Neuroscience/Objectives/Lectures 1-3

From PhysioWiki

Basic structure and function of the CNS

Diagram and compare the basic organization of gray matter and white matter in the cerebral hemispheres and the spinal cord.

In the cerebral cortex, white matter is generally deep to the superficial gray matter. That is, neuron cell bodies are superficial while fiber tracts are deep. Exceptions to this rule include the substantia nigra, putamen, and globus pallidus, which are deep clusters of cortical cell bodies.

The reverse is true in the spinal cord, which has white matter superficial to the deep gray matter.

List the major anatomical divisions of the CNS. Note that you should be able to do this in serveral ways.

• Brain
  • Telencephalon
    • Cerebral cortex
    • Limbic system (including the amygdala, hippocampus)
    • Basal ganglia
    • White matter includes the corpus callosum, fornix, and anterior and posterior commissures
  • Diencephalon
    • Thalamus
    • Hypothalamus
  • Brainstem
    • Mesencephalon (midbrain)
    • Metencephalon (pons and cerebellum)
    • Myelencephalon (medulla)
• Spinal cord
  • Afferents
  • Efferents

List the cell types in the CNS and their general functions.

• Neurons process information
• Glia provide support and some function
• Vascular cells produce the blood-brain barrier
• Stem cells are neuronal and glial progenitors located (for example) in the subventricular regions

List the general categories of CNS neurons in terms of their functions. Note that you should be able to do this in several ways.

• Projection neurons connect the PNS to the CNS. They may be a type of CNS interneuron or may extend from or to CNS
• Relay interneurons relay information across long distances
• Local interneurons process information within local circuits

Distinguish between the following terms:

Sensory and motor pathways
Sensory pathways are afferent pathways carrying sensory input from the periphery to the CNS. Motor pathways are efferents that carry motor input from the CNS to the periphery.

Ipsilateral and contralateral
Ipsilateral means "same side"; an ipsilateral axonal projection starts and ends on the same side of the body. Contralateral means "opposite side"; a contralateral axonal projection starts on one side of the body and ends on the other. This crossing over of projections is called a decussation.

Tonotopic and somatotopic
Tonotopy is the physical organization of tissue participating in the perception of sound (eg, cochlea, auditory cortex). Somatotopy is the arrangement of tissues responsible for motor and sensory pathways (eg, cortex).

Primary and secondary neurons
Primary neurons are those whose cell bodies are in the dorsal root ganglia. They synapse on secondary neurons in the ventral horn of the spinal cord.

Primary and association cortex
Primary cortex is the principal processing area for a given set of stimuli, such as the primary visual cortex which processes visual inputs. Association cortices are integrative areas which take and process inputs from multiple primary, secondary, etc. cortices.

Hippocampus and thalamus
The hippocampus is a medial temporal lobe structure responsible for memory recall and consolidation, especially as it pertains to spatial memory. The thalamus is an area of the diencephalon that relays sensory information from the sense organs to the primary sensory cortices.

Define and understand the functional meaning of the following terms:

A segmental reflex is a simple behavior controlled by direct connections between the sensory and motor systems. The knee-jerk reflex is an example of a segmental reflex.

Ascending pathways are afferent pathways that typically originate in the peripheral nervous system and end in the central nervous system. For example, the anterolateral system is an ascending pathway.

A descending pathway (such as the corticospinal tract) carries information from the central nervous system to the periphery.

Decussation is the crossing over of a tract from the ipsilateral to the contralateral side. This occurs in both ascending (eg, the medial lemniscus system) as well as descending (eg, corticospinal tract) pathways.

Lateral inhibition is the presence of inhibitory input to a single layer of neurons. The neurons in a given layer will send feedback inhibition to other neurons in the same layer, and the amount of inhibitory input depends on the activity of a given neuron. Thus the most active neurons will simultaneously send the most inhibitory input to other neurons within the same layer. Consequently, only a few neurons are active at any one time, resulting in very fine signals that are easy to discriminate. Without lateran inhibition, signals are spread wide across neurons of a single layer, resulting in a low signal-to-noise ratio.

