Neuroanatomical vocabulary and concepts1

Authored by Henry Strick van Linschoten

The nervous system

The human nervous system works closely together with the endocrine system to channel, process and integrate information about the environment and about the body as a whole, and to take action or not, on a macroscopic or microscopic, external or internal scale, as a result.

All animals (with the exception of sponges) have at least a rudimentary nervous system. The modern view is that all vertebrates (i.e. including fish, amphibians, reptiles and birds) rely on a central nervous system (CNS) that includes the spinal cord, hindbrain, midbrain, forebrain and optical nerve. The brain and spinal cord together are called the central nervous system (CNS), and all other parts, mainly nerves leading to and coming from the CNS, are termed the peripheral nervous system (PNS).

1The majority of specialised words used here designate locations in the nervous system. These are typically locations that are characterised by having a common function or functions. Where possible a short description of this function will be given, as well as an idea of which other parts of the nervous system the particular location is connected with. Some terms are different and do not specify a location, but by the use of the term make clear something about the type or shape of the group of (almost always) neurons or parts of neurons that they refer to. In particular cortex and neocortex are in the latter category, and refer to a type of sheet of cells which is present in various locations, although they are regularly used as shorthand for the cerebral cortex.

Focusing on the control and processing of information, the following major divisions of the nervous system can be distinguished:

  • the sensory division of the peripheral nervous system that obtains information through the senses, which in general is passed to the central nervous system
  • the central nervous system that processes information leading to “conclusions” or “decisions” and in turn sends signals to the motor division of the PNS, and/or to the endocrine system
  • the motor division of the peripheral nervous system, that executes the “instructions” it is given by passing them on to muscles or glands

Neurons, synapses and neurotransmitters

A neuron (alternative spelling sometimes “neurone” – sometimes referred to as nerve cell or even brain cell) is a special kind of cell. There are many types and shapes of neurons; they have in common that they are part of the nervous system of animals. Neurons process and transmit information contained in incoming and outgoing signals through electrochemical means. A neuron typically consists of a cell body (soma), dendrites and an axon. A neuron often has many dendrites (dendrite tree), but usually it has only one axon, although that axon can branch many times. The dendrites receive inward signals, the axons send them out.

Sensory neurons typically receive their information through a number of dendrites, which then locally connect to an axon to take the information further in the direction of the neuron body.

When two neurons are connected an axon of one neuron is very close to a dendrite of the other. Where they (almost) touch is called a synapse.

When multiple neurons are connected, this is called a neural network. Axons and dendrites can touch each other or interface with other types of cells, e.g., muscle, skin or glandular cells, either for obtaining or sending information.

Neurons do not reproduce. The generation of human neurons – neurogenesis – starts in the womb and peaks around birth time. In adults far fewer new neurons are generated, though the process never ceases completely.

The distribution of neurons over the human brain seems somewhat skewed: a recent detailed update (Azevedo et al., 2009) concluded that the brain in total has about 86 billion neurons, of which the cerebellum has 69 billion, the cerebral cortex 16 billion, and the rest of the brain, including the whole brainstem, in the order of 1 billion. From different sources there is a global estimate of around 100 trillion synapses.

At the synapses, one neuron transfers information to another by passing special molecules called neurotransmitters. The main neurotransmitters are divided into groups:

  • Smaller molecules – amino acids. Key examples: glutamate (the principal excitatory neurotransmitter); aspartate; GABA (the principal inhibitory neurotransmitter); glycine
  • Smaller molecules – others. Key examples: acetylcholine; nitric oxide
  • Biogenic amines – catecholamines. Key examples: (nor-)adrenaline; dopamine (reward motivation)
  • Biogenic amines – others. Key examples: histamine; serotonin (usually inhibitory)
  • Peptides. Key examples: substance P; endogenous opioids; encephalins, endorphins

A group of neurons in the body can be described as follows:

  • grey matter, the most general term, only used for the central nervous system (CNS)
  • nucleus, when the group is clearly distinguishable, usually deep in the brain
  • cortex, when the collection of neurons forms a thin sheet
  • ganglion, a group or collection outside the CNS (exception: the “basal ganglia” despite the name are part of the CNS; they would more appropriately be called basal nuclei)

A collection of axons can be described as follows:

  • nerve, when outside the CNS
  • white matter, a general term for a collection inside the CNS
  • tract, a group in the CNS with a common origin and destination
  • bundle, axons running together with different origins and / or different destinations

The cerebrum – the cerebral cortex

The cerebrum is the name for a range of essential brain parts which in their size and functioning distinguish the human brain from those of mammals and even primates. It contains the two hemispheres of the cerebral cortex, the corpus callosum connecting them, the hippocampi and amygdalae, the basal ganglia, the olfactory bulbs and the optical nerve.

