Nervous System: Overview and Anatomy

Overview of the Nervous System

• The nervous system controls perception and experience of the world
• It directs voluntary movement
• It's the seat of consciousness, personality, learning, and memory
• It regulates homeostasis with the endocrine system, affecting respiratory rate, blood pressure, body temperature, sleep/wake cycle, and blood pH

Anatomical Divisions of the Nervous System

• The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS)
• The CNS includes the brain and spinal cord
• The brain contains billions of nerve cells (neurons) and is protected by the skull bones
• The spinal cord begins at the foramen magnum and continues through the vertebral foramina from the first cervical to the first or second lumbar vertebra
• Millions of neurons are in the spinal cord, which communicates with the brain to the rest of the body below the head and neck
• The PNS includes all nerves outside the skull and vertebral column
• Nerves are axons of neurons bundled with blood vessels and connective tissue; they carry signals to and from the CNS, classified by origin or destination
• There are 12 pairs of cranial nerves traveling to or from the brain
• Spinal nerves consist of 1 pair traveling to or from the spinal cord

Functional Divisions of the Nervous System

• Sensory division (afferent division) gathers information about internal and external environments
• Input from somatic and visceral sensory divisions is carried from receptors to the spinal cord and/or brain via spinal and cranial nerves
• Somatic sensory division transmits signals from skeletal muscles, bones, joints, skin, and organs of vision, hearing, taste, smell, and balance
• Visceral sensory division transmits signals from viscera (heart, lungs, stomach, kidneys, and urinary bladder)
• Integrative functions analyze and interpret incoming sensory information and determine an appropriate response
• About 99% of integrated sensory information is subconsciously disregarded as unimportant
• Remaining sensory stimuli that the CNS responds lead to a motor response

Motor Functions of the Nervous System

• Motor functions are actions performed in response to integration by the motor (efferent) division of the PNS
• The motor division is subdivided into somatic and autonomic divisions based on the organs the neurons contact
• Motor neurons carry out motor functions and travel from the brain and spinal cord via cranial and spinal nerves
• Effectors are organs that carry out the effects of the nervous system
• The somatic motor division transmits signals to skeletal muscle, providing voluntary control
• The autonomic nervous system (ANS) or visceral motor division carries signals to thoracic and abdominal viscera, critical for maintaining homeostasis
• The ANS regulates secretion of certain glands, contraction of smooth muscle, and contraction of cardiac muscle, operating involuntarily

Neurons

• Neurons are excitable cells responsible for sending and receiving signals as action potentials
• Neurons consist of three parts
• The cell body (soma) is the most metabolically active region, manufacturing all proteins needed for the whole neuron
• Organelles within the cell body support a high level of biosynthetic activity
• Free ribosomes and rough endoplasmic reticulum (protein synthesis)RER visible with microscope (Nissl bodies) are present
• Neurons contains Golgi apparatus (vesicular transport)
• Neurons comprise Large or multiple nucleoli (ribosomal RNA)
• Neurons need Mitochondria to supply energy
• The cytoskeleton consists of microtubules that provide structural support and chemical transportation between the cell body and axon
• Neurofibrils, made of intermediate filaments of the cytoskeleton, offer structural support extending into neuron processes
• Dendrites are short, branched processes that receive input from other neurons and transmit it toward the cell body as electrical impulses; each neuron can have many dendrites
• Each neuron has only one axon (nerve fiber) that generates and conducts action potentials, with distinct regions
• Axon hillocks are where the axon originates from the cell body
• Axon collaterals are branches extending from the main axon
• Telodendria are small branches arising from the axon and axon collaterals near where extensions end
• Axon terminals or synaptic bulbs arise from telodendria and communicate with the target cell
• Axolemma is the plasma membrane surrounding the axon and its cytoplasm (axoplasm)
• Substances travel through axoplasm by slow and fast axonal transport (flow)
• Slow axonal transport transports substances from the cell body through the axon
• Fast axonal transport requires motor proteins and ATP
• Fast axonal transport vesicles and membrane-bound organelles travel back toward (retrograde) or away from (anterograde) the cell body, occur at rates of 200 mm/day and 400 mm/day, respectively
• Neurons have three main functional regions: a receptive region consisting of dendrites and cell body, a conducting region (axon), and a secretory region (axon terminal)
• Neurons can be classified according to structural features such as Multipolar, Bipolar, or Pseudounipolar
• Multipolar neurons have a single axon and multiple dendrites and make up 99% of all neurons
• Bipolar neurons have one axon and one dendrite, with the cell body between them, and are found in the eye and olfactory epithelium (nasal cavity)
• Pseudounipolar neurons possess only one fused axon extending from the cell body, which divides into two processes, with one carrying sensory information from sensory receptors to the cell body, and the other carrying sensory information (pain, touch, and pressure) from the cell body to the spinal cord
• Neurons are also classified into three functional groups: sensory (afferent), interneurons (association), and motor (efferent)
• Sensory (afferent) neurons carry information toward the CNS; neuron cell bodies in the PNS receive information from sensory receptors and relay it via axons to the brain or spinal cord; usually pseudounipolar or bipolar
• Interneurons (association) neurons relay information within the CNS between sensory and motor neurons; most neurons in the body are multipolar and communicate with many other neurons
• Motor (efferent) neurons carry information away from the cell body in the CNS to muscles and glands and are mostly multipolar
• Specific neuron components group together in the CNS and PNS
• In the CNS, clusters of neuron cell bodies are called nuclei, and bundles of axons are called tracts
• In the PNS, clusters of neuron cell bodies are called ganglia, and bundles of axons are called nerves

