Introduction to the Nervous System

• The nervous system is the body's main controlling and communication network.
• Thoughts, actions, and emotions originate from the nervous system.
• Chemical and electrical signals are the means of communication.

Overlapping Functions of the Nervous System

• Sensory input is gathered by millions of sensory receptors to monitor internal and external changes.
• Sensory input integration involves processing and interpreting sensory information to determine a response.
• Motor output is initiated as a response to sensory input.

Structural Organization of the Nervous System

• The nervous system has two main parts; the Central Nervous System (CNS) & Peripheral Nervous System (PNS).
• The Central Nervous System (CNS) comprises the brain and spinal cord.
• The CNS functions as the integration and control center.
• The Peripheral Nervous System (PNS) consists of nerves and ganglia.
• The PNS receives information and carries out actions.

Organizing Principles

• The organization is hierarchical, with lower levels conveying messages to and from higher levels.
• Example: Spinal cord (lower) and cortex (upper).
• Neurons with similar functions tend to be located in the same area due to structural and functional patterns.
• Neurons are organized topographically to match the body's organization.
• The CNS is plastic and changes constantly due to both internal and external factors.

Functional Organization

• The sensory nervous system, also called the afferent nervous system, conveys information (impulses) to the CNS.
• Somatic sensory input is consciously perceived from sight, touch, etc.
• Visceral sensory input isn't consciously perceived from blood vessels and internal organs.
• The motor nervous system, also called the efferent nervous system, conveys information (impulses) from the CNS to effector organs.
• Somatic motor output is consciously or voluntarily controlled with the effector as skeletal muscle.
• Autonomic motor output is not consciously or is involuntarily controlled, with effectors such as glands, smooth, and cardiac muscle.
• The autonomic motor system is further divided into sympathetic and parasympathetic systems.

Cell Types

• Neurons are excitable cells of the nervous system that transmit electrical signals.
• Supporting cells are smaller and support neurons and they are collectively referred to as neuroglia or glial cells.

Neurons

• The basic functional structure of the nervous system is the neuron.
• Neurons are excitable cells that initiate and transmit electrical signals.
• Neurons can detect a stimulus and transduce it to an electrical signal.
• Conduct electrical signals from one part of the body to another
• Neurons can communicate by secreting neurotransmitters (NT) in response to conductive activity.
• Neurons have extreme longevity, are amitotic.
• They require an abundant supply of O₂ & glucose.

Neuron Cell Body

• The cytoplasm is called perikaryon.
• Contains the normal organelles except centrioles.
• Protein-making machinery is highly active with ribosomes called Nissl bodies (chromatophilic substances).
• Dendrites and axons are focal points for neural outgrowth.
• Clusters of cell bodies have specific names such as nuclei in the CNS, while the PNS consists of ganglia.

Neural Processes

• Neural Processes extend from the cell body.
• The CNS has both neuron cell bodies & their processes, called tracts.
• The PNS primarily has processes, called nerves.

Dendrites

• Dendrites are the main receptive (input) regions of the neuron.
• More dendrites results in more input.
• Convey incoming messages towards the cell body and generate graded potentials, but not action potentials (nerve impulses).

Axons

• Each neuron has only one axon that arises from the axon hillock
• The axon may divide along its length (collaterals) with branches (terminals or telodendria) at the end with 100 – 1000s termini.
• The end of the terminal is an axon terminal or synaptic knob

Axon Structure and Function

• Functionally, the axon is the conducting region of the neuron, generating and transmitting the nerve impulse away from the cell body to the terminal.
• Structurally, the axon lacks Nissl bodies & golgi apparatus, and depends on the cell body for protein production.
• The cytoplasm is called axoplasm and the plasma membrane is called the axolemma.

Neuron Transport

• Movement of materials occurs along the cytoskeletal element.
• Anterograde movement goes toward the axon terminal.
• Retrograde movement goes to the cell body.
• Anterograde and retrograde movement serves as a means of intracellular communication and moving cellular debris back to the cell body machinery for processing in retrograde direction.
• Fast axonal transport happens at ~400 mm/day, moves along microtubules using ATP and motor proteins, and uses both anterograde and retrograde movement.
• Slow axonal transport happens at ~0.1 to 3 mm/day, involves movement of the axoplasm (axoplasmic flow), and uses only anterograde movement.

