Neuroanatomy

by

Basic Macroscopic Organization of the Nervous System

  • The CNS arises from a sheet of ectodermal cells that folds over during embryological development to form the neural tube
    • CNS = Brain and spinal cord
    • PNS = Cranial nerves and ganglia
    • Spinal nerves and dorsal root ganglia
    • Sympathetic and parasympathetic nerves and ganglia
    • Enteric nervous system

Main divisions and subdivisions of the human CNS (saggital view of divisions p.15)

  • Prosencephalon (forebrain)
    • Telencephalon
      • Cerebral hemispheres
      • Cerebral cortex
      • Subcortical white matter
      • Basal ganglia
      • Basal forebrain nuclei
    • Diencephalon
      • Thalamus
      • Hypothalamus
      • Epithalamus
  • Mesencephalon (midbrain)
    • Cerebral peduncles
    • Midbrain tectum
    • Midbrain tegmentum
  • Rhombencephalon (hindbrain)
  • Metencephalon
    • Pons
    • Cerebellum
  • Myelencephalon
    • Medulla
  • Spinal cord
  • cerebrospinal fluid (CSF) is formed mainly by vascular tufts lying within the ventricles and choroid plexus
  • CSF circulates from the lateral ventricles to the third ventricle, and then leaves the ventricular system via

foramina in the fourth ventricle, to percolate around the outside surface of the brain and spinal cord; once it leaves the ventricular system it travels in the space between the arachnoid and pia layers and is ultimately reabsorbed into the venous system

  • The CNS is covered by three membranous protective layers called meninges
  • Meninges (from inside to outside): pia, arachnoid, and dura (“PAD”)

Orientation and Planes of Section

  • Ventral (“belly”) – always toward the earth
  • Dorsal (“back”) – toward the sky
  • Rostral (“beak”) – toward the snout
  • Caudal (“tail”) – toward the tail

Note: in humans, below the midbrain these terms apply as if the person is on all fours like dog, but above the midbrain the terms

apply to the brain as if they are standing upright – see diagram p. 16

  • Anterior – forward, front
  • Posterior – rear
  • Superior – above
  • Inferior – below
  • Planes of section, as in CT and MRI scans (diagram p. 17):
    • Horizontal/Axial/Transverse – perpendicular to long axis of the body
    • Coronal – approximating a plane as that of a tiara-like crown
    • Sagittal – in the direction of an arrow shot, like the profile of an archer, as in the constellation “Sagittarius”
      • midsagittal: passing through the midline
      • parasagittal: just off the midline

