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Ch. 8 The Nervous System (8.1 & 8.2) (8.2 Neurons & Neuroglia…
Ch. 8 The Nervous System (8.1 & 8.2)
8.1 Anatomical Divisions
2 major anatomical divisions
Central Nervous System (CNS)
= brain & spinal cord
Peripheral Nervous System (PNS)
= nerves that extend from the brain (cranial nerves) and spinal cord (spinal nerves)
2 divisions of PNS
Sensory ( Afferent ) division picks up sensory stimuli
Motor ( Efferent ) division sends directions from the brain to the muscles and glands.
This division is further broken down into:
Somatic
rules skeletal muscles
Autonomic
controls involuntary
This division is further broken down still:
Sympathetic
mobilizes the body into action
Parasympathetic
calms body down
3 General functions
motor input (PNS)
motor impulses are carried from CNS to responsive body parts called effectors;
a motor impulse is carried on a motor neuron;
Involves the response of a body part;
Effectors= 2 types:
Glands ( that secrete a hormone )
Muscles ( that contract )
sensory input (PNS)
Once stimulated, sensory rec. transmit a sensory impulse to CNS
Sensory impulse is carried on sensory neuron
Sensory receptors (located at the ends of peripheral neurons) detect changes occurring in their surroundings;
integration (CNS)
A motor impulse begins
integration occurs in interneuron
Involves interpretation of an incoming sensory impulse decision is made concerning what's going to happen next, based on sensory impulse
8.2 Neurons & Neuroglia
Neuron = the structural & functional unit of the nervous system; a nerve cell
Neuron structure
Each neuron in composed of a cell body and many extensions from the cell body called neuron processes or nerve fibers
cell body = central portion of neuron; contains usual organelles, except centrioles; identify; nucleus, prominent nucleolus, & many Nissl bodies = RER
Neuron processes / Nerve Fibers = extensions from cell body ; two types:
Dendrites
Many per neuron, short and branched, receptive portion of a neuron, & carry impulses toward cell body
Axons
one per neuron, long, thin process, carry impulses away from cell body, and note terminations of axon branch = axonal terminals; synaptic knobs
Axons in PNS (sensory and motor)
Large axons are surrounded by a myelin sheath produced by many layers of Schwann Cells (neuroglia cell)
myelin = lipoprotein
interruptions in the myelin sheath between Schawnn cells = Nodes of Ranvier
"myelininated nerve fibers"
Small axons do not have a myelin sheath
"unmyelinated nerve fibers"
however all axons in PNS are associated with Schwann cells
Axons in CNS (brain & spinal cord)
Myelin is produced by an oligodendrocyte rather than Schwann cells
A bundle of myelinated nerve fibers= white matter
PNS = nerve
CNS = tract
Gray matter
PNS = ganglia
CNS = neucleous
Regeneration of Nerve Fibers
Cell body injury = death of neuron; damage to an axon may allow for regeneration
Neuroglia Cells = accessory cells of nervous system form supporting network for neurons; "nerve glue"
CNS = 4 types; provide bulk of brain and spinal cord tissue:
Astrocyte
star-shaped
Function: nourishes neuron
Microglia
looks like spider
Function: phagocytes
Ependymall cells
epithlial-like layer
Function: lines spaces in CNS
brain= ventricles
Spinal cord= central canal
Oligodendrocyte
looks like eyeball
Function: produces myelin
PNS = schwann cells produces myelin and satellite cells nourish neurons
Classification of neurons : Functional classification : sensory neurons & Structural classification
afferent neurons
carry sensory impulses from sensory receptors to CNS
Input information to CNS
PNS
Location of receptors= skin & sense organs
Somatic receptors
External receptors ( touch, pressure, temp, sight, smell, hearing)
proprioceptors (position & movement)
Visceral receptors (digestive, respiratory, cardiovascular, urinary, reproductive, taste, deep pressure and pain
Interneurons - CNS, link other neurons together
motor neuron- PNS, efferent neurons, carry motor impulses away from CNS and to effectors, output info from CNS
effectors= muscles & glands
somatic
Visceral
Structural c.
Bipolar N.
2 extensions, one fused dendrite toward, one axon away
Unipolar N.
most PNS, one process from cell body, forms central and peripheral processes, only distal ends are dendrites
Multipolar Neuron
most common in CNS, many extensions, many dendrites lead toward cell body , one axon leads away
Ch. 8 The Nervous System (8.3-8.6)
8.4
Structure of a synapse
site where a neuron communicates with another cell- is composed of presynaptic and postsynaptic cells whose plasma membranes are separated by a narrow synaptic cleft
Synaptic function & neurotransmitters
convergence permits both conscious and subconscious control of the same motor neurons
Blocking calcium channels at the presynaptic terminal of a cholinergic synapse would prevent the influx of calcium that triggers the release of acetylcholine, so no communication
8.3
steps in generation of action potentials
2-activation of voltage-gated sodium channels and rapid depolarization
3- inactivation of sodium channels & potassium channels
1- depolarization to threshold
4- return to normal
If the voltage-gated sodium channels in a neuron's plasma membrane were blocked, the neuron would not be able depolarize because sodium ions could not flow into the cell
If the extracellular potassium concentration were to decrease, more potassium ions would leave the cell , increasing the electrical gradient across the membrane
8.6
You would find polio virus in the anterior gray horns of the spinal cord, because that is where the cell bodies of somatic motor neurons are located
All spinal nerves are classified as mixed nerves because they contain both sensory and motor fibers
Damage to the ventral root of a spinal nerve, which is composed of visceral and somatic motor fibers, would interfere with motor function
8.5
3 types of meninges surrounding CNS
Arachnoid
subarachnoid space contains a delicate network of collagen & elastin fibers through which cerebrospinal fluid circulates
subdural space separtes inner surafce
Pia mater
found in blood vessels servicing the spinal cord
underlying neural tissue
Dura mater
outermost covering
2 fibrous layers
8.4-8.6
blood vessels servicing the spinal cord are found in pia mater
dural sinuses are located in the dura mater
Specialized membranes that protect the spinal cord are termed spinal meninges
Receptor, sensory neuron, inter neuron, motor neuron, and effector
Adrenergic synapses release the neurotransmitter, norephinephrine
Subarachnoid space contains network of collagen..
Calcium needed to imitate the release of acetylcholine into the synaptic cleft
Diffusion across the arachnoid villi returns excess CSF to venous circulation
Chemical synapse= nervous system
Projections of gray natter toward the outer surface of the spinal cord are horns
Neurotransmitters are released from synaptic terminals/knobs
white matter of the spinal cord contains bundles of axons
Synaptic knobs occur at the ends of axons
COMMISSURES
Masses of myelinated nerve fibers appear white
Gray matter = interneurons
31 segments control limbs
Posterior horns of the spinal cord contain sensory nuclei