Biology 120 Presentation - Chapter 12 - Nervous system

1. Nervous System

2. The nervous system and the endocrine system together
integrate one communication function of the human body
The nervous system is subdivided in a variety of ways
according to its structure, the direction of information flow,
and the control of effectors

4. Central nervous system
• Structural and functional center of the entire nervous system
• Consists of the brain and spinal cord
• Integrates incoming information from the senses, evaluates the
information, and initiates an outgoing response

5. Peripheral nervous system
• Consists of the nerve tissues that lie on the periphery or
regions outside the CNS
• Cranial nerves – nerves that originate from the brain (or through the
skull)
• Spinal nerves – nerves that originate from the spinal cord

6. A-fferent division – consists of all
incoming sensory or afferent
pathways
E-fferent division – consists of all
outgoing motor or efferent pathways

7. Somatic nervous system – carry information to the
somatic effectors (skeletal muscles)
• Somatic motor division – efferent pathways
• Somatic sensory division – afferent pathways
• Integrating centers – receive the sensory
information and generate the efferent response
signal

8. Autonomic nervous system – carry information to the
autonomic (visceral) effectors
• Sympathetic division – pathways that exit from the middle
portions of the spinal cord; involved in preparing the body to deal
with immediate threats: the fight-or-flight response
• Para-sympathetic division – exit from the brain or lower
portions of the spinal cord and coordinate the body’s normal
resting activities; “rest-and-repair” division

9. Neurons consist of a cell body, one or more dendrites, and one axon
• Distal ends of dendrites of sensory neurons are called receptors
because they receive the stimuli that initiate nerve signals
• Axon hillock – tapered portion of the cell body; “decides” whether to
send the impulse any farther in the neuron
• Axons with larger diameters conduct nervous impulses faster than
those with smaller diameters
• Synaptic knobs – release neuro-transmitters

12. Classification of neurons – three distinct structural types of
neurons
• Multi-polar neurons – have only one axon but several dendrites
• Bi-polar neurons – have only one axon and also only one highly
branched dendrite
• Uni-polar neurons – sensory neurons with a single process extending
from the cell body

14. Classification of neurons – according to the direction in
which they conduct impulses
• A-fferent (sensory) neurons – transmit nerve
impulses to the spinal cord or brain
• E-fferent (motor) neurons – transmit nerve impulses
away from the brain or spinal cord to or toward
muscles or glands
• Inter-neurons – conduct impulses from afferent
neurons to or toward motor neurons

16. Reflex arc – automatic signal conduction route to and
from the CNS
• Most common form of reflex arc is the three-
neuron arc; consists of an:
Afferent neuron
Interneuron
Efferent neuron
Synapse – junction between the synaptic knobs of one
neuron and the dendrites (or cell body) of another
neuron

17. Neur/o-gli/o - al(Neuroglial)– pertaining to the support cells, glial cells, of nerves.
Five basic types:
• Astro-cytes – largest and most numerous type of glia; help form the blood-brain
barrier (BBB)
• Microglia – serve a protective function when the brain is under attack by
microorganisms
• Ependymal cells – produce the fluid that fills the cavities in the brain and spinal cord
• Oligo-dendro-cytes – help hold nerve fibers together and also produce the vitally important
myelin sheath in CNS
• Schwann cells – support nerve fibers in the PNS and sometimes form a myelin sheath around
them

20. Neur/o – al – (Neural) – Pertaining to nerves.
Nerves - bundles of peripheral nerve fibers (axons) held together by
several layers of connective tissues
• Endo-neurium – delicate layer of fibrous connective tissue
surrounding each nerve fiber
• Peri-neurium – connective tissue layer surrounding each bundle
of nerve fibers (fascicles)
• Epi-neurium – fibrous coat surrounding numerous fascicles

21. Tracts – bundles of nerve fibers within the CNS
White matter – bundles of myelinated fibers
Gray matter – cell bodies and unmyelinated fibers
Nucleus – distinct regions of gray matter within the
CNS
Ganglia – distinct regions of gray matter within the PNS