Basic Cellular and Neurochemical Organization of the Nervous System

  • Neuron support cells: glial cells or glia
  • A typical neuron consists of:
    • cell body – containing the nucleus
    • dendrites – short processes that receive most inputs to the cell
    • axons – long processes carry most outputs
  • Review diagram of “Typical Mammalian Neuron” – p. 18
  • multipolar – has several dendrites and several axons (most mammalian neurons are multipolar)
  • axon collaterals – axons that branch off the main axon to reach different targets
  • bipolar – single dendrite and single axon arises from the cell body; bipolar neurons are often sensory neurons, such as those involved in vision or olfaction
  • unipolar – occur mainly in invertebrates, both axons and dendrites arise from a single process coming off the cell body
  • synapse – gap or space between two neuron structures in which communication occurs
    • Typically, from an axon terminal of one neuron to the dendrites of another neuron
    • However, there are also axo-axonic, dendro-dendritic, and in some cases communication can even occur in reverse, from dendrites to axon
    • chemical neurotransmitter molecules, stored mainly in synaptic vesicles, are released from presynaptic terminals. They then bind to neurotransmitter receptors on the postsynaptic neuron, giving rise to either excitation or inhibition of the postsynaptic neuron
    • in some cases, communication also takes place at electrical synapses where direct electrical coupling of neurons occurs through specialized junctions
  • action potential – a transient voltage change that occurs when excitatory synaptic inputs combine with endogenous transmembrane currents to sufficiently excite a neuron, lasts about 1 millisecond and can travel rapidly throughout the length of a neuron at rates of up to around 60 meters per second. Classically, they travel from the dendritic end of a neuron along its axon to reach presynaptic terminals
  • myelin sheath – insulating lipid layer of an axon formed by specialized glial cells; speeds the rate of action potential conduction
  • myelin-forming glial cells in the CNS are oligodendrocytes, in the PNS they are called Schwann cells
  • nodes of Ranvier – short exposed segments of axon where voltage-gated ion channels are concentrated; conduction from node to node occurs rapidly by a process called saltatory conduction
  • Two general types of functions of chemical neurotransmitters:
    • mediate rapid communication between neurons through fast excitatory or inhibitory electrical events called excitatory postsynaptic potentials (EPSPs)
    • inhibitory postsynaptic potentials (IPSPs); fast EPSPs and IPSPs occur in tens of milliseconds, and rapidly move the membrane voltage of the postsynaptic neuron between states more or less likely to fire an action potential. The postsynaptic neuron summates EPSPs and IPSPs arising from many presynaptic inputs
  • neuromodulation, generally occurring over slower time scales; it includes a broad range of cellular mechanisms involving signaling cascades that regulate synaptic transmission, neuronal growth, and other functions; it can either facilitate or inhibit subsequent signaling

Important Neurotransmitters summarized in a Table on p. 20

  • Most common excitatory CNS neurotransmitter: glutamate
  • Most common inhibitory CNS neurotransmitter: GABA (gamma-aminobutyric acid)
  • Main transmitter at neuromuscular junctions (PNS): acetylcholine
  • Both acetylcholine and norepinephrine are important in the autonomic nervous system

CNS Gray Matter and White Matter; PNS Ganglia and Nerves

  • white matter: made up of mainly myelinated axons (communication over large distances)
  • gray matter: made up of mainly cell bodies (most local communication between neurons in the CNS occurs in gray matter
  • cerebral cortex: unique mantle of gray matter over surface of cerebral hemispheres
  • nuclei: large clusters of cells (gray matter) located deep within the hemispheres and brainstem (includes the basal ganglia, thalamus, and cranial nerve nuclei
  • Note: in the cerebral hemispheres, the gray matter is on the outside and white matter on the inside, but in the spinal cord the reverse is true (white outside, gray inside); in the brainstem – grey and white are found on inside and outside, but most of outside is white matter
  • White matter pathways have several different names: tract, fascicle, lemniscus, and bundle
  • commissure – white matter pathway connecting identical structures on right and left sides
  • Axons in the PNS form bundles called peripheral nerves
  • Clusters of PNS cell bodies: ganglia
  • afferent – pathway carrying a signal toward a structure (afferents “arrive”, efferents “exit”)
  • efferent – pathway carrying a signal away from a structure

Spinal Cord and Peripheral Nervous System

  • Throughout the nervous system, motor systems tend to be more ventral, or anterior; sensory systems more dorsal, or posterior. The same is true for the spinal cord.
  • dorsal nerve roots convey mainly afferent sensory information into the dorsal spinal cord
  • ventral nerve roots carry mainly efferent motor signals from the ventral spinal cord to the periphery
  • See Figures 2.8, 2.9 (pp. 22-23)
  • cervical, thoracic, lumbar, and sacral nerve roots are displayed in the figures
  • plexus, as in brachial plexus (arm) and lumbosacral plexus (leg), is an elaborate meshwork of peripheral nerves
  • control of the arms and legs require more signal flow than does control of the chest and abdomen, thus the thickness of the spinal cord increases in those areas, producing the cervical enlargement and lumbosacral enlargement
  • autonomic nervous system is divided into sympathetic and parasympathetic
  • sympathetic – arises from thoracic and lumbar spinal levels and releases norepinephrine onto end organs, and is involved in “fight or flight” functions
  • parasympathetic – arises from the cranial nerves and from the sacral spinal levels (S2 – S4), it releases acetylcholine onto end organs and is involved in sedentary functions, such as increasing gastric secretions and peristalsis, slowing heart rate, and decreasing pupil size

