Neuroanatomy exam 1

Neuron Doctrine (1890s)

theorized that the brain was composed of individual, specialized cells called neurons that were not connected directly (like electrical wires) but separated by functional space (synapse) through which they communicate with eachother using neurochemical signals 

• It was originally believed that the brain was similar to an electrical circuit

Synapse

 the space where signaling/communication occurs between neurons 

Interneurons

small neurons that terminate locally 

Neurons

cells in brain, vary in size and complexity, motor neurons are larger than sensory neurons, axon and dendrites extend off of soma, neurons are characterized by how many neurites branch off of them

Dendrites

branch off from the cell body, considered the “receptive area of the cell” 

• Branches of neurons that extend from cell body 

• Provide receptive pole of the neuron 

• Long and thin, act as resistors, isolating electrical events such as postsynaptic potentials from one another

• Branching pattern can be complex and determines how the neuron integrates synaptic inputs from various sources 

• Can produce dendritic spines 

• Conduct electrical signals (information) to the cell body 

• Receptive area of neuron

Perikaryon

cell body or soma, metabolic center, much like other cells in the body 

• Metabolic and genetic center of a neuron 

• Cell body and dendrites = receptive pole of neuron 

• Synapses cover the whole cell body

Axon

single neurite originating from cell body, considered the “output structure” of the neuron 

• Neurite that conducts an electrical signal from cell body to presynaptic terminal 

• The electrical signal arrives at pre-synaptic terminal and is the stimulus that produces neurotransmitter release from the pre-synaptic terminal 

• Conducts electrical signals (action potentials) from rge initial segment to the synaptic terminals 

  • Contains high density of sodium channels which allow for it to act as a trigger zone where action potentials are generated to travel fully down teh axon 

  • Does not contain Nissl substance 

  • Initial segment arises from axon hullock 

• Cylindrical tube of cytoplasm covered by a membrane called axolemma 

• Cytoskeletion with neurofilaments and microtubules runs through axon 

  • Provides a framework for axonal transport 

  • Specialized molecular motors (kinesin molecules) which bind to vesicles (which contain NT’s for transport) via a series of ATP consuming teps along the microwaves 

• axons conduct action potentials and transport materials from cell body to the synaptic terminals (anterograde transport)  and from teh synaptic terminals to the cell body (retrograde transport) 

• Axons don’t have ribosomes, so new proteins must be synthesized and transported there 

• PNS axons can regenerate but not in the CNS

• Collects electrical signals 

• Output structure of the neuron 

• Electrical signals/info: dendrite → soma → axon → presynaptic terminal/bouton → synapse

Myelin sheath

fat and protein covering the axon, acts as an insulator for the electrical signal 

• Produced by schwann cells in PNS 

• Produced by ogliodendrocytes in CNS 

• Myelin = white (white matter = tracts of axons) 

• Insulator that keeps electrical signal in axon 

• Consists of multiple layers of lipid-rich membrane produced by schwann cells in PNS and ogliodendrocytes in CNS

• Functions as an insulator 

• Divided by 1mm long segments separated unmyelinated gap called nodes of ranvier

Presynaptic terminals, branches or boutons

bulbous structure at the end of the axon 

• Contains synaptic vesicles which store and release neurochemical signals (called neurotransmitters and neuropeptides) that are essential for neuronal communication bewteen neurons 

• Contain little “water balloons” of NTS or neuropeptides called vesicles

Neurites

Cellular fibers emerging from the soma, ex. dendrites/axon 

• Extensions that branch off of soma 

• Dendrites and axons

Dendritic spines

small mushroom shaped dendritic branches, dendritic spines increase the surface area and the contact points of dendrites 

• small, mushroom-shaped projections that receive synaptic inputs 

• Thicker its neck, teh greater its influence on the synapse 

• Dynamic (shape can change) → can strengthen synapses and memory formation 

• Small individual extensions that extend the surface area of the dendrite, allowing for more synaptic contact 

Neuronal action at the synapse

neurotransmitters are released from the pre-synaptic terminal on neurons, and they cross the synaptic space to bind with proteins called neurotransmitter receptors that are embedded in the membrane of the next neuron - forming the communicating link between neurons 

• Functional space between neurons 

• Axon terminal of transmitting neuron → receptive region of receiving neuron

Neuronal action at the synapse steps

when neurotransmitters bind to neurotransmitter receptors → they open a channel allowing electrically charged ions like sodium [Na+] 

