Planes and Directions in Nervous Systems

Planes and Directions

• Nervous systems are studied by slicing them up (literally or virtually)
• Three sets of planes are most commonly used: coronal (frontal), axial (transverse, horizontal), and sagittal
• Coronal planes are parallel to the front of the face
• Axial planes are perpendicular to the long axis of the body
• Sagittal planes are parallel to the one that divides the body into two symmetric halves
• Dorsal means "toward the back", the same direction as superior ("toward the sky")
• Dorsoventral axis is perpendicular to the anteroposterior axis
• Rostral ("toward the beak") and caudal ("toward the tail") have roughly the same meaning as anterior and posterior
• Superior means toward the sky, and anterior continues to be the direction in which one is heading
• Dorsal is equivalent to posterior in the brainstem and spinal cord but becomes equivalent to superior in the cerebrum
• Caudal means toward the sacral spinal cord/midbrain, and rostral means toward the nose

Cerebral Hemispheres

• Each cerebral hemisphere has a cerebral cortex covering of a few millimeters thick
• Contains white matter interconnecting cortical areas with each other and with various subcortical structures
• Includes gray matter areas that are major components of the basal ganglia and limbic system
• Developed from a hollow epithelial tube, fluid-filled remnants extend through most parts of the adult CNS, with the portion in each cerebral hemisphere being a lateral ventricle

Cerebral Cortex

• Covers nearly the entire surface of the cerebral hemisphere
• Almost all of it is neocortex, named because it has a six-layered structure found only in mammals
• Humans have much more neocortex than most other mammals do
• Brain is prominently corrugated into a series of gyri and sulci to accommodate this increased area
• Three prominent sulci: central, lateral, and parieto-occipital, are landmarks to divide each hemisphere into frontal, parietal, occipital, and temporal lobes
• The corpus callosum is a thick fiber bundle interconnecting the two hemispheres
• Insula is a separate area of cortex buried in the lateral sulcus

Neocortical Structure

• Two main classes of cortical neurons: pyramidal cells and nonpyramidal cells
• Pyramidal cells are more numerous and often fairly large with conical apexes pointing toward the cortical surface
• All pyramidal cells make excitatory (glutamate) synapses on their targets.
• Many nonpyramidal cells are small, multipolar neurons with relatively short axons
• Nonpyramidal cells are the principal interneurons of the cortex
• They make inhibitory (GABA) synapses on their targets
• Cortical neurons arranged anatomically in layers parallel to the cortical surface, and functionally into columns about 50 to 500 µm wide
• The most superficial cortical layer contains few neurons meaning interactions occur

Neocortical Connections

• Each area of neocortex receives inputs both from other cortical areas and from subcortical sites
• Inputs from cortical areas in the same hemisphere arrive from neighboring areas via short U-fibers and from faraway areas through longer association bundles
• Inputs from cortical areas in the contralateral hemisphere arrive through two commissures, the largest of which being the corpus callosum
• Subcortical inputs arise primarily in the thalamus and in a series of small nuclei, which provide widespread modulatory inputs
• Cortical pyramidal cells project to other cortical areas and many other areas of the CNS (basal ganglia, limbic system, thalamus, numerous brainstem nuclei, and spinal cord)

Cortical Maps

• The structure of neocortex varies in different areas due to the area's specific function like areas that give rise neurons with particularly long axons have populations of large pyramidal cells making the area thicker
• Mapping systems parcel the neocortex into a series of Brodmann's areas, based on a numerical system

Functional Localization

• Neocortex is functionally divided it into primary areas, unimodal association areas, multimodal association areas, and limbic areas
• Primary areas deal most directly with elemental functions
• Unimodal association areas work on more complex or refined aspects of the same function
• Multimodal association areas receive major inputs from unimodal areas of multiple types
• Two major expanses of multimodal association cortex are the parts of the frontal lobe anterior to motor association areas (prefrontal cortex) and the parts of the parietal and temporal lobes bordered by somatosensory, visual, and auditory association cortex

Limbic System

• Limbic areas of cortex, together with the hippocampus, the amygdala, and some diencephalic structures are centrally involved in drives, emotions, and some forms of memory
• The hippocampus is a strip of three-layered cortex rolled into the medial part of the temporal (or limbic) lobe but still continuous with the parahippocampal gyrus
• The amygdala is a collection of nuclei at the anterior end of the hippocampus

Basal Ganglia

• The term basal ganglia is also, and more appropriately, called the basal nuclei
• A group of nuclei and their interconnections located partially in the cerebrum and partially in the brainstem, are prominently involved in the initiation and control of movement and broader cerebral functions
• Major elements of the basal ganglia situated in each cerebral hemisphere are the caudate nucleus, putamen, and globus pallidus
• Putamen and globus pallidus together are referred to as the lenticular nucleus because of their physical adjacency
• The caudate nucleus and putamen are the major input structures
• The globus pallidus is the major output structure
• The circuit of basal ganglia connections does not cross the midline, it winds up affecting cortex on the ipsilateral side.
• Because each cerebral hemisphere controls muscles on the contralateral side of the body, unilateral damage to the basal ganglia affects movements on the contralateral side

Diencephalon

• Includes the pineal gland, thalamus, hypothalamus, and a few other nuclei not visible in sections
• Portion of the ventricular system is the third ventricle
• Pineal is a neuroendocrine gland involved in circadian rhythms and seasonal cycles
• The thalamus and hypothalamus are groups of nuclei, each group having a distinctive function

