Biology 2401 Anatomy and Physiology I notes - Nervous System 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; dendrites and 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 sensory, 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
- channels can be always open “leak” cannels or they can be gated
channels that can open and close
- gated channels 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 60 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, in peripheral 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 patterns:
Fig 9.16
divergence - 1 stimulates several; allows impulse to beamplified
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 -60 mvolts called subthreshold, no action
potential
- if membrane potential depolarizes to threshold (-60 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?
Anatomy of the nervous system
the central NS - composed of brain and spinal cord
- enclosed in bony cavity - brain in cranium, spinal cord in vertebral canal.
- enclosed in 3 connective tissue membranes called the meninges
Fig 9.22
- dura mater is tough fibrous outer layer,
- arachnoid is middle layer composed of thin cells with delicate web of
collagen and elastic fibers, suspend brain and spinal cord;
subarachnoid space is filled with cerebrospinal fluid,
(the cerebrospinal fluid absorbs vibrations, shock absorber)
- pia mater is a thin inner layer that is anchored firmly to nervous tissue.
- blood-brain barrier is formed by extensions of the astrocytes - regulates what
substances leave the blood vessels into the brain.
- circumventricular organ - areas in hypothalamus where blood-brain
barrier is lacking, allows brain to “sample” blood for certain substances
*Describe three ways that the brain is protected.
*List the connective tissues enclosing the brain and spinal cord. Which acts as a shock
absorber?
*What is the importance of the circumventricular organ?
Brain - 98% of all nervous tissue in body; 100 billion neurons with 100+ trillion synapses and
900 billion glial cells
- brain is 2 % of body mass but uses 20% of energy in body
- in CNS groups of neuron cell bodies (neuronal pools) called centers, or nuclei;
surface layer of neuron cell bodies called cortex; in PNS called ganglia;
gray matter, no myelin
- in CNS bundles of axons called tracts, in PNS called nerves; white matter due
to myelin; no synapses
- brain and spinal cord form hollow tube; 4 wider areas of passageway in brain
called ventricles which are connected and continuous with central canal of
spinal cord
9.30 – 9.31
- choroid plexus in each ventricle; capillary beds that produce cerebrospinal fluid
which circulates through hollow passageway and arachnoid; resorbed into
blood in veins in dura mater of brain
- cerebrospinal fluid floats brain and spinal cord and absorbs vibrations; transports
nutrients and waste products
Brain composed of 4 major divisions:
cerebrum
Fig 9.27 – 9.29
- left and right hemisphere, separated by longitudinal fissure; control opposite side
of body
- each hemisphere divided into distinct lobes: frontal, temporal, parietal, occipital
and insula
- corpus callosum is band of white matter that connects left and right hemispheres
- cerebrum is covered by cerebral cortex (gray matter) and contains deeper centers
of gray matter (basal nuclei). White matter is myelinated axons connecting
cortex and centers and other brain regions
- cerebral cortex folded into ridges (gyri) and grooves (sulci) - increases surface
area
- 2 lateral ventricles, one in each hemisphere
- cortex areas specialize
sensory areas receive stimuli from receptors
association areas analyze and interpret sensory information - perform
higher functions
motor areas initiate skeletal muscle movements
- higher neural functions occur in the cerebrum: learning, reasoning, language,
memory, emotions
*What are the two major structures that form the cerebrum?
*What are the three primary functions of the cerebrum?
*What are the two cavities in the cerebrum called?
*What is the term for the surface gray matter of the cerebrum?
*What is the term for the deeper gray matter areas?