The somatosensory cortex is the cortical area (posterior to the central sulcus) that receives sensory input from the body. It is arranged predictably with, for example, the lower body being represented medially, and the head being represented laterally and inferiorly.

Wernicke's area is a structure in the auditory cortex responsible for understanding speech. Broca's area is a frontal structure important in speech generation. Wernicke's and Broca's areas communicate via the arcuate fasciculus.

The visual cortex is a topographically organized structure in the occipital lobe that receives and processes sensory input indirectly from the retina via the lateral geniculate nucleus of the thalamus.

Thalamic relay nuclei include the lateral and medial geniculate nuclei, which are responsible for relaying visual and auditory information from the retina and cochlea, respectively.

Understand how the following aspects of CNS cell biology are important in medical neuroscience.

• Axonal transport
  • Anterograde (eg, mitochondrion and vesicle trafficking; fast; mediated by kinesin)
  • Retrograde (eg, neurotransmitter recycling; slow; mediated by dynein)
• Synaptic vesicle fusion
  • Vesicles filled with neurotransmitter
  • Dependent in part on calcium influx once action potential reaches terminal bouton
• Roles of non-neuronal cells in CNS function and dysfunction
  • Glia (support, scavenging, blood-brain barrier, glioma)
  • Vascular cells (blood-brain barrier, hemorrhage)
  • Stem cells (neuronal and glial regeneration)

Define the components of the blood-brain barrier and its importance.

The blood-brain barrier is chiefly composed of endothelial cells joined by tight junctions. Astrocytic processes surround some segments of the endothelium. Ultimately, non-diffusible drugs and nutrients from the bloodstream must pass at least the inner plasma membrane, cytoplasm, and outer plasma membrane of the endothelial cell. In parts of the vasculature covered by astrocytic processes, nutrients and drugs must cross these processes as well.

This barrier constitutes the main means by which the brain is protected by blood-borne agents.

Not all regions of the brain are protected by the blood-brain barrier. For example, the organum vasculosum of the lamina terminalis (OVLT), a structure anterior to the hypothalamus, does not possess a blood-brain barrier, nor do several other circumventricular organs.

Diagram the PNS-CNS junction and be able to indicate where it occurs in the spinal cord and the brainstem.

The junction between PNS and CNS occurs deep to the dura mater at the level of the pia mater in both the brainstem and spinal cord. At nodes of Ranvier there is an abrupt transition between CNS and PNS marked by a change in cell type responsible for myelination of axons (CNS uses oligodendrocytes, PNS uses Schwann cells).

List at least two specific functions with each of the five major regions of the CNS.

• Telencephalon
  1. Conscious perception and motor control
  2. Special sensory systems (vision, audition, etc.)
  3. Learning and memory
  4. Higher control of autonomic responses
  5. Higher functions (eg, language)
  6. Deep structures involved in motor behavior (basal ganglia)
• Diencephalon
  1. Autonomic and endocrine regulation (hypothalamus)
  2. Cortical relay center (thalamus)
• Brainstem
  1. Segmental motor, sensory, and reflex function
  2. Intersegmental reflexes relative to head and neck via cranial nerves
  3. Cardiovascular control center
  4. Repiratory center
  5. Other visceral centers
  6. Pathways connecting spinal cord, cerebellum, diencephalon, telencephalon, etc.
  7. Relay nuclei for special senses such as vision and hearing
• Cerebellum
  1. Motor behavior
  2. Learned motor patterns
  3. Coordination
  4. Timing
  5. Accuracy
• Spinal cord
  1. Segmental motor, sensory, and reflex function
  2. Intersegmental reflexes
  3. Transmission of sensory information to higher centers
  4. Execution of motor commands from higher centers