Nerves Neuroanatomy

In order to interpret the physical symptoms seen in the hospital or clinic, it is important to have some understanding of the function of the different parts of the brain most commonly damaged in stroke. A basic familiarity with the following parts of the brain will assist in identifying potential problems and enhance the overall plan of care

• Cerebral cortex
• Basal ganglia
• Thalamus
• Brainstem
• Cerebellum

The Cerebral Cortex

The cerebral cortex is a thin (1.4 to 4mm) layer of mostly nerve cell bodies that cover the outer surface of each hemisphere of the brain. The gray matter of the cortex has a high metabolic requirement, using six times more blood than the white matter of the brain

The cortex was sub-divided by Brodman in 1909 into 6 layers called lamina. Each cortical cell layer contains a different type of nerve cell and is named according to the predominant cell type found in that layer. Axons arising from the estimated 100 billion cell bodies of the cortex run both horizontally and vertically, allowing each neuron to interconnect with thousands of other neurons, creating a highly complex and interrelated neural network. The interconnectedness of the nerve cells within the CNS creates a flexible system with redundancy of function that allows recovery of function following injury to the brain

Beneath the thin layer of the cerebral cortex lies the white matter of the brain — vast networks of axons extending to and from the cortex. Under the white matter lie the basal portions of the brain — the basal ganglia that interact with the motor portions of the cortex and cerebellum to control movement and the thalamus and adjoining structures that are responsible for sensory processing

Lobes of the Cerebral Cortex - The cerebral cortex of each hemisphere is divided into four lobes — the frontal, parietal, temporal and occipital lobes. The physical symptoms seen in the hospital or rehabilitation setting will depend upon which part of the brain has been damaged. Symptoms are varied, often confusing and frequently layered on top of or intertwined with other complicated medical problems

Damage to the cerebral cortex causes disruption of higher processing or programming of motor and sensory tasks. A confusing and inexact method of defining cortical disorders leads to difficulty in understanding the mechanism underlying the physical and cognitive symptoms seen in the hospital or clinical setting. Lack of understanding or inaccurate diagnosis of the underlying cause of a problem leads to inadequate and sometimes non-existent treatment options

• Aphasia
• Apraxia
• Agnosia
• One-side neglect

“Neurology’s favorite word is ‘deficit’, denoting an impairment or incapacity of neurological function: loss of speech, loss of language, loss of memory, loss of vision, loss of dexterity, loss of identity and myriad other lacks and losses of specific function (or faculties). For all of these dysfunctions (another favorite term), we have privative words of every sort — aphonia, aphemia, aphasia, alexia, apraxia, agnosia, ataxia — a word for every specific neural or mental function of which patients, through disease or injury, or failure to develop, may find themselves partly or wholly deprived.”