Diencephalon
9.32
- contains the third ventricle
- is an important link between brain stem and higher brain, beginning of
consciousness
- thalamus forms upper 2/3, receives and relays most sensory input, maintains
cerebral arousal; beginning of consciousness
- hypothalamus forms lower 1/3 - primary area for maintaining homeostasis
control center for autonomic nervous system, coordinates sympathetic and
parasympathetic nervous systems
centers for emotions and primary drives such as rage and pleasure, thirst,
hunger, sex
regulates body temperature; fluid and electrolyte balance
regulates endocrine system, linking endocrine and nervous systems
- limbic system is series of nuclei, cortical areas and tracts of cerebrum and
diencephalon - “emotional brain” - involved in determining emotional
state (fear, anger, pleasure, sorrow) and appropriate behavior
brain stem composed of midbrain, pons and medulla oblongata
- connects spinal cord and diencephalon
- composed of several nuclei and many tracts
midbrain
- superior brain stem, composed of tracts connecting higher and lower brain
- surrounds cerebral aqueduct that connects third and fourth ventricles
- centers control eye movements reflex, auditory reflex
pons
- composed of tracts that connect brain stem, cerebellum, spinal cord, cerebrum
- contains control centers for several functions: chewing, saliva secretion, some
respiration control
medulla oblongata
- continuous with spinal cord; contains fourth ventricle
- composed of tracts that connect brain with spinal cord
- contains several control centers that regulate vital functions: cardiac centers,
vasomotor, respiratory centers, swallowing, coughing, sneezing, vomiting
reticular formation is network of centers and tracts throughout brain stem, reticular activating
system - filters incoming stimuli and stimulates cerebrum to alert awareness
cerebellum
- composed of white matter tracts (arbor vitae) and gray matter cortex, connected
to brain stem via tracts
- rapidly adjust muscle movements to maintain position and balance (equilibrium)
- coordinates and fine-tunes muscle movements (does not initiate movements)
Spinal cord
Fig 9.22 - 9.25
- hollow tube (central canal) with a shallow posterior medial sulcus and deeper
anterior median fissure
- extends from the brain to the lumbar region (Li-L2)
- 31 pairs of spinal nerves branch along its length, emerge through intervertebral
foramen between vertebra
- each spinal nerve has 2 roots
- a dorsal root that contains the axons of sensory nerves and a dorsal root ganglion
that contains the cell bodies of the sensory neurons.
- a ventral root that contains the axons of motor neurons (the cell bodies of
the motor neurons are in the spinal cord)
- the dorsal and ventral root merge to form the spinal nerve before it exits the
intervertebral canal
- the spinal cord ends at lumbar vertebra 2, but spinal nerves continue through the
vertebral canal through the sacrum; cauda equina is term for this extension.
- the spinal cord is composed of gray matter (cell bodies and dendrites of neurons)
and white matter (myelin covered axons).
- gray matter composes the central “butterfly-shaped” areas (posterior, lateral and
anterior gray horns and gray commissure)
- reflex arc occur here, spinal reflexes
- white matter surrounds the gray matter and is made of ascending and descending
tracts (posterior, lateral and anterior white columns)
- ascending tract carry sensory impulses to brain
- descending tracts carry impulses from brain to muscles and glands
- cervical and lumbar enlargements are areas that are slightly thicker because
more cells bodies are located here to supply nerves for the limbs.
*What parts of the neurons form gray matter? What part forms white matter?
*What type neurons are in the dorsal roots of the spinal nerves? ventral roots?
*What causes the cervical and lumbar enlargements?
Anatomy of the nervous system
peripheral nervous system - composed of sensory receptors, sensory nerves and motor nerves
nerves - groups of neuron axons wrapped in dense fibrous connective tissue
Fig.9.17
- endoneurium enclosing each neuron (fiber), perineurium enclosing groups of
neurons called a fascicle and epineurium enclosing a nerve
- each neuron is either a sensory afferent neuron that carries action potential
towards cns or motor efferent neuron that carries action potential away
from cns (only one or the other, not both)
- nerves can be sensory (containing only sensory neurons), motor (containing only
motor neurons) or mixed (containing sensory and motor neurons)
*What is the difference between a mixed nerve and a sensory nerve?
*What is the difference between a neuron and a nerve?
