Overview of the Nervous System

Overview of the Nervous System

• The endocrine and nervous systems both coordinate internal bodily functions
• The endocrine system uses chemical messengers called hormones, that are secreted into the blood
• The nervous system uses electrical and chemical signals to send messages from cell to cell
• The nervous system completes tasks in three essential steps:
  • Sense organs receive data regarding internal and external changes before transmitting it to the spinal cord and brain
  • The CNS then processes the data, relates it to existing knowledge, and decides on the right response
  • The CNS sends commands to muscle and gland cells to produce that response
• The basic pathway involves receptors detecting a stimulus, transmitting it to a sensory neuron which relays the signal to the central nervous system for processing
• Next a motor neuron then carries the response to an effector, like a muscle or gland
• The system consists of two main divisions: the central nervous system (CNS) and the peripheral nervous system (PNS)
• The CNS consists of the brain and spinal cord
• The PNS is the rest of the nervous system (minus the brain and spinal cord); made up of nerves and ganglia
• Nerves refer to axon bundles wrapped in fibrous connective tissue, which may be spinal or cranial
• Ganglia are knot-like nerve swellings with concentrated neuron cell bodies
• The PNS has sensory (afferent) and motor (efferent) divisions
• The somatic sensory division carries signals from the skin, skeletal muscles, and joints
• The visceral sensory division sends signals from the viscera (heart, lungs, stomach, bladder, etc.)
• The motor (efferent) division transmits signals away from the CNS
• Somatic fibers target the skeletal muscle as part of the somatic nervous system
• Visceral fibers target smooth muscle, cardiac fibres or glands in the autonomic nervous system
• The somatic nervous system enables voluntary control with just one motor neuron, faciliating somatic reflexes that result in involuntary muscle contractions
• The autonomic nervous system is involuntary, targeting smooth and cardiac muscle or endocrine glands, using a motor neuron pair for it's autonomic or visceral reflexes

Autonomic System Subdivisions

• The visceral motor division, also known as the autonomic nervous system features two subdivisions
• The sympathetic division tends to arouse the body facilitating action, originating in the thoracolumbar region for 'fight or flight' i.e. ‘E’ responses that include excitement, emergency, exercise, and embarrassment
• The parasympathetic division supports a calming effect on the body, originating out of the craniosacral region

Properties of Neurons and Glial Cells

• Neurons and neuroglia are the main cell types in nerve tissue
• Neurons are able to be excited and transmit electrical signals and are the functional units of the nervous system
• Neuroglia are the supporting cells
• Neurons share 3 main properties:
  • Excitability enables them to respond to stimulus changes in their environment
  • Conductivity enables electrical signals to pass to other cells
  • Secretion permits them to release neurotransmitters that impact the next cell
• Neurons fall into three functional classes:
  • Sensory (afferent) neurons that detect and transmit data to the CNS
  • Interneurons (association neurons) exist only in the CNS, linking sensory and motor pathways, making decisions—about 90% of neurons are in this class
  • Motor (efferent) neurons that send signals to gland and muscle cells (effectors)

Neuron Anatomy

• The soma, (or neurosoma/cell body), is the neuron's control centre
• Most neuron cell bodies do not have the ability to divide
• The nuclei contain one nucleolus and a cytoplasm
• They do not contain mitotic spindles
• Neurons can have extreme longevity
• Dendrites are soma branches that receive signals from other neurons and more dendrites mean more data
• Axons (nerve fibers) originate at the axon hillock and carries signals from the soma
• Neurons generally have one axon (or none) that splits into axon collaterals
• Axons contain axolemma and may be enclosed by a myelin sheath
• Axon terminals (terminal buttons) at the end of axons have synaptic vesicles containing neurotransmitters
• Soma generated proteins are transported to the axon and axon terminal to fix axolemma and organelles by microtubules using motor proteins

Neuroglia in the CNS

• There are about 1 trillion neurons in the nervous system
• Neuroglia cells vastly outnumber neurons at least 10 to 1
• Neuroglia (glial cells) protect and ensure neuron function
• They hold neurons firmly in place providing a nervous tissue framework via glial covering in the absence of neuron contact
• CNS contains fourtypes of neuroglia:
  • Oligodendrocytes supporting myelin sheaths that enable fast signal conduction
  • Ependymal cells in brain cavities circulating cerebrospinal fluid (CSF)
  • Microglia (macrophages) that search for debris and damage
  • Astrocytes (the most abundant type of glial cell) which ensheathe surfaces and nonsynaptic areas in/on the brain that also use blood-brain barriers with perivascular feet

Two Types of Neuroglia in PNS

• Schwann cells surround axons to produce the myelin sheath, in a similar fashion to CNS oligodendrocytes assisting in damaged fibre regeneration
• Satellite cells surround somas in the PNS’s ganglia cells allowing for electrical insulation and regulated chemical levels for neurons

