Biology 2401 Anatomy and Physiology I notes - Nervous System Exam 4 Ch 9
Functions of nervous system: monitor internal and external environment (sensory)
integrate sensory input
(integration)
coordinate responses of the body
(motor)
General pathway:
Fig 9.2, 9.7
stimulus --------------------------------------------------------------------------------> response
receptor cells ----> afferent sensory ----> interneurons ----> efferent motor ----> effectors
neurons
neurons
(muscles,
glands)
I------peripheral N.S. -----------------I ---central N.S. ----I --peripheral N.S.--I
*List the cells in a neural pathway between the stimulus and response.
*Can the nervous impulse in a neural pathway travel back and forth or only one way?
*What parts of the neural pathway between the stimulus and response form the peripheral
nervous system?
Cells that compose nervous tissue: connective tissues, blood, specialized nervous tissue:
neurons - communicate
neuroglia, or glial cells -support and protect neurons
Neuroglia are cells that support and protect neurons.
Fig 9.3
glia = “glue”
- more numerous than neurons (about 1 trillion, 10 X more than neurons)
- derived from embryonic connective tissue, function as connective tissue
- retain the ability to divide (cancers in nervous tissue from these cells)
(neurons become specialized and lose the ability to divide)
5 types of glial cells:
astrocytes - only in central ns; anchor neuron for support; cover blood
capillaries to form blood brain barrier.
oligodendrocytes - only in central ns; form wrapping around the axons of
neuron (these wrapping contain myelin) called myelin sheath.
microglia - only in central ns; derived from white blood cells; destroy
bacteria and foreign material by phagocytosis.
ependymal cells - only in central ns; act like epithelial cells which line
canals and ventricles in the brain and spinal cord; secrete cerebrospinal fluid.
Schwann cells - in peripheral ns only; form myelin sheath and
neurolemma around axons of neurons outside of brain and spinal
cord. (more on myelin sheaths later)
*List five types of glial cells and tell what the function is of each.
*Which glial cells are only in the central nervous system (brain and spinal cord) and which are
only in the peripheral nervous system.
*Why are malignant cells in nervous tissue from glial cells and not neurons?
*Which two glial cells produce myelin for neurons and where is each located?
Neurons are the communicating cells of the nervous system; link sensory receptors with brain
and spinal cord, integration within brain and spinal cord and link of brain and spinal cord with
effectors.
- specialize and lose the ability to divide
General structure of neurons:
see figs 9.1, 9.4, 9.5
cell body (soma) - contain most cytoplasm, nucleus and cell organelles
dendrites - usually numerous cellular extensions, receive incoming signals
axon - one large elongate cellular extension, transmit outgoing signal
(may have branches, called collaterals)
synaptic knobs of axon terminal - expanded tips of axon that form synapses
with other neurons, glands and muscle cells. An axon may have many
synaptic terminals.
chromatophilic substance (Nissl bodies) - densely staining areas that contain large
number of ribosomes (ribosomes produce proteins, many of which are
neurotransmitters, the chemicals released at axons synaptic terminals).
neurofibrils - filaments of cytoskeleton that extend from cell body through axon to
synaptic terminals; quickly transport products from cell body to synaptic terminals;
300 times faster than by diffusion; axonal transport. 15 cm/day
Schwann cells - form myelin sheath and neurilemma along axon only
- gaps between adjacent Schwann cells are nodes (nodes
of Ranvier )
*Draw a neuron and label each part listed above.
*What is the important function of the neurilemma and why can neurons in the brain not
regenerate?
*What is the function of the neurofibrils?
Neurons can be classified by the number of extension:
Fig 9.6
multipolar - many dendrites, one axon. Very numerous in brain and spinal cord.
unipolar - one extension, which places cell body to side; single axon continuous. In
sensory nerves.
bipolar - one dendrite and one axon with cell body between. In some sensory organs.