The human cerebral cortex is estimated to contain between 10 and 20 billion neurons. It has two hemispheres, which have small structural differences, and a degree of specialisation.

Cortex is not the name for a location, but for a kind of special (neural) tissue. In the human cerebrum most of it is composed of six layers and usually referred to as neocortex (sometimes isocortex). The neocortex is covered underneath (“ventrally”) by a cortex composed of three or four layers. The hippocampus is covered on the outside by a three-layered cortex, and there are a number of other cortex structures in the brain. The 2 to 4 mm thick sheet of the neocortex is folded in a characteristic structure of ridges (ridge = gyrus) and furrows (furrow = sulcus).

It covers the whole top and back parts below the human skull. To distinguish locations, it is divided into four main parts (“lobes”), which also largely have different special functions, the frontal, parietal, occipital and temporal lobes. In addition, inside these major lobes there are other parts with special names such as the cingular cortex, the insular cortex and the operculum. For smaller locations that are not cortex but groups of nuclei, a range of specialised terms are used.

The cerebral cortex is the seat of complex cognition. It plays a key role in perception (using all five external senses), conscious awareness, cognition and thought, memory, language and speech, and voluntary action. It is connected with almost every other part of the nervous system, directly or indirectly, but that does not mean that it is actively involved in every bit of human movement and activity. Substantial functions necessary for survival, including important homeostatic functions such as regulating heartbeat and breathing, upright balance, and the complete digestive system, operate with a great degree of independence from the cerebral cortex.

A different way of describing parts of the cortex is naming them by function. All these functional areas are present in both hemispheres, and are linked with the respective body parts they relate to in mirror form, left to right and vice versa (although for instance information from the eyes is sent to both sides in roughly a 60/40 proportion).

The functional parts of the cerebral cortex can be divided into three groups:

  • The sensory cortices (primary cortices for all the senses, and the somatosensory cortex in the parietal lobes)
  • The motor cortices, mainly in the frontal lobes
  • The association areas in the parietal, occipital and temporal lobes

NOTE: The association areas constitute a clear majority of the cerebrum. It is believed that these link the sensory and motor components, and mediate the “higher order” functioning of the brain. Neuro-scientists usually summarise this with the word cognition which includes the ability to:-

(a) Pay attention to external stimuli and internal motivation

(b) Give these meaning and put them in context

(c) Initiate responses (planning and decision-making)

These three main capabilities are especially focused in (a) the parietal cortices, (b) the temporal lobes, and (c) the frontal lobes.

Almost no functions or brain parts operate in isolation. The challenge is in understanding how the parts work together, how action arises, how much of brain activity is unconscious, and what brain parts participate.

It is recommended that you listen to Dr Ruth Lanius’s audio presentation for further information on the connection between different modes of consciousness.

Other parts of the cerebrum

The corpus callosum connects the two cerebral hemispheres, a bundle of over 200 million axons running in both directions between the two halves of the cerebrum through which signals, chemical and / or electrical, are transmitted or communicated.

The hippocampi (used here in plural, as there are two of them on the left and right side) lie directly underneath the cortex of the medial part of the temporal lobes. They play a major role in the formation and retrieval of (long-term) memories about experienced events, and in spatial navigation. The hippocampi are the first to be damaged in the Alzheimer’s form of neurocognitive disorder. Damage to the hippocampus leads to anterograde amnesia, the inability to form new memories that last. It appears that the hippocampus is one of the few regions where, during adult life, new neurons can be created and incorporated. The hippocampi can be larger or smaller, and a considerable numbers of studies try to draw conclusions from the significance of their size.

The amygdalae (in plural as there are two on each side of the brain) are two complexes of grey matter on the left and right, buried in the front to middle portion of the temporal lobe just above and in front of one end of the hippocampi. They are groups of nuclei amongst which at least seven areas can be separately identified. The amygdalae play a major role in the processing and storage of memories with emotional associations – best studied in animals. It appears that the amygdalae have a major role in the consolidation of long-term memories, the emotional strength and valence of these memories, and in (longer-term) fear conditioning. There may be a relationship with anxiety disorders, and there may be some degree of specialisation between the left and right amygdalae. For a useful short review article about the amygdalae, with a focus on their role in emotions, see LeDoux (2008).

