Assigned Reading:  Jenkins pp. 347-355.

N.B.		Material which appears in italics or within [brackets] will not be covered on the exam; it is simply for explanatory purposes.

   Two centuries of experimental research have brought us to the realization that elements of both theories are correct. It is now well established that discrete areas of the brain have discrete functions. However, these functions typically involve a single component of a behavior, and usually a relatively elementary one at that (e.g., primary motor cortex). Instead, the full behavior emerges from serial and parallel interconnections between several brain regions. Thus, when we speak of the function of specific cortical areas or nuclei, we are in actuality saying that these areas are involved in or concerned with certain behaviors. It does not imply that the behavior is wholly subsumed to this area, nor does it imply that activity of this area is limited to the identified behavior .

I. Review of Nervous System

• Nervous system consists of all the neural tissue in the body; 2 anatomical subdivisions:
  • 1) central nervous system (CNS) - brain and spinal cord; derived from neural tube
  • 2) peripheral nervous system (PNS) - derived from both neural crest and neural tube; consists of:
    • (a) all the nerves that emanate from CNS (spinal and cranial; neural tube origin)
    • (b) the peripheral ganglia and the nerves originating from them; neural crest origin
  - CNS and PNS can be separated anatomically but functionally are unitary

II. Useful definitions

• neuron - a nerve cell; the fundamental element of the nervous system; in vertebrates comprised of a cell body and generally two distinct cytoplasmic processes:
  • 1) dendrites receive and convey information toward the cell body
  • 2) axons convey information (typically an action potential) away from the cell body
• nerve - a collection of axons bound together with connective tissue located in the PNS
• tract - a collection of axons in the CNS; form white matter (due to myelination)
• plexus- collection of nerves in PNS (redundant) from multiple sources (e.g., ventral rami) serving as an area of redistribution (e.g., brachial plexus)
• ganglion - collection of nerve cell bodies outside the CNS; e.g., spinal (dorsal root) ganglion
• nucleus - collection of nerve cell bodies inside the CNS; form gray matter

III. Development of the CNS

• Key to understanding anatomy of CNS is to recognize that CNS starts as a straight tube (i.e., a hollow cylinder) with walls of uniform thickness. The CNS ends up as a bent tube with a split at its rostral end and walls of varying thickness. Nevertheless, it remains a tube, albeit a highly complex one.
• Nervous system derives from ectoderm which also gives rise to epidermis of skin
  • - notochord induces formation of neural plate, a dorsal midline thickening of the ectoderm
  • - in a process known as neuralation, neural plate folds up creating a hollow neural tube which forms the CNS
  • - during neuralation neural crest cells pinched off and migrate to form peripheral ganglia of PNS (and other structures, e.g., melanocytes)
• Brain development
  • Following neural tube formation, differentiation of brain marked by three rostral swellings, the primary vesicles (I-III), which subsequently elaborate to form the five (1-5) anatomical divisions of the brain
    • I) forebrain (prosencephalon)
      • - gives rise to (1) telencephalon (cerebral hemispheres) and (2) diencephalon
    • II) midbrain - (3) mesencephalon
    • III) hindbrain (rhombencephalon)
      • - gives rise to (4) metencephalon (pons and cerebellum) and (5) myelencephalon (medulla oblongata)
  • - cavity of neural tube in the brain elaborates as ventricles through which cerebro-spinal fluid (CSF) flows
    • - in spinal cord cavity persists as central canal

prosencephalon - prosos + enkephalos = forward + brain; forebrain

telencephalon - tele + enkephalos = distant + brain

diencephalon - dia + enkephalos = through + brain

mesencephalon - mesos + enkephalos = middle + brain; midbrain

rhombencephalon - rhombo (G < IE) + enkephalos = base + brain; hindbrain

metencephalon - meta + enkephalos = after or hindmost + brain

myelencephalon - myel + enkephalos = marrow + brain; medulla (L. marrow) oblongata