Neuron Classification

• Neurons are are classified structurally according to the number of processes.
• Functionally classified according to the roles they have.

Structural Classification of Neurons

• Multipolar neurons have 3+ processes and are most common in the CNS.
• Bipolar neurons have 2 processes, an axon and dendrite extends from opposite sides of the cell body and are rare and are specific to the special sense organs (retina + olfactory bulb).
• Unipolar neurons have a single process (T-like, distal portion often associated with a sensory receptor (peripheral process)) that emerges from the cell body and is chiefly located in the PNS.
• Axon extends centrally into the CNS.
• Anaxonic neurons have only dendrites and no axon, only produce local electrical signals, and produce no action potentials.

Functional Classification of Neurons

• Sensory (afferent) neurons transmit impulses to the CNS from sensory receptors and are typically unipolar (except special senses).
• Cell bodies located in ganglia in PNS.
• Interneurons are located between afferent & efferent pathways, shuttle signals within the CNS, and are the most abundant (99% of all neurons).
• Most are multipolar with structural variation.
• Motor (efferent) neurons transmit impulses away from the CNS to the effector organs and are multipolar.
• Cell bodies are located in the CNS.

Nerves

• A nerve is a bundle (or cord) of peripheral neuronal axons and components of the PNS.
• Nerves consists of the axon portion of the many neurons and are typically macroscopic.
• Each individual fiber enclosed by a layer of connective tissue
• Three successive CT wrappings surround nerves in connective tissue: epineurium, perineurium, endoneurium (outside → inside)
• The cell bodies of nerves are located in collections called ganglia.
• Posterior (dorsal) root ganglia house the cell bodies of sensory (afferent) neurons.

Nerve Classification

• Nerves are structurally classified based on which CNS component the nerve extends from.
  • "Cranial nerves" extend from the brain.
  • "Spinal nerves" extend from the spinal cord.
• Nerves are functionally classified on the nerve's functionality.
  • Sensory nerves only transmit sensory fibers to the CNS.
  • Motor nerves only transmit motor fibers from the CNS.
  • Mixed nerves contain both sensory and motor neurons. sensory only transmits sensory , motor only transmits motor, bundles are simply for convenience.

Synapses

• A synapse is the functional junction between two neurons or a neuron and its effector.
• There are two types of synapses: electrical and chemical.

Electrical Synapses

• Electrical synapses are the least common synapse type and have gap junctions.
• The gap junctions allow the flow of ions between cells.

Chemical Synapses

• A more common synapse type, neurotransmitters are released and received by this synapse.
• When an action potential reaches the axon terminal, its purpose is to signal the release of neurotransmitter from the synaptic knob
• Neurotransmitter released from the axon terminal (pre-synaptic cell) serves as a signal and induces a response (graded potential) in the post-synaptic cell.

Electrical Synapse Communication

• Gap junctions create electrical continuity between cells.
• There is no synaptic delay.
• Cannot be modulated.
• Found in cardiac and smooth muscles.
• Bidirectional communication, meaning no pre- or post-synaptic cells.

Synaptic Signals

• Information (a signal) is sent from one side of the synapse and received on the other side.
• The pre-synaptic neuron is the sender, and the post-synaptic neuron is the receiver.
• Neurons are separated by a fluid-filled space called the synaptic cleft.
• With regard to the synapse: the pre-synaptic junction is towards the synapse (sending side), and a post-synaptic junction is away from the synapse (receiving side).
• Transmission is the release of neurotransmitter (NT) from synaptic vesicles into the synaptic cleft.
• NT diffuses across the synaptic cleft and binds to a receptor on the post-synaptic cell, which elicits a response.
• Axodendritic synapse: axo somatic synapse location of neuron.
• Axo axonal: Synapse between axons.