Cerebral Cortex: Basic Organization and Primary Sensory and Motor Areas

  • I won’t list the four main lobes of the brain, cuz if ya don’t know ‘em yet, ya may wanna rethink this whole ABPP thing
  • sulci – crevices/infolds of brain; deep sulci: fissure
  • gyri – bumps or ridges between sulci
  • insular cortex (Figure 2.24B – p. 42)
  • operculum – “lip”; frontal operculum and parietal operculum (Figure 2.24B – p.42)
  • longitudinal fissure (or interhemispheric fissure) – midline fissure separating the two hemispheres

Surface Anatomy of the Cerebral Hemispheres in Detail

  • STUDY Figure 2.11 (pp. 26-27)…….Learn It, Love It, Live It
  • corpus callosum – main bridge/pathway between the two cerebral hemispheres
  • divisions of the corpus callosum, anterior to posterior: rostrum, genu, body, splenium
  • cingulum (cingulate gyrus) means “girdle” or “belt”
  • lingula (“little tongue”) – portion of the medial occipital lobe below the calcarine fissure
  • cuneus (“wedge”) – portion above the calcarine fissure
  • gyrus rectus (“straight gyrus”)

Primary Sensory and Motor Areas

  • Study Figures 2.12 and 2.13 (pp. 28-29)
  • Note the location of the primary auditory cortex on Heschl’s gyrus
  • Note that sensory and motor pathways are topographically and somatotopically organized in the form of a homunculus (“little man”)
  • Similarly, retinal areas are mapped onto the cortex in a retinotopic fashion
  • Also, adjacent areas of the cochlea sensing different frequencies have a tonotopic representation
  • Of course, primary motor and somatosensory cortex are lateralized to the opposite side of the body
  • Primary visual cortex represent visual inputs from the opposite visual field
  • However, auditory cortex is less lateralized, and represents more of a mixture of inputs from both ears (input from the opposite ear is slightly stronger, but not usually clinically detectable)

Cell Layers and Regional Classification of the Cerebral Cortex

  • The neocortex has six cell layers (listed from the surface inward)
  • Layer I (molecular layer)
    • dendrites and axons from other layers
  • Layer II (small pyramidal layer)
    • cortical-cortical connections
  • Layer III (medium pyramidal layer)
    • cortical-cortical connections
  • Layer IV (granular layer)
    • receives inputs from thalamus
  • Layer V (large pyramidal layer)
    • sends outputs to subcortical structures (other than thalamus)
  • Layer VI (polymorphic layer)
    • sends outputs to thalamus
  • The relative thickness of the cell layers varies according to the function of that area of the cortex. For example, primary motor cortex has a thicker layer V, because there are many more cell bodies than, say, layer IV. However, layer IV is thicker in a sensory area, such as primary visual cortex
  • You should know that Korbinian Brodmann published a map of the cortex in 1909, based on cytoarchitectonic areas (areas based on the microscopic appearance of different regions of the cerebral cortex). Often referred to as “Brodmann’s Areas”. Table 2.4 (p.31) lists them all.
  • You should know at least the following Brodmann’s areas: 1-3, 4-8, 9-12, 17-19, 22, 39, 41, 44,

Motor Systems

Main Motor Pathways

  • Most important motor pathway: corticospinal tract (pathway involving primary motor cortex to spinal cord)
  • pyramidal decussation – where the majority of the motor fibers (~85%) crossover to control movement of the opposite side of the body; this occurs at the medulla/spinal cord junction
  • lesions above the decussation result in contralateral (opposite side) weakness: lesions below the decussation result in ipsilateral (same side) weakness
  • upper motor neurons – motor neurons projecting from cortex to spinal cord or brainstem
  • lower motor neurons – their axons project out of the CNS via anterior spinal roots (or cranial nerves) to reach muscle cells in the periphery (see Fig. 2.16, p.33)

