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Biology 55
Nervous System
FUNCTIONS OF THE NERVOUS SYSTEM
sensory function – senses stimuli (changes in internal or external environment)
integration function – processes sensory inputs and decides on appropriate
responses
motor function – sends signals to effectors, which respond to the stimuli
Neurons – functional cells of nervous system, receive and send electric signals
cell body – contains the nucleus and cellular organelles
dendrites – short, branched receiving portion of neuron
axon – single, long sending portion of neuron
synapse – site where neuron communicates with another cell
releases a chemical neurotransmitter (eg. acetylcholine)
sensory neuron – axon sends signals to the CNS
motor neuron – axon sends signals away from the CNS
interneurons – neurons in CNS that integrate sensory and motor signals
neuroglia – nervous cells that help neurons perform their function
astrocytes – form a blood-brain barrier that protects neurons
Schwann cells & oligodendrocytes – form a myelin sheath around axons
helps axons transmit signals faster
DIVISIONS OF THE NERVOUS SYSTEM
Central Nervous System (CNS) – brain and spinal cord
main integration center
white matter – contains many myelinated axons sending signals
gray matter – contains many cell bodies integrating information
Peripheral Nervous System (PNS) – all nervous tissue outside the CNS
nerves – bundles of axons sending electrical signals
cranial nerves (12 pairs) – arise from the brain
spinal nerves (31 pairs) – arise from the spinal cord
ganglia – small clusters of neuron cell bodies outside the CNS
sensory receptors – detect changes in internal or external environment
Functional Divisions of the PNS
somatic nervous system (SNS) – voluntary
controls skeletal muscle
autonomic nervous system (ANS) – involuntary
controls cardiac muscle, smooth muscle, glands
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NEURON PHYSIOLOGY – Production of Electrical Impulses
resting membrane potential – cell membrane is polarized when at rest
(inside of cell is negative, outside is positive)
action potential – flow of charged particles (electric current) when neuron is
stimulated
depolarization – stimulation of neuron opens protein channels that let
positive ions into cell (inside becomes positively charged)
repolarization – inside of cell becomes negative again
returns to resting membrane potential
conduction of the action potential – an action potential starts at the beginning
of the axon and, once started travels to the end of the axon (axon terminal)
refractory period – after action potential begins the cell can't produce
another for a brief period of time
ensures one-way conduction of the action potential
saltatory conduction – faster, jumping conduction in myelinated axons
COMMUNICATION AT THE SYNAPSE
synapse – site of communication between a neuron and another cell
neuromuscular junction – synapse between neuron and muscle fiber
neuroglandular junction – synapse between neuron and gland
most synapses are between one neuron and another neuron
Synapses Between Neurons
presynaptic neuron – sending neuron (axon terminal)
postsynaptic neuron – receiving neuron (dendrite or cell body)
synaptic cleft – small space between 2 communicating neurons
an action potential in the presynaptic neuron triggers release of a chemical
neurotransmitter – chemical released by presynaptic neuron that binds to
protein receptors in postsynaptic cell's membrane
excitatory neurotransmitter – stimulates postsynaptic neuron to produce
an action potential of its own
inhibitory neurotransmitter – makes it less likely that the postsynaptic
neuron will produce its own action potential
summation of all of the excitatory and inhibitory synapses determines whether the
postsynaptic neuron produces an action potential
How Drugs and Toxins Modify Nervous System Function
stimulate or inhibit neurotransmitter synthesis
stimulate or inhibit neurotransmitter release
block or activate neurotransmitter receptors
agonists activate receptors (mimic neurotransmitter)
antagonists block receptors
stimulate or inhibit neurotransmitter removal
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THE CENTRAL NERVOUS SYSTEM
Meninges – 3 connective tissue membranes around brain & spinal cord
pia mater – thin inner membrane on surface of brain & spinal cord
contains blood vessels which supply the brain and spinal cord
arachnoid mater – middle, web-like membrane connected to pia mater
subarachnoid space – space between arachnoid and pia that
contains cerebrospinal fluid (CSF)
cushions brain and spinal cord
dura mater – outer, tough double membrane of fibrous connective tissue
splits in some regions around the brain to form blood-filled
dural venous sinuses
epidural space – space between dura mater and vertebrae filled
with adipose tissue that cushions spinal cord
site for anesthetic injections
Cerebrospinal Fluid (CSF) – clear fluid which circulates through cavities in
brain, spinal cord, and in subarachnoid space
ventricles – 4 cavities in brain where CSF is produced by filtering blood
ependymal cells lining ventricles regulate content of CSF
CSF circulates to central canal of spinal cord and the subarachnoid space
returns to blood in the dural venous sinuse
hydrocephalus – excess accumulation of CSF resulting in pressure
THE BRAIN – has 4 main divisions; brainstem, diencephalon, cerebellum, cerebrum
1) Brainstem – connects to the spinal cord
sensory and motor axon tracts pass through these regions
Medulla oblongata – inferior brainstem
cardiovascular center – regulates heart and blood vessels