The basal ganglia (or nuclei) are a group of different nuclei located underneath the main cerebral cortex and more or less grouped around the thalamus and hypothalamus. Main components of the basal ganglia are the striatum (caudate nucleus and putamen), the globus pallidus, the substantia nigra, the nucleus accumbens, and the subthalamic nucleus. All these have a left and right-hand copy. The basal ganglia are involved in (the modulation of) voluntary motor control and the formation of procedural learning in the form of habits, and they probably co-operate with the prefrontal cortex in the selection of actions. The role of the basal nuclei in motor control is especially evident in the movement problems of people with Parkinson’s disease, where the basal nuclei are the brain area most affected.

The olfactory bulbs and the optical nerve are counted as part of the central nervous system. It is clear from evolutionary and embryological evidence that the cerebral cortex has evolved from what originally was an olfactory nerve system.

Hypothalamus and thalamus
Together often referred to as the diencephalon

The thalamus is a fundamental part of the brain. It is complex, and has about 50 subdivisions. The thalamus is in an evolutionary sense close to the cerebral cortex. Most of the information that reaches the cerebral cortex from the rest of the CNS reaches it via the thalamus. There is speculation about what difference the thalamus makes to all the signals passing through. Localisation of the thalamus, though less well developed than that of the cerebral cortex, tends to follow the cortical map, i.e. if there is a part for something in the cerebral cortex, there will also be a part in the thalamus that it passes through or is processed by. It may be that the thalamus plays a role in the regulation of consciousness, sleep and alertness, and possibly in epileptic seizures.

The hypothalamus is almond-size, but composed of a large number of well-described nuclei. It is strongly connected with the body’s endocrine system (see its role in different endocrine “axes”), controls a number of homeostatic functions (e.g. blood flow, blood glucose level, response to threat / stress, body temperature, hunger and thirst, circadian rhythms), is involved in the integration of emotions, and mainly controls the sexuality and reproductive system. It may play a role in social soothing and the attachment and caregiving systems (Coan, 2008). Although physically it clearly is a part of the cerebrum, functionally it plays a bridging role between the human nervous system and the endocrine system, partly through the pituitary gland (hypophysis).

The brainstem

The brainstem is positioned between the diencephalon and the spinal cord. It can be divided between midbrain (mesencephalon) and hindbrain (rhombencephalon), and sometimes (but not here) taken to include the cerebellum.

Parts of the brainstem have many specialised functions, but three overall general functions can be identified:

  • The cranial nerves that deal with the sensory and motor function of the head and neck start and finish in the brainstem
  • A number of tracts (bundles of axons with a common origin or destination) pass through the brainstem, especially sensory tracts ascending from the spinal cord, head and neck, and descending motor tracts from the forebrain and the optical nerve
  • Regulation of the level of consciousness through connections from the reticular formation to the cerebrum

These functions are so generally critical for the functioning of the nervous system that substantial damage to the brainstem tends to be more completely life-threatening than damage to the cerebrum, the cerebellum or the spinal cord.

The midbrain includes the tectum, tegmentum, cerebral aqueduct and the cerebral peduncles. Inside the tegmentum, around the cerebral aqueduct, is found the periaqueductal grey (PAG), which plays a major role in pain regulation, and in emotions. The role of the PAG in emotions is strongly brought out by Panksepp (Panksepp, 1998Panksepp & Biven, 2012).

The hindbrain mainly consists of medulla (oblongata) and pons. The medulla contains centres responsible for maintaining breathing, controlling the heart rate, and for digestion (note that the heart and the digestive system also have considerable local nerve centres steering their functioning, but the medulla commands the main brain input and involvement). The pons is involved with transferring information about the body’s movements between the cerebral hemispheres and the cerebellum.

The reticular formation is a distributed (sparse) neuronal network extending from the medulla to the thalamus, and is involved in arousal (being awake; it plays the key role in the “reticular activating system”), temperature regulation, the direction of eye movements and motor control.

