Human Brain and Senses

Outline for today Levels of analysis Basic structure of neurons How neurons communicate Basic structure of the nervous system

Levels of analysis Organism Brain Cell Synapses Membrane channels

Basic structure of neurons Dendrites: site of inputs to neuron Soma or cell body integration of inputs and initiation of action potentials Axon: output pathway Neurons communicate with electrical pulses (action potentials or spikes ) that travel from the cell body down the axon. Axon terminals: make synapses with other neurons by releasing neurotransmitter

Neurons come in a great variety of shapes and sizes Purves et al., 2001

How do they differ? Retinal neurons Cortico-spinal neuron

Dendrites and their synaptic inputs Cerebellar Purkinje cell show great variability Cochlear nucleus bushy cell

Example of aggregations of neurons to form nuclei

Glial cells Glial cells outnumber neurons by about 10:1! They serve to provide support and nourishment to neurons Astrocytes: help to regulate extracellular ionic concentrations Microglia: serve as garbage collectors to clean up dead tissue Oligodendroglia: in the CNS to form myelin around axons for electrical insulation which speeds conduction Schwann cells: in the PNS to form myelin

How neurons communicate Ionic basis for resting membrane potential Action potentials Synaptic transmission: chemical

Action potentials propagate down their axon Larger diameter axons have less resistance to ion flow Speed of conduction is faster in large diameter axons

Saltatory conduction in myelinated axons Myelinated axons conduct action potentials up to 10X faster Large myelinated axons can conduct up to 150 m/sec, 330 mph

Action potential propagation animation Saltatory conduction animation

In Out In Out If K channels open: K will follow concentration gradient Inside negative If Na channels open: Inside positive Equil. potential when elect. potential = concentration grad. E K = -58 mv E Na = +58 mv At rest, K permeability is much higher than Na permeability Resting potential is near E K

Ionic basis of resting membrane potential The inside of neurons is about -60 mv relative to the outside Cell membrane is semi-permeable Ionic concentrations inside and outside are different [Na] is high outside, low inside [K] is high inside, low outside Sodium/potassium pump helps maintain this imbalance Much of the energy (70%) of the brain is expended here At rest K permeability is about 40x higher than Na Resting potential: when electrical potential balances concentration gradient

Action potentials: arise from changes in Na and K permeability Out In At rest K channels are open Depolarization Na channels open briefly to produce surge of inward Na K channels open again to repolarize Increase inward Na current Increase Na permeability A.p. is regenerative Properties of a.p.: All-or-none, not graded in amplitude Membrane potential must be depolarized above a threshold of about -55 mv

Synaptic transmission at a chemical synapse Action potential in presynaptic terminal triggers release of neurotransmitters from synaptic vesicles Neurotransmitter diffuses across synaptic cleft to bind with receptors for transmitter Activation of receptors opens ion channels leading to excitatory (EPSP) or inhibitory post-synaptic potentials (IPSP)

Synaptic transmission animation

Synaptic vesicles come in two flavors

Neurotransmitters Excitatory Inhibitory Small, clear vesicles (specific) Dense core vesicles (modulatory) Acetylcholine (Ach) Glutamate Catecholamines (epi-, norepi-, dopamine) Serotonin Histamine Peptides (many!!) Glycine GABA Peptides

Bear 5.13 and 5.14 Excitatory depolarize (glutamate) Inhibitory hyperpolarize (GABA) Bear et al., 2001

Synaptic vesicles Small, clear vesicles Associated with pre- (release sites) and post-synaptic specializations Fast-acting Dense core vesicles Released into extracellular space Slow, modulatory effect

Cell body integrates inputs EPSPs can summate spatially or temporally Bear et al., 2001

Action potentials: neurons communicate with other neurons by electrical pulses They are more or less identical The membrane potential must be depolarized above a threshold of about -55 mv They are all-or-none, not graded in amplitude They are followed by an absolute and relative refractory period

Divisions of the nervous system

Cytoarchitectonic maps of the cerebral cortex Korbinian Brodmann (1868-1918)

Levels of analysis Organism Brain Cell Synapses Membrane channels

Anatomical reference terms

281 MSC