IV. Anatomical directions in the brain

• In most vertebrates CNS is organized along a straight line with long axis running rostro-caudally
  • - in humans rostral-caudal axis curves due to rotation of viscerocranium under the neurocranium, thus causing shift in orientation of axes relative to body
  • - therefore directions in human CNS are intrinsic and refer to original embryonic positions, independent of their position in the adult body.
• rostral - toward the muzzle (rostrum) along axis defined by original neural tube cavity (ventricles and central canal of adult CNS)
• caudal - towards the tail along axis defined by original neural tube cavity
• dorsal - orthogonal (at right angle) to rostral-caudal axis; toward site of neural plate fusion
• ventral - orthogonal to rostral-caudal axis; towards site of notochord

V. Anatomical subdivisions of the CNS

• CNS can be divided into 6 anatomical regions, each of which develops from a distinct division of the neural tube
  • 1) spinal cord (medulla spinalis) - develops from caudal end of neural tube; motor and sensory supply of body
  • 2) medulla oblongata (myelencephalon) - caudal-most portion of brain; see brainstem
  • 3) metencephalon - rostral to medulla; divisible into:
    • a) pons (L. bridge) - ventral portion; see brainstem
    • b) cerebellum (L., little brain)
      • - integrates motor and sensory information to coordinate planning, timing and patterning of movement and posture
      • - cerebellar deficit marked by intention tremor
  • 4) midbrain (mesencephalon) - rostral to pons and cerebellum
    • a) dorsal [(tectum (L., roof)] - inferior & superior colliculi; important parts of visual and auditory pathways
    • b) ventral [tegmentum] - contains nuclei important in motor control including substantia nigra (e.g., Parkinson's disease); also see brainstem
  • 2-4) brainstem = midbrain and hindbrain ventral to ventricular system (i.e., sans colliculi and cerebellum); contains:
    • a) nuclei which give rise to cranial nerves III-XII, i.e., motor and sensory innervation of head (including autonomic and special senses)
    • b) tracts running between forebrain and spinal cord
    • c) reticular system - network of nuclei in rostral portion which mediate arousal (mental alertness)
    • d) pattern generators for locomotion, breathing and mastication
  • 5) diencephalon- caudal division of forebrain (prosencephalon); 4 divisions
    • a) thalamus - important relay nuclei to cortex of both motor and sensory systems
    • b) hypothalamus - loci for control of homeostasis = constancy of internal environment; controls ANS and hormonal secretion of adjacent endocrinal hypophysis (pituitary gland)
    • (c) epithalamus - contains endocrinal epiphysis (pineal gland) which secretes hormone (melatonin) of circadian rhythm
    • (d) subthalamus - contains nuclei important in motor control (e.g., hemiballismus)
  • 6) telencephalon - cerebral hemispheres
    • - rostral division of forebrain (prosencephalon); divisible into:
      • a) cerebral cortex - forms bulk of human brain covering dorsal and lateral sides of brainstem and diencephalon
      • b) subcortical nuclei - includes corpus striatum which are nuclei important in motor control (e.g. Huntington's disease)
    • - bilateral hemispheres separated dorsally by longitudinal fissure; communicate via corpus callosum
    • - concerned with so-called higher functions (non-reflexive): perception, cognition, emotion, memory and motor control

