the nervous system

functions:

• four basic functions of the central nervous system:
  • voluntary movement
  • perception
  • homeostasis
  • cognition

voluntary movement

• driven by the brain; voluntary and involuntary expressions, such as:
  • facial expressions
  • raising your hand
  • jumping up and down
  • dance steps
• is lost in people with locked-in syndrome for example

perception

• perception is what is consciously appreciated about sensation - not the same as sensation
• sensory brain monitors CO₂ in the blood, organ chemical, muscle lengths etc., i.e. all the physical and chemical characteristics of the body
  • but this is at a sub-system level
  • sense and receptors
  • the actual amounts don’t reach the conscious perception
• perception is vision and hearing, smell and taste, the sense of balance, the sense of position in the world
  • “Where are we? Where is our head in the world? A sense of equilibrium”

homeostasis

• homeostasis is what we use to keep our body in physiological limits, which the body can tolerate
  • homeostasis examples
• there are life cycle events that are challenges to our being
  • these include giving birth, and nursing a newborn, suckling from a mother - all of these things are part of homeostasis’ effort to keep the body in equilibrium
  • another part of homeostasis is rhythm
    • daily rhythm: the wake and the sleep
    • seasonal rhythm: has a little bit less of an obvious influence
    • the cycle of life: we go from infancy, to childhood, to adolescence, early adulthood, to middle age, to being older

cognition

• these are abstract functions
  • thinking
  • feeling emotions, and
  • motivation
  • language, and
  • memory, and
  • learning
  • interaction with others

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neurons

• lung cells, liver cells, kidney cells, etc. might be of five, ten different types but after that - out of variation
• however, neurons are unique; they could be categorized into a million different types
• they are the longest cells in the body
  • the longest neuron starts at the big toe and ends at the back of the spinal cord; in really tall people, that one neuron can be 6 ft

parts of the neuron

• four parts:
  • cell body: ‘soma’ - the center of the cell body
  • dendrites: they receive information from synaptic terminals of other cells and relay it to the cell body
  • arbor: the tree of branching of dendrites, locally dense
  • axon: relays messages out of the cell body, only one axon per neuron and can go far distances in the body

• synapse: communication center between neurons
  • space terminating one axon connecting to dendrite of next neuron
  • information travels out along the axon, ends up at the the synapse
  • at the synapse, the axon gives the information to the next dentrite in line; synapse = point of information transfer
  • a synapse can interface with:
    • other neurons
    • muscles
      • skeletal muscles
      • intestinal smooth muscles
      • cardiac muscles
    • glands
• uniqueness: each neuron is different from each other
  • in anatomy:
    • the dendritic arbor collects information from a volume
    • each neuron and it’s arbor manifests a little differently
  • in excitability:
    • how sensitive it is to actually fire
    • some keep firing all the time with little triggers, but some have to be pushed hard to trigger
    • comparable to sensor sensitivity in engineering, variability in speed and amount of response
  • in neurotransmittance quality:
    • some only say “yes”, others only say “no”
    • some “maybe”, “probably”, “questionably”
    • akin to logic gates

glial cells

• if neurons are like the stars of the show, glial cells are the support cast

• glia are needed for running the brain (the shit-show that it is), there are 10 times more glia than there are neurons; some other reach show there are slightly less number of them compared to neurons (research under progress)

• astrocytes:

  • sanitation workers of the brain that clean up the refuse of the neurons including excess ions, excess non-transmitters and their metabolites

  • allow neurons to reach the location from where they are born while they develop as an earlier stage progenitor cell, hence are vehicles for neuron transportation

  • synapses are enveloped in the processes of astrocytes and are supported by glia in their maintenance
  • 20% of glia cells
  • oligodendrocytes and schwann cells are two kinds of astrocytes that make myelin
    • oligodendrocytes:
      • make myelin in the central nervous system (CNS)
      • 75% of central glia cells
    • schwann cells:
      • make it in the peripheral nervous system (PNS)
• microglia are the exception to that rule that cells of the nervous system come from the ectoderm (in the embryo); these come from the blood

  • they are immune cells which quietly monitor for problems

  • when there is a problem, they try to rectify things and bring some attention to areas of damage

  • sometimes they go overboard, and they start to participate in making the problem as well as solving the problem (hot topic)

myelin

• neurons can be myelinated or un-myelinated; myelin is a fatty wrap on some axons

• myelin sheaths made of layers and layers of myelin cover the axon body; there are gaps between adjacent myelin wraps for the signal to jump over as they travel down the axon

• axon information is transferred as binary code (zeros and ones)
  • the ones in the code are the activation potential spike
  • the temporal pattern (timing) of these spikes carries the information
  • (electric) signal jumps across the myelin wraps like hoops to increase speed
  • absence on myelin doesn’t allow jumps of signals and hence slows it down
• information speeds are different along the two types of axons:
  • un-myelinated: 0.2 - 1 m/s (1.5m in 1.5s to 7s)
  • myelinated: 2 - 120 m/s (1.5m in 0.012s - 0.75s)