And potassium [K+] to enter or exit the neuron → changes the electrical charge inside the post-synaptic neuron → this change in electrical charge either excites or inhibits the post-synaptic neuron 

• Electrical signal stimylates neurotransmitter release 

• NT then moves across synapse and binds to NT receptors on postsynaptic cell

• Binding of NT opens up a pore for charged ions to move into cell 

  • That movement of ions gives the cell its electrical charge 

  • Produces electricity which can excite or inhibit the next cell

Synaptic vesicle

located in the terminal boutons of a sending neuron release neurotransmitters into the synaptic space 

• Some are large, some are small, some with a dense core (contain catecholamines), some are flat (contain inhibitory mediators) 

• Vesicle release is dependent on calcium or other transporter molecules

Neurotransmitters

chemical messengers between nerve cells 

• Acetylcholine 

• Glutamate 

• Gamma aminobutyric acid (GABA) 

• Biogenic Amines 

  • Catecholamines 

    • Dopamine 

    • Norepinephrine 

  • Indolamines 

    • Serotonin 

    • Melatonin 

- some have very specific locations, others are spread throughout 

- one NT can be excititatory or inhibitory depending on what receptor it binds to 

- agonists = things that bind to them

Neuropeptides

 bigger molecules, gene products 

• DNA in nuclei encodes for genes which encode into proteins, some proteins can be broken down into peptides, packaged into vesicles and released into neurons 

• NS has expansive ability to use chemicals 

• Anatomy doesn’t change, but chemicals do

Types of neurons in the nervous system:

unipolar, bipolar, pseudo-unipolar, multipolar 

Unipolar neurons

neurons with one neurite

Bipolar neurons

 neurons with two neurites

Pseudo-unipolar neurons

neurons with one neurite that splits

Multipolar neurons

 neurons with many neurites, many dendrites, 1 axon with presynaptic terminal  

Axodendritic synapse

between axon terminal and dendrite → usually excitatory

Axosomatic synapse

between axon terminal and cell body → usually inhibitory

Axo-axonal synapse

between axon terminal and another axon terminal → presynaptic inhibition (modulates transmitter release in postsynaptic axon)

Axo-synaptic synapse

synapses onto synapses, terminal onto other synaptic terminal, can create a synaptic shunt and that terminal can top teh effect of the input from the previous synapse

Dendrodendritic synapse

between dendrite and denderite, local interactions (may be excitatory or inhibitory) in axonless neurons such as the retina

Neuronal groups and connections

neuronal cell bodies are grouped into clusters or groups within the brain called nuclei, each “nucleus” contains projection neurons and interneurons, nucleus = when alike neurons are clustered together

Projection neurons

the neuron’s axons project to other parts of the brain

• Axons project as tracts (fasciculi or leminisci in brain) or columns (in the spinal cord) 

• Project out of nucleus as part of a tract 

• Tracts = bundles of axons like highways 

• The axons of these neurons carry impulses to other parts of the nervous system

Interneurons

connect one neuron with another within a nucleus 

• These are short relays of information within a brain region 

• How projection neurons communicare 

• Connects the different neurons in a nucleus 

• In PNS, these compact groups of cell bodies are called ganglia

Neuroglia

macroglia and microglia

• Cells in nervous system that do not form synapses 

• Dominate the nervous system 

• Play an important role in myelin formation, controlling ion balance, guidance of developing neurons, reuptake/metabolism of neurotransmitters and neuroimmunity 

• Glia outnumber brain and spinal cord 10:1 

• Important roles such as: maintaining extracellular potassium levels, myelin formation of developing neurons, NT reuptake

• Glia = glue 

• Glia do not form synapses, but they dominate the nervous system 

• They produce myelin, developmental pathways, radial glial cell guides, maintain ion balance, NS immune system

Macroglia

astrocytes and ogliodendrocytes, important for neuronal development and function 

• Maintenance of health and function of life 

• Feet of astrocyte → blood brain barrier 

• Both astrocytes and ogliodendrocytes are derived from the ectoderm and can regenerate in some circumstances

• Blood brain barrier → selective filter system that makes it difficult for external toxin can get into brain 

Astrocytes

• Provide structural support 

• Act as glial guide wires for neurons during development 

• Maintain ion balance around neurons 

• Participate in reuptake and metabolism of neurotransmitters 

• Participate in blood brain barrier 

• Migrate to side of neuronal injury to aid in repairing damaged neuronbal tissue 