22. Mixed nerves – carry both sensory (afferent) and
motor (efferent) fibers
Sensory nerves – contain mostly afferent fibers
Motor nerves – contain mostly efferent fibers
Nerve fibers can sometimes be repaired if the damage is
not extensive

23. Nerve impulse – wave of electrical energy that travels along the
plasma membrane of the nerve
Cell membrane potential – difference in electrical charge
across their plasma membranes
• Resting membrane potential (RMP) – mechanism that maintains the
potential voltage; when the neuron is not in an excited state, we say
that the neuron is “at rest”; typically –70 mV
• The slight excess of positive ions on the outer surface is produced by ion
transport mechanisms and the membrane’s permeability
characteristics
• The membrane’s selective permeability characteristics help maintain a
slight excess of positive ions on the outer surface of the membrane

26. Synapse – place where signals are
transmitted from one neuron,
called the pre-synaptic neuron, to
another neuron, called the
post-synaptic neuron

27. Types of synapses
• Electrical synapse – where two cells are joined end-to-end by gap junctions; as a result, an
action potential simply continues along the postsynaptic plasma membrane as if it belonged to
the same cell
• Chemical synapses – use a chemical neuro-transmitter to send the message to the postsynaptic
cell:
• Synaptic knob – contains many small sacs (vesicles) filled with neurotransmitter
molecules
• Synaptic cleft – fluid-filled space (about one millionth of an inch in width) between a
synaptic knob and the plasma membrane of a postsynaptic neuron
• Post-synaptic neuron – has protein molecules embedded in it, each facing toward the
synaptic knob and its vesicles

29. Neuro-transmitters – means by which neurons “talk” to
one another; more than 50 compounds are now
known to be neurotransmitters
Neuro-transmitters are commonly classified by their
function or by their chemical structure
• Excitatory neurotransmitters
• Inhibitory neurotransmitters

36. •Excitation – occurs when a stimulus
triggers the opening of stimulus-gated
Na+ channels
•Inhibition – stimulus triggers the
opening of stimulus-gated K+ channels

37. Action potential – an electrical signal that travels along the surface
of a neuron’s plasma membrane
• When an adequate stimulus is applied to a neuron, the Na+ channels
open at the point of stimulation. Na+ diffuses rapidly into the cell at
the site of this local depolarization
• If the magnitude of the local depolarization exceeds a limit called the
threshold potential (about –59 mV), then additional Na+ channels are
opened
• As more Na+ rushes into the cell, the membrane moves rapidly
toward 0 mV and then continues in a positive direction to a peak of
+30 mV
• The action potential is an all-or-none response
• Once the peak of the action potential is reached, the membrane
potential begins to move back toward the resting potential of –70 mV
in a process called repolarization
• Because the K+ channels often remain open as the membrane reaches
its resting potential, too much K+ may rush out of the cell;
hyperpolarization

38. Absolute refractory period – very
brief period when a local area of an
axon’s membrane resists
restimulation

39. Conduction of the action potential
• The action potential never moves backward,
restimulating the region from which it just came
• In myelinated fibers, the insulating properties of the
thick myelin sheath resist ion movement and the
resulting flow of current
• The rate at which a nerve fiber conducts an impulse
depends on its diameter and also on the presence
or absence of a myelin sheath

41. Severe psychic depression occurs when a
deficit of norepinephrine, dopamine,
serotonin, and other amines exists in certain
brain synapses
• Anti-depressant drugs – some of these
inactivate dopamine and serotonin; others
called SSRIs (serotonin-specific reuptake
inhibitors) produce antidepressant effects by
inhibiting the uptake of serotonin

42. Cocaine – produces a temporary feeling
of well-being in cocaine abusers by blocking
the uptake of dopamine
Anesthetics – produce their effects by
inhibiting the opening of sodium channels in
the nerve cell membrane, thus blocking the
initiation and conduction of nerve impulses