Basic Neuroanatomy of Vertebrates

  1. Vertebrate nervous system
    • Central nervous system (CNS): brain and spinal cord
    • Peripheral nervous system (PNS): Autonomic and Somatic nervous systems.
    • Also enteric nervous system.  This is a complex of nerves that regulate the activity of the stomach. When you get sick to your stomach or feel butterflies when you get nervous, you can blame the enteric nervous system.
    • Made up of sensory (afferent), inter, and motor (efferent) neurons.
    • Neuron: dendrites = input; axon = output;
  2. Cells of the NS
    • Neurons: electrical signalling, transmission
    • Glial cells: Microglia and Macroglia
    • Microglia: mainly phagocytes - protect clean up, repair brain (equivalent to immune cells but only found in the brain).
    • Macroglia: Oligodendrocytes, Schwann cells, Astrocytes
    • Oligodendrocytes: Found in CNS; Can myelinate many different neurons simultaneously
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    • Schwann cells: Found in PNS; Only wraps around one neuron axon
    • Astrocytes: 
    • Located between neurons and help space them out
    • Regulate K+ concentration in vicinity of neurons
    • Can take up neurotransmitters that diffuse from synapses
    • Release growth factors to nourish neurons
    • Help form the blood-brain barrier
    • Astrocytes don't have VGIC's (so not electrically-excitable cells) but have non VG potassium channels, ‘Leak channels’ that are constantly open.
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  3. Blood Brain Barrier
    • Permits water, oxygen, and lipid soluble compounds to cross.
    • Prevents charged molecules (ions) and large molecules (e.g. proteins) from crossing.
    • Blood vesicles in the brain are lined by endothelial cells that form tight junctions together.
    • The outer layer of the BBB is formed by specialised structures of astrocytes called ‘end-feet’
    • Pericytes regulate BBB-specific gene expression patterns in endothelial cells and 
    • Induce formation of astrocyte end-feet 
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  4. Axon Guidance and the Growth Cone
    • Moves forward (about 1 mm per day)
    • Steers the tip
    • Trails a single axon (or dendrite) behind it
    • Receptors in the growth cone membrane Detect the external signals: Consequently actin filaments are either assembled or disassembled

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    • Guidance: long range cues (chemoattraction/repulsion); short range cues (contact attraction/repulsion).
  5. Development of nervous system: Neurulation
    • Neurulation: when the neural plate forms into the neural tube.
    • The neural plate forms at the cranial end of the embryo and grows in a craniocaudal direction (cranial end = future brain; caudal end = future spinal cord).
    • The lateral edges of the neural plate become elevated and move together to form the neural folds (3rd week). The space created by the folding of the neural plate is called the neural groove
    • The neural folds fuse together forming the neural tube (future CNS).
    • Fusion of the neural tube usually begins in the middle of the embryo. Extending in both cranial and caudal directions.
    • Cells on the crest of the neural folds detach forming a new cell population called the neural crest (future PNS).
    • Once the neural tube have been completely fused the process of neurulation is complete.
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    • Occlusion of the neural tube: where the lower, future spinal cord region is closed of to allow the initial expansion of the brain (3 day chick). It is later reopened.
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  6. Vertebrate Brain Anatomy and Function: Brainstem
    • Sensation & motor control of head, neck, & face = 12 cranial nerves output to muscles of head
    • Specialised neurons for parasympathetic reflexes: cardiac output, blood pressure, gut peristalsis, constriction of pupils.
    • Afferent and efferent pathways carry sensory & motor information to other parts of CNS
    • Medulla oblongata: Regulates blood pressure and respiration; input area for hearing, balance, taste.
    • Pons: 2 parts = ventral and dorsal
    • Ventral pons includes ‘pontine nuclei’ - relay motor & sensory info from cerebral cortex to cerebellum
    • Dorsal portion involved in sleep, taste & respiration
    • Midbrain: Links between nodes in motor system, especially cerebellum, cerebral cortex and basal ganglia
    • Other midbrain areas involved in hearing, vision and eye control
    • -> brainstem maintains breathing and heartbeat; major damage= death
  7. Vertebrate Brain Anatomy and Function: Cerebellum
    • Key area for motor control; does not initiate movements, but:
    • > maintains posture
    • > coordinates head/eye/arm movements
    • > fine-tunes precision movement
    • > facilitates motor learning
    • Very dense – more neurons than rest of brain, but only 10% of total volume
    • Very few types of neurons, highly organised 
    • Massively dense network of highly organised and arborised ‘Purkinje neurons
    • Enormous processing power for accurate coordinating of incredibly precise movements
    • Cerebellar lesions: cause disruption of precise movements, due to failure of motor error control – ataxia, intention tremor
    • Effects of alcohol on movement: excessive ethanol disrupts signalling in complex cerebellar networks, leading to uncoordinated movement
  8. Vertebrate Brain Anatomy and Function: Cerebrum - Diencephalon
    • Thalamus & Hypothalamus have unrelated functions
    • Thalamus: key processing node for sensory input periphery to cortex
    • > Critical for all senses except olfaction
    • > Specific sensory input from sensory cortex is processed then sent to specific motor areas in cortex
    • > Involved in generating and monitoring of movements
    • Pineal gland: produces melatonin - modulates sleep patterns in both circadian and seasonal cycles
    • Pituitary gland: 2 lobes - anterior and posterior, interacts with hypothalamus
    • Hypothalamus: key endocrine site within the brain
    • > Chemical (secretory) & electrical roles (autonomic nervous system)
    • > Controls homeostasis: body temp, blood oxygen, glucose concentration, blood volume, pressure, salinity & acidity.
