BIOB30 Ch8 (247-258).docx

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BIOB30 Chapter 8 (247-258)
Neurons: Cellular and Network Properties
1
The brain and spinal cord are integrating centers for homeostasis, movement
and many other body functions
Nervous system a network of billions or trillions of nerve cells linked
together in a highly organized manner to form the rapid control system of
the body
o Controlled by the brain and spinal cord
Neurons uniquely shaped cells that carry electrical signals rapidly and,
sometimes, over long distances
o Also called nerve cells
o Processes long thin extensions possessed by some neurons
Can extend up to a meter in length
Neurotransmitters chemical signals released into the extracellular fluid by
most neurons
o Alternative: Gap functions links between neurons that allow
electrical signals to pass directly from cell to cell
Single-celled protozoa and plants also use electrical signaling mechanisms
similarly
Emergent properties complex processes that cannot be predicted from
what we know about the properties of individual nerve cells
o Nervous system examples: consciousness, intelligence, emotion
Organization of the Nervous System
The nervous system has two parts
o Central nervous system (CNS) the brain and the spinal cord
The integrating center for neural reflexes
o Peripheral nervous system (PNS) afferent (or sensory) neurons and
efferent neurons
Sensory receptors monitor conditions in internal and external environments
o Information is sent along afferent neurons to the CNS
o The CNS integrates this information and determines response
o CNS uses efferent neurons to signal a response
Targets of efferent neurons are usually muscles and glands
Efferent neurons can be subdivided
o Somatic motor division controls skeletal muscles
Motor neuron is a clinical synonym but is sometimes used to
describe all efferent neurons
o Autonomic division controls smooth and cardiac muscles, exocrine
glands, some endocrine glands, and some types of adipose tissue
Synonym: visceral nervous system (because it controls
contraction and secretion in various internal organs or viscera)
Autonomic neurons are divided into
o Sympathetic and parasympathetic branches
o The difference is the anatomical organization and the chemicals used
o Many internal organs have both types (they exert antagonistic control
over a single target))
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2
Enteric nervous system a network of neurons in the walls of the digestive
tract
o Third division of the NS
o Usually controlled by the autonomic division of the NS but can
function autonomously as its own integrating center
The CNS has processes with no input or output from the PNS
o It can initiate activity without sensory input or measurable output
E.g. thinking, dreaming
Cells of the Nervous System
NS has two main types of cells
o Neurons the basic signaling units of the NS
o Glial cells support cells
Neurons Carry Electric Signals
Functional unit the smallest structure that can carry out the functions of a
system
o A neuron is the functional unit of the NS
Neurons usually have long processes extending form the cell body
o Dendrites receive incoming signals
o Axons carry outgoing information
Can be classified structurally or functionally
o Structurally: classified by the number of processes that originate from
the cell body
Pseudounipolar axon and dendrites fuse during development
to create one long process
Bipolar a single axon and dendrite
Multipolar many dendrites and branched axons
Anaxonic lacking an identifiable axon
o Functionally
Sensory (afferent) neurons, interneuron’s, and efferent
(somatic motor and autonomic) neurons
Sensory neurons carry information about temperature, pressure, light, and
other stimuli from sensory receptors to the CNS
o Peripheral sensory neurons have cell bodies near the CNS with long
processes extending to the limbs
o Sensory neurons close to the CNS (smell, vision) have two long
processes so that the signal passes from the dendrite, through the cell
body and to the axon
Interneurons neurons that lie entirely within the CNS (interconnecting
neurons)
o Have complex branching processes (for communication with other
neurons)
Efferent neurons: motor and autonomic
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BIOB30 Chapter 8 (247-258)
Neurons: Cellular and Network Properties
3
o Autonomic
Varicosities enlarged regions along the axon that store and
release neurotransmitter (not all neurons have it)
Nerves cordlike fibers made from long axons of afferent and efferent
peripheral neurons bundled together with connective tissue
o Extend from CNS to targets of the component neurons
o Sensory nerves carry afferent signals only
o Motor nerves carry efferent signals only
o Mixed nerves carry signals in both directions
The Cell Body Is the Control Center of the Neuron
Cell body (or soma) contains a nucleus and all organelles needed to direct
cellular activity
o The cytoskeleton extends into the axon and dendrites
o It is usually small (1/10th of the cell volume)
Innervated controlled by a neuron
An innervated muscle is paralyzed permanently when the axon of the
attached motor neuron is severed
o If it’s a sensory neuron, there is a loss of sensation
Dendrites Receiving Incoming Signals
Dendrites thin, branched processes that receive incoming information
from neighboring cells
o Increases cell surface area allowing for more communication
Dendritic spines expand a dendrite’s surface area
o Different shapes (e.g. spikes, mushroom shaped knobs)
PNS dendrites
o The primary function is to receive incoming information and transfer
it to an integrating region within the neuron
CNS dendrites
o Dendritic spines can function as independent compartments sending
signals back and forth with other neurons in the brain
Many dendritic spines contain polyribosomes and can make
their own proteins
o Dendritic spines can change their size and shape in response to input
from neighboring cells
Changes in spine morphology are associated with learning and
memory; various pathologies (e.g. genetic disorders and
degenerative diseases)
Axons Carry Outgoing Signals to the Target
Axon hllock a specialized region of the cell body where axons on most
peripheral neurons originate from
Length varies from a meter to a few micrometers
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