Cerebellum and Basal Ganglia

  • the cerebellum and basal ganglia act by modulating the output of the corticospinal and other descending motor systems; they also receive inputs from the brainstem and spinal cord
  • they project back to the motor cortex via the thalamus
  • lesions of the cerebellum: ataxia (disorder of coordination and balance)
  • lesions in the basal ganglia cause:
  • hypokinetic movement disorders such as Parkinsonism
  • hyperkinetic movement disorders such as Huntington’s disease

Somatosensory Systems

Main Somatosensory Pathways

  • There are 2 main pathways in the spinal cord for somatic sensations:
  • posterior column pathway (dorsal column/medial lemniscus) – proprioception, vibratory sense, and fine touch (Fig 2.18 – p.35)
  • anterolateral pathway (anterolateral system) – pain, temperature, and crude touch (Fig 2.19 – p. 36)

since some aspects of touch are carried by both pathways, touch sensation will not be eliminated by isolated lesions in either pathway

  • Thalamus – important relay center consisting of multiple nuclei – (discussed in Chp.7)

Brainstem and Cranial Nerves

  • STUDY Fig. 2.22 (pp. 38-39)……….learn the locations of the main structures of the brainstem
  • MEMORIZE the 12 Cranial Nerves by number and name so they are interchangeable in your mind and develop an understanding of their function (Table 2.5 p.40)
  • reticular formation – extends throughout the central portions of the brainstem from the medulla to the midbrain; the rostral portion plays an important function in regulating the level of consciousness

Limbic System

  • limbus = “fringe” (medial edge of the cerebral cortex)
  • olfaction and regulation of emotions, memory, appetite drives, and autonomic and neuroendocrine control
  • includes medial and anterior temporal lobes, anterior insula, inferior medial frontal lobes, and cingulate gyri; also includes the hippocampal formation, amygdala, medial thalamic nuclei, hypothalamus, basal ganglia, septal area, and brainstem
  • these areas are interconnected by a variety of pathways, including the fornix, a paired, arch-shaped white matter structure connecting the hippocampal formation to the hypothalamus and septal nuclei
  • lesions can cause: deficits in encoding, behavioral changes and psychiatric disturbance, and epileptic seizures

Association Cortex

  • unimodal – association cortex where higher-order processing takes place mostly for a single sensory or motor modality; it’s usually located adjacent to a primary motor or sensory area
  • heteromodal – association cortex involved in integrating functions from multiple sensory and/or motor modalities
  • Examples of heteromodal cortex:
    • Wernicke’s area – in the dominant (usually left) hemisphere; lesions cause deficits in language comprehension
    • Broca’s area – located in the frontal lobe (usually left); lesions cause deficits in production of language, with relative sparing of language comprehension
    • Inferior parietal lobule – lesion can produce Gerstmann’s syndrome: acalculia, right/left confusion,

finger agnosia, and agraphia

Other lesions of association cortex can cause:

  • apraxia – poor sequencing of motor movements
  • hemineglect – neglect of one side of space (and sometimes body), usually left
  • anosognosia – impaired self-awareness, especially of deficits
  • frontal release signs – primitive reflexes that are normal in infants, such as grasp, root, suck, and snout reflexes
  • perseveration
  • disinhibition
  • abulia – tendency to stare passively, respond after long delay, poor initiation
  • magnetic gait – feet shuffle close to the floor
  • urinary incontinence
  • prosopagnosia – inability to recognize faces
  • achromatopsia – inability to recognize colors
  • palinopsia – persistence or reappearance of an object viewed earlier