VI. Cerebral cortex

• Cerebral cortex is divided into 5 bilateral lobes each involving multiple functions
  • - surface of cortex convoluted consisting of elevations called gyri and depressions or fissures called sulci (L. grooves)
  • - 4 lobes named for overlying cranial bones
    • 1) frontal lobes
      • - lie in anterior cranial fossa anterior to central sulcus
      • - contains primary motor cortex (precentral gyrus) and language production center (see below)
      2) parietal lobes
      • - lie posterior to central sulcus just below parietal bones
      • - contains primary sensory cortex (postcentral gyrus)
      3) temporal lobes
      • - lie in middle cranial fossa inferior to parietal lobes and separated by lateral fissure
      • - contains primary auditory cortex and language comprehension center (see below)
      4) occipital lobes
      • - posterior to parietal and temporal lobes, supported by tentorium cerebelli
      • -contains primary visual cortex (medial surface)
    • 5) insular lobes
      • - "island" of cortex deep to lateral fissure
• lateralization = asymmetrical cortical function
  • - unlike rest of CNS, many functions in cerebral cortex not bilateral
  • - e.g., language centers located in left hemisphere of most individuals,even sinister (left handed) ones
    • a) language comprehension (Wernicke's) area
      • - adjacent to primary auditory cortex in temporal lobe
      • - lesions produce inability to understand language; not deaf; can speak, sign and write normally
      b) language production (Broca's) area
    • - in frontal lobe adjacent to primary motor cortex
    • - individuals with lesions understand language but cannot speak, sign or write grammatically
• Primary motor and sensory cortex
  • - both primary motor (precentral gyrus) and sensory (postcentral gyrus) cortex have representations of the body (homunculi (L., dim. of man) such that stimulation will produce movement or create sensation, respectively
  • - homunculi are contralateral; note extension of hindlimbs and perineum along longitudinal fissure
  • - not all body parts equally represented and representation proportional to sensitivity / motor control
    • - e.g., lips and thumb of sensory homunculus; hand, face and tongue of motor
    • - see Arterial blood supply of the cerebral cortex below for clinical significance

VII. Cranial cavity

• - brain resides within cranial cavity formed by neurocranium
• - floor of cranial cavity forms 3-tiered cranial fossae
  • 1) anterior: formed by frontal, ethmoid, lesser wing and body of sphenoid; supports frontal lobe
  • 2) middle: formed by sphenoid (body & greater wing) and temporal anterior to crest of petrous; supports temporal lobe
  • 3) posterior: formed by temporal posterior to crest of petrous and occipital; supports cerebellum and caudal brainstem

VII. Meninges (Gr. membrane)

• - brain, like spinal cord, covered by 3 concentric membrane which support and protect CNS; from superficial to deep:
  • 1) dura mater (L., tough mother) - divisible into 2 layers
    • external = periosteum of cranial cavity; limited to cranial cavity
    • internal - fibrous membrane continuous with dura mater of spinal cord
  • 2) arachnoid mater (L., spider-like mother)
  • 3) pia mater (L., tender mother) - adherent to brain
• - as in spinal cord, CSF flows in subarachnoid space
• Dural Septa
  • - divide cranial cavity into compartments providing support
    • 1) falx (L., sickle) cerebri - divides cerebral hemispheres; extends into longitudinal fissure
    • 2) tentorium cerebelli - separates occipital lobe from cerebellum, supporting latter
      • - brainstem passes through aperture
    • 3) falx cerebelli - extends inferiorly from tentorium along midline; divides cerebellar hemispheres
    • 4) diaphragm sellae - attaches to clinoid processes; separates hypophysis and hypothalamus; infundibulum of hypothalamus passes through aperture
• Arterial blood supply of dura mater
  • - primarily by middle meningeal a, br. of maxillary (terminal br. of external carotid; enters cranial cavity via foramen spinosum
• Nerve supply of dura mater
  • - primarily by CN V (but also CN X and C1-3); headaches

IX. Dural Venous Sinuses

• - veins within dura mater which drain blood from brain, primarily to internal jugular
• 1) superior sagittal - dorsal border of falx cerebri
• 2) inferior sagittal - ventral (free edge) border of falx cerebri
  • - joins with great cerebral vein (from brain) to form straight sinus
• 3) straight sinus - runs in junction of falx cerebri and tentorium cerebelli; drains to confluens of sinuses
• 4) confluens of sinuses - junction of occipital, superior sagittal and straight sinuses at internal occipital protuberance
• 5) transverse sinuses - bilateral; run laterally (transversely) from confluens
• 6) sigmoid sinuses - bilateral; continuation of transverse to jugular foramen
• 7) occipital sinus - within attached border of falx cerebelli; drains to both vertebral plexus and confluens
• 8) cavernous sinus - located on either side of sella turcica; drains posteriorly to internal jugular or anteriorly to tributaries of facial vein
  • - running within the cavernous sinuses are the following structures:
    • a) CN VI (abducens)
    • b) internal carotid a - countercurrent circulation
• 9 & 10) superior and inferior petrosal sinuses - drain cavernous sinusto internal jugular
• - dural sinuses are valveless, hence bi-directional; infections from the face, scalp or vertebral column can enter the CNS via the cavernous sinus, emissary veins or occipital sinus, respectively

