Chapter 8:

INTRODUCTION The central nervous system is totally dependent on a continuous supply of blood; viability of the neurons depends on the immediate and constant availability of both oxygen and glucose� Interruption of this lifeline causes immediate loss of function followed very quickly by death of the nervous tissue (the neurons and axons)� Study of the nervous system must include a complete knowledge of the blood supply and the structures (nuclei and tracts) situated in the vascular territory of the various arteries� Failure of the blood supply to a region, because of either occlusion or hemorrhage, causes the expected functional deficits�

Areas of gray matter, where the neurons are located, have a greater requirement for blood supply than white matter� Loss of oxygen and glucose supply to these neurons leads to loss of electrical activity almost immediately (in the adult) and, if continued for several minutes, to neuronal death� Although white matter requires less blood supply, loss of adequate supply leads to destruction of the axons in the area of the infarct and an interruption of pathways� After loss of the cell body or interruption of the axon, the distal portion of the axon (the part on the other side of the lesion separated from the cell body) and the synaptic connections degenerate, resulting in a permanent loss of function�

Every part of the nervous system lies within the vascular territory of an artery, sometimes with an overlap from adjacent arteries� Visualization of the arterial (and venous) branches can be accomplished using the following:

• Arteriogram: By injecting a radiopaque substance into the arteries (this is a procedure which is done by a neuroradiologist), and

following its course through a rapid series of x-ray studies, a detailed view of the vasculature of the brain is obtained (see Figure 8�3)� This is an invasive procedure carrying a certain degree of risk�

• Magnetic Resonance Angiogram-MRA: Using neuroradiology imaging with MRI, the major blood vessels (such as the circle of Willis) can be visualized (see Figure 8�2)� A similar visualization can be obtained with computed tomography (CT) scanning, called a CTA�

It is extremely important to know which parts of the brain are located in the territory supplied by each of the major cerebral and brainstem blood vessels and to understand the functional contribution of these parts� This is fundamental for clinical neurology�

A clinical syndrome involving the arteries of the brain is often called a cerebrovascular accident (CVA) or a stroke� The nature of the process, whether a blood vessel occlusion through infarction or embolus or a hemorrhage, is not specified by the use of this term, nor does the term indicate which blood vessel is involved� The clinical event is a sudden loss of function; the clinical deficit depends on where the occlusion or hemorrhage occurred�

Occlusion is more common than hemorrhage and is often caused by an embolus (e�g�, from the heart)� Hemorrhage may occur into the brain substance (parenchymal), thereby causing destruction of the brain tissue (discussed with Figure 7�2) and at the same time depriving areas distally of blood�

The arterial circle (of Willis) is a set of arteries interconnecting the two sources of blood supply to the brain, the vertebral and common carotid arteries� It is located at the base of the brain, surrounding the optic chiasm and the hypothalamus (the mammillary nuclei) (review Figure 1�5A and Figure 1�5B)� Within the skull, the circle of Willis is situated above the pituitary fossa (and gland)� The major arteries to the cerebral cortex of the hemispheres are branches of this arterial circle� This illustration is a photographic view of the inferior aspect of the brain, including brainstem and cerebral hemispheres, with the blood vessels (as in Figure 1�6)� Branches from the major arteries have been added to the photographic image�

The cut end of the internal carotid arteries is a starting point� Each artery divides into the middle cerebral artery (MCA) and the anterior cerebral artery (ACA)� The MCA courses within the lateral fissure� The anterior portion of the temporal lobe has been removed on the left side of this illustration to follow the course of the MCA in the lateral fissure� Within the fissure, small arteries are given off to the basal ganglia, called the striate arteries (not labeled; see Figure  8�6)� The artery emerges at the surface (see Figure 1�3) and courses upward, dividing into branches that are distributed onto the dorsolateral surface of the hemispheres (see Figure 8�4)�

By removing (or lifting) the optic chiasm, the ACA can be followed anteriorly� This artery heads into the interhemispheric fissure (see Figure 2�2A) and is followed when viewing the medial surface of the brain (see Figure  1�7, Figure 2�2A, and Figure 8�5)� A very short artery connects the ACAs of the two sides, the anterior communicating artery (see also Figure 8�2 and Figure 8�3)�

The vertebro-basilar system supplies the brainstem, the cerebellum, and the posterior part of the hemispheres� The two vertebral arteries unite at the lower border of the pons to form the midline basilar artery, which courses in front of the pons� The basilar artery terminates at the midbrain level by dividing into two posterior

cerebral arteries (PCA)� These arteries supply the inferior aspect of the brain and particularly the occipital lobe (see Figure 8�5)�

The arterial circle is completed by the posterior communicating arteries (normally one on each side, approximately of the same size-note the size difference here), which connects the internal carotid artery (or MCA) artery, often called the anterior circulation, with the posterior cerebral artery, the posterior circulation�

Small arteries directly from the circle (not shown) provide the blood supply to the diencephalon (thalamus and hypothalamus), some parts of the internal capsule, and part of the basal ganglia� The major blood supply to these regions is from the striate arteries (see Figure 8�6)�

The branches from the vertebral artery and basilar artery supply the brainstem� Small branches directly from the vertebral and basilar arteries (not shown), known as paramedian arteries, supply the medial structures of the brainstem (further discussed with Appendix Figure A�9)� There are three major branches from this arterial tree to the cerebellum-the posterior inferior cerebellar artery (PICA), the anterior inferior cerebellar artery (AICA), and the superior cerebellar artery (SCA)� All supply the lateral aspects of the brainstem, including nuclei and tracts, en route to the cerebellum; these are often called the circumferential branches�

The vascular territories of the various cerebral blood vessels are shown in color in this diagram� The most common clinical lesion involving the cerebral blood vessels is occlusion, often resulting from an embolus originating from the heart or the carotid bifurcation in the neck� These clinical deficits are described with each of the major branches to the cerebral cortex (with Figure  8�4 and Figure 8�5)�

In the eventuality of a slow occlusion of one of the major blood vessels of the circle of Willis, sometimes one of the communicating branches becomes large enough to provide sufficient blood to be shunted to the area deprived (see Figure 8�3)�

Advances in technology have allowed for a visualization of the major blood vessels supplying the brain, notably the arterial circle of Willis, without injection of a radiopaque substance (usually gadolinium based)� In this instance, as with the magnetic resonance venogram (see Figure 7�7), no contrast agent was used� Although the quality of such images cannot match the detail seen after an angiogram of select blood vessels (shown in Figure 8�3), the non-invasive nature of this procedure and the absence of risk to the patient clearly establish this investigation as desirable to provide some information about the state of the cerebral vasculature�

This arteriogram shows the circle of Willis as seen as if looking at the brain from below (as in Figure  8�1)� The carotid artery goes through the cavernous (venous) sinus of the skull and forms what is called the carotid siphon� It then divides into the anterior cerebral artery, which goes anteriorly, and the middle cerebral artery, which goes laterally� The basilar artery is seen at its termination, as it divides into the posterior cerebral arteries� The anterior communicating artery is present, and there are two posterior communicating arteries completing the circle, joining the internal carotid artery with the posterior cerebral artery on each side�

This is the same angiogram, but displayed at a different orientation-an anterior (somewhat tilted) view� The two vertebral arteries can be seen, joining to form the basilar artery; it is not uncommon to see the asymmetry in these vessels� The posterior inferior cerebellar artery can be

seen; this is a branch of the vertebral artery (it is also labeled in the upper image)� The basilar artery gives off the superior cerebellar arteries and then ends by dividing into the posterior cerebral arteries� The internal carotid artery can be followed through its curvature in the petrous temporal bone of the skull before it divides into the anterior and middle cerebral arteries�

One of the characteristic vascular lesions in the arteries that make up the arterial circle of Willis is a type of aneurysm, called a berry aneurysm� This is caused by a weakness of part of the wall of the artery that causes local ballooning of the artery� These aneurysms often rupture when they reach a certain size (>5 mm), particularly if there is accompanying hypertension� This sudden rupture occurs into the subarachnoid space and may also involve nervous tissue of the base of the brain� The whole event is known as a subarachnoid hemorrhage, and this diagnosis must be considered when one is faced clinically with an acute major cerebrovascular event, without trauma, accompanied by intensely severe headache and often loss of consciousness (discussed also with Figure 7�2)�

Sometimes, these aneurysms leak a little blood, and this causes irritation of the meninges and accompanying symptoms of headache a so-called “Sentinel Bleed�” A computed tomography angiogram (CTA) or magnetic resonance angiogram (MRA) can, at the minimum, visualize whether there is an aneurysm on one of the vessels of the circle of Willis and whether the major blood vessels are patent�

Note to the Learner: One of the best ways of learning the circle of Willis and the arterial supply to the brain is to actually make a sketch drawing, accompanied by a list of the areas supplied and the major losses that would follow a sudden occlusion� The blood supply to the brainstem and the most common vascular lesions affecting this area are discussed with Figure 8�1 and Figure 8�5�

This radiograph was done by injecting radiopaque dye into the internal carotid artery� Normally, the procedure is to thread a catheter from the groin up the aorta and into the carotid artery, an invasive procedure not without some risk�

In this particular case, a slow occlusion of the right internal carotid artery had allowed time for the anterior communicating artery of the circle of Willis to become widely patent; therefore, blood was shunted into the anterior and middle cerebral arteries on the affected side� This is not usual, and in fact this radiogram was chosen for this reason�

The middle cerebral artery goes through the lateral fissure and breaks up into various branches on the dorsolateral surface of the hemisphere (shown in Figure 8�4)� The striate arteries-also called the lenticulostriate

arteries-are given off en route to supply the interior structures of the hemisphere (discussed with Figure 8�6)�

This radiograph shows the profuseness of the blood supply to the brain and the hemispheres, and it is presented to give the student that notion, as well as to show the appearance of an angiogram�

Visualization of the blood supply to the brain is required for the accurate diagnosis of aneurysms and occlusions affecting these blood vessels�

It is important to once again note the radiological convention for radiographs of the brain-the orientation of the laterality (right/left) is as if one is looking at the patient�

Had there been a sudden total occlusion of the blood supply to the right hemisphere of the brain, the patient would have experienced motor and sensory deficits affecting the face and the body-on the opposite side-with an extensor plantar response (see Clinical Cases)�

This illustration shows the blood supply to the cortical areas of the dorsolateral aspect of the hemispheres; it has been created by superimposing the blood vessels onto the photographic view of the brain (the same brain as in Figure 4�5)�

After coursing through the depths of the lateral fissure (see Figure 8�1 and Figure 8�3), the middle cerebral artery emerges and breaks into a number of branches that supply different parts of the dorsolateral cortex-the frontal, parietal, and temporal areas of cortex� Each branch supplies a different territory, as indicated; branches supply the precentral and postcentral gyri, the major motor and sensory areas for the face and head, and the upper limbs (as indicated)� On the dominant side, this includes the language areas�

The vascular territories of the various cerebral blood vessels are shown in color in this diagram� The branches of the middle cerebral artery extend toward the midline mid-sagittal fissure, where branches from the other cerebral vessels (anterior and posterior cerebral) are found, coming from the medial aspect of the hemispheres (see Figure 8�5)�

A zone remains between the various arterial territories-the arterial borderzone region (a watershed area)� This area is poorly perfused and prone to  infarction, particularly if there is a sudden loss of blood pressure (e�g�, with cardiac arrest or after a major hemorrhage)�

The most common clinical lesion involving these blood vessels is occlusion, often caused by an embolus originating from the heart or the carotid bifurcation in the neck� This results in infarction of the nervous tissue supplied by that branch-the clinical deficit depends on which branch or branches is or are involved� For example, loss of sensory or

motor function, or both, to the arm and face region (of the opposite side) is seen after the blood vessel to the central region is occluded� The type of language loss that occurs depends on the branch affected in the dominant hemisphere: a deficit in expressive language is seen with a lesion affecting Broca’s area, whereas a comprehension deficit is found with a lesion affecting Wernicke’s area�

Acute strokes are now regarded as an emergency with a narrow therapeutic window� According to current evidence, if the site of the blockage can be identified and the clot (or embolus) removed within 4�5 hours-preferably less-there is a reasonable chance that the individual will have significant if not complete recovery of function� The therapeutic measures include a substance that will dissolve the clot such as tPA (tissue plasminogen activator), combined with interventional neuroradiology whereby a catheter is threaded through the vasculature and into the brain and the clot is either dissolved with the tPA and/or removed� Major hospitals now have a protocol called “Stroke Code” to alert the stroke team and investigate these people immediately when brought to emergency, including a plain CT scan and CT angiogram, so that therapeutic measures can be instituted within the therapeutic window�

A clinical syndrome has been defined in which a temporary loss of blood supply affects one of the major blood vessels� Some would limit this temporary loss to less than 1 hour, whereas others suggest that this period could extend to several hours� This syndrome is called a transient ischemic attack (TIA)� Its cause could be blockage of a blood vessel that resolves spontaneously or perhaps an embolus that breaks up on its own� Regardless, people are being educated to look at this event as a “brain attack,” much like a heart attack, and to seek medical attention immediately� The statistics indicate that many of these people experiencing TIAs have a high risk of stroke within 30 days if not properly treated�

Note to the Learner: The clinical cases at the end of the atlas will present clinical problems involving occlusion of the various cerebral branches� Students are encouraged to work through these cases-answers will be provided on the Web site�

In this illustration, the blood supply to the medial aspect of the hemispheres has been superimposed onto this view of the brain (see Figure 1�7)� Two arteries supply this partthe anterior cerebral artery and the posterior cerebral artery� The vascular territories of the various cerebral blood vessels are shown in color in this illustration�

The anterior cerebral artery (ACA) is a branch of the internal carotid artery from the circle of Willis (see Figure 8�1, Figure 8�2, and Figure 8�3)� It runs in the interhemispheric fissure, above the corpus callosum (see Figure 2�2A and Figure 9�1B), and it supplies the medial aspects of the frontal lobe and the parietal lobe; this includes the cortical areas responsible for sensory and motor function of the lower limb�

The posterior cerebral artery (PCA) supplies the occipital lobe and the visual areas of the cortex, areas 17, 18, and 19 (see Figure 6�5 and Figure 6�6)� The posterior cerebral arteries are the terminal branches of the basilar artery from the vertebral or posterior circulation (see Figure 8�1)� The demarcation between these arterial territories is the parieto-occipital fissure�

Both sets of arteries have branches that spill over to the dorsolateral surface� As noted in Figure 8�4, there is a gap between these and the territory supplied by the middle cerebral artery that is known as the arterial borderzone or watershed region�

The blood supply to the brainstem and cerebellum is shown from this perspective and should be reviewed with Figure 8�1� The three cerebellar arteries-posterior inferior, anterior inferior, and superior-are branches of the vertebro-basilar artery that supply the lateral aspects of the brainstem en route to the cerebellum�

The deficit most characteristic of an occlusion of the anterior cerebral artery (ACA) is selective weakness and spasticity of the contralateral lower limb� Clinically, the control of micturition seems to be located on this medial area of the brain, perhaps in the supplementary motor area (see Figure 5�8), and symptoms related to voluntary bladder control may also occur with lesions in this area but usually these have to be bilateral�

The clinical deficit found after occlusion of the posterior cerebral artery on one side is a loss of one half of the visual field of both eyes-contralateral homonymous hemianopsia� The blood supply to the calcarine cortex, the visual cortex, area 17, is discussed with Figure  6�6� Depending of the size of the occlusion, there can be deficits of the medial temporal lobe, the thalamus and the parietal lobe�

Note to the Learner: This is an opportune time to review the optic pathway and to review the visual field deficits that are found after a lesion in different parts of the visual system�

Studies indicate that the core of tissue that has lost its blood supply is surrounded by a region where the blood supply is marginal but that is still viable and may be rescued-the penumbra, as it is now called� In this area surrounding the infarcted tissue, the blood supply is reduced below the level of nervous tissue functionality and the area is therefore “silent,” but the neurons are still viable�

These studies have led to a rethinking of the therapy of strokes:

• In the acute stage, if the patient can be seen quickly (preferably within 3 hours and now up to 4�5 hours) and investigated immediately, the site of the lesion may be identified� Some hospitals have a “stroke code” to expedite the management of such patients� If there is an occlusion, then therapeutic measures can be instituted immediately, either with powerful drugs to dissolve the clot and/or the use of interventional neuroradiology (in large centers)� If treatment is administered soon enough after the stroke, it may be possible to avert any clinical deficit�

• There may be an additional period beyond this time frame when damaged neurons in the penumbra can be rescued through the use of neuroprotective agents-specific pharmacological agents that protect the neurons from the damaging consequences of loss of blood supply� Such agents are currently an area of active research�

As loss of function and diminished quality of life are the end result of strokes, it is clear that this is a most active area of neuroscience research as to what can be done in the way of preventative measures, for example, the control of high blood pressure (hypertension), and the regulation of the level of blood cholesterol� This takes on added significance in view of the increasing number of people in the “senior” age range�

