OVERVIEW OF THE CNS: Evolutionary considerations [Return to Lecture Schedule]

The vertebrate CNS develops embryonically as a dorsal plate of neurectodermal cells which invaginate to form a closed, cylindrical neural tube above the notochord (which later forms the basis of the vertebral column). Go to Fig. 1. (Neural Plate Formation)

Figure 1

Figure 2: The cells of the neural tube proliferate to form neurones and glial cells (Go to Fig.2, Neural Tube Differentiation). A single layer of ciliated cells the ependymal cells, lines the inner cavity of the tube (the central canal) which is filled with a fluid, the cerebrospinal fluid. Two major layers form the wall of the neural tube: an inner layer of grey matter, next to the ependyma, and an outer layer of white matter. The grey matter is made up of the cell bodies of neurones (somata) and their interacting processes (dendrites), along with supporting and insulating glial cells, the astrocytes. Many neurones have long thin axons which leave the grey matter and run along the tube for varying distances, to different levels. These axons are myelinated by glial cells termed oligodendrocytes, and run together in bundles or tracts which travel from a given point of origin to a given destination. These myelinated axons, running together in tracts, form the superficial white matter, which is white because of the fatty myelin sheaths on the axons.

[Go to Neurons] [See also Glia] [See also Multiple Sclerosis]

The neural tube receives sensory input from, and generates patterned motor output to the segmentally arranged muscle blocks or myotomes along the body. The movements generated are in part, determined endogenously by the patterns of activity and connectivity of the cells in the spinal grey matter; and in part by the pattern of sensory inputs.

The neural tube is divided into a dorsal sensory part, and a ventral motor part. In each segment, sensory neurones, originating from the cells of the dorsal neural crest, aggregate to form a dorsal root ganglion on each side. These neurones have no dendrites, but give off a single neurite which branches into two: one branch runs out to the periphery and terminates in sensitive endings (the receptors). The other branch penetrates the dorsal aspect of the neural tube, to carry sensory information into the gray matter or along the tube in the white matter (or both).

The axons of the sensory neurones running together from the dorsal root ganglion, form the dorsal (sensory) root on each side. The "first order" sensory neurones synapse on sensory cells in the dorsal grey matter, which help to integrate the incoming (afferent) sensory information and pass it on to the ventrally located motor neurones in order to modify their activity patterns. These connections at the segmental level, give rise to more or less simple segmental reflexes, whereby feedback from sensory receptors in a segment can modify the motor activity in that segment.

Motor neurones in the ventral grey matter in each segment, on each side, send out-going (efferent) axons to the muscles. These axons bundled together, form the segmental ventral (motor) roots. The ventral and dorsal roots run peripherally, meet and fuse to form the bilateral, segmentally arranged, spinal nerves. Sensory and other information passing from one level to other levels of the tube, mediate intersegmental reflexes, ensuring co-ordination between segments - important in the generation of effective motor patterns.

Complex neural control and skeletomotor development, allow animals to move rapidly, headlong through the environment. The anterior end rapidly encounters important changes, to which fast and critical responses must be made: To approach or avoid? To accelerate or decelerate? What form of evasive action to take? To discern predator from prey, mates from meals, and to respond accordingly. In response to these challenges, whether by the action of careful design or blind forces, organisms evolved rostrally located sense organs for rapidly detecting changes at a distance, ahead in the environment. Three sensory "distance detection" systems evolved: the visual system (eyes) for discerning light, dark, shape and pattern; the acoustico-vestibular system (ears etc.) for detecting vibrations of different sorts and sources, and for recognizing orientation, and linear and rotational acceleration; and the olfactory system (nose) for recognizing chemicals and chemical gradients in the environment.

Inputs from these complex sense organs had to be processed: that is, received & discriminated (sensation), recognised & interpreted (perception), compared with the inputs from other sensory systems (integrated) and routed to appropriate motor output pathways in order to generate suitable responses. Processing is achieved through the complex interconnections between large numbers of neurones of different sorts. Three rostral expansions of the neural tube, developed in order to process the inputs from the major sensory systems: the fore-brain for olfactory inputs; the mid-brain for visual inputs; and the hind-brain for acoustico-vestibular inputs. These three domains form the brain, now demarcated from the still quite simply organised caudal part of the tube, the spinal cord. [Go to Fig.4, Brain Divisions - Development].

Progressively more derived groups of vertebrates, show increasing development of cerebral dominance, with increasing size, complexity and degree of control by more "anterior" portions of the brain. The hindbrain initially may have had the most important descending tracts. It is divided into a rostral part with the dorsal cerebellum and ventral pons, and a caudal part, the medulla oblongata. The central grey matter of the medulla, the upgraded homologue of the spinal cord grey matter, receives sensory input from the head and from ascending spinal tracts. Outputs arouse and energize neural systems in general, and regulate autonomic systems in particular. These "reticular" grey matter areas in the hindbrain (and midbrain) give rise to descending reticulo-spinal tracts, which help to regulate muscle tone. The cells of the vestibular nuclei receive input from the vestibular apparatus, and give rise to the vestibulo-spinal tracts projecting to spinal centres, to regulate balance and orientation. The cerebellum receives input from the vestibular and other sensory systems, and feeds output to the vestibular nuclei and other motor control systems, to ensure coordinated muscle action. As animals became more complex and moved onto the land the demands of balance and coordination were magnified, and the complexity of the cerebellum grew. We will see this in more detail when we study the cerebellum. .