Other Cell Types

• Neurons are excitable and transmit electrical signals.
• Supporting cells, collectively called neuroglia or glial cells, are smaller and support cells.

Glial Cells

• Glial cells are also called neuroglia, are smaller; they're also responsible for mitosis.
• These cells do not transmit nerve signals; rather, they support neuronal functions.
• Glial cells typically make up half the mass/volume of the nervous system
• Outnumber neurons approximately 10 to 1.
• Glial cells consists of six types: includes Astrocyte, Ependymal, Microglial, Oligodendrocyte cell and Satellite cell & Schwann cell.
• 4 in the CNS and Two are located in the PNS

Astrocytes

• Most abundant and star-shaped.
• Astrocytes establish the physical structure of the brain by scaffolding (directing neurons to their proper destination) and physically supporting neurons in their proper spatial relationships.
• Astrocytes control the chemical environment by glycogen storage and glycogenesis, providing glucose and lactate to neurons, and controlling ion concentrations in the extracellular space.
• Play a role in exchanges between capillaries and neurons.
• Astrocytes repair damage to nerve cells and act as scar tissue after neuronal damage.
• They also release ATP, which stimulates oligodendrocytes to produce the myelin sheath.

Microglial Cell

• Approximately 10-15% of the CNS.
• Act as resident macrophages, related to monocytes.
• Resting state: release growth factors promoting brain cell survival.
• Active state: mobile moving to site of injury to phagocytize damaged tissue or foreign substances.
• Functions as Immune defense cell of the CNS.
• Facilitates Removal of damaged neurons or neuroglia and infectious agents.
• Functions to Recognise infectious agents and action as antigen-presenting cells.
• Prevents inflammation within the CNS with removal of infectious agents.

Ependymal Cells

• Ependymal cells line the internal cavities of the CNS.
• Ciliated and help move the cerebrospinal fluid through the ventricles of the brain.
• The basal membrane is attached to astrocytes.
• Ependymal cells create a permeable barrier between cerebrospinal fluid & the tissues.
• Ependymal cells contribute towards formation of Cerebrospinal Fluid.

Oligodendrocyte

• Oligodendrocytes provide support and insulation to axons within the CNS.
• Form myelin sheaths around the axons in the CNS (can myelinate up to 50 axons).
• Oligodendrocytes inhibit neurons' ability to regenerate.
• They also produce growth-inhibitory proteins.
• Produce a variety of peptide factors that control neuronal cell function.

Neurolemmocyte

• Also known as the Schwann cell.
• Functionally similar to oligodendrocytes.
• Form myelin sheaths around the axons in the PNS.
• Neurolemmocytes wrap nerves & form myelin sheaths and are vital to regeneration of damaged peripheral nerve fibers.

Satellite Cell

• Satellite cells are found in ganglions of the PNS.
• Satellite cells are usually arranged around the cell bodies of the sensory neurons.
• Physically separate the cells and regulate nutrient exchange.

Myelin Sheaths

• Not part of the neuron itself, but made from "support" cells that wrap themselves around the axons.
• Repeating concentric layers of plasma membrane.
• Only can be associated with axons: it is majorly composed of lipids because it acts as an electrical insulator.

Nodes of Ranvier

• Oligodendrocytes (CNS) have multiple processes that wrap around multiple axons.
• Neurolemmocytes (PNS) individually wrap around the axon to form internodes.
• Current only flows at the Nodes of Ranvier, which exposes the axon to the ECF.
• Distance between each node is short enough to allow current to flow between adjacent nodes.

Axon Regeneration

• Mature neurons cannot divide, but regeneration is possible if damage occurs.
• Conditional, if damage occurs near or at the cell body, the cell will die.
• If distance between the two severed endings is too great, surrounding tissue can impede successful regeneration (occurs at a rate of 2 - 5 mm/day).
• Stages: severed by trauma, sealing of the proximal portion of the axon and Wallerian degradation of the downstream axon and myelin sheaths (neurolemma remains intact), formation of the regeneration tube, regeneration and myelination, and restoration of innervation.