• Aphasia - An acquired communication disorder in people who previously had normal ability. Aphasia affects speaking, comprehension, reading and writing. It is associated with damage to the left frontal and/or left temporal lobes
• Wernicke’s Aphasia - Sometimes called receptive or fluent aphasia because the person is able to speak but the words carry no meaning — sentence length is more than 7 words. This type of aphasia is caused by damage to area 22 of the lateral surface of the left temporal lobe
• Broca’s Aphasia - Sometimes called expressive or non-fluent aphasia because the person is unable to communicate but sentences are short and choppy — less than 7 words. This type of aphasia is caused by damage to areas 44, 45 and 47 of the lateral surface of the left frontal lobe
• Global Aphasia - A combination of Wernicke’s and Broca’s aphasia - unable to understand the spoken word or communicate with speech. A severe stroke may begin with global deficits then slowly resolve to a lesser deficit
• Anomia Word-finding Difficulty
• Apraxia - A disorder of the execution of movement that cannot be attributed to weakness, incoordination, sen sory loss, poor language comprehension or attention deficit. Apraxia is a weakening of the top-down formulation of an action, an inability to sustain the intent to complete a movement. As a result, the nervous system is easily influenced by irrelevant input — a sort of pathological absent-mindedness. Apraxia can affect all modalities including speech, writing, gesturing, dressing and all activities of daily living. Examples of apraxia are: picking up a telephone and beginning to talk without dialing, lighting a candle and trying to smoke it as if it were a cigarette, using a knife to brush one’s hair, using a pencil to butter bread. In these examples it is thought that the command to perform a movement fades before the movement is completed. The performer completes the movement in the best way he or she is able — probably by guessing
• Agnosia - A disorder of the sensory system in which a person is unable to recognize an object by sight, touch or hearing in the absence of defects in the sensory apparatus of these systems. The person can touch, hear and see but cannot recognize or identify the object. Agnosia is usually tested by asking a person to identify a series of objects that are placed out of sight in a bag or behind a partition. The person with agnosia will be unable to name an object by touch alone but will be able to identify the object using vision.
• Anosognosia (hemi-neglect) - A sensory disorder caused by damage to the parietal lobe in which a person is unaware of the contralateral side of the body including half of the visual field. There is a disruption of a person's body schema and spatial orientation. The person is often unaware that half of his or her body exists and will deny that anything is wrong. A person with hemi-neglect may ignore food on the left side of a plate, walk into objects in the left half of the visual field and completely ignore the left extremities. He or she may even claim that the affected arm or leg belongs to another person
• Subcortical Structures (Basal Ganglia or Extrapyramidal System)
• The basal ganglia are 3 large masses of cells (ganglia) that lie at the base of the cerebral cortex and surround the thalamus. Confusion arises due to the many names that are used for the basal ganglia. The 3 masses that compose the basal ganglia are called

• Caudate nucleus
• Putamen
• Globus pallidus

These 3 names are combined in various ways but the most common is to refer to the caudate nucleus and the putamen together as the striatum. The globus pallidus and the putamen together are referred to as the lentiform nucleus. The basal ganglia are also sometimes referred to as the “extrapyramidal system” to differentiate them from the pyramidal or corticospinal tract which is responsible for voluntary movement especially voluntary control of the hand. Disorders affecting the basal ganglia are sometimes called extrapyramidal disorders

• The basal ganglia, together with the cerebellum and the motor cortex, are responsible for motor control. A motor command initiated by the cortex is modified and processed by the basal ganglia. This part of the brain helps the cerebral cortex to execute subconscious, learned movements. It scales movement and determines how large or small and how fast or slow a movement needs to be for optimum performance. The basal ganglia works in conjunction with the substantia nigra as part of the dopamine circuit which is damaged in Parkinson's disease. Blood supply to the basal ganglia is via the middle cerebral artery
• A stroke affecting the basal ganglia usually causes motor control problems rather than motor weakness (hemiparesis). Apraxias are often associated with Basal Ganglia disorders. Damage will cause too much movement (hyperkinesia) as in chorea and athetosis or too little movement (hypokinesia) as in Parkinson's disease
• Hyperkinesia “What then is the opposite of deficit - an excess or superabundance of function? Neurology has no word for this — because it has no concept. A function, or functional system, works — or it does not; these are the only possibilities it allows. Thus a disease which is ‘ebullient’ or ‘productive' in character challenges the basic mechanistic concepts of neurology, and this is doubtless one reason why such disorders — common, important and intriguing as they are — never have received the attention they deserve. And this alone suggests that our basic concept or vision of the nervous system — as a sort of machine or computer — is radically inadequate, and needs to be supplemented by concepts more dynamic, more alive.”
• Symptoms caused by damage to the basal ganglia are distinctly different from damage t o the motor portion of the cortex. Both systems help to control movement but damage to the cortex causes hemiparesis while damage to the basal ganglia causes motor dysfunction. A variety of hyperkinetic movement disorders are associated with damage to the basal ganglia
• Chorea - A hyperkinetic movement disorder characterized by arrhythmic, rapid, involuntary movement that flows from one part of the body to another. The most common type of non-drug related chorea is Huntington's chorea
• Dystonia - A hyperkinetic movement disorder characterized by involuntary movement that is twisting, sustained and repetitive. Over time, the affected body part may assume a fixed posture involving 1 joint (focal dystonia), 2 joints (segmental dystonia) or several joints (generalized dystonia)
• Athetosis - A hyperkinetic movement disorder characterized by spontaneous writhing movements of the hand, arm, neck or face
• Tardive Dyskinesia - A slow-onset, drug-induced hyperkinetic movement disorder characterized by rhythmic, unwanted movements of the face and extremities such as facial grimacing, tongue movements and pill-rolling motions with the fingers.
• The thalamus or “inner chamber” is located at the base of the cerebral hemispheres between the third ventricles and is a small ovoid mass about 3cm long. The thalamus is closely integrated with the cerebral cortex and is responsible for the initial processing of all sensory information except olfaction
• The thalamus accepts and sifts sensory information and is the part of the brain where sensation is first consciously experienced or felt. The lateral part of the thalamus is involved in the integration of sensory information and sends nerve impulses to the parietal, temporal and occipital lobes for further processing. The anterior part of the thalamus is involved with attention, memory and learning. The medial part is involved with sensory integration necessary for abstract thinking and long-term, goal oriented behavior
• After a stroke, a person may experience thalamic pain or “central pain syndrome” due to damage to the spinal tracts that carry pain and temperature sensation from the periphery to the thalamus. Damage to the spin othalamic or trigeminothalamic tract result in severe, spontaneous pain in the parts of the body connected to the damaged tracts. Thalamic pain starts several weeks after the stroke and presents as an intense burning pain on the side of the body affected by the stroke and is often worsened by cutaneous stimulation. Blood supply to the thalamus is via the anterior or carotid arterial system