The cerebellum

The cerebellum is a gigantic unit, evolutionarily old, that is estimated to contain almost 70 billion neurons (Azevedo et al., 2009). It does not occupy a commensurate amount of space, as many of its neurons are rather small granule cells. Much of the cerebellum is in the form of cortex. It has four nuclei, hidden deep inside the cortical hemispheres, and three peduncles. The cerebellum is not connected with the body in mirror image: the left hemisphere of the cerebellum is connected with the left side of the body. Most of its connections with the rest of the brain travel through the pons, which lies just in front.

The two hemispheres of the cerebellum have between them a formation called the vermis, which is strongly connected with all parts of the cerebellum. The cerebellar cortex is divided into lobes. Certain parts of it are designated with functional designations. The vermis is especially associated with body posture, while in general the cerebellum is associated with fine motor control.

In addition to its long-known involvement in fine motor control, the cerebellum is thought to be involved in language, attention, and probably associative learning, classical as well as operant, especially when this has a motor aspect.

Evolutionary understanding of the forebrain

In the 1930s an American neuroanatomist, James Papez, published an article about a pathway in the brain that he thought might be the main brain system involved with emotions. This was picked up by Paul MacLean (1913-2007), a doctor and professor in physiology whose presentation about the topic was published in 1949. MacLean devoted much of his life to developing his theory about the brain, and summarised much of his conclusions in a 1990 monograph (MacLean, 1990). He divided the parts of the forebrain (=telencephalon / cerebrum + diencephalon) in three groups, which he called the:

  • Protoreptilian formation
  • Limbic system
  • Neomammalian formation

His theory has remained very popular and is much quoted but there are a number of problems with it and its use.

  • The reptilian formation, commonly referred to as the “reptilian brain” is often assumed to refer to midbrain and or hindbrain (the brainstem). However, MacLean intended it to refer to a major part of the basal ganglia. The term reptilian brain is now hardly used, except in popular literature. The limbic brain refers to a collection of cortical and subcortical parts of the cerebrum. Although as a locational description it is still used, it is not seen as useful anymore as it encompasses too many different structures which do not share functions. Only parts of the limbic system are involved in emotional processing, and parts are not.
  • MacLean’s supporting theories were, to a great extent, speculations. He suggested that because certain brain parts were evolutionarily older than others they were bound to function differently, and with a considerable degree of autonomy. There is no basis for that, and those types of reasoning have been largely abandoned (Striedter, 2005).

There is no doubt that MacLean was a very creative thinker who did much to encourage a positive kind of global functional thinking about the brain. In that respect he has been a positive influence on researchers. LeDoux (1998, chapter 4), who has studied MacLean in some detail, believes that MacLean’s conceptualisation of emotions and his evolutionary linkages remain fruitful, as do his ideas about somatising disorders, epilepsy and mental disorders and their neurobiological background. However, he also concludes that the grouping together of the parts of the limbic brain serves no purpose.

A broader critique of MacLean’s evolutionary speculations can be found in Striedter (2005), where he continues to set out the modern views that have superseded MacLean’s pioneering attempts. Kötter & Stephan (1997) engage in a helpful discussion specifically focused on the limbic system concept.

The spinal cord

The spinal cord receives and processes sensory information from all over the body. It has cervical, thoracic, lumbar and sacral regions, and reaches down from the medulla; it finishes in the lumbar region, and does not entirely extend down to the bottom of the spine. It is a substantial part of the nervous system. Many of the signals it receives are transmitted to the brain, and many signals are received back, but the spinal cord also has interneurons of its own, and takes care independently of a number of reflex actions, and it modulates a number of rhythmic patterns through “central pattern generators”, which are used in particular for locomotion, swimming and breathing.

The peripheral nervous system

The peripheral nervous system consists of spinal nerves, some of the cranial nerves, the lumbosacral nerves, and other nerve cell groups with a considerable degree of autonomy from the central nervous system, such as the enteric nervous system.

The autonomic nervous system

It should not be forgotten that the autonomic nervous system is really an autonomic motor system, i.e. it is composed of motor neurons, and its role is to let these motor neurons take action by passing instructions to a very wide range of muscles, glands and organs of the body. The word or idea of the “autonomic nervous system”, or “sympathetic autonomic system” might lead one to believe that it is one overall “system”, which at any one time has one “state”. This is not the case. It is a system with countless elements, composed of large numbers of nerves and neurons, which all at the same time are engaged in different activities in different parts of the body, and have different connections throughout the nervous system.

