Cells in the Nervous System 

Structure & Function of a Neuron

Soma (body): produces neurotransmitters (the messages to be transmitted)

Dendrites: receive information through the synapse of another cell

Axons: carry output message …from here the message travels through the axon hillock

Presynaptic Terminal: transfers information through the secretion of neurotransmitters

Axoplasmic Transport

  • This is the transportation of substances along the axon to/from the synapse
    • Anterograde: SOMA → PRESYNAPTIC TERMINAL (new info)
    • Retrograde: PRESYNAPTIC TERMINAL → SOMA (old info)

The Synapse

  • Synaptic cleft: site for interneuronal communication
  • Postsynaptic Terminal (dendrites of the next cell): area containing specific receptors
  • Components of a chemical synapse:
    • action potential (involving Na+ flowing in and K+ flowing out) takes place in the presynaptic neuron → voltage gated Ca2+ channels open
    • increase of Ca2+ →pushing out (exocytosis) of neurotransmitter molecules into the synaptic cleft
    • The released neurotransmitters interact with receptors on the postsynaptic cell membranes → to cause an excitatory response (depolarizing) or inhibitory response (hyper-polarizing).

Types of Neurons

  • Bipolar cells: composed of
    • One dendrite & One axon
    • Repeats the same function over
    • Found in the retinal cells in the eye
  • Pesudounipolar cells:
    • SUBCLASS OF BIPOLAR CELLS
    • One dendrite & Two axons
    • Single projection dividing into 2 axonal roots
    • Sensory neurons
    • Found in olfactory receptor and retina
  • Multipolar Cells
    • Multiple dendrites & one axon
    • essentially multiple areas of input and it has to balance all the incoming signals
    • Found in spinal motor neurons
    • MOST COMMON, majority of the neurons in our body
    • Bring in a wide range of information

Signaling & Action Potentials

  • Sodium-Potassium Pump = basis for depolarization underlying the action potential
    • Voltage sensitive Na+ channels open when the membrane is depolarized
    • increase in Na+ permeability → further depolarization, opening of other Na+ channels
    • ALL-or-NONE response/depolarization →the action potential (NOT A GRADED RESPONSE)
  • Nerve cell at rest
    • Na+ and K+ ions flow through the membrane
    • Na+/K+ pump is fueled by ATP→ADP
    • Na+ flows out, K+ flows inward (at rest…Na+ flows inward, K+ flows out, during an Action Potential)
  • When the actual depolarization of the presynaptic membrane occurs when the action potential is “fired” following depolarization
  • The action potential is only propagated downstream
  • In myelinated axons, action potentials propagate faster by “jumping” from one Node of Ranvier to the next node by saltatory conduction (versus continuous conduction)
    • Myelination of axons = fast conduction, we see in our longer nerves and areas of white matter

Support Cells (aka Glial Cells)

  • Central Nervous System:
    • Astrocytes (Grey matter)
    • activated neurons or mechanical changes
    • communicate to adjust astrocytes by gap junctions, “share award” (direct communication)
    • remove potassium ions and neurotransmitters w/in the extracellular matrix
    • assist in forming the blood-brain barrier
    • route of nutrients to neurons (capillary to neuron)
    • Ependymal Cells
    • manage (produce & secrete) and control movement of CSF
    • Microglia
    • CNS immune system
    • activated following injury, infection, disease
    • migrate to areas and remove debris
    • Oligodendrocytes: myelinate CNS axons
    • if 1 oligodendrocyte is damaged or dies → multiple nerves are affected
  • Peripheral Nervous System:
    • Schwann Cells: myelinate PNS axons
    • Satellite Cells
    • help create a healthy/happy environment for information to be relayed
    • Surronds neurons in ganglia (sensory, sympathetic and parasympathetic ganglia)
    • communicate via paracrine signaling (communicating to neighbor)
    • regulates ion concentration (outside of the cell..environment)
    • Recycles neurotransmitters
    • vital for communication in the pancreas
    • Do not see them in motor systems, mainly found in regulatory systems

Myelin

  • Myelination occurs on the axon of the nerve and influences the rate of conduction of the signal (“travel time”)
  • Myelinated: usually conduct info regarding survival and large pain signals
  • Unmyelinated: higher chance of distorting the original signal being sent (travels along every point of the axon~telephone game)
  • CNS Demyelination →(MS) Multiple Sclerosis

  Multiple Sclerosis

  • HOW: antibodies attack oligodendrocytes → demyelination & formation of plaque
  • IT CAUSES: slowing or blockage of signal transmission
  • SIGNS & SYMPTOMS:
    • weakness
    • lack of coordination
    • impaired vision
    • impaired sensation
    • disruption of memory and emotions
  • Inflammatory disease of the CNS (<-Demyelination)
    • chronic disease
    • immune-mediated
    • variable course (location)
    • different types
    • several pathological processes
  • Pathophysiology: immune related & Chronic
    • The components: inflammation, demyelination, axonal damage, neurodegeneration
    • Eventually if untreated will lead to brain atrophy

  Therapy in MS

  As PT’s we can help the patient better manage the symptoms, aid in decreasing the rate of onset

  • Self-care activities
    • functional mobility
    • dressing
    • bathing
    • grooming
    • eating
  • Productive Activities
    • Work
    • Home Management
  • Leisure Activities
    • Social
    • Recreational
  • Body Temperature/Heat management

Schwann Cells: Myelin in the PNS

  • Wrap around one or several cells
  • Unmyelinated
  • Myelinated
  • Pain sensors and some pressure sensors are unmyelinated

PNS Demyelination

  • atrophy of muscles (especially in the distal limbs) affect the longest nerve fibers, and the feet and hands are affected in early stages of the disease
  • Affecting the most sensitive nerves
  • Symptom associated with diabetic muscular atrophy

Guillain-Barre Syndrome

  • autoimmune disease (short effect time, is not life long fro individuals)
  • involves damage to Schwann cells & axons of PNS
  • most common cause of acute flaccid paralysis in the US (occurring to 1 to 3 every 100,000)
  • bacterial/viral illness 2-4 weeks prior to onset of weakness
  • 40% have positive C. jejuni serum antibodies/stool culture
  • ascending progression
  • Diaphragmatic and cranial nerves can be involved…autonomic involvement causing bradycardia and hypotension (SEVERE CASE)
  • Within 4 weeks you'll see worsening conditions and then a plateau
  • 85% achieve a fully functional recovery
  • 20% residual disability (nerve is dead and gone)
  • remyelination and healing of axon may not fully recover if cell death has occurred
  • 3% of patients have one or more clinical similar relapses

  Where does PT come in…

  • Acute phase
    • stretching & splinting
    • ROM exercises
    • Comorbidity prevention
  • Recovery Phase
    • Strengthening exercises
    • Assertive technology
    • Avoid overworking muscles

Neural Stem Cells

These cells that we see in the embryonic developmental are pluripotent cells

  • Immature
  • Undifferentiated
  • Self-renewing
  • Multipotent
  • Precursors to neurons and glial cells
  • Populate developing regions of the CNS

  In Adult Brains …

  • potential for new development resides in the hippocampus = forming new memories & learning new tasks
  • Cells lining ventricles also have stem-cell potential