• Processes radiate in all directions from the cell body, surrounding blood vessels in NS and cover exterior surface of brain and spinal cord below pia 

• The 2 broad classes of atrocyres are protoplasmic and fibrous

Reactive astrocytes

 specialized set of astrocytes, a subset of protoplasmic astrocytes that migrate to the site of neuronal injury and proliferate and aid in repairing damaged neuronal tissue, this can lead to gliosis/glial scarring if unsuccessful 

• May also play a role in synaptic transmission 

• React to injury by processing protective factors that help neurons heal → if damage is too big, they act as fibroblasts and produce a glial scar  (unfixable, neurons can’t heal) gliosis = formation of glial scar 

• Undergo change in response to injury 

• Migrate to site of injury and try to repair it 

• If damage is too big → glial scar which contains collagen

Protoplasmic astrocytes

delicate astrocytes with many branched processes 

• Exist in gray matter of the CNS 

• Provide structural support 

• Act as glial guide wires for neurons during development 

• Maintain ion balance around neurons 

• Participate in reuptake and metabolism of neurotransmitters 

• Sent astrocytic feet that surround blood vessels and act as an integral part of the blood brain barrier 

• Very delicate, but bushy-looking

• Gray matter → cell bodies 

• Pool of astrocytes from which reactive astrocytes form 

Fibrous astrocytes

more fibrous and robust astrocytes 

• Radiate long, straight processes in all directions 

• Exist in white matter areas of CNS 

• Oriented longitudinally in the plane of the fiber bundles 

• Provide axonal support 

• Act as glial guide wires for neurons during development 

• Sed astrocytic feet that surround blood vessels and act as an integral part of the blood brain barrier

• More spindle/spiny structure

• In white matter 

• Always oriented longitudinally 

• Role: provide support for axon, especially during development (glial guide wires)

• Feet to blood muscles → blood brain barrier

Ogliodendrocytes

in the CNS, are the myelinating glia, ONE ogliodendrocyte extends multiple arms to myelinate MANY (30-40) axons 

• CNS myelinating glia 

• One of these provide many arm to many different axons 

• Predominate in white matter 

• Extend arm-like processes which wrap tightly around axons, extruding the ogliodendroglial cytoplasm to form a compact heath of myelin

• One axon gets arms from many different ogliodendrocytes myelin sheaths in PNS are longer, larger and thicker 

• CNS is more compact, less space for cells to do the myelinating 

Schwann cells

in PNS, ONE schwann cell myelinates ONE axon, axons have many schwann cells 

• It takes more space to myelinate in PNS 

• Doent means one axon has one schwann cell, one axon ha many schwann cells but one schwann cell can only myelinate one axon 

• Remyelination can occur after injury in peripheral nerves

Microglial cells

specialized macrophages (scavengers) 

• Mobile glial cells that survey teh CNS to combat infection 

• Activate and infiltrate injured zones of CNS to scavenge for infection and neural damage 

• Some exist within the CNS and others infiltrate from the blood 

• Specialized macrophages, survey the system to combat infection 

  • Endocyosis of neural damage 

  • Goal = protect NS from invaders 

  • Some exist in CNS, some from blood 

  • Cell is already dead so they remove/scavenge dead cells and “trash” 

• Development, health and mainteneance in CNS and PNS 

• Activate and move to site of injury when brain or spinal cord is damaged 

• Always present in brain but infiltrate to other areas through blood vessels after damage or infection

Wallerian degeneration

if axon is cut → the distal end of the axon degenerates (because materials (mostly proteins)) formed in cell body can no longer be transported down the axon (asoplasmic transport) 

• Cell body reacts (chromatolysis) - swelling and breakdown of “Nissl substance” 

• Axon reacts - loss of structural integrity (degeneration), detachment of synapses

• Occurs in PNS and CNS 

• Severing of axons 

• Evered axons in PNS can be “sewn” back together       

• Cut axon → everything distal to cut/downstream degenrates and synapses detach 

• Proximally → chromatoslsis / break down of proteins and ER 

• It takes 1.5-2 days to start seeing an axon break down into its component pieces after transection → a lot of the breaking down is due to microglial cells/ macrophages, antiinflammatory medication and icing could prevent this