Thalamus

• All specific functional pathways to the cerebral cortex pass through the thalamus
• The thalamus acts as a bank of switches that regulate the access of these functional pathways to their cortical targets
• All thalamic nuclei are constructed and behave similarly
• Most of the neurons are thalamocortical neurons whose axons synapse directly in the cerebral cortex
• The switch function is based on the two stable physiological states in which these neurons can exist: tonic mode and burst mode
• Two kinds of inputs for each thalamocortical neuron: specific inputs conveying the information that can be passed accurately to the cortex, and regulatory inputs that collectively determine the physiological state of the neuron
• Relay nuclei receive specific inputs from subcortical structures and project to primary and unimodal association areas
• Association nuclei receive specific inputs primarily from association cortex and project back to association cortex
• Regulatory inputs are provided to both kinds of thalamic nuclei by the thalamic reticular nucleus, by diffuse modulatory projections from the brainstem, and by the cerebral cortex itself

Thalamic Nuclei

• The thin layer of white matter (the internal medullary lamina) partitions the thalamus into anterior, medial, and lateral divisions
• Larger lateral division includes three nuclei arranged in an anteroposterior sequence: the ventral anterior (VA), ventral lateral (VL), and ventral posterior (VP) nuclei, followed by the pulvinar
• Appended to the bottom of the pulvinar is a little curved extension containing the medial geniculate nucleus (MGN) and lateral geniculate nucleus (LGN)
• Each of these nuclei has a specific pattern of connections
• The dorsomedial nucleus and pulvinar are the association nuclei for prefrontal cortex and for parietal-occipital-temporal association areas, respectively.
• The rest are relay nuclei: VP, MGN, and LGN for the somatosensory, auditory, and visual systems; VA and VL for the basal ganglia and cerebellum; and the anterior nucleus for the hippocampus

Internal Capsule

• Thalamocortical axons, together with the axons of cortical pyramidal cells on their way to subcortical sites, funnel into a small cleft bordered laterally by the lenticular nucleus and medially by the thalamus and caudate nucleus where they form the internal capsule

Hypothalamus

• Major control center for the autonomic nervous system (ANS) and for drive-related behavior

Brainstem

• Structures packed together in the brainstem are long tracts on the way to the thalamus, long tracts on their way downstream from the cerebral cortex and other places, and tracts and nuclei related to cranial nerves, all embedded in a neural matrix called the reticular formation

External Anatomy of the Brainstem

• The brainstem is subdivided into the midbrain, pons, and medulla
• Each subdivision is characterized by a set of surface landmarks and cranial nerve (CN) attachment points
• Dorsally Midbrain has two pairs of bumps called the superior and inferior colliculi
• Ventral surface has two massive fiber bundles, the cerebral peduncles, separated by a cleft into which the oculomotor nerve (CN III) emerges
• Forth ventricle funnels into the cerebral aqueduct which traverses the midbrain
• Pons has a large bulge on its anterior that looks as if it interconnects the two halves of the cerebellum, three cerebellar peduncles tether the cerebellum to the brainstem
• Medulla has two longitudinal elevations on the anterior surface, in line with the cerebral peduncles but smaller called the pyramids

Internal Anatomy of the Brainstem

• a combination of the neural structures underlying the surface features just described, long tracts, and sensory and motor nuclei dealing with CNs III to XII, all surrounding or embedded in the reticular formation

Cranial Nerves

• Categorized as special sensory nerves, somatic motor nerves, and branchiomeric nerves
• CN I (olfactory): axons of olfactory receptor neurons in the olfactory musoca, projects directly to a cerebral hemisphere without passing through the thalamus
• CN II (optic): ganglion cells, cross in optic chiasm, and the crossed and uncrossed fibers regroup as the optic tract and project to the lateral geniculate nucleus; a tract of the CNS
• CN III (oculomotor): motor nerve for four of the six extraocular and principal muscle
• CN IV (trochlear): motor nerve for the superior oblique, one of the extraocular muscles
• CN V (trigeminal): somatosensory nerve for the face and the motor nerve for the muscles used for chewing
• CN VI (abducens): motor nerve for the lateral rectus,
• CN VII (facial): motor nerve for facial muscles and the sensory nerve for taste buds on the anterior two thirds of the tongue
• CN VIII (vestibulocochlear): sensory nerve for hearing and balance.
• CN IX (glossopharyngeal): innervates test buds on the posterior third of the tongue
• CN X (vagus): motor axons for striated muscles of the larynx and pharynx as well wanders throughout the thoracic and abdominal cavities
• CN XI (accessory): motor nerve for muscles that shrug the shoulder and turn the head
• CN XII (hypoglossal): motor nerve for most tongue muscles
• Arranged in a set of nuclei in the brainstem

Reticular Formation

• Has a network of neurons and connections occupying much of the core of the brainstem
• Different regions are specialized for particular functions
• Prominent among these are the locus ceruleus and midline raphe nuclei which distribute norepinephrine and serotonin

Cerebellum

• Has a distinctive cerebellar cortex covers its surface and receives most of the input, and a series of deep nuclei embedded beneath the cortex provide the output
• Involved in the regulation of movement largely by affecting the output of the cerebral cortex and also cerebral functions
• One side of the cerebellum affects the ipsilateral side of the body

Spinal Cord

• Simplified version of the brainstem, containing spinal nerves, and a reticular formation
• The basic cross-sectional anatomy of the spinal cord includes a central H-shaped core of gray matter surrounded by white matter. -Horns of gray matter subdivide the white matter into posterior, lateral, and anterior funiculi
• Spinal nerves define a segmentation of the spinal cord. Human spinal cords have 31 segments: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal
• Each dorsal root conveys information from a restricted area of skin (a dermatome), and each ventral root innervates muscles in a restricted part of the body