The Frontal Lobe

The frontal lobe is by far the largest lobe of the cerebral cortex comprising nearly 2/3rds of the total surface of the cortex. The frontal lobe represents the highest level of neural evolution and is larger in humans than in any other primate

The prefrontal lobe is concerned with personality, emotional coloring, foresight and complex decision making. The premotor and primary motor areas of the frontal lobe are concerned with voluntary movement. The primary motor area of the frontal lobe is often affected by stroke and damage in this area will present as one-sided weakness (hemiparesis) in the hand, arm, face, throat, trunk and/or leg on the opposite side of the body from the brain damage. Also found in the frontal lobe is the motor-speech area called Broca's area

Blood supply to the lateral surface of the frontal lobe is via the middle cerebral artery. Blood supply to the medial surface and anterior portion of the frontal lobe is via the anterior cerebral artery

Damage to the frontal lobes causes the following damage

• Broca’s Aphasia - Broca’s motor-speech area is located in the frontal lobe and is the main speech output area of the brain. Damage to this area causes a condition in which the person is unable to speak fluently but is able to understand the spoken word of others. Because damage occurs on the lateral surface of the frontal lobe, the person with Broca’s aphasia may have contralateral hemiparesis of the face and upper extremity
• Hemiparesis - The frontal lobe contains the primary motor cortex and is the origin of the pyramidal or corticospinal tract which is responsible for voluntary movement — especially find hand control. The neurons responsible for control of the face and upper extremity are located on the lateral surface of the frontal lobe — an area of the brain perfused by the middle cerebral artery. The neurons responsible for control of the trunk and lower extremities are located on the medial surface of the frontal lobe — an area perfused by the anterior cerebral artery
• Emotional disturbances - The areas of the frontal lobe in front of the motor and premotor areas are critical areas for attention, short-term memory, motivation and problem solving. Damage to this part of the frontal lobe can cause disruption of social behavior, disinhibition, lack of initiative and short-term memory problems

The Parietal Lobe

The parietal lobe is located just behind the frontal lobe and is separated from the frontal lobe by the central sulcus. The main function of the parietal lobe is processing of somatosensory information such as touch, vibration, pain and position sense (proprioception). Located in this part of the cortex are the prima ry somatosensory area, which is the final destination of the spinothalamic tract, and other sensory tracts that transmit information from the peripheral nervous system to the cortex via the thalamus

