Impulses and Neurotransmitters

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

1. Sodium ions flow into the cell through channels causing depolarization
2. Polarity is reversed as the interior of the cell becomes more positive than that outside in that region of the cell (or axon)
3. 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
4. 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