Anatomical Organisation

Spinal Cord | Brain Stem & Cerebellum | Diencephalon | Cerebral Hemispheres | Table of structures listed in the atlas

Superficially, the wallaby brain consists of several distinct structures, of which the cerebral cortex accounts for the largest part (Fig. 8). It is a convoluted structure, comprising four lobes: frontal, parietal, temporal and occipital, and some of its surface area lies within sulci. The olfactory bulb is considered an extension of the cortex. Caudal to the cerebral hemispheres lie the brain stem (including cerebellum) and spinal cord.

The mammalian central nervous system is commonly divided into six anatomical sectors: spinal cord, medulla, pons and cerebellum, mesencephalon (midbrain), diencephalon (between brain) and the cerebral hemispheres. In summarising the anatomical organisation of the wallaby brain, we have deviated from this convention by dividing the brain as follows: spinal cord, brain stem (medula, pons, and midbrain) and cerebellum, diencephalon (thalamus and hypothalamus) and the cerebral hemispheres. A brief anatomical and functional summary of these regions follows.

Spinal Cord

The most caudal part of the central nervous system, the spinal cord extends from the base of the skull to the second lumbar vertebra. Enveloped in three layers of membrane (meninges), it consists of nerve cells and bundles of nerves connecting the brain with the internal organs, skeletal joints, skin and muscles of the trunk and limbs.

Brain Stem & Cerebellum

The enlarged, rostral extension of the spinal cord known as the brain stem comprises the medulla, pons and midbrain. The pons and medulla are together known as the bulb or bulbar area. Cranial nerves which carry sensory inputs and motor outputs are analogous to spinal nerves in the spinal cord.

The medulla, which is formed by an enlargement of the spinal cord operates in conjunction with the pons to regulate blood pressure and respiration. Immediately rostral to the medulla lies the pons which also functions to relay inputs from the cerebral hemispheres to the cerebellum.

Although the cerebellum is usually considered separate from the brain stem, there are good reasons for including it in a description of this part of the brain. In the first place, many motor functions of the pons and cerebellum are closely related. In addition, it is formed from the dorsal aspect of the hindbrain. Lying dorsal of the pons and medulla, the cerebellum is a highly laminated structure wrapped around the brain stem. It comprises several functionally independent lobes which receive somatosensory afferents from the spinal cord, motor inputs from the cerebral cortex and data about balance from vestibular nuclei of the inner ear. The cerebellum integrates these data, co-ordinating the planning, timing and patterning of skeletal muscle responses. In addition, the cerebellum is implicated in posture maintenance and the co-ordination of head and eye movements.

Lying rostral to the pons is the midbrain, which also participates in eye movement and motor control of skeletal muscle. Important relay nuclei of the auditory and visual systems are found in the midbrain.

Diencephalon

The diencephalon, which comprises the thalamus and hypothalamus, lies between the cerebral hemispheres and midbrain. Almost all sensory and motor information destined for the cerebral cortex is processed by, and passes through, the thalamus. Awareness and emotional aspects of sensory data are also believed to be mediated by this structure. Major thalamic nuclei are the lateral geniculate body, pulvinar (or posterior thalamic nucleus), midline nuclei, stria medullaris thalami and posterior thalamic peduncle.

The hypothalamus, which is located ventral to the thalamus on each side of the third ventricle, regulates the autonomic nervous system and hormonal secretions of the pituitary gland. The major structures of the hypothalamus include the paraventricular nucleus, periventricular zone, supraoptic nucleus, nucleus basalis, fornix lateral hypothalamic area, ventromedial nucleus, arcuate nucleus, optic tract, mammillary body and median eminence.

Cerebral Hemispheres

The cerebral hemispheres make up the bulk of the mammalian brain. Separated by the interhemispheric fissure, they comprise the cerebral cortex and underlying white matter, the basal ganglia, hippocampal area and amygdala.

Divided into functional areas (sensory cortex, association cortex and motor cortex), the cerebral cortex is that part of the brain most directly concerned with consciousness and voluntary activity; and has direct or indirect connections with all parts of the body. It plays an essential role in perception, memory, mental ability and, in higher mammals, original thought and intellect.

The basal ganglia comprise the globus pallidus, corpus striatum (consisting of caudate nucleus and putamen), subthalamic nucleus and basal forebrain (substantia innominata). Although the structure receives data from the four lobes of the brain, its projections innervate only the frontal cortex, by way of the thalamus. The basal ganglia are important in regulating movement and play a role in cognition.

The corpus striatum, which consists of the caudate nucleus and putamen, is the largest component of the basal ganglia and is located deep within each hemisphere. It is a large “C” shaped body which lies medial to the internal capsule and is related throughout its length to the surface of the lateral ventricle. The caudate nucleus is not completely separated from the putamen by the internal capsule, although histologically, the two structures are not homogeneous.

The hippocampal formation and amygdala form part of the limbic system which is involved in the expression of instinct and mood in activities of the body's motor and endocrine systems. The limbic system co-ordinates visceral responses such as blood pressure, heart rate and pupillary size with that of motivation by moderating the activity of the autonomic nervous system. Through its regulation of the release of hypothalamic hormones, it is able to control the body's endocrine system.

The hippocampal formation is one of the brain’s most primitive parts. It is formed by a curved band of cortex found within each cerebral hemisphere and is involved with complex physical aspects of behaviour governed by emotion and instinct, as well as memory storage.

The amygdala is an almond-shaped nucleus of grey matter lying in the dorsomedial part of the temporal lobe and consists of several sub-nuclei. It has extensive connections with the olfactory system and it also innervates the hypothalamus. The amygdala's role is to co-ordinate the functions of the autonomic and endocrine systems, and is thought to be concerned with mood, feeling, instinct and possibly, memory of recent events.

Although this has been a very brief anatomical overview, the following table relates structures we have identified in the wallaby with major divisions of the mammalian brain. By placing these structures in an overall anatomical context, it is hoped the reader will gain greater insight into the wallaby brain.