One of the most important sets of branches of the middle cerebral artery is found within the lateral fissure (this artery has been dissected in Figure 8�1)� These branches are known as the striate arteries, also called lenticulostriate arteries� They supply most of the internal structures of the hemispheres, including the internal capsule and the basal ganglia (discussed with Figure 4�4)�

These branches are seen with the arteriogram in Figure 8�3�

In this illustration, a coronal section of the brain (as shown in the small photographs above, from both the lateral and medial perspectives), the middle cerebral artery is seen traversing the lateral fissure� The artery begins as a branch of the circle of Willis� Several small branches are supplying the area of the lentiform nucleus and the internal capsule, as well as the thalamus� The artery then emerges, after passing through the lateral fissure, to supply the dorsolateral cortex (as previously shown)�

These small blood vessels are the major source of blood supply to the internal capsule and the adjacent portions of the basal ganglia (head of caudate nucleus and putamen), as well as the thalamus (see also Figure 3�6A and Figure 3�6B)� Additional blood supply to these structures comes directly from small branches of the circle of Willis (discussed with Figure 8�1)�

These small-caliber arteries are functionally different from the cortical (cerebral) vessels� First, they are end arteries and do not anastomose� Secondly, they develop degenerative changes due to a chronically elevated blood pressure

(hypertension) by a process of degeneration of the muscular wall of the blood vessels-called fibrinoid necrosis�

Following this there are two possibilities:

• These blood vessels may occlude, causing small infarcts in the region of the internal capsule that are seen radiographically as small holes-often called lacunes (lakes)� Hence, they are known as lacunar infarcts, commonly known as a stroke� Lacunes are therefore small areas of white matter infarction and can be seen anywhere in the central nervous system� The extent of the clinical deficit with this type of infarct depends on its location and size in the internal capsule� A relatively small lesion may cause major motor or sensory deficits, or both, on the contralateral side� This may result in devastating incapacity of the person, with contralateral paralysis�

Note to the Learner: At this time, the student should review the major ascending and descending tracts and their course through the internal capsule�

• The other possibility is that these weakened blood vessels can rupture, leading to hemorrhage deep in the hemispheres� (Brain hemorrhage can be visualized by computed tomo graphy; reviewed with Figure 3�6A�)

Although the blood supply to the white matter of the brain is significantly less (because of the lower metabolic demand), this nervous tissue is also dependent on a continuous supply of oxygen and glucose� A loss of blood supply to the white matter results in the loss of the axons (and myelin) and hence interruption of the transmission of information� This type of stroke may lead to a more extensive clinical deficit because the hemorrhage itself causes a loss of brain tissue, as well as a loss of the blood supply to areas distal to the site of the hemorrhage�

The whole spinal cord is shown, from an anterior perspective (see Figure 1�1 and Figure 1�11), with the anterior spinal artery highlighted� Beside it is a higher-magnification photographic image of the cervical region of the spinal cord� Most of the attached roots are the motor (ventral) roots, coming from the ventral horn of the spinal cord (discussed with Figure 1�12 and Figure 7�3); a few of the dorsal (sensory) roots can be seen, entering the dorsal horn of the spinal cord�

The vertebral arteries enter the skull and join to form the basilar artery� Each drops a branch (shown in Figure 8�8), and the branches unite to form the anterior spinal artery� This somewhat tortuous artery courses down the midline of the cord anteriorly, within the ventral median fissure (see Figure 3�9)� This artery is the major blood supply to the ventral portion of the cord, its anterior two thirds-shown in the axial (cross-sectional) illustration (on the lower right)—including the ventral horn and all the tracts in the anterior and lateral funiculi (see also Figure  6�10)� Along its way, the anterior spinal artery receives supplementary branches from the aorta, called radicular arteries, that follow the nerve roots�

The anterior spinal artery tapers as it descends, thereby creating a vulnerable area of the spinal cord blood supply around the lower thoracic (spinal cord) level� This is very important clinically (see Clinical Aspects below)�

The vertebral arteries also give off branches on the posterior aspect of the spinal cord, the posterior spinal

arteries (not shown in Figure 8�8), and these course the full length as separate arteries, supplemented en route by the radicular arteries� These vessels supply the gray matter of the dorsal horn and the region of the dorsal columns (see Figure 5�2 and the cross-sectional view and also Appendix Figure A�11)�

As noted, the blood supply to the lower thoracic spinal cord is tenuous; in fact the anterior spinal artery is supplemented in this area by a branch from the thoracic aorta, the artery of Adamkiewicz (shown in Figure 8�8)� A dramatic drop in blood pressure, such as occurs with cardiac arrest or excessive blood loss, may lead to an infarction of the lower spinal cord, affecting primarily the territory of the anterior spinal artery� The result can be just as severe as if the spinal cord was (partially) severed by a knife� The most serious consequence of this would be the loss of voluntary motor control of the lower limbs, known as paraplegia� In addition, pain and temperature sensation would be lost below the level of the lesion, on both sides; dorsal column sensation (e�g�, vibration) would not be affected� The clinical picture is based on an understanding of the sensory and motor tracts of the spinal cord (discussed in Section 2)�

The pia is attached directly to the spinal cord� Sheets of pia are found in the subarachnoid space, between the ventral and dorsal roots, and can be seen attaching to the inner aspect of the arachnoid-these are called denticulate ligaments (see also Figure 1�11)� These ligaments, which are located at intervals along the spinal cord, are thought to secure the cord, perhaps to minimize its movement�

This illustration of the brain, brainstem and spinal cord (modified from the Integrated Nervous System) shows the blood supply to the spinal cord, notably the anterior spinal artery formed by branches from each vertebral artery� (The posterior spinal arteries are not shown�) A particularly important branch off the aorta supplies the critical region of the spinal cord and supplements the blood supply to the spinal cord� The great radicular artery of Adamkiewicz, a branch from the thoracic aorta, joins the anterior spinal artery supplying the spinal cord at a variable level between T8 and L1 and supplementing the flow to the lower spinal cord� Of some interest, this artery enters most frequently on the left side of the person (note orientation)�

This obliquely oriented arteriogram done with computed tomography, called a CTA (discussed in the Introduction to this chapter) is a composite view of the artery of Adamkiewicz entering the vertebral canal and joining with the anterior spinal artery� Careful viewing shows the filling of the anterior spinal artery both upward and downward for a short stretch in both directions� Capturing this view is not routine�

Surgeons who operate on the abdominal aorta (e�g�, for aortic aneurysm) must make every effort to preserve the small branches coming off the aorta because these are critical for the vascular supply of the spinal cord� One would not want the end result of an aneurysmal repair to be a paraplegic patient�

Infarction of the cord can also occur from a small embolus occluding the anterior spinal artery via the artery of Adamkiewicz�

The term limbic is almost synonymous with the term emotional brain-the parts of the brain involved with our emotional state� In 1937, Dr� James Papez initiated the limbic era by proposing that a number of limbic structures in our brain formed the anatomical substratum for emotion (see Figure 10�1A)�

EVOLUTIONARY PERSPECTIVE Dr� Paul MacLean (1913-2007) postulated the triune model of brain evolution� The pre-mammalian (reptilian) brain has the capacity to look after the basic life functions, and behaviorally it has organized ritualistic stylized patterns of behavior� In higher species including mammals, neocortical structures evolved that are adaptive, allowing for a modification of behavior depending on the situation�

Dr� MacLean introduced the term limbic system (in  1952)� He conceived of a set of functional interconnected structures which arose in early mammals that were responsible for behaviors which are thought of as motivational and emotional, including feeding, reproduction, and parenting�

Hence, we now view the limbic system as those parts of the brain that are involved in regulating the “emotional” state (see definition) of the animal in relation to the external and internal worlds�

DEFINITION Most of us are quite aware or have a general sense of what we mean when we use the term emotion or feelings, yet the concept is somewhat difficult to explain or define precisely� A medical dictionary (Stedman’s) defines emotion as “a strong feeling, aroused mental state, or intense state of drive or unrest directed toward a definite object and evidenced in both behavior and in psychologic changes�” Thus, emotions involve the following:

• Physiological changes: These changes include basic drives involving thirst, sexual behavior, and appetite� They are often manifested as involving the autonomic nervous system or endocrine system, or both�

• Behavior: The animal or human does something, that is, performs some type of motor activity (e�g�, feeding, fighting, fleeing,

displaying anger, mating activity); in humans, this may include facial expression�

• Alterations in the mental state: These can be understood as subjective changes in the way the organism “feels” or reacts to the state of being or to events occurring in the outside world� In humans, we use the term psychological reaction.