The Nervous System Explained

Nervous System

• Master control and communication system in the body
• Cells communicate via electrical signals

Functions of the Nervous System

• Monitors changes that occur inside and outside the body via sensory input
• Processes, interprets, and makes decisions about the changes via integration
• Responds by activating various organs via motor output

Branches of the Nervous System

• Central nervous system
• Peripheral nervous system

Peripheral Nervous System Divisions

• Sensory (afferent) division
  • Somatic sensory includes touch, pain, pressure, vibration, temperature and proprioception in skin, body wall, and limbs
  • Visceral sensory includes stretch, pain, temperature, chemical changes, and irritation in viscera, nausea, and hunger
• Motor (efferent) division
  • Somatic motor (voluntary) includes innervation of all skeletal muscles
  • Visceral motor (involuntary) – ANS includes motor innervation of smooth muscles, cardiac muscles and glands

The Nervous System

• Made of neurons and glial cells
• Neurons: nerve cells that conduct electrical signals
• Glial cells: neuroglial/support cells

Characteristics of Neurons

• Longevity
• Do not divide, losing their mitotic ability
• High metabolic rate

Neurons

• Neurons include dendrites, soma (cell body or perikaryon), axon, and axon terminals
• Soma diameter ranges from 5-140 micrometers, containing abundant protein synthesis organelles
• Nissl bodies (chromatophilic bodies) are present
• Many extension from cell bodies, receptive sites for electrical signal
• Generate and conduct signals away from soma.
• A neuron only has one axon, and initial branching is axon hillock
• Abundant microtubules transport substances.
• Axon collaterals terminate in many axon terminals (end bulbs, boutons, or telodendria)
• Contain neurotransmitter-filled vesicles

Classification of Neurons

• Classified based on their structure and function
• Structural types are multipolar, bipolar, and unipolar
• Functional types are sensory, interneurons, and motor neurons

Structural Classification of Neurons

• Multipolar neurons have many dendrites and one axon, 99% of all neurons like motor neurons and interneurons
• Bipolar neurons have two extensions only, found in ears, nose and eyes, function as sensory neurons
• Unipolar neurons have a short single process and divide into two long branches that function as sensory neurons

Functional Classification of Neurons

• Sensory neurons (afferent) transmit electrical signals towards the CNS from PNS
• Interneurons (association) are entirely in CNS, facilitate signals between sensory and motor neurons,make up 99% of neurons
• Motor neurons (efferent) transmit signals from CNS to organs, muscles, and glands, and located partly in CNS and PNS

How Neurons Communicate

• Neurons form synapses, specialized junctions
• Synapses allow for electrical signal transmission from one neuron to another
• Include presynaptic neuron, postsynaptic neuron
• Types include axosomatic and axodendritic

Synapse

• Presynaptic neuron contains axon terminals and neurotransmitter-filled vesicles
• Postsynaptic neuron contain dendrites

Glial Cells

• Supporting cells without electrical properties, smaller than neurons with a 10:1 ratio, non-nervous tissue
• Make up half the brain mass
• In CNS, include astrocytes, microglia, ependymal and oligodendrocytes
• In PNS, include satellite and Schwann cells

Astrocytes

• Most abundant glial cell
• Regulate neurotransmitter availability.
• Connect to capillaries and neurons
• Modulate the chemical environment, participate in nervous tissue repair, maintain the blood-brain barrier, and guide neuronal development

Microglia

• Smallest and least abundant glial cell
• Macrophages of the CNS (immune)
• Originate from monocytes.
• Non-nervous tissue

Ependymal Cells

• Glial cell type
• Line the ventricles
• Modified epithelial cells (columnar)
• Form and circulate CSF

Oligodendrocytes

• Glial cell
• Produce myelin that wraps around axons to insulate and speed up electrical signals
• Can coil over several axons.
• Spaced nodes of Ranvier

Satellite Cells

• Amphicytes, that surround neuron cell bodies
• Regulate the exchange of nutrients and waste products between the cell body and the extracellular fluid
• Help in insulate the cell body

Schwann Cells

• Form myelin around axons
• External layer of axons where the Schwann cell is located is called neurilemma
• Wrap around unmyelinated axons (~15) without forming myelin
• Wrap around one axon

Myelin in CNS vs PNS

• In PNS, Schwann cells' ratio is 1:1 with nodes of Ranvier
• In CNS, oligodendrocytes’ ratio is 1:many with nodes of Ranvier

Terminology

• Cell bodies in the CNS are called nuclei
• Cell bodies in the PNS are called ganglia
• Axons in CNS are called tracts
• Axons in PNS are called nerves
• Nerves contain many axons bundled together - each bundle is known as a fascicle

Nerves

• Outer layer is called the epineurium made of tough dense network of collagen fibers (dense irregular connective tissue)
• Each fascicle is surrounded by the perineurium from dense irregular connective tissue Each axon is surrounded by Schwann cells and covered with an endoneurium

How Nervous Tissue Works Together

• Reflex arc includes stimulus, receptor, sensory neuron, interneuron, motor neuron, and effector

Disorders of the Nervous System: Multiple Sclerosis

• Affects 1 in 1000
• Appears in early adulthood and affects more women than men
• It's an autoimmune disease that destroys the myelin in the CNS