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
  • 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

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

Neuron Parts

  • 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

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

Meninges

  • 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

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

relevant terms

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

reference