Neuroglia

• Neuroglia (neuroglial) cells provide structural support and protection for neurons and maintain their environment
• They can divide and fill in space left behind when a neuron dies
• The form of each type of neuroglial cell is specialized for its function
• 4 types of neuroglia reside in the CNS such as Astrocytes, Oligodendrocytes, Microglia, and Ependymal cells
• 2 types reside in the PNS called Schwann cells and Satellite cells
• Astrocytes are large star shapes and anchored to neurons and blood vessels; help to define and maintain the three-dimensional structure of the brain
• Astrocytes transport nutrients and gases between blood vessels and neurons, regulates the extracellular environment of the brain
• Astrocytes form of the blood brain barrier, a protective structure and surrounds capillary endothelial cells
• Microglia are small and become phagocytic when ingest disease-causing microorganisms, dead neurons, and cellular debris
• Ependymal line hollow spaces within the CNS (brain and spinal cord) and secrete cerebrospinal fluid
• Oligodendrocytes are also in CNS that form myelin with flattened sacs

Myelin Sheath

• The myelin sheath consists of layers of plasma membrane of Schwann cells or oligodendrocytes in the PNS and CNS respectively
• Myelin is neuroglial that wrap multiple layers of membrane around the axon
• Lipid content of myelin sheath insulates the axon and prevents ion movements like rubber around a copper wire
• It increases the speed of action potential conduction
• Myelinated axons conduct action potentials about 15-20 times faster than unmyelinated axons
• In PNS, neurolemma exist on outer surface of myelinated axons
• Multiple processes are myelinated versus just the one axon the Schwann produces
• Myelination begins early in fetal development in the PNS and much later in CNS and not very much present in the brains of newborns
• Axons in both the CNS and PNS are generally longer than neuroglial cells, so multiple cells must provide a complete myelin sheath
• Internodes are segments of the axon covered by neuroglia
• Nodes of Ranvier are gaps between adjacent neuroglia where the myelin sheath is absent
• Small axons in the CNS and PNS are usually unmyelinated
• White matter is composed of myelinated axons that appear white
• Gray matter is composed of neuron cell bodies, unmyelinated dendrites, and axons, appearing gray

Regeneration of Nervous Tissue

• Regeneration or replacement of damaged tissue is nearly nonexistent in the CNS
• It's limited in the PNS, where neural tissue regenerate only if the cell body remains intact
• Axon and myelin sheath degenerate distal to the injury which Is Wallerian degeneration and facilitated by phagocytes
• After that, growth processes form from the proximal end of the axon
• Schwann cells and basal lamina form a called a regeneration tube
• A single growth process grows into the regeneration tube, directing the new axon toward its target cell
• Then the new axon reconnects to its target cell