• action potential spike transfer along an axon - myelinated vs. un-myelinated

• if the myelin sheaths are damaged, they disrupt the temporal patterning of action potential; some activation potentials never reach the end of the axon
  • diseases from demyelination/myelinating glial cells affect
    • central nervous system:
      • multiple sclerosis/oligodendrocytes
      • symptoms affects motor functions
    • periphery nervous system:
      • charcot-marie-tooth/schwann cells
      • guillian-barre/schwann cells
      • symptoms affect either sensation, control of skeletal muscles, and internal organs

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central nervous system (CNS) vs. peripheral nervous system (PNS)

• neurons are classified to belong to either CNS or PNS based on where their cell body is

• there is an organic demarcation between the central and the peripheral system called meninges
  • all the neurons of the CNS are contained within the meninges
• axons of motors neurons go out through meninges:
  • to go into the PNS, and rest of the body to enable motor function control
  • the outgoing motor neurons also connect to autonomic neurons that control glands, cardiac muscles, smooth muscles and guts
• sensory neurons are located peripherally and carry information into the CNS

meninges

• the meninges is made of three membranes
  • the pia mater: weak and tender film
    • closest to the brain
  • the arachnoid: spidery webbed layer
    • between the pia and the dura
  • the dura mater: tough layer
    • protects from having concussions all the time, floats brain in a fluid, takes hard hits

• the meninges protects the central nerves from the diseases of the periphery nerves
  • the neurons in the periphery like the sensory neurons and the autonomic neurons in the autonomic ganglia are vulnerable to the diseases that don’t affect the CNS
    • e.g. congenital insensitivity to pain
  • the meninges forms barrier against toxins, viruses, and damage

vulnerability and diseases

• diseases that affect the nervous system affect either the CNS or PNS, they tend to not affect both at the same time

• periphery nervous system is far more vulnerable than the central nervous system

• although the periphery system is more vulnerable, their neurons have a far greater capacity to repair - that doesn’t occur in the well-protected central neurons
  • atomic area of research to find healing environments for CNS
• polio virus enters the synapse of an axon that connects with a motor muscle and enters the CNS through the meninges, piggy-backing on that axon
  • the host motor neuron will die when the virus enters axon’s cell body in the CNS
  • control for the muscle that the virus entered through will be weak or lost as the virus kills the associated motor neuron

• botulinum toxin affects only the PNS

• herpes zoster causes shingles, where the virus enters the synapse of an axon of the sensory neuron connected to the skin
  • the virus travels up the axon and lives in an appendage cavity on the axon located outside the meninges
  • may stay harmless unless it decides to blossom
  • if it does reproduce (an outbreak), it sends back the virus on the axon to the skin, causing a rash
  • affects only one side of body, disease goes away eventually

brain tumors

• a cell in some part of the body might lose it and divide without limits, becoming immortal
  • this causes a tumor, this can be removed sometimes
• but sometimes, the cells from those tumor go rouge and travel to other parts of the body (they metastasize)
  • if they enter the brain, they cause brain tumors
  • as the tumor expands in the cranium, it can pressurize the insides and cause problems
• once a neuron is born, it grows and it dies - it does not replace itself
  • neurons don’t make tumors because they don’t divide at all; glia cells, meningial cells and cells of the brain glands can cause tumors
• glia cells can be another source of brain tumors if they divide uncontrollably; their tumors are called gliomas
  • myelin producing glial cells surrounding the cranial nerves can grow tumors, producing dysfunction of facial expression, balance and sensory function

• meningial cell division can cause tumors called meningiomas, and they cause pressure within the cranium leading to problems

• glandular cells of pineal gland and pituitary gland might divide to cause tumors
  • pineal gland produces melatonin which influences sexual development and daily sleep-wake cycles
  • pituitary gland is often called the master gland because it controls several other hormone glands, including the thyroid and adrenals, the ovaries and testicles
    • pituitary adenoma are tumors erupting from this gland

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brain and spinal cord

• brain and spinal cord are the two main units of the nervous system along with the nerves
  • the brain is in the cranium
  • the spinal cord is contained in the vertebra

• foramen magnum is the hole at the bottom of the cranium that lets the top of the spinal cord connect to the bottom of the brain

• the dura mater of the meninges wraps the brain and the entirety of the spinal cord
  • it is separated from the brain by a layer of fluid, encasing it in a bag of fluid, providing it cushioning
  • there are folds within the dura to add strength
  • the dura and inside of the skull cranium are plastered together, with blood vessels in-between
  • however, the dura is not plastered up against the insides of the vertebra
    • the spinal cord can move a bit within the vertebra
  • so pressure buildup within the brain due to tumors is more riskier than pressure buildup within the spinal cord

• the arachnoid wraps the brain bulk but doesn’t cover the hills and valleys

• the pia mater covers the hills and valleys of the outer surface of the cerebrum

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relevant terms

• “somatic” mutation: present in only some parts of the body
• “germline” mutation: is present in all cells of body

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reference

• Understanding the Brain: The Neurobiology of Everyday Life
• central nervous system - structure and function
• congenital insensitivty to pain (CIP)
• brain-facts: interactive brain
• society for neuroscience

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