Myelin Sheaths

• This provides insulation around axon nerve fibers
• Oligodendrocytes (in the CNS) and Schwann cells (in the PNS) create it
• Consists of a plasma membrane of glial cells containing 20% protein and 80% lipids
• Myelination (myelin sheath production) starts in the 14th week of fetal development, proceeding rapidly during childhood and is complete by late adolescence at which time dietary fat becomes essential for CNS development
• Within the PNS individual Schwann cells create many membrane layers lacking any cytoplasm—the outermost membrane layer is named neurilemma, containing the bulk of their cytoplasm that has a basal lamina outside it plus a very thin fibre layer
• An oligodendrocyte myelinates several nerve fibers at the same time but is unable to migrate from any one Schwann cell
• Myelination builds up pushing newer layers in because there is no neurilemma present

Myelin Sheath Segmentation

• For nerve fiber coverages, there is a need for multiple Schwann cells/oligodendrocytes
• Myelin sheaths are segmented
• There are gaps between them referred to as Nodes of Ranvier
• The myelin-covered internodes, trigger zone and the axon hillock, as well as the initial segment, are important regarding nerve signal initiation

Nerve Fibre Conduction Speed

• The speed of a nerve signal along a fiber's surface is proportionate to its diameter and its presence or absence of myelin
• Wider fibre allows for a quicker transmission on account of the greatly increased area which allows larger signals to transfer
• Additionally, myelin plays a role via speeding up said signal
• Slow signals are often sent to the GI tract where speed matters less
• Fast signals are normally sent to skeletal muscles where speed matters a lot more for balance and body movement coordination

Nerve Regeneration

• Nerve fiber regeneration may materialize if the some remains undamaged
• Fibre destruction beyond an injury allows for axon stump sprouting and Schwann glial or basal-lamina regeneration at the injury zone
• New fibre reconnection allows the muscle to regrow with a slow annual regrowth from a previously damaged location
• CNS fibre damage does not repair, instead a scar is formed

Electrophysiology of Neurons

• Electrical potential is the difference in concentration of charged particle between a point and another
• Electrical current is a flow of charged particles from one point to another
• Within the body, currents manifest through ion transmission—for instance, travelling via plasma membranes
• Gated channels regulate these ion flows via stimulus or closure
• Cells may switch electrical currents either towards an on or off configuration
• Negative charge exists inside cells and relative positivity exists outside cells - thus the cell is polarized, resulting in a potential energy difference of -70mV
• High concentrations of intracellular potassium and high concentrations of extracellular sodium lead to the resting potential
• The combination of three factors leads to the resting membrane potential (RMP): ion diffusion pursuant to gradient concentration around the membrane, plasma selectively permeable to certain ions and attraction between opposite ion charges (cations to anions)
• Sodium measures 145 mEq/L and Potassium is approximately 4 mEq/L
• Sodium measures 12 mEq/L intracellularly while Potassium reaches around 150 mEq/L—in addition to large impermissible large anions that cannot egress intracellular space

Sodium and Potassium Factors

• Potassium has a large influence because the plasma membrane is more permeable to it compared to sodium
• Potassium slowly exits (down the concentration gradient) until anions from the cytoplasmic portion attract it causing an equilibrium resulting in a slightly higher potassium reading internally (ICF) vis-à-vis extracellular fluid (ECF)
• Sodium influences membrane permeability ever so slightly due to constant plasma leakage
• Sodium leaks into cells, which reduces negativity vis-à-vis if solely determined via Potassium
• The sodium/potassium pump moves three sodium ions outside for each two potassium ions inside
• The unequal charge balance permits approximately -3mV potential

Membrane Potential Change

• Three events result in membrane voltage shifts:
  • Depolarization that allows the inner membrane portion to become less negatively charged and reverses polarity
  • Repolarization allows return to a resting membrane voltage
  • Hyperpolarization allows increasing interior membrane negativity
• It is important to note the 2 main categories of signals: local (graded) potentials and action potentials
• Neurons see potential shift in a part of the cell nearby or on account of it being stimulated
• These occur across short ranges that occur very quickly specifically dendrites including some components that are cell material
• Sodium gates are open pursuant to mechanical stimulation and changes such as heat, light and chemical activity
• Stimulus strength affects signal size - action is proportionate to stimulus
• A cell that allows stimulation will see membrane depolarization, leading to action potential
• A cell that rejects stimulation will see membrane hyperpolarization that prevents action potential

Action Potentials

• Occurs when there are changes in membrane polarity caused by voltage-gated ion channels
• Travel greater distances and retain strength on axons of both muscular and neural cellular types
• Voltage changes brought about on account of electrical changes to the cells or voltage-gated effects trigger Sodium gates to release
• An 'all or nothing' principle underlies responses
• Action potentials are a rapid string of actions that result in an up/down voltage reading caused by step by step activity
• During axonal change at axon hillock for instance, membrane depolarization requires the action to then attain -55mV and that triggers action that enables voltage-release of regulated gates
• Opening of channels for voltage-gates causes entry and change that makes voltage climb in a rapid positive
• During upward potential shift beyond 0mV, gate closures begin which max around -35mV
• Slower channels make cell outflow result toward a phase called repolarization for cell's channels remain a bit more elongated beyond
• Extracellular astrocytes are released