respiratory center – controls muscles for breathing
reflex centers for coughing, sneezing, swallowing, vomiting
Pons – superior to medulla
regulates rate of breathing
Midbrain – superior brainstem
contains reflex centers for vision and hearing
2) Diencephalon – between brainstem and cerebrum
thalamus – 80% of diencephalon
relay station for most sensory impulses traveling to the cerebrum
hypothalamus – found below thalamus
link between the nervous and endocrine systems
regulates the autonomic nervous system
produces hormones
regulates eating and drinking – thirst center, feeding center
regulates body temperature
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3) Cerebellum – attached to dorsal brainstem
coordinates skeletal muscle movements
receives voluntary motor impulses from cerebrum
receives sensory impulses related to body position and balance
compares intended movements with movements actually occurring
4) Cerebrum – largest, most superior portion of brain
origin of voluntary actions, site of conscious perceptions, center of intellect
longitudinal fissure – deep groove that divides cerebrum into 2 hemispheres
corpus callosum – axon tracts connecting the 2 cerebral hemispheres
Cerebral Cortex – outer gray matter
divided into frontal, parietal, temporal, and occipital lobes by sulci
contains neuron cell bodies that integrate all conscious functions
Sensory areas – posterior cerebrum
primary somatosensory area – receives sensations of pain, touch,
temperature from opposite side of the body (parietal lobe)
visual area – receives visual sensations (occipital lobe)
Motor areas – frontal lobe
primary motor area – controls movements of skeletal muscles
on the opposite side of the body
Association areas – located within or near motor and sensory areas
allow recognition of sensations
control complex, learned motor skills
Processing centers – integrate information from other areas and perform
complex function such as analysis, reasoning, comprehension of
spoken or written language, creativity, etc.
Limbic system – neural pathways in cerebrum that link to the diencephalon and
control emotions and function in learning and forming memories
SPINAL CORD – found within the vertebral canal and surrounded by meninges
central gray matter integrates spinal reflexes
peripheral white matter contains axon tracts
sensory (ascending) tracts carry impulses to the brain
motor (descending) tracts carry impulses from the brain
central canal – cavity in center containing cerebrospinal fluid
PERIPHERAL NERVOUS SYSTEM
spinal nerves – 31 pairs emerging from the spinal cord
detect sensations and control functions of trunk and limbs
mixed nerves – carry sensory and motor axons
dorsal nerve root – sensory axons running to spinal cord
dorsal root ganglion – contains cell bodies of sensory neurons
ventral nerve root – motor axons running from spinal cord
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cranial nerves – 12 pairs emerging from brainstem
mainly detect sensations and control functions of head and neck
(vagus nerve controls functions of thoracic and abdominal organs)
may be mixed nerves or only sensory or motor
SOMATIC VS AUTONOMIC NERVOUS SYSTEM
Somatic Nervous System – controls movements of skeletal muscles
voluntary movements or reflexes which can be voluntarily suppressed
Autonomic Nervous System – regulates activities of smooth muscle, cardiac
muscle, and glands
involuntary, operates mainly by reflexes
REFLEXES – fast, automatic responses to specific stimuli
somatic reflexes – involve skeletal muscle
protective reflexes, can be consciously overridden
autonomic reflexes – involve smooth muscle, cardiac muscle or glands
maintain homeostasis in the body
Reflex Arc – pathway for nerve impulses of a reflex
1) sensory receptor – detects a stimulus
2) sensory neuron – generates a sensory impulse and carries it to the CNS
3) integrating center in brain or spinal cord
4) motor neuron – carries impulse from CNS to an effector
5) effector – part of body that responds to the motor impulse (muscle, gland)
the effector’s automatic response to the stimulus is called a reflex
knee-jerk reflex – suddenly stretching skeletal muscle by striking the tendon
makes it contract to protect the muscle from overstretching
somatic, spinal reflex
pupillary light reflex – shining a light in the eye causes the pupil to contract
to reduce the amount of light entering
autonomic, cranial reflex
DIVISIONS OF AUTONOMIC NERVOUS SYSTEM
many organs are receive motor innervation from 2 divisions of the ANS
one division is usually excitatory, the other inhibitory
Sympathetic division – motor neurons that trigger fight-or-flight responses
caused by physical or emotional stress
motor neurons release norepinephrine at synapse with effector
also triggers release of the hormone epinephrine by the adrenal gland
pupils dilate
increased heart rate and force
increased blood flow to skeletal muscles, lungs
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airways dilate, respiratory rate increased
energy released from storage in liver and adipocytes
increased metabolism and sweating
inhibition of non-essential activities – digestive & urinary function
Parasympathetic division – motor neurons that trigger rest-and-digest
activities when no stress is occurring
motor neurons release acetylcholine at synapse with effector
pupils constrict
decreased heart rate and force
airway constriction
stimulation of digestive and urinary functions
increased nutrient absorption in cells
increased energy storage
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