X. Ventricular system

• - elaboration of neural tube cavity in brain
• ventricles 1 & 2 = lateral ventricles (bilateral); (stubby) pitchfork-shaped cavities within each cerebral hemisphere; communicates with 3rd via interventricular foramen
• 3rd ventricle - midline; between halves of the diencephalon
• cerebral aqueduct - flows through mesencephalon; connects 3rd and 4th ventricles
• 4th ventricle - between pons and medulla (ventrally) and cerebellum (dorsally); median and lateral apertures connect ventricular system to subarachnoid space; continuous caudally with central canal of spinal cord

XI. Cerebro-spinal fluid (CSF)

• - produced by choroid plexus within ventricles
• - circulates internally within ventricles (brain) and central canal (spinal cord) and externally in subarachnoid space; communication occurs at median and lateral apertures of the fourth ventricle
• - absorbed by arachnoid villa which project into dural venous sinuses (hypertrophy with age forming arachnoid granulations and associated osseus depressions)
• - protein-free filtrate of blood plasma; contains a few lymphocytes; clear, colorless
• - replaced several times a day (80 - 120 ml in volume]
• - functions:
  • 1) removal of brain metabolites (including heat)
  • 2) mechanical cushioning - CNS suspended in water bath
  • 3) circulation for hypothalamic hormonal peptides
  • 4) homeostasis - pH effects pulmonary ventilation and cerebral blood flow

Total blood flow to the brain is about 750-1000 ml/min (70% via carotid system; 30% via vertebral). The high metabolic rate of the brain renders it vulnerable to disturbances in its blood supply; anoxic and ischemic episodes lasting a few seconds can produce neurological symptoms, those lasting a few minutes can produce irreversible neuronal damage.

XII. Arterial blood supply of brain

• - 2 pair of bilateral arteries (internal carotids and vertebrals)
  • 1) internal carotid - terminal branch of common carotid tortuous route: ascends within carotid canal; runs through cavernous sinus and terminates adjacent to posterior clinoids; major branches:
    • A) ophthalmic - small; runs within optic nerve to supply internal eye
    • B) anterior cerebral - terminal br; runs within longitudinal fissure
      • i) anterior communicating a - connects anterior cerebral aa
    • C) middle cerebral - terminal br; runs through lateral fissure
  • 2) vertebral a - branch of subclavian; ascends through transverse foramina and enters via foramen magnum; unites with opposite at caudal end of pons to form basilar; major branches:
    • A) posterior inferior cerebellar
    • B) basilar - formed by union of L & R vertebral
      • i) anterior inferior cerebellar
      • ii) superior cerebellar
      • iii) posterior cerebral
        • a) posterior communicating - joins internal carotid

XIII. Cerebral arterial circle (pentagon) (of Willis)

• - series of anastomoses between vertebral (basilar) and internal carotid systems
• - capacity decreases with age
• - which vessels participate in the cerebral arterial circle?

XIV. Arterial blood supply of the cerebral cortex

• 1) anterior cerebral - br of internal carotid; supplies medial aspects of frontal and parietal lobes including hindlimb and perineal portions of primary motor and sensory cortex.
• 2) middle cerebral - br of internal carotid; supplies most of temporal lobe and lateral aspects of frontal and parietal lobes including both motor and sensory primary cortex (excluding hindlimb and perineal regions); also languagecenters (left side only)
• 3) posterior cerebral - br of basilar; supplies occipital lobe (including primary visual cortex) and postero-inferior temporal lobe