*Describe the fibrous connective tissue wrappings in a nerve.
Nerve pathways
Figs 9.17 and 9.18
- reflexes are simplest pathways, simplest integration; fast, predictable (particular stimulus
causes particular response) and automatic
- can involve only spinal cord (spinal reflex) or only brain (cranial reflex)
- more complex integration can involve many neuronal pools
- relays action potential from sensory cells to brain and spinal cord (sensory), and
cranial nerves - emerge from the brain; there are 12 pairs (left and right); some are
sensory (contain only sensory neurons), some are motor (contain only motor
neurons) and some are mixed (contain both sensory and motor neurons)
spinal nerves - emerge from the spinal cord and pass through the intervertebral foramen
- there are 31 pairs, each passing between adjacent vertebra
- each spinal nerve has a dorsal root where sensory neurons enter the spinal cord
and a ventral root where motor neurons exit the spinal cord
- these merge together before the nerve exits the intervertebral foramen, all spinal
nerves are mixed (both sensory and motor)
- spinal nerves named for the vertebral region (cervical 1-8, thoracic 1-12, lumbar
1-5, sacral 1-5, coccygeal 1)
- sensory neuron cell bodies are in the dorsal root ganglia; motor neuron cell
bodies are in the anterior gray horn
- spinal nerves form plexuses, networks of nerves that are bundled together. These
are axons only so no synapses occur here. Nerves then leave plexus to
body region
*What structures compose the peripheral nervous system?
*What is the difference between cranial nerves and spinal nerves?
*What are plexuses? *Why can integration (summation) not occur in white matter?
Somatic and autonomic nervous systems
- the motor (efferent) portion of the peripheral ns is composed of 2 systems, the somatic and the
autonomic ns.
- the autonomic ns is divided into the sympathetic and the parasympathetic systems
- the somatic n.s. always has only one neuron between the brain or spinal cord and the
effector cell
- the effector cells are always skeletal muscle cells
- these neurons use only 1 neurotransmitter (acetylcholine) and it is always excitatory
- the somatic (soma means body) ns allows the body to respond to external environment
- the autonomic (means self-governing) ns always has 2 neurons between the brain or spinal cord
and the effector cell.
- the preganglionic neuron has its cell body in the brain or spinal nerve and its synaptic
terminal in a ganglion
- the postganglionic neuron has its cell body in the ganglion and its synaptic terminal at the
effector cell
- the effector cells are smooth muscle, cardiac muscle, glands
- preganglionic neurons release only 1 neurotransmitter (acetylcholine) and it is always
excitatory
- postganglionic neuron release 1 of two different neurotransmitters (acetylcholine or
norepinephrine) which may be excitatory or inhibitory
- the autonomic ns is composed of the parasympathetic and the sympathetic divisions
- in the sympathetic ns the ganglia are closer to the spinal cord and the postganglionic
neurons diverge to several effectors
- in the parasympathetic ns the ganglia are in, or very near, the specific organ where the
effector cells are located
- in the parasympathetic ns the postganglionic neuron releases acetylcholine which may have an
excitatory or inhibitory affect, depending on the receptor
- in the sympathetic ns the postganglionic neuron releases norepinephrine which may have an
excitatory (usually) or an inhibitory affect
- the parasympathetic and the sympathetic ns innervate many of the same visceral organs
(some exceptions: adrenal gland, sweat glands only sympathetic.)
- the sympathetic ns prepares the body for an emergency, the “fight or flight response”
- the parasympathetic ns slows the body into an energy producing and conserving response
- sympathetic neuron from spinal nerves T1-L2, parasympathetic neurons from cranial nerves
and S2-4
*What is the value of having dual innervation to many organs?
*List several effects of the sympathetic and parasympathetic ns on the body. Table 9.7
*Compare the number of neurons between the brain or spinal cord and the effector cells
in the somatic, sympathetic and parasympathetic ns.
*List the type effector cells for each of the three.