Neurons can be classified by their function: sensory neurons, interneurons and motor neurons
Neurophysiology - electrical activity of neurons
Cell structures that are important:
Fig 9.11-9.13
- pumps are plasma membrane proteins that force ions across the
membrane against their concentration gradient and require ATP energy
- specific (one type ion) (examples are Na+ / K+ pump and Ca++ pump)
- channels are plasma membranes proteins that are passive (no energy required)
that allow ions to cross the membrane
- are specific (one type ion) and work with concentration gradient
- leak channels are always open
- gated channels are closed and can be opened by:
-a certain chemical that fits into a receptor that is part of their
structure (chemically gated channels),
-a nearby electric current (electrically gated channels),
-mechanical stress (mechanically gated channels)
- several of the ions that are important to neuron and muscle physiology
are potassium (K+), sodium (Na+), calcium (Ca++), chlorine (Cl-)
resting membrane potential
Fig 9.11 – 9.13
- neurons (and muscle cells) build electrical potential across cell membrane
- this is a difference in concentration of positively (+) and negatively (-) charged
ions and molecules on either side of the membrane (chemical
concentration gradient and electrical gradient = electrochemical gradient)
- sodium-potassium pumps move 3 sodium (Na+) ions out and 2 potassium (K+)
ions in (these pumps require ATP energy)
- some K+ leaks out through K+ leak channels (open channels)
- most Na+ cannot move back into the cell because the Na+ channels are gated
- many of the large protein molecules in the cell have negative charges
- the result is a positively charged intercellular fluid and a more negatively
charged cell cytoplasm
- this is called a membrane potential or a resting membrane potential and is
typically measured at about minus70 millivolts (-70 mV) in neurons
- it is also called polarization because the cell membrane has + and - “poles”
- the cell remains like this until it is stimulated (pumps work to offset “leakage”)
*Why is the term “resting” membrane potential misleading?
*Compare membrane pumps and channels. What causes each to work?
*Compare gated channels and leak channels.
*Describe the events that cause the cell membrane to become polarized.
*Neurons consume a considerable amount of energy. Describe one of the primary uses of energy
in neurons.
action potential
Figs 9.13 - 9.15
- depolarization is the movement of ions across the membrane so that the potential
is decreased (to 0 mV maybe)
- gated Na+ channels open in response to several types of stimuli on the
membrane of the cell body and dendrites in neurons, such as stimulus
from other neurons, pressure, some chemicals, light
- the stimulus is graded (of various strengths) ;if only a few sodium gates open the
membrane potential will change slightly, but not enough to cause an action
potential, and will die out (subthreshold)
- summation of graded potentials can occur on the cell body and dendrites of
neurons - more on this later
- the base of the axon is thicken area called axon hillock, functions as “trigger zone”
- if enough Na+ channels open in the trigger zone, and the membrane potential
depolarizes to about 55 mV, then an action potential will begin
- chemically gated Na+ channels opened by the stimulus allow a flow of Na+
across the membrane - this is an electric current, which causes nearby
electrically gated Na+ channels to open causing an electric current, which
causes nearby electrically gated Na+ channels to open causing an electric
current, which .... . .
- this chain reaction of opening of electrically gated Na+ channels travels the
entire length of the cell as a wave, the action potential
- an action potential that starts at the base of the axon (where it leaves the cell
body – the trigger zone) will travel the length of the axon with one strength no
matter what graded potentials started it (all-or-none with constant maximum
strength)
- the Na+ channels open and close very quickly and the K+ channels open more
slowly, allowing more K+ to move out, quickly rebuilding charge
- the refractory period is the time required to rebuild the resting potential so that
the action potential can occur again; the cell cannot be stimulated; in
neurons this time period is 1-2 milliseconds (that’s fast my friends)
- on unmyelinated axons the action potential travels from electrically gated Na+
channel to electrically gated Na+ channel, as in muscle cells - this is called
continuous conduction
- on myelinated axons the action potential jumps from node of Ranvier to node of
Ranvier, with the membrane beneath the Schwann cell and myelin not
being depolarized - this is called saltatory conduction
- saltatory conduction is much faster (7 to 10 times faster) and is more efficient
because only the membrane of the nodes needs to be repolarized
(remember Na+/K+ pumps require energy)
- larger axons are also faster (a large myelinated axon can transmit an action
potential 200 times faster than a small unmyelinated axon)- 120m/sec to 0.5m/sec
*Describe the events that occur to initiate and propagate an action potential.