Specific parts of the parietal lobe are responsible for tactile perceptions such as recognizing objects by touch (stereognosis) and for tactile guidance of limb movement and visual control of eye and hand movements. The areas of the parietal lobe that lie closest to the frontal, temporal and occipital lobes form transition areas which function to integrate visual, somatosensory and motor commands

The parietal lobe of the non-dominant hemisphere (right side) is concerned with spatial awareness and the ability to carryout and understand spatial relationships. Blood supply to the parietal lobe is via the anterior or carotid arterial system

General damage to the sensory cortex is referred to as agnosia. Agnosia is the inability to understand the meaning of a sensory stimulus despite being alert, non-aphasic and having intact sensation. Agnosia can affect any sensory modality including vision, touch, sound, taste and proprioception. Damage to the right parietal lobe may cause a disorder called contralateral neglect in which the person is unaware of the opposite side of the body and unaware of objects in the left half of the visual field

Temporal Lobe

The temporal lobe is located behind and below the frontal and parietal lobes in the vacinity of the temple and the ears. As might be guessed by the location, the temporal lobe is concerned with auditory processing — especially basic perception and comprehension of auditory information. As with the other lobes of the cerebral cortex, the left (dominant) and the right (non-dominant) temporal lobes differ somewhat in function

The left temporal lobe contains Wernicke's area that is responsible for the interpretation and organization of language — especially understanding and responding to verbal input. The right temporal lobe is specialized for processing and interpretation

Damage to the temporal lobes causes both visual and language comprehension deficits. Visual field deficits occur because the part of the visual pathway called the optic radiation's pass through the lower part of the temporal lobes on the way to the occipital lobes. Damage to Wernicke's area in the left temporal lobe affects the ability to comprehend written and spoken language and impairs the ability to solve math and logic problems

Occipital Lobe

The occipital lobes are located posterior to the parietal lobes and are responsible for processing of visual input and coordination of eye movement. They work closely with the temporal and parietal lobes to process visual images for tasks such as reading. Cells of the occipital lobes involved with visual processing break images into components such as form, color and movement and transmit the images to the temporal lobe (form and color) and the parietal lobe (analysis of movement). Most of the visual cortex is devoted to the analysis of central vision

The Brainstem

The brainstem is located above the spinal cord and beneath the thalamus and consists of 3 parts

1. Medulla oblongata
2. Pons
3. Midbrain

The brainstem contains many well-defined clusters of nerve cell bodies or nuclei that receive sensory input from the cranial nerves and send output to the thalamus for further processing. Additionally, the brainstem has an ill-defined central core called the brainstem reticular formation (RF) that contains the respiratory and cardiovascular "centers" that influence breathing, respiration, BP, circulation and vasomotor tone. Blood supply to the brainstem is via the vertebral-basilar arterial system

Cerebellum

The cerebellum or “little brain” is located behind and above the brainstem. It is connected to the brainstem via 3 pairs of peduncles or “little feet” which are actually bundles of nerve fiber tracts entering and exiting the cerebellum.

The cerebellum is responsible for several aspects of movement and works in conjunction with the basal ganglia and the motor portion of the cerebral cortex to control movement. Specifically, the cerebellum coordinates the planning and timing of movement and corrects deviations in movement by comparing one movement with another and fine-tuning subsequent movements. The cerebellum also sends and receives information from the vestibular system and helps to control balance by controlling the axial muscles of the body. Blood supply to the cerebellum is via the posterior or vertebrobasilar arterial system

A stroke affecting the cerebellum will cause disruption in balance, timing and force of movement. The following symptoms are commonly seen in cerebellar stroke

• Ataxia — motor incoordination due to irregularities in timing, rate and force of muscular contraction
• Unsteady gait — grossly uncoordinated or “drunken” gait
• Loss of balance and a tendency to fall
• Dizziness — A sensation of spinning and nausea
• Intention tremor — Tremor which worsens with movement
• Dysdiadochokinesia — Inability to perform rapid, alternating movements
• Pendular swinging of a joint — A large swinging movement after joint displacement (for example — after a patellar tendon tap)
• Nystagmus — Conjugate drift of the gaze followed by rapid return to the center