HUMAN PHYSIOLOGY FOR NURSES II
Physiology is the scientific discipline that deals with process or the functions of the living things. It also examines how the parts of the body work and the ways in which they cooperate together to maintain life and health of the individual.
One outstanding quality of physiology is that it integrates the individual functions of all the body’s different cells and organs into a functional whole, the human or animal body. Indeed, life in the human being relies upon this total function, not on functions of the single parts in isolation from the others.
Human Physiology is a three credit course for students in the BNSc. programme. The course is broken to 4 modules with 12 study units. It will introduce the students to Physiology. At the end of the course, the learner is expected to demonstrate clear understanding of physiology and its application to nursing and holistic patients’ care. This course guide provides you with what to expect in the course, how to work through the course material as a distance learner saddled with the responsibility of studying on your own and your overall responsibilities and expectations. Tutorial sessions are also linked up with the course to provide the needed support you required.
The Nervous system is one of the two major control systems of the body in addition to the endocrine system. The central nervous system (CNS) is composed of the brain and spinal cord. These receive input about the external and internal environment from afferent neurons. The CNS sorts, processes and transmits this information to the efferent neurons which carry the instruction to glands or muscles to bring about the desired response. The nervous system acts by means of its electrical signals to control the rapid responses of the body. The brain, the first part of the central nervous system is arranged in regions and subdivisions. This unit will examine the functions of the various regions of the brain and some higher brain functions.
The central nervous system (CNS) consists of the brain and spinal cord. They receive input from sensory neurons, and direct the activity of motor neurons. Association neurons are present to “associate” appropriate motor responses with sensory stimuli.
The early embryo contains an embryonic tissue layer known as ectoderm, on its surface that will eventually form the epidermis of the skin and the nervous system. As development continues, a groove appears in the ectoderm along the dorsal midline of the embryo’s body. The groove deepens and by the twelfth day after conception has fused to form a neural tube.
The part of the ectoderm where the fusion occurs becomes a structure separate from the neural tube and is called the neural crest. The neural tube becomes the CNS later while the neural crest eventually becomes the ganglions of the peripheral nervous system and other structures. By the middle of 4th week three distinct swellings are evident on the anterior end of the neural tube which will form the forebrain, midbrain and hindbrain.
In the 5th week those three areas are modified to form five regions. The forebrain divides into the telencephalon and diencephalon. The midbrain is unchanged and the hindbrain divides into the metencephalon and myelencephalon. These regions later become greatly modified but the terms described are still used to indicate the general regions of the brain.
The telencephalon grows disproportionately in humans forming the two enormous hemispheres of the cerebrum that cover the diencephalon, the mid brain and some portion of the hindbrain. The CNS that began as a hollow tube remains hollow even as the regions are formed. The hollow parts divide into cavities called ventricles in the brain. These become filled with cerebrospinal fluid. These cavities are connected to themselves and are continuous with the central canal of the spinal cord.
The central nervous system is composed of grey and white matter.
Grey matter is composed of high concentration of neuronal cell-bodies and dendrites that do not have myelin sheath. Grey matter is found in the surface layer (cortex) of the brain and deeper within the brain in neuronal aggregations called nuclei. White matter consists of high concentration of axon tracts. Because axons are usually myelinated they acquire the white colour of myelin sheath. White matter lies under the cortex and also surrounds the nuclei.
The adult brain contains about 100 billion neurons, weighs about 1.5kg and receives about 20% of the total blood flow to the body per minute. The human brain is mostly water (75%) and has the consistency of gelatin. It needs support for its existence. The brain is therefore protected by the scalp, hair etc and by the reinforced bony cranium which is one of the strongest structures in the body. It also floats shock-proof in cerebrospinal fluid and is encased in three layers of cranial meninges – the dura mater, arachnoid mater and pia mater.
Functions of the Cerebrum
The cerebrum is the only structure of the telencephalon, and the largest of the brain regions (accounting for about 80% of the brain mass). It is believed to be the centre for higher brain functions and the most sophisticated region of the brain. The cerebrum is divided into two halves the right and left cerebral hemisphere, and these two hemispheres are connected to each other by a thick band of neuronal axons called corpus callosum. The outer layer of the cerebrum is the cerebral cortex which caps an inner core of white matter that houses the basal nuclei.
The Cerebral Cortex
The cerebral cortex is composed of a thin (2 to 4mm thick) outer covering of grey matter overlying a thick central core of white matter. The cortex characteristically contains numerous folds and grooves called convolutions. The elevated folds are called gyri (singular-gyrus-) and the depressed grooves are called sulci (singular sulcus).
Each cerebral hemisphere is divided by deep sulci or fissures into lobes – 4 major ones and probably an additional minor fifth lobe. The lobes are the frontal, parietal, temporal and occipital lobes, which are visible from the surface and deep and fifth insula lobe.
The frontal lobe is the anterior portion of each cerebral hemisphere. It is separated by a deep fissure called the central sulcus from the parietal lobe. The precentral gyrus is located in the frontal lobe just in front of the central sulus. This gyrus is responsible for voluntary motor control. Other functions of the frontal lobe are that it is also responsible for speaking ability and for elaboration of thought. The primary motor cortex in the precentral gyrus on each side of the brain primarily controls muscles on the opposite side of the body.
The parietal lobes situated directly on the top of the head behind the frontal lobe and separated from the frontal lobe by the deep central sulcus. The parietal lobes are primarily responsible for receiving and processing sensory input such as touch, pressure, heat, cold and pain from the surface of the body.