*Compare continuous conduction and saltatory conduction. Explain what each is and tell which
is fastest and which is most efficient and why.
*Which of these events are graded and which are all-or-nothing? What do these terms mean?
Synapses - junctions between neurons and other cells
Figs 9.8 - 9.10
- synapse between neuron and other cells called neuro____ junction (for example
with muscle cell it is called neuromuscular junction)
- axons branch and each branch ends in an expanded tip called a synaptic terminal
or synaptic knob ( maybe as many as 1,000)
- synaptic knobs contain membrane sacs called vesicles that are filled with
molecules of a chemical messenger called a neurotransmitter
- when the action potential reaches the synaptic knob electrically gated Ca++
channels open, allowing Ca++ to enter the cell, causing the vesicles to
merge with the membrane and release the neurotransmitter molecules into
the synaptic cleft
- the synaptic knob of the presynaptic neuron is separated from the postsynaptic
neuron (or cell) by a narrow space called the synaptic cleft
- the neurotransmitter molecules diffuse across the synaptic cleft
- the postsynaptic neuron (or cell) membrane has chemically gated channels with
receptors that the neurotransmitter molecules fit into
- the postsynaptic neuron membrane is stimulated
- neurotransmitter molecules are either broken down or reabsorbed by the
presynaptic neuron (or surrounding cells) and the stimulation ends
- receptor part of chemically gated channel; channels that are opened determine if
the neurotransmitter is excitatory (depolarization, Na+ channels) or
inhibitory (hyperpolarization, K+ or Cl-- channels)
- postsynaptic neuron may have several types of receptors, each specialized for a
different neurotransmitter
- inhibitory and excitatory stimulation combined to make action potential or not
(threshold or subthreshold)
- the synaptic arrangement has two major consequences:
1. synapses work only in one direction (since the presynaptic neuron has
the neurotransmitter and the postsynaptic neuron has the receptors)
2. synapses are slow compared to the speed of the action potential
- neurotransmitters - chemicals released by presynaptic neuron, received by postsynaptic
neuron. (most are modified amino acids or short proteins)
- 100 + identified, most in central nervous system
- Acetylcholine and norepinephrine in PNS and CNS - usually excitatory
- GABA, dopamine, and serotonin in CNS - usually inhibitory
*How do synapses affect the speed and direction of nerve impulses (action potential)
through the nervous system?
*In view of the question above, what is the significance of nerve cells having very long
extensions (instead of being small round cells without extensions)?
*Make a drawing of a synapse and label the important structures described above.
*What ends the stimulation of the postsynaptic neuron?
*Explain how a drug that blocks Ca++ channels could be a depressant and how a drug that
makes membranes more permeable to Ca++ could be a stimulant.
*What is the value of having two different neurotransmitter receptors at a synapse?
* How can a neurotransmitter be excitatory in one place and inhibitory in another?
Neuronal pools - small groups of neurons that act together to perform a specific function.
- several inputs and output
- several patterns:
Fig 9.16
divergence - 1 stimulates several; allows impulse to be amplified
convergence - several stimulate 1, facilitation and summation may result
- summation at postsynaptic neuron results from combining excitatory and
inhibitory input
- resting membrane potential is -70 mvolts; inhibitory stimulus increases potential
(-80 mvolts), excitatory decreases potential
- partial depolarization to about -55 mvolts called subthreshold, no action
potential
- if membrane potential depolarizes to threshold (-55 mvolts) then action potential
begins
- all excitatory and inhibitory inputs added over surface of neuron (spatial
summation) and during brief time span of about 15 msec (temporal
summation)
- this is summation within a neuronal pool
- action potential travels all-or-none with the same speed and strength.
- neuron pools connected in complex ways to regulate and coordinate activities
within body
*Tell what a neuronal pool is and describe two types. Which can lead to summation?
*Describe the process of summation. What types of channels are involved in each?
*What is the advantage of divergence in a neuronal pool?
*What is the difference between spatial and temporal summation?