The autonomic nervous system (ANS) has three divisions, the sympathetic, parasympathetic and enteric divisions. Although it can be and often is influenced by the CNS, much of its activity is automatic, “autonomous”, and is not only unconscious but unaffected by the CNS. The sympathetic division is composed of spinal nerves including nerves from the lumbar section of the spinal cord. The parasympathetic division consists of most of the cranial nerves (some cranial nerves form part of the CNS), and of the spinal nerves coming from the sacral section of the spinal cord. Most body organs receive innervation from both the sympathetic and the parasympathetic branch, but not all. Those organs (for which in some circumstances both sympathetic and parasympathetic signals need to be given in a synchronised way) are clearly the most challenging to control.

The sympathetic nervous system orchestrates the fight of flight response (described in Confer’s Trauma and Dissociation module), which is an organised set of responses over a whole range of systems.

The parasympathetic division can be analysed via each of the cranial and sacral nerves that are part of it. It has an important role in regulating the heartbeat, which involves the dorsal motor nucleus of the vagus nerve (cranial nerve X), as discussed at length in Porges (2011).

The enteric (sometimes: “intrinsic”) nervous system is located mainly in the walls of the gastrointestinal section, from oesophagus to anus but especially the small intestine, in a special tissue layer. A very readable and useful book about the enteric system is Gershon (1998).

Resources

Neuroanatomy is essential for following and evaluating talks or writings about neurobiology. The descriptions given above are basic and concise but a lot of useful information is available by reading some of the references. To find one’s way it is well worth studying at least one major recent textbook about neuroscience, such as Breedlove & Watson (2013)Purves et al. (2012) or Bear et al. (2007), of which Breedlove and Watson is perhaps the closest to focusing on matters of interest to psychologists. Miller Miller (2010) offers a longer article on the interface between neurobiology and psychology. It is also highly recommended to read this paper with Wikipedia on the side, which provides good articles about almost every keyword.

References

Azevedo, F.A.C., Carvalho, L.R.B., Grinberg, L.T., Farfel, J.M., Ferretti, R.E.L., Leite, R.E.P., Filho, W.J., Lent, R. & Herculano-Houzel, S. (2009). Equal numbers of neuronal and non-neuronal cells make the human brain an isometrically scaled-up primate brain. The Journal of Comparative Neurology 513: 532-541.

Bear, M.F., Connors, B.W. & Paradiso, M.A. (2007). Neuroscience: Exploring the Brain (3rd edition).Philadelphia, PA: Lippincott, Williams & Wilkins.

Breedlove, S.M. & Watson, N.V. (2013). Biological Psychology: An Introduction to Behavioral, Cognitive, and Clinical Neuroscience (7th edition). Sunderland, MA: Sinauer.

Coan, J.A. (2008). Toward a neuroscience of attachment. In J. Cassidy & P.R. Shaver (Eds.), Handbook of Attachment: Theory, Research, and Clinical Application (2nd edition). New York: Guilford Press.

Gershon, M.D. (1998). The Second Brain: A Groundbreaking New Understanding of Nervous Disorders of the Stomach and Intestine. New York: HarperCollins.

Kötter, R. & Stephan, K.E. (1997). Useless or helpful? The “limbic system” concept. Reviews in the Neurosciences, 8: 139-145.

LeDoux, J. (1998). The Emotional Brain: The Mysterious Underpinnings of Emotional Life. London: Phoenix.

LeDoux, J.E. (2008). Amygdala. Scholarpedia, 3(4): 2698. Available at www.scholarpedia.org (accessed 3 January 2014).

MacLean, P.D. (1990). The Triune Brain in Evolution: Role in Paleocerebral Functions. New York: Plenum.

Miller, G.A. (2010). Mistreating psychology in the decades of the brain. Perspectives on Psychological Science, 5: 716-743.

Porges, S.W. (2011). The Polyvagal Theory: Neurophysiological Foundations of Emotions, Attachment, Communication, and Self-Regulation. New York: W W Norton.

Purves, D., Augustine, G.J., Fitzpatrick, D., Hall, W.C., LaMantia, A-S. & White, L.E. (Eds.) (2012). Neuroscience. (5th edition). Sunderland, MA: Sinauer.

Striedter, G.F. (2005). Principles of Brain Evolution. Sunderland, MA: Sinauer.