    • > Regulates physiology & behaviour through hormones released from pituitary.
  9. Chemical Hypothalamus
    • Magnocellular neurons in hypothalamus project to posterior pituitary
    • Synthesise hormones & release them directly into circulation
    • > Vasopressin: anti-diuretic hormone; water retention, blood pressure
    • > Oxytocin: uterine contractions, milk let down
    • These peptide hormones have different functions when released into the brain or the body on behaviour and physiology respectively. Blood brain barrier.
    • = 1 stage endocrine control
    • Parvocellular neurons in hypothalamus project to anterior pituitary.
    • Hypothalamic hormones regulate pituitary cells.
    • Pituitary cells synthesise hormones & then release into circulation
    • Indirect control by parvocellular neurons
    • = 2-stage endocrine control
    • HPA Axis = Hypothalamus-Adrenal Axis
    • HPG axis = hypothalamus, ant. Pituitary, gonads
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  10. Electrical Hypothalamus
    • Overall integration of autonomic nervous system (the Boss) - controlling and integrating centre
    • Major caudal outputs to brain stem (parasympathetic nuclei), intermediolateral cell column of the spinal cord (sympathetic preganglionic cells), and indirectly or directly to the sacral part of the parasympathetic system. Also enteric outputs.
    • Receives regulatory input from the limbic system.
    • Behaviour controlled directly by electrical stimulation + indirectly by hormones)
  11. Vertebrate Brain Anatomy and Function: Cerebrum - Telencephalon
    • Hippocampus (seahorse shape): 
    • essential for formation (but not storage) of
    • declarative memories (and not procedural memories) (Henry Molaison)
    • Amygdala: Part of 'limbic system' = key circuit regulating emotion, hormonal behaviour, motivation & olfaction
    • > Anger
    • > Fear
    • > Aggression
    • Lesions of the amygdala: Klüver-Bucy syndrome = Lack of fear & flat affect; Inability to recognise fear; Hypersexuality
    • Integrates both expression and recognition of emotion
    • (parts of) Basal ganglia
    • Cerebral cortex: Frontal lobe; separated from Parietal lobe by Central Sulcus; Ocipital lobe; Temporal lobe; Insula - underneath temporal and frontal lobes (gustatory cortex)
    • Olfactory bulb
  12. Cerebral cortex - Functional divisions
    • Sensory cortex: processes purely sensory information from each sense
    • Each sense has dedicated brain areas for analysing and processing information from sensory receptors
    • > Visual cortex (occipital lobes)
    • > Auditory cortex (temporal lobes)
    • > Gustatory cortex (insula)
    • > Somatosensory cortex (parietal lobes)
    • > olfactory cortex (temporal lobes)
    • All sensory cortices except for olfaction receive  their input from the thalamus – relay region  from primary sensory circuits
    • Olfactory cortex receives input directly from olfactory bulb
    • Somatosensory cortex: Rostral part of parietal lobe

    • Association cortex: carries out the intermediate calculations and processing between sensory input, and motor output = does NOT receive direct input from sensory organs
    • > Integration of input from sensory areas (cortex), and planning of appropriate motor outputs
    • > Many different functions, related to sensory processing, perception, attention, motor planning etc.
    • Two stages:
    • Unimodal association cortex
    • Multimodal association cortex
    • Integrates information from different senses, as well as stored information (memory, previous associations etc)
    • Higher order association involved in complex integration and processing of sensory input (e.g. language, thinking, abstract planning)
    • Multimodal association areas integrate sensory information for higher order processing:
    • 1. Anterior association area: memory, planning, decision making
    • 2. Posterior association area (temporal/proprietal): perception, language, attention
    • 3. Limbic association area: learning, memory, emotional responses

    • Motor cortex: generates coordinated output to all skeletal muscles to produce appropriate action
    • Premotor cortex
    • – Planning of movements
    • – Active (far) in advance of actual movements
    • – Integrate coordinated movement across muscle groups
    • Primary motor cortex
    • – Actual decision to move
    • – Triggers signals to muscle groups
    • – Tell each skeletal muscle to contract/relax
    • > PrimMC same layout as sensory cortex
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    • Information in -> process it -> action out
  13. Cerebrum - Basal Ganglia
    • A collection of distinct masses of gray matter lying deep in the brain that lie to the side of and surround the thalamus.
