Neurons and Glia Three basic neurons: ∼ Multipolar: Neurons by

Neurons and Glia
Three basic neurons:
∼ Multipolar:
◊ Neurons by far the most common
◊ They possess an axon and a number of dendrites
∼ Bipolar:
◊ Neurons with a centrally placed cell body
◊ 1 axon extends away from cell body
◊ 1 dendrite extends from axon
◊ Occur in afferent pathways of the visual, auditory
and vestibular systems
∼ Unipolar:
◊ Possess a single process from the cell body
◊ This usually divides into dendritic and axonal
branches
◊ Consists of primary afferents of the spinal (and
some cranial) nerves
Golgi Neurons:
∼ Type I – projecting neurons eg. pyramidal cells
∼ Type II – interneurons (local projections) eg. stellate or granule neurons, with smooth
dendrites
Types and Functions of Glia:
∼ Astrocytes
◊ Physical and metabolic support
◊ Blood-brain barrier by wrapping
around blood vessels
◊ Regulation of blood flow
◊ Uptake, recycling and release of
some neurotransmitters
◊ Buffering of K+, pH
◊ Maintenance of extracellular space
◊ Response after injury, scarring
∼ Oligodendrocytes
◊ Myelination in CNS
◊ Growth-inhibitory factors
◊ May play a role in
ion/neurotransmitter balance in
extracellular space
∼ Ependyma: lining of ventricles
∼ Microglia
◊ Activated microglia express a
Oligodendryocytes vs. Schwann Cells
number of immune-related
- CNS
- PNS
molecules, divide and migrate to the
- myelinates many - myelinates a
site of injury
neurons
single internode
◊ Can become phagocytic, destroying
dysfunctional neurons/synapses,
removal of debris
Synaptic transmission can be rapid and point-topoint or slow and often diffuse. Neuromodulators
determine which.
Neurochemistry
Types of Neurotransmitters/Neuromodulators:
∼ Amino Acids
◊ Glutamate – Most common excitatory neurotransmitter
◊ GABA & Glycine – Most common inhibitory neurotransmitters. GABA is brain, Glycine is
SC
∼ Monoamines: (involved in attention, cognition, emotion)
◊ Serotonin
Neurotransmitter Receptors: ◊ Histamine
∼ Ionotrophic: ◊ Dopamine
◊ Involves transmitter gated ion channels ◊ Epinephrine
◊ Involves fast synapses, point-­‐to-­‐point ◊ Norepinephrine
◊ Responsible for EPSP’s and IPSP’s ∼ Peptides:
∼ Monotrophic: ◊ Endorphin – Perception of pain
◊ Activates G proteins and second ∼ Other:
messengers, that then go on to alter gene expression ◊ Acetylcholine – Important role in ANS
◊
Slow synapses and released when motor neurons
◊ Diffuse, long acting effects synapse onto skeletal muscle to
contract.
Types
∼
∼
∼
∼
∼
∼
∼
of Synapses:
Axosecretory: Axon terminal secretes directly into bloodstream
Axoaxonic: Axon terminal secretes into another axon
Axodendritic: Axon terminal ends on a dendritic spine
Axoextracellular: Axon with no connection secretes into extracellular fluid
Axosomatic: Axon terminal ends on soma
Axosynaptic: Axon terminal ends on another axon terminal
Autapse: Neuron synapses on itself; a negative feedback mechanism.
Electrical Synapse
∼ Transfer of electrical impulse, no
neurotransmitter involved
∼ Cells are close together
∼ Very fast, but cannot gain or lose
magnitude of signal
∼ Movement of ions and small-medium
sized molecules
Chemical Synapse
∼ Contains pre- and post-synaptic elements
separated by a synaptic cleft
∼ Release of neurotransmitters/neuromodulators
∼ Neurotransmitters bind to receptors
∼ Multiple cells are synapsing on a single cell
∼ Other chemicals released other than
neurotransmitters
Excitatory Post-Synaptic Potential (EPSP)
∼ Temporary depolarization of post-synaptic
membrane
∼ Flow of positive ions into cell via ligand gated
ion channels
∼ Makes an action potential more likely
Inhibitory Post-Synaptic Potential (IPSP)
∼ Hyperpolarizes the cell
∼ Inflow of negative ions or outflow of positive
ions
∼ Reduces chance of action potential
Sites of Drug Action:
∼ Synthesis of neurotransmitter /
neuromodulator
∼ Presence and release of vesicles /
neurotransmitter from pre-synaptic site
∼ Binding to post-synaptic receptor
∼ Change the effect in the post-synaptic
cell
∼ Inactivation: by either removal,
inactivation or degradation by neurons
and glia
Ventricular System and CSF
Flow of CSF:
Produced by choroid plexus in
lateral and fourth ventricles
ê
Through intraventricular foramen
(of Monro)
ê
Flows from lateral to third ventricle
ê
Through cerebral aqueduct
ê
Enters 4th ventricle
ê
Through the Foramina of Luschka
and Magendie
ê
Subarachnoid Space
ê
Through arachnoid granulations
ê
Reabsorbed in the sagittal sinus
Hydrocephaly
∼ Noncommunicating:
◊ Blockage of intraventricular foramen (of Monro)
preventing flow from lateral to 3rd
◊ Could be due to tumours, congenital, meningitis,
scarring, etc.
◊ Body is making the right amount of CSF
◊ Brain gets compressed, medulla starts to go through the floor of the skull
◊ Treatment by surgery
∼ Communicating:
◊ Overproduction of CSF – papilloma tumour of the plexus
◊ More rarely due to poor absorption / obstruction of superior sagittal sinus or congenital
absence of arachnoid villi
◊ Gradual increase in CSF becomes like a balloon and skull expands and cortex is
compressed against skull
◊ Can be fatal in babies, but ultrasounds can pick it up in embryos
Thalamus
Nucleus
Ventroposterolateral
(VPL)
Ventroposteromedial
(VPM)
Medial Geniculate
(MGN)
Lateral Geniculate
(LGN)
Pulvinar
Ventrolateral (VL)
Input(s)
Output(s)
Sensory Nuclei
∼ Spinothalamic
Primary Sensory
Cortex
∼ Medial Lemniscus
∼ Trigeminothalamic
Primary Sensory
Cortex
∼ Pontine taste area
Branchium of the
Primary Auditory
inferior colliculus
Cortex
Optic Tract
Function
Somatic sensation for
body
Somatic sensation for
face, taste
Hearing
Primary Visual Cortex
LGB, MGB, sup. and
Visual Association
inf. colliculi
Cortex
Motor Nuclei
∼ Cerebellum
Primary Motor Cortex
∼ Basal Ganglia
Ventroanterior (VA)
Basal Ganglia
Premotor Cortex
Ventrointermedial (VI)
Cerebellum
Primary Motor Cortex
Anterior (Ant)
Mediodorsal (MD)
Centromedian (CM)
Limbic and Nonspecific Projection Nuclei
Mammillothalamic
Cingulate Cortex
Tract
∼ Temporal Lobe
Prefrontal Cortex
∼ Amygdala
∼ Hippocampus
Nonspecific cortical
Slow pain pathways
projections
Vision
Visual Processing
Modulation/Coordination
of movement
Initiation/Planning of
movement
Coordination of
movement
Memory storage and
emotion
Motivation, drive,
emotion
Emotional content of
pain
Thalamic Function
∼ Not really known
∼ Input filter to discard
information?
∼ Seems to be all about
cortical/basal
ganglia/reticular system
feedback loops but there is
little evidence to prove this