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Neuroscience Book Notes Chapter I

9/29/16 3:18 PM

Genetics
Gene: sequence of DNA nucleotides A, T, C, G
Exons: segments in a gene that are transcribed into messenger RNA, then
into a chain of amino acids that specifies for a protein
Introns: removed from final gene transcript but influence how genes are
expressed
Cells of the Nervous System
Two categories: (1) neurons or nerve cells and (2) neuroglia
Nerve cells: electrical signaling over long distances
Neuroglia or Glial Cells: not capable of significant electrical signaling;
however they have essential functions in the developing and adult brain
Neurons
Dendrites: primary target for synaptic input from other neurons, they have
a high ribosome content as well as specific cytoskeletal proteins
• Some cells lack dendrites all together while some have a complex
branched system of them
• Nerves with a large number of dendrites can be innervated bya
large number of other neurons
• Convergence: the number of inputs to a single neuron
• Divergence: the number of targets innervated by any one neuron
The presynaptic terminal is immediately adjacent to the post synaptic
specialization of the target cell
Synaptic cleft: space between the pre and post synaptic elements, site of
extracellular proteins that influence diffusion, binding, and degradation of
molecules secreted by the pre synaptic
Axon: where information conveyed by synapses is integrated and read out,
portion of the nerve that is specialized for relaying electrical signals
• Has distinct cytoskeletal elements for functional integrity
• Short axons are features of local circuit neurons or interneurons
• Long axons are features of projection neurons that extend to
distant targets

•

Action potentials carry signals over long distances in a self
regenerating wave of electrical activity
o Initiated at axon hillock to axon terminus

Synaptic transmission is the chemical and electrical process by which info
is encoded by AP’s and passed on at the synaptic contents to the next cell
• Presynaptic terminals (aka synaptic endings, axon terminals, or
terminal boutons) and their post synaptics specializations are
usually chemical synapses (most abundant synapse in NS)
• Electrical Synapses are rare and facilitated by gap junctions
• Synaptic vesicles are the secretory organelle of the chemical
synapse… they are filled with neurotransmitters
o Proteins regulate the positioning of these vesicles and their
fusion to initiate release
o Neurotransmitters bind to neurotransmitter receptors of the
post synaptic cell
Neuroglial Cells of Glial Cells
More numerous than neurons in the brain (3:1) ratio
Roles à
•
•
•
•
•

maintaining ionic environment of nerve cells
modulating the rate of nerve signal propagation
modulating synaptic action by controlling the uptake of
neurotransmitters @ or near the synaptic cleft
providing a scaffold for some aspects of neural development
aiding recovery (or impeding) from neural injury

Three Types of Glial Cells:
• Astrocytes, Oligodendrocytes, Microglial Cells
Astrocytes
• restricted to the CNS, star like appearance
• major job is to maintain (in a number of ways) an appropriate
chemical environment for neuronal signaling
• Form the blood brain barrier

Oligodendrocytes
• restricted to CNS
• lay down a lipid rich myelin sheath around some but not all axons
• In PNS, these are Shwann cells
Microglial Cells
• Derived primarily from the hematpoietic precursor cells
• Share many properties with macrophages
• Scavenger cells that remove ceullular debris from sites of injury or
normal cell turnover
• Secrete signaling molecules that can modulate local inflammation of
cell survival/death
Neural circuits
• Neurons cant function alone for they are organized into neural
circuits providing the foundation of sensation, perception, and
behavior
• Neurophil is region between nerve cell bodies where most synaptic
connectivity occurs
• Afferent neurons carry information TOWARDS the brain
• Efferent neurons carry information AWAY from the brain
• Interneurons only participate in local aspects of a circuit based on
short distances which their axons extend
à example of neural circuit
•
•

Myotatic spinal reflex (knee jerk)
(1) hammer tap stretches tendon which stretches sensory receptors
in leg extensor muscle à (2A) sensory neuron (afferent) synapses
with and excites motor neuron (efferent) in spinal cord (2B)
sensory neuron excites spinal interneuron (2C) Interneuron synapse
inhibits motor neuron to flexor muscle à (3A) motor neuron
conducts AP to synapses on extensor muscle fibers causing
contraction (3B) Flexor muscle relaxes because the activity of its
motor neurons have been inhibited (4) Leg extends

To measure the electrical activity of a nerve cell you can use:
extracellular recording (electrode placed near the nerve cell of interest)
• detect Aps, temporal patterns of AP activity

intracellular recording (electrode placed inside the cell of interest)
• smaller graded potentials so more detailed communication in a
circuit
• graded potentials can arise at either sensory receptors or synapses
and care called receptor potentials or synaptic potentials
* Using these recordings we can see that the sensory neurons created a high
frequency of APs in both extensor and flexor motor neurons; however the
interneuron causes the APs to decline in the flexor motor neuron

The Organization of the Human Nervous System
Neural systems are systems that process similar types of information
Sensory Systems acquire and process information from the environment
Motor systems respond to such information by generating movements and
other behaviors
Associational systems are systems in between
In the cortex there are sheet like arrays of nerve cells
Axons in the CNS are divided into tracts
• Tracts that cross the midline of the brain are commissures
o Gray matter à accumulation of cell bodies
o White matter à axon tracts and commissures
Peripheral motor neurons in autonomic ganglia innervate smooth muscle,
glands, and cardiac muscle controlling voluntary movement

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