    • > Striatum (aka caudate and putamen)
    • > Nucleus accumbens
    • > Globus pallidus
    • > Substantia nigra (part of midbrain in brainstem)
    • > Subthalamic nucleus
    • Receives input from sensory/association & motor parts of cortex
    • Sends output back to premotor cortex via thalamus
    • Basal ganglia is a loop circuit, primarily involved in movement planning
    • Responsible for executing a skill pattern (something learned; riding a bike) that we can do with very little thought.
    • Complementary, antagonistic pathways within basal ganglia appear to:
    • – help select appropriate motor actions
    • – inhibit inappropriate motor actions
    • – evidence BG fine tune movements
    • Basal ganglia nuclei that regulate movement are highly dopaminergic (use dopamine neurotransmitters)
    • – Substantia nigra & nucleus accumbens (afferent dopamine projections)
    • – Ventral tegmental area (efferent dopamine projections) closely linked to these
    • Parkinson’s disease (1% of >60’s) involves dopamine depletion in striatum
    • Progressive, no cure, but symptoms can be alleviated by increasing dopamine release from remaining neurons
    • Overactive basal ganglia = hypokinesia (Parkinson’s)
    • Lesioned basal ganglia = hyperkinesia (Huntington’s) -> disorder affecting only one side
    • Chorea: Spontaneous, jerky, uncontrollable movements.
    • > Due to damage/loss of function in subthalamic nucleus
    • > Normally: subthalamic nucleus excites globus pallidus, which then inhibits thalamus = normal movement control
    • > Damage to subthalamic nucleus reduces excitatory input to globus pallidus, leading to reduced inhibition of thalamus
    • > Thalamus is involved in control of movement initiation, loss of inhibition = spontaneous movement
    • > Chorea also part of symptoms of Huntington’s disease – also involves loss of basal ganglia neurons
  14. Limbic system
    • Emotional motor system
    • The limbic system supports a variety of functions; Emotion, behavior, motivation, long-term memory, olfaction
    • Operates by influencing the endocrine system and ANS
    • A collection of structures from the telencephalon, diencephalon and mesencephalon including; olfactory bulbs, hippocampus,hypothalamus and amygdala
    • Amygdala: gateway to the limbic system and passes sensory input on to the hypothalamus
    • Hypothalamus: control center of LS; connected to the pituitary gland & ANS
    • > Responsible for the bodily expression of emotional responses, such as fear and anger
  15. Meninges
    • Layers of protective tissue between skull and brain (also around spinal cord - continuous through foramen magnum)
    • Dura mater (outer layer): This tissue forms several structures that separate the cranial cavity into compartments and protect the brain from displacement.
    • > The falx cerebri separates the hemispheres of the cerebrum.
    • > The falx cerebelli separates the lobes of the cerebellum.
    • > The tentorium cerebelli separates the cerebrum from the cerebellum.
    • The dura mater also forms several vein-like sinuses that carry used blood back to the heart.
    • The epidural space is a potential space between the dura mater and the skull. If there is hemorrhaging in the brain, blood may collect here.
    • The subdural space is another potential space. It is between the dura mater and the arachnoid mater. Blood may collect here and push down on the lower layers of the meninges. If bleeding continues, brain damage will result from this pressure.
    • Arachnoid mater: middle layer
    • Subarachnoid space between Arachnoid mater and Pia mater is filled with cerebrospinal fluid. All blood vessels entering the brain, as well as cranial nerves pass through this space.
    • Pia mater: innermost layer, adheres closely to the brain, running down into the sulci and fissures of the cortex. It fuses with the ependyma, the membranous lining of the ventricles to form structures called the choroid plexes which produce cerebrospinal fluid.
  16. Ventricles of the Brain
    • Four interconnected cavities in the brain where cerebrospinal fluid (CSF) is produced
    • A ‘choroid plexus’ (collection of cells) in each ventricle produces the CSF
    • CSF provides nutrients and drainage for the deeper parts of the brain and spinal chord
    • (Blood flow provides nutrients and drainage to surface parts of the brain)
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Author
charl_drogo
ID
344158
Card Set
Basic Neuroanatomy of Vertebrates
Description
Nervous system: cells, blood brain barrier, axon guidance, development. Functional divisions of the brain
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