, personality, learning, and memory

• Maintains homeostasis (respiratory rate, blood pressure, body temperature, sleep/wake cycle, blood pH) in conjunction with the endocrine system

11.1 Structural Divisions of the Nervous System

Central Nervous System (CNS)

• Brain: ~100 billion neurons; regulates all body functions

• Spinal Cord: ~100 million neurons; merges with the brain at the foramen magnum, passes through vertebral foramen, continues to the first or second lumbar vertebra

• Enables communication between brain and body below head/neck; some independent functions

Peripheral Nervous System (PNS)

• Nerves: Bundles of long neuron axons ("arms") with blood vessels, surrounded by connective tissue sheaths

• Cranial Nerves: Originate from/travel to the brain; 12 pairs

• Spinal Nerves: Originate from/travel to the spinal cord; 31 pairs

11.1 Functional Divisions of the Nervous System

Major Functions

• Sensory Functions: Gather information about internal/external environments (Sensory/Afferent Division of PNS)

• Integrative Functions: Analyze/interpret sensory stimuli, determine response (CNS, mostly brain)

• Motor Functions: Actions performed in response to integration (Motor/Efferent Division of PNS)

PNS Sensory Division

• Sensory Receptors: Detect stimuli; range from neuron tips to complex receptors

• Somatic Sensory Division: Signals from skeletal muscles, bones, joints, skin; includes special senses (vision, hearing, taste, smell, balance)

• Visceral Sensory Division: Signals from viscera (heart, lungs, stomach, intestines, kidneys, urinary bladder)

CNS Integrative Function

• Integrates sensory input to form a complete picture

• Disregards ~99% of unimportant data

• Responds with motor output if necessary

PNS Motor Division

• Effectors: Organs that carry out motor effects (muscle contraction, gland secretion)

• Somatic Motor Division: Signals to skeletal muscles (voluntary)

• Visceral Motor Division (Autonomic Nervous System): Signals to thoracic/abdominal viscera; regulates glands, smooth/cardiac muscle (involuntary)

11.2 Nervous Tissue

Composition and Cell Types

Nervous tissue consists of about 80% cells and 20% extracellular matrix.

• Neurons: Excitable cells that transmit signals

• Neuroglial Cells: Smaller, more prevalent; supportive functions, do not transmit signals

11.2 Neurons

General Properties

• Transmit signals via action potentials

• Long-lived, generally amitotic (do not divide after development)

• Size varies: CNS neurons can be 1 mm, PNS neurons over 1 meter

• Structure: Cell body (soma), dendrites (input), axon (output)

Cell Body (Soma)

• Diameter: 5–100 μm; metabolically active

• Contains free ribosomes, rough ER (Nissl bodies), Golgi apparatus, nucleoli, mitochondria

• Neuronal cytoskeleton: Neurofibrils (intermediate filaments), microtubules for support/transport

Dendrites and Axons

• Dendrites: Short, highly branched; receive input, transmit to cell body; can grow/prune with development

• Axons (Nerve Fiber): Generate/conduct action potentials

• Axon Hillock: Origin of axon from cell body

• Axon Collaterals: Branches of axon

• Telodendria: Fine branches at axon end

• Axon Terminals (Synaptic Knobs): Communicate with target cells; typically 1,000+

• Axolemma: Axon plasma membrane

• Axoplasm: Axon cytoplasm; lacks protein-making organelles

Axonal Transport

• Slow Axonal Transport: Cytoskeletal proteins move away from cell body (1–3 mm/day)

• Fast Axonal Transport: Vesicles/organelles move via motor proteins, uses ATP

• Retrograde: Toward cell body (up to 200 mm/day)

• Anterograde: Away from cell body (up to 400 mm/day)

Functional Regions of Neurons

• Receptive Region: Dendrites/cell body

• Conducting Region: Axon

• Secretory Region: Axon terminals

Structural Classification of Neurons

• Multipolar Neurons: Single axon, multiple dendrites; >99% of neurons

• Bipolar Neurons: One axon, one dendrite; sensory (retina, olfactory epithelium)

• Pseudounipolar Neurons: Single axon with peripheral/central processes; sensory (touch, pressure, pain)

Functional Classification of Neurons

• Sensory (Afferent) Neurons: Toward CNS; pseudounipolar/bipolar

• Interneurons (Association Neurons): Within CNS; multipolar; relay messages

• Motor (Efferent) Neurons: Away from CNS to muscles/glands; multipolar

Table: Neuron Classification

Structural Class	Multipolar Neurons	Bipolar Neurons	Pseudounipolar Neurons
Structural Features	One axon, two or more dendrites; highly branched	One axon, one dendrite	Single short process splits into two axons (no dendrites)
Typical Functional Class	Motor (efferent) neurons, interneurons	Sensory (afferent) neurons	Sensory (afferent) neurons
Location	Most neurons in CNS; motor neurons in PNS	Special sense organs (retina, olfactory epithelium)	Sensory neurons in PNS (touch, pain, pressure, vibration)

Groups of Neuron Cell Bodies and Axons

• Nuclei: Clusters of cell bodies in CNS

• Ganglia: Clusters of cell bodies in PNS

• Tracts: Bundles of axons in CNS

• Nerves: Bundles of axons in PNS

11.2 Neuroglia

Neuroglial Cells

Neuroglia are supportive cells that maintain the environment, protect neurons, and assist in their functions. They can divide and fill gaps when neurons die. There are six types: four in CNS, two in PNS.

Neuroglia in the CNS

• Astrocytes: Star-shaped; anchor neurons/blood vessels, facilitate nutrient/gas transport, regulate extracellular environment, remove excess potassium/neurotransmitters

• Blood Brain Barrier Formation: End-feet ensheath capillaries, induce tight junctions, separate blood from extracellular fluid

• Repairing Damaged Tissue: Divide rapidly, may impede neuron regrowth

• Oligodendrocytes: Form myelin sheath by wrapping axons; increase impulse speed

• Microglia: Small, branching; act as phagocytes, ingest pathogens/dead cells, stimulate inflammation

• Ependymal Cells: Ciliated; circulate cerebrospinal fluid, form/monitor its composition

Table: Neuroglial Cells of the CNS

Neuroglial Cell Type	Function
Astrocyte	Anchor neurons/blood vessels, regulate environment, form blood brain barrier, repair tissue
Oligodendrocyte	Form myelin sheath in CNS
Microglial Cell	Act as phagocytes, remove debris/pathogens
Ependymal Cell	Circulate and produce cerebrospinal fluid