The mid-brain received input mainly from the eyes and was perhaps the chief co-ordinating centre in the primitive brain, forming prominent optic lobes in fish and birds. Visual and other inputs were relayed to the dorsal (sensory) roof, the optic tectum, for processing, and crossed, descending tracts (the tecto-spinal tracts) carried output to the contralateral cord. Ventrally, in the walls and floor of the mid-brain (the tegmentum) major motor nuclei developed...the red nucleus and the substantia nigra (black nucleus). The red nucleus gives rise to the crossed, descending rubro-spinal tract (quite small in man), whilst the substantia nigra works in conjunction with forebrain nuclei to regulate movement (we will see its importance in Parkinson's disease later). The old central grey matter in the mid-brain remains important in regulating arousal and responses to noxious stimuli (see above), and is referred to, with its continuation in the hindbrain, as the reticular formations.

The primitive forebrain consists of an unbranched part (the diencephalon) with bilateral, dorsal outpocketings rostrally, the cerebral hemispheres (telencephalon). The roof of the diencephalon, the epithalamus, gives rise to the pineal body (the third eye) which regulates circadian activity primitively, but in man helps in regulating the onset of puberty. It synthesizes the hormone melatonin. The walls of the diencephalon form the thalamus, grey matter nuclei which relay information from lower areas to the telencephalon, and between forebrain areas. The floor forms the hypothalamus, which connects to the pituitary body, and helps to regulate both endocrine activity, through the pituitary, and autonomic activity via the midbrain and medullary centres. Primitively, the dorsal grey matter in the cerebral hemispheres was concerned with olfactory processing. A specialised posterio-medial region was involved in generating approach-avoidance tendencies (emotions- like/dislike?) in response to the olfactory and other inputs. The importance of olfaction in food, mate and family recognition, probably explains the involvement of the adjacent diencephalon in hormonal and autonomic control. The ventral grey matter, typically, is involved in motor regulation: forming in more advanced species, the basal ganglia, which regulate motor activity in conjunction with the diencephalic sub-thalamic nuclei, and the midbrain substantia nigra.

In mammals several major developments occur. The dorsal grey matter areas of the cerebral hemispheres expand and move away from the central cavity, to form a superficial, three-layered cortex. The olfactory area is now termed the paleocortex, and the emotional (?) area, the archicortex. A new, six-layered area, the neocortex develops between the two and expands tremendously, because sensory input from all areas are routed via the thalamus to regions of the neocortex for processing. Visual inputs are shunted to the neocortex via the thalamus... away from the optic lobes, which remain important for processing visual reflexes, but not in pattern recognition, and diminish in relative size. [Go to Fig.5, Cortical Development].

The development of an ear with a long, coiled cochlea, leads to new, complex sensory inputs, which relay in the medulla, and pass to the midbrain, to bilateral, swollen areas, behind the diminished optic lobes. The mid-brain roof thus becomes a four-lobed structure, the corpora quadrigemina - two superior colliculi (visual) and two inferior colliculi (auditory). From the inferior colliculi the auditory inputs relay via the thalamus to the neocortex. Sensory inputs from the receptors in the body and head also relay through the thalamus to the neocortex.

Rostral areas of the neocortex also become specialised to generate motor output, receiving processed information from the sensory areas, and working in conjunction with the basal ganglia, and associated motor nuclei. This neocortical motor area gives rise to a major, mainly crossed, descending tract, the corticospinal tract. As a result of the expansion of the neocortex, the cerebral hemispheres overshadow the remainder of the brain in size. In man because of the upright posture, the hemispheres not only overgrow and fuse with the diencephalon, but also become flexed back on themselves, curving around the midbrain and hindbrain. The corrugated neocortex forms the sulci and gyri of the cerebrum. The olfactory cortex is pushed laterally to the rostral underside of the hemispheres (pyriform cortex) separated from the neocortex by the rhinal fissure; the "emotional" cortex gets pushed medially to form the cingulate gyrus and hippocampal formations. Portions of the basal ganglia also elongate and loop around with the curving hemisphere, to form the caudate nucleus. Axon tracts running to and from the neocortex, traverse the basal ganglia, separating blocks, and giving them a striated appearance (corpora striata) .

The complex structure of the human brain, derives from simple origins by a series of integrated additions and reorganizations. Each stage of development preserves and even upgrades earlier stages, to give a set of hierarchically related, but harmoniously integrated systems, designed to respond effectively to the demands of our environment. Complex stimuli are recognized, integrated and used to regulate behavioural output, based upon the neuronal circuitry of the CNS. This circuitry in turn is developed through dynamic interactions between genetic endowment, and environmental influences. Genetic programming sets broad limits on the types and numbers of neurones, and their interconnections in the CNS. Developmental history modifies the way in which the genetic programme is expressed, depending upon individual experiences and environmental influences. The patterns of central interconnections undergo continual reorganisation, due to the ongoing process of learning, modifying the responses elicited by given senrory cues, on the basis of the results of past experiences. [Return to start]