Gliomas and Astrocytomas

• Primary brain tumors originate in the brain and are usually gliomas caused by abnormally high rates of division of glial cells
• Predisposing conditions for brain tumors include exposure to ionizing radiation and certain diseases
• Astrocytes are most commonly affeced by the tumor called astrocytoma and is ranges in severity including being very aggresive with poor prognoses
• Treatment varies with tumor type, age, and health of patient, but generally involves the surgical removal of mass with chemotherapy and perhaps radiation therapy

Introduction to Electrophysiology of Neurons

• All neurons are excitable (responsive) in the presence of chemical signals, local electrical signals, and mechanical deformation
• Stimuli generate electrical changes across the neuron plasma membrane, which are rapidly conducted (conductivity) along the entire length of the membrane
• Two forms of electrical changes occur in neurons: local potentials, which travel short distances, and action potentials, which travel the entire length of the axon

The Resting Membrane Potential

• There is a thin layer of negatively charged ions exists in the cytosol
• There is a thin layer of positively charged ions existing on the outside of the cell.
• Voltage is the electrical gradient established by the separation of charges between two locations (across the plasma membrane)
• Membrane potential is the electrical potential across the cell membrane and the source of potential energy for the cell
• A typical neuron has a resting membrane potential (RMP) of -70 mV

Ion Channels and Gradients

• Electrical changes across neuron plasma membranes rely on ion channels and resting membrane potential
• Ions cannot diffuse through the lipid component of the plasma membrane and must rely on specific protein channels
• Leak channels are always open, continuously allowing ions to flow down concentration gradients between the cytosol and ECF
• Gated channels are closed at rest and open in response to a specific stimulus
• Ligand-gated channels open in response to the binding of a specific chemical (ligand) to a specific receptor
• Voltage-gated channels open in response to changes in voltage across the membrane
• Mechanically-gated channels open or close in response to mechanical stimulation (pressure, stretch, or vibration)
• Ions moving against electrochemical gradients move via ATP-consuming pumps; one important pump is the sodium-potassium ion pump (Na+/K+ ATPase)
• The sodium-potassium ion pump moves three Na+ ions out and two K+ ions into the cell, per ATP hydrolyzed
• The pump maintains a high concentration of Na+ in the extracellular fluid and a lower concentration in the cytosol, while the opposite is true for K+

Changes in Membrane Potential: Ion Movements

• Cell starts with a negative resting membrane potential (negative with respect to the extracellular fluid)
• There is unequal distribution of ions across the plasma membrane
• Gated channels and pumps maintain gradients
• When gated channels for a specific ion open, ions will follow the electrochemical gradient into or out of the cell
• Opening gated channels and causing ions to flow into or out of the cell can alter a cell's membrane potential
• Repolarization has occurred when cell returns to the resting membrane potential

Local Potentials

• Local potentials are small local changes in the neuron's plasma membrane potential and are triggers for long-distance action potentials
• They can result in depolarization, where positive charges enter the cytosol, making the membrane potential less negative (e.g., changing from -70 to -60 mV)
• Or hyperpolarization, where either positive charges exit or negative charges enter the cytosol, making the membrane potential more negative (e.g., changing from -70 to -80 mV)
• They are also sometimes called graded potentials where they vary greatly in size
• Changes in membrane potential depend on the length of stimulation, the number of open ion channels, and the types of ion channels
• It's is reversible where when the stimulus stops, the neuron quickly returns its resting potential
• It has decremental effect, where changes in membrane potential are small; current is lost across the membrane over a few millimeters, and cannot send signals over great distances; useful for short-distance signaling only (local potentials)

