Central Nervous System
brain and spinal cord
Peripheral Nervous System
neural tissue outside the CNS
sensory and motor neurons
Sensory Nervous System
contains receptors
transmits information from receptors to CNS
Motor Nervous System
transmits information from CNS to rest of body
sends motor information to effectors
Somatic Nerves
usually to skeletal muscle
voluntary
Autonomic Nerves
usually to smooth muscle of body organs or glands
involuntary
Neuron
reception, transmission, and processing of stimuli
triggering of certain cell activities
release of neurotransmitters
extremely variable in shape and size
Multipolar Neurons
have more than two processes
one axon and many dendrites
majority of neurons in the body
Bipolar Neurons
one axon and one dendrite
found in cochlear and vestibular ganglion, retina and olfactory mucosa
Pseudounipolar Neurons
single process, close to cell body
divides into two branches
nerve impulse bypasses cell body
found in spinal and cranial ganglia
Glial Cells
several types of cells that support and protect neurons
about 10x more abundant in mammalian brain than neurons
surround cell bodies, axons and dendrites
occupy interneural spaces
Oligodendrocytes
produce myelin sheath that provides electrical insulation of neurons in the CNS
have long processes that wrap around neurons
Schwann Cells
have the same function as oligodendrocytes but are located in the PNS
one cell forms myelin around a segment of one axon
spaces between adjacent cells are nodes of Ranvier
Nerve Fibers
consist of axons enveloped by a special sheath
exhibit differences related to whether they belong to central or peripheral nervous system
axons of small diameter usually unmyelinated
thicker axons have increasingly numerous concentric sheath around them (myelinated fibers)
Multiple Sclerosis
results from the destruction of myelin
can be in the brain or spinal cord
range of symptoms depending on what nerves are affected
cause in unknown
treatments designed to prevent attacks and improve function
medication is used to increase astrocytes
Astrocytes
star shaped cells with radiating processes
bind to capillaries (and elsewhere)
2 types protoplasmic and fibrous
most numerous glial cells
provide structural support for neurons
regulate ionic and chemical environment of neurons
important in blood-brain barrier
involved in repair process
Protoplasmic Astrocytes
shorter, more numerous processes
found in grey matter
Fibrous Astrocytes
long processes
found in white matter
Ependymal Cells
columnar epithelial cells that line the ventricles of the brain and spinal cord canal
involved in the production of cerebrospinal fluid in ventricle of the brain
cilia on apical end used for moving cerebrospinal fluid around elsewhere
may also serve as a reservoir for new neurons and a first line of defense against viral infections
Microglial Cells
found throughout the brain and spinal cord
make up 10-15% of all cells in the CNS
small, elongated cells with short processes
phagocytic cells derived from precursors from bone marrow
involved in inflammation and repair of the CNS
Membrane Potential
all eukaryotic cells have a difference in electrical charge between the inside and outside of the cell
this is potential energy that can be used
Sodium Potassium Pump
creates a membrane potential because Na+ and K+ move against their concentration gradients
the pump pushes 3 sodium ions out of the cell for every 2 potassium ions going in
some potassium ions leak out passively down the concentration gradient
there are more positive ions outside the cells meaning the ICF is more negative than the ECF
Membrane Resting Potential
voltage across a cell membrane when that cell is at rest and not engaging in any activity other than the normal maintenance of the cell
Action Potentials
when a small area of the axon membrane is stimulated the sodium gates are activated in the membrane
facilitated diffusion of sodium ions into the cell reducing resting potential (ICF becomes more positive)
if the resting potential is reduced from -70mV to -50mV or 55mV an action potential is generated
Action Potential Process
Sodium ions flow into the cell through channels causing depolarization
Polarity is reversed as the interior of the cell becomes more positive than that outside in that region of the cell (or axon)
Membrane potential reaches +30mV and the sodium channels close while the potassium channels open causing potassium to rush out of the cell leading to repolarization
Action potential is completed and the pump extrudes any extra sodium and recovers potassium, the membrane potential is re-established
Important Notes About Action Potentials
occurs very quickly over a very small part of the membrane
active transport processes not involved in the production of an action potential as it is the result of sodium and potassium flowing down the concentration/electrical gradient
the pump is needed to maintain membrane potential
it is an all or none process
the gates are open for a fixed period of time
Conduction of Nerve Impulses
depolarization of membrane opens up sodium channels in adjacent parts of the membrane
the wave moves along the cell (axon) creating a nerve impulse
Conduction in an Unmyelinated Axon
every patch of membrane that has sodium and potassium gates can produce an action potential (AP)
APs must be produced at every micrometer along the axon
conduction is “relatively” slow compared to myelinated axons
Conduction in an Myelinated Axon
myelinated sheath prevents sodium and potassium ions from crossing the membrane
short gaps in sheath called nodes of Ranvier
APs leap from node to node and signals travel much faster
Synapse
responsible for the unidirectional transmission of nerve impulses
in CNS another neuron; in PNS another neuron, muscle, cell or glandular cell
can make contact with cell bodies, dendrites or other axons
the transmission is known to be mostly chemical
Synaptic Occurrences
chemicals released at presynaptic endings
synaptic cleft between cells is so narrow that it can only be observed with an electron microscope
neurotransmitter molecules are enclosed within the synaptic vesicles, fuse with the membrane and then released
the number that fuse depends on the number of action potentials
What happens when the action potential arrives at the synapse?
calcium ion gates open and calcium enters the cell causing neurotransmitter vesicles to fuse with the membrane
the contents are released by exocytosis
neurotransmitter moves across the synaptic cleft to post synaptic cell and binds to the membrane
this causes the sodium channel to open, letting sodium flow in
if the threshold is reached the AP is initiated
Excitatory Synapses
normally due to the flow of positive ions into the postsynaptic cell (usually sodium)
increased the probability of an action potential occurring
Inhibitory Synapses
usually due to the opening of potassium or chloride channels
potassium ions leak out of the cell or chloride ions leak into the cell
drops the negative membrane potential and decreases the likelihood of an action potential
What determines if an action potential is generated?
sum of excitatory and inhibitory inputs
Neurotransmitters
brain chemicals that relay signals between nerve cells
more than 50 different kinds
can be excitatory, inhibitory or both depending on the receptor
tell your heart to beat, lungs to breathe and stomach to digest
also affect mood, sleep, concentration and weight
Acetylcholine
released at all neuromuscular junctions
triggers muscle contraction
also stimulates release of certain hormones
excitatory at neuromuscular junctions in skeletal muscle
inhibitory in cardiac muscle (autonomic nervous system)
Dopamine
inhibitory and excitatory neurotransmitter
vital roles in movement, cognition, pleasure or motivation
plays a central role in positive reinforcement and motivation
important that levels are right as it can affect psychological functioning
GABA
inhibitory neurotransmitter that is widely distributed in the brain
contributes to motor control, vision and many other cortical functions
major inhibitory/calming neurotransmitter
Seratonin
contributes to regulating body temperature, sleep, mood, appetite and pain
affects most cells of brain
low levels are often associated with anxiety, panic attacks, obesity, insomnia and fibromyalgia
Norepinephrine
neurotransmitter and hormone
important for attentiveness, emotions, sleeping, dreaming and learning
as a hormone causes blood vessels to contract and heart rate to increase