Regeneration in the PNS:

each schwann cell myelinates a single axon 

• When axon is damaged or severed, the schwann cell forms a guidance tube to guide the regenerating end of the axon to the target end of the axon 

• Therefore you get regeneration and the axon damage is temporary 

• Damage can be temporary 

• Won't see much more recovery after a year 

• Smaller injury = faster healing 

• One schwann cell is 100% comitted to one axon and it protects environment of damaged axon, allowing it to heal 

• Schwann cells also produce trophic factor to help PNS cells to heal itself and “restring” axon together 

• Nerve growth factors 

• Axons are made of microtubules that span length of axon

Neuronal regeneration in the CNS

each ogliodendrocyte myelinates multiple axons 

• When axon is damaged or severed, ogliodendrocytes fail to respond 

• Ogliodendrocytes withdraw remaining myelin support from the damaged axon 

• Therefore it degenerates and damage is permanent 

• Glial scar 

• Molecules such as NoGo act as “stop signs” for regeneration 

• Ogliodendrocytes in CNS can’t regenerate axons like schwann cells in PNS 

• Doesn’t produce repairing factors, instead it produces NoGo which inhibits other repairing factors 

• It still has other neurons to myelinate 

• Ogliodendrocyte ignores severed axon, o severed axon degenerates over time

Collateral sprouting

regenerative sprouting in CNS and PNS occur in response to partial severing axons from neighboring neurons form new collaterals that try to rwinnervate end organ 

• Occurs when an innervated structure has been partially deneverated 

• Wallerian degeneration → loss of input to axon 

• Brain is plastic, you get regeneration in CNS but its not axonal, its collateral 

• Only neurons with same function can take part in collateral sprouting 

• You get a drop infunction and then a recovery in function that never reaches 100% 

Structure division of the nervous system

• CNS = brain and spinal cord 

• PNS = crinal and spinal nerves that innervate body

Functional division of the nervous system

• Somatic nervous system = controls the body structures, soma = body, voluntary action 

• Autonomic nervous system = controls smooth muscles, glands, blood vessels, ex) heart rate, breathing, stomach → digestion 

• Function applies to PNS

Central nervous system

comprised of brain and spinal cord 

Peripheral nervous system

 carries messages to and from the CNS comprised of somatic and autonomic nervous system (which compete and fight with eachother)

Somatic nervous system

controls voluntary muscles and transmits sensory information to the CNS

Autonomic nervous system

controls involunteay body functions, comprised of sympathetic and parasympathetic nervous systems

Sympathetic nervous system

arouses body to expand energy

• Fight or flight 

• Awake and aware

Parasympathetic nervous system

calms body to conserve and maintain energy 

Rest and digest

Axes of orientation

use spatial reasoning to determine structure position

Quadripetal CNS orientation

straight from tip of the brain to end of spinal cord

Rostral (quadrapedal)

 towards the head region oo the brain and spinal cord

Caudal (quadrapedal)

towards the tail region of the brain and spinal cord

Dorsal (quadrapedal)

 towards upper-facing surface of the brain and spinal cord

Ventral (quadrapedal)

 towards the bottom-facing surface of the brain and spinal cord

Bipedal (human) CNS orientation

makes 90 degree turn from tip of brain to end of spinal cord, so the axes of orientation rotate 90 degrees 

Rostral (bipedal)

towards the head rehion of the brain but now the upper facing portions of the spinal cord 

Caudal (bipedal)

towards the tail region of the brain but lower-facing portions of spinal cord 

Dorsal (bipedal)

towards upper-facing surface of brain and the back-facing surface of spinal cord

Ventral (bipedal)

towards the lower-facing surface of brain and front-facing surface of brain and front-facing surface of brain and front-facing surface of spinal cord

Medial

close to the midline

Lateral

farther from the midline

Planes of section

(saggital, horizontal, coronal, transverse) - frequently used in the description of the brain and spinal cord

Horizontal:

cut like a burger bun, typically has a huge, deep separation in the back

Coronal

 also called frontal section, sections brain is from front to back

Saggital

side to side, typically midsaggital

Transverse

in same plane as horizontal section of brain

• Spinal cord 

• Divides it into “pepperoni slices”

CNS - a general overview

 comprised of brain (encephalon) and spinal cord

Brain has tiered structure with 3 main subdivisions: 