Action Potentials

• Action potentials are uniform, rapid depolarization and repolarization of membrane potential
• Action potential generated only in trigger zones (axolemma, axon hillock, and initial segment of axon)
• Voltage-gated channels open action potentials (sodium ions and potassium ions)
• They are the most abundant in axolemma of neuron; only axons have action potentials
• Voltage-gated potassium channels have two phases in their state a closed an open phase
• Resting state the channels are closed, allowing no potassiums to passes
• Activated stated the channels are open and flowing
• Voltage-gated sodium channels have two gates (activation and inactivation gates) with three states: resting, activated, and inactivated
• Resting state means inactivation gate and open activation is closed
• Activated state that has a Voltage change opens activation, with everything being open
• During innactivation, the inactivation gate is closed and activation gate
• Neuronal action potential has three general phases lasting milliseconds
• Depolarization which bring the menbrane potential rises to near zero and to a little bit of positivety
• Repolarization to a negative value then
• Hyperpolarization of the membrane temporarily than resting membrane potential
• A local potential must be able to depolarize the axon strongly enough to reach a level called threshold (usually -55 mV)
• Onced the threshold is activated, voltage-gated sodium channel activating

Refractory Period

• The neurons creates the inability to generate an action potential after it has just one that's divided into two phases
• Absolute refractory period is when no additional stimulus is strong enough to have additional potential (no matter how strong)
• This is caused by voltage gated sodium channels and the failure to return to rest state when channels return

Propagation of Action Potentials

• Action potentials must be conducted (propagated) along the entire length of the axon to serve as a long-distance signaling service
• Action potentials occur because they are self-propagating in only one direction, which begins with the trigger zone and the ending with exon terminals
• Action potentials have to continue In one directions as the axon goes into the refactory period as the next section depolarizes
• Action potential propagation down the nerve is called nerve impulses
• Condution speed is how action potentails propgate is determined on two things axon diameter and melantation that dermine how fast singals can occur
• Axons with larger diamters ahve fasters speeds so bigger axons with lower resistance
• presences of what's called saltatory conduction with insulated properties that helps increase the signal corductions
• Saltatory conducts has depolarized modes running as "jumps"
• Continuous conduction does it in unmyelated axons however it propagates slowly because there is no myeline and the exon must depolarize

Overview of Neuronal Synapses

• Communication with other cells, especially other neurons, is essential for neurons to carry out their functions
• Synapse (where the neuron meets it target) can be electrical or chemical
• Neuronal synapses consists of axon of one neruron connecting with another location on another neruons _ These places consists of:
  • Axodendritic synapse between one exon of the neruon and denderite
  • Axosomatic synapse which binds to cell body
  • Axoaxonic synapse to the axon terminal spot
• The neuron doing the sending is referred to as Presynaptic
• The nuerons that is receiving will be called the Postsynaptic
• The transfer of signals between neurons is synaptic transmission
• Synaptic transmission is very fundamental like cognitions, volunteering and sensative
• average presynaptic neuron will send into synapse of 1000 presynaptic neruons
• Postsnyaptic nuerons ahve 10000 connectoins of neruons

Electrical Synapses

• Electrical synapse occur when the cell joins together _ Axolemms connect in the cells and connect into one thing _ Brain is responsible for making automatic beahovuios such as breathing _ Cardiac adn viscoseral connects to allow more coordianted muscles
• Electical current can occur when the neruons connect such as creatng twu unique phases
• One direction in the nueron to nueron connections
• Transmittance happens so fast even less then seconds

chemical Synapses

• Chemicals are part of the majoirty and are more effetent than electeical _ The siagnials and don't lose signal or strength
• Occrus when the syanps recvieves transmitters and they bind to receptors
• They open up leading to loacl potential and have potetnial energy for an actual action
• Transmissions can be terminated by ending nerouns effcets throuhg many means _ Nueron transmittiosn are broken down and reabsorned to the neuron _ nuerons can be reubtakened ( reabsorned)

Nuerotransmatters

• Transmitters undergo almost same patterns
• Made in the cell body/ or exon termina abd packed into transportes
• realse dafter being crossed
• binding to the specifi receptoros of the post synaptse
• the effets offten rapidli throuhg dgeradations

Majoir neuroTrasnmatterss

• Transmittyers can lead to an ehancemtn of excitability and the lowering in inhibitory
• Transmitters can have both depeidngo nthe receptos thst they bind and having seceral types _ majoir trasnmittyes have chmeical structues