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: Contains about 100 billion neurons (nerve cells) that regulate all body functions

• Spinal Cord: Contains about 100 million neurons; merges with the brain at the foramen magnum, passes through the vertebral foramen of the cervical vertebrae, and continues to the first or second lumbar vertebra

• Enables communication between the brain and body below the head/neck; can carry out some functions independently

Peripheral Nervous System (PNS)

• Nerves: Bundles of long neuron axons ("arms") with blood vessels and connective tissue sheaths; carry signals to and from the CNS

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

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

11.1 Functional Divisions of the Nervous System

Major Functions

• Sensory Functions: Gather information about internal and external environments; performed by the Sensory (Afferent) Division of the PNS

• Integrative Functions: Analyze and interpret sensory stimuli to determine an appropriate response; performed by the CNS (mainly the brain)

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

PNS Sensory Division

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

• Somatic Sensory Division: Neurons carry signals from skeletal muscles, bones, joints, skin, and special sensory organs (vision, hearing, taste, smell, balance)

• Visceral Sensory Division: Neurons carry signals from internal organs (heart, lungs, stomach, intestines, kidneys, urinary bladder)

CNS Integrative Function

• Integrates multiple types of sensory input to form a complete picture

• Disregards about 99% of unimportant data

• If a response is needed, leads to a motor response

PNS Motor Division

• Neurons carry out motor functions (muscle contractions, gland secretion); target organs are called effectors

• Somatic Motor Division: Transmits signals to skeletal muscles (voluntary)

• Visceral Motor Division (Autonomic Nervous System): Transmits signals to thoracic/abdominal viscera; regulates gland secretion, smooth muscle, and cardiac muscle (involuntary)

11.2 Nervous Tissue

Composition and Cell Types

Nervous tissue consists of about 80% cells and 20% extracellular matrix. The two main cell types are:

• Neurons: Excitable cells that transmit signals

• Neuroglial Cells: Smaller, more prevalent cells that support neurons but do not transmit signals

11.2 Neurons

General Properties

• Transmit signals via action potentials; responsible for sensory, integrative, and motor functions

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

• Vary in length: CNS neurons may be 1 mm, PNS neurons may exceed 1 meter

• Structure: Cell body (soma) with organelles, one or more dendrites (carry signals to cell body), and one axon (carries signals away)

Cell Body (Soma)

• Diameter: 5–100 μm; most metabolically active part

• Contains free ribosomes and rough endoplasmic reticulum (Nissl bodies), Golgi apparatus, nucleoli, and many mitochondria

• Neuronal cytoskeleton: Intermediate filaments (neurofibrils) and microtubules support the cell and transport chemicals

Dendrites and Axons

• Dendrites: Short, highly branched; receive input and transmit electrical impulses toward the cell body; can grow and be "pruned" during development

• Axons (Nerve Fiber): Generate and conduct action potentials; features include:

  • Axon Hillock: Origin of the axon from the cell body

  • Axon Collaterals: Branches of the axon

  • Telodendria: Fine branches at the end of axon/collaterals

  • Axon Terminals (Synaptic Knobs): Communicate with target cells; typically 1,000 or more per neuron

  • Axolemma: Axon plasma membrane

  • Axoplasm: Axon cytoplasm; contains mitochondria, filaments, vesicles, lysosomes (no protein-making organelles)

• Axonal Transport: Movement of substances through axoplasm

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

  • Fast Axonal Transport: Vesicles/organelles move via motor proteins using 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 and cell body

• Conducting Region: Axon

• Secretory Region: Axon terminals (release chemicals)

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 and central processes; sensory (touch, pressure, pain)

Functional Classification of Neurons

• Sensory (Afferent) Neurons: Carry signals toward CNS; pseudounipolar/bipolar

• Interneurons (Association Neurons): Relay messages within CNS; multipolar; most numerous

• Motor (Efferent) Neurons: Carry signals 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)

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

General Functions

• Hold neurons together

• Maintain environment around neurons

• Protect and assist neurons

• Can undergo mitosis and fill gaps when neurons die

• 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 K+ and neurotransmitters), assist in blood-brain barrier formation, repair damaged tissue (may impede regrowth)

• Oligodendrocytes: Radiating processes wrap axons, form myelin sheath (increase impulse speed)

• Microglia: Small, branching; activated by injury, act as phagocytes (ingest pathogens, dead neurons, debris), stimulate inflammation

• Ependymal Cells: Ciliated; circulate cerebrospinal fluid in brain/spinal cord cavities, some form fluid, others monitor composition

Table: Neuroglial Cells of the CNS

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, stimulate inflammation
Ependymal Cell	Circulate and monitor cerebrospinal fluid