• Cerebrum 

• Cerebellum 

• Brainstem

• Each subdivision is further divided into discrete regions

Cerebrum (forebrain)

most phylogenetically advanced brain region 

• Responsible for complex functions such as cognition 

• Combrised of telencephalon and diencephalon 

• Layers: gray (cells) → white (fibers) → gray (cells) 

Telencephalon

• cerebral cortex gray mater (neuronal cell bodies, dendrites and axons) 

• Subcortical white matter (myelinated axon fiber tracts) 

• Basal ganglia (subcortical collection of neuronal cell bodies (gray matter) , dendrites and axons) 

  •  Cerebral cortex (high order level function) 

  • Ubcortical white matter 

  • Commissures → fibers that cross the brain 

  • Basal ganglia →  subcortical gray matter, helps control motor movement

Diencephalon

• Thalamus = uppermost portion of diencephalpn 

• Hypothalamus 

• Epithalamus 

• Subthalamus 

  • Thalamus → center of brain, sensory info. → sends info to right part of brain 

  • Hypothalamus → controls growth, ability to maintain water, menstrual cycle, birth 

  • Epithalamus → DMT production 

  • Subthalamus 

- more centered in brain 

Large bilateral structure

Cerebellum

collections of gray matter and white matter tracts 

• Vermis = central/middle portion of the cerebellum 

• Lateral lobes (2) 

• Motor coordination 

• Bilateral structure 

• White matter = subcerebellular fibers 

• Gray matter = subcerebellylar nuclei

Brainstem

collections of gray matter and white matter tracts 

• Midbrain (mesencephalon) 

• Pons (bulbous) 

• Medulla oblongata 

Ventricles

hollow spaces within the vrain and spinal cord filled with cerebral spinal fluid (CSF) 

• Fluid-filled spaces throughout the brain that produce CSF (produced by choroid plexus) 

• High in salts, sugars and ions 

• Highway for energy and nutrients throughout brian 

• Disease with CSF pressure cna limit cortical development 

• White matter = fiber tracts, hundreds of muelinated axons together

Spinal cord

 conists of neuronal somata (cell bodies) and axon tracts 

• Neuron cell  bodies lie in center of the spinal cord in an area called the central gray (gray matter) 

• Connections (pathways) to/from neurons in CNS exist as axon fiber tracts (white matter) 

• White matter on outside, gray matter on inside 

• Opposite orientation than in the brain: white in surround, gray in center “central gray”

Tracts and commissures in the brain and spinal cord

connections (pathways) bewteen neurons in CNS exit as fiber bundles or tracts called faciculi 

• Many fasciculi connect different area of the cerebral cortex

Columns

 aggregates of fasciculi in the spinal cord are called columns 

• Vertical columns may remain on the same sife of the spinal cord or cross the midline (decussate) to the opposite side of the spinal cord 

• Brain = fasiciculi 

• Spinal cord = columns/leminisci 

Ipsilateral:

 on same side

Contralateral

on the opposite side, decussates contralateral to where cells are 

Bilateral:

on both sides

Symmetry of the nervous system

the nervous system is bilaterally symmetrical, organized into left and right hemispheres 

• Organized into right and left hemispheres 

• Some higher cortical functions such as language are represented more stronger in one hemisphere than the other

Functional maps exist within the brain

at many levels, the brain maps the outside world 

• Maps of function that can be laid down to very specific neurons 

• Cortex is plastic → can change with experience during development, you dont get more neurons dedicated to an area, teh power of those neurons becomes stronger

Sensory and motor homunculus

the brain’s sensory and motor representation of the body surface 

• Topographically faithful map 

• Body relationships preserved 

• Size of part reflects disproportional sensitivity

Peripheral nervous system (PNS) - a general overview

• Spinal and cranial nerves and ganglia (collections of neuronal cell bodies) outside of the CNS 

• PNS axons (fibers) that conduct information to the CNS are called afferent fibers (incoming fibers)

• PNS axons (fibers) that conduct information from the CNS are called efferent fibers (outgoing fibers) 

• PNS axons (nerves) connect to spinal cord by dorsal (sensory) and ventral (motor) roots

Spinal nerves

 branch off from the spinal cord 

• Each nerve is split into dorsal and ventral nerve roots 

• Dorsal roots are sensory (afferent) (coming through back of spinal cord)

• Ventral motes are motor (efferent) (goes OUT of the front of the spinal cord) 

• Ventral horn of spinal cord = lower motor neurons