The post-central gyrus, directly behind the central sulcus in the parietal lobe is the primary area of the cortex responsible for the perception of these sensations collectively called somaesthetic sensations. (i.e. those arising from cutaneous, muscle, tendon, and joint receptors).
The temporal lobes are located at the sides of the head and contain the auditory centres that receive sensory fibres from the cochlea of the ear. The lobe is also involved in the interpretation and association of auditory and visual information. The occipital lobes are located at the back of the head and they are primarily responsible for vision and coordination of eye movements.
The insula lobe is small and buried deep in the central sulucs. It is supposed to be associated with memory, and visceral activities.
The Basal Nuclei
The basal nuclei is also known as basal ganglia. They are masses of grey matter composed of neuron cell bodies located deep within the white matter of the cerebrum. A nucleus (plural nuclei) refers to a functional aggregation of neuronal cell bodies. The most prominent part of the basal nuclei is the corpus stratum.
The major function of the basal nuclei is a complex role in the control of voluntary movement in the following ways – (i) inhibiting muscle tone throughout the body (to balance excitatory inputs); (ii) selecting and maintaining purposeful motor activity while suppressing useless or unwanted patterns of movement, and (iii) helping to monitor and coordinate slow sustained contractions, especially motor neurons but they modify ongoing activity in motor pathways.
The importance of the basal nuclei in motor control is evident in diseases involving this region; the most common of which is Parkinson’s disease. In this condition there is deficiency of dopamine, a neurotransmitter in the basal nuclei. This lack makes the basal nuclei unable to perform normal roles therefore the characteristic features of Parkinson’s disease manifest. These are (1) increased muscle tone or rigidity (2) involuntary, useless or unwanted movements like resting tremors and (3) slowness in initiating and carrying out motor behaviours.
Lateralization and Kabgyage Language Function of the Cerebral Cortex
Function of the cerebral cortex. Each cerebral hemisphere controls the movements of the opposite side of the body through motor fibres that originate in the precentral gyrus. In the same way, somaesthetic sensations from each side of the body projects to the opposite side on the post central gyrus due to crossing over (decussation of fibres). However, the two hemispheres can receive information from both sides of the body because they communicate with each other via the corpus callosum.
The cortical areas described so far appear to be equally distributed. However, studies seen to indicate that a task can be successfully performed by one side but not by the other. For example, the left hemisphere has been shown to be the one in which most of the language and analytical abilities reside as well as handedness. Fine motor control seems to be more under the control of the left hemisphere.
This is evidenced by the fact that most people are right handed and the left hemisphere controls the right side of the body. It is these findings that led to the concept of cerebral dominance. That is since these various obvious activities are controlled by the left hemisphere, then the left hemisphere must be the dominant one.
However, further studies have shown that the right hemisphere is also specialized along different less obvious lines. Therefore, rather than one hemisphere being dominant and the other, being subordinate, the two hemispheres appear to have complimentary functions. The term cerebral lateralization, or specialization of function in one hemisphere or the other, may be preferred, more recently to the term cerebral dominance.
Unlike the sensory and motor regions of the cortex, which are present in both hemispheres, the areas of the brain responsible for language ability are found in the left hemisphere in most people.
Language is a complex form of communication in which written or spoken words symbolize objects and convey ideas. It involves the integration of two distinct capabilities namely, expression and comprehension – each of which is related to a specific area of the cortex. The two areas are the Broca’s area and Wernicke’s area. Broca’s area is responsible for speaking ability and articulation of speech.
Broca’s aphasia is the result of damage to the Broca’s area. Common symptoms include weakness of the right arm and right side of the face. People with Broca’s aphasia are reluctant to speak, and when they speak, their speech is slow and poorly articulated. Speech comprehension is not affected. They can understand a sentence but have difficulty repeating it.
The Wernicke’s area is located in the parietal lobe almost at the junction of the parietal, occipital and temporal lobes. It is concerned with language comprehension. It plays a critical role in understanding both spoken and written messages. It is responsible for formulating coherent patterns of speech that are transferred via a bundle of fibres to the Broca’s area for articulation. Wernicke’s aphasia results in speech that is rapid but without meaning. People with Wenicke’s aphasia produce what has been described as “word salad’ e.g. real words chaotically mixed together or made up words. Language comprehension whether written or spoken is destroyed in Wernike’s aphasia.
Together with the telencephalon (cerebrum) the diencephalon constitutes the forebrain. The diencephalon is almost completely surrounded by the cerebral hemispheres and contains a cavity called the third ventricle. It can be said to be the deep part of the cerebrum connecting the midbrain with the cerebral hemispheres. It is composed of the thalamus, epithalamus and hypothalamus. The pituitary gland is connected to the hypothalamus.
The thalamus is composed of two egg-shaped masses of grey matter covered by a thin layer of white matter. It is located in the centre of the cranial cavity directly beneath the cerebrum and above the hypothalamus. It forms the lateral walls of the third ventricle.
The thalamus acts as an intermediate relay point and processing centre for all sensory impulses (except smell) on their way to the cerebral cortex from the spinal cord, brain stem, cerebrum, basal ganglia and other sources. It screens out insignificant sensory impulses to appropriate areas of the somato sensory cortex as well as to other regions of the brain. The thalamus working with the brain stem and cortical association areas is important in our ability to direct attention to stimuli of interest and so forth.
The epithalamus is the dorsal portion of the diencephalon that contains a choroid plexus over the third ventricle where cerebrospinal fluid is formed. It also contains the pineal gland or epiphysis.
The hypothalamus is a small portion of the diencephalon located below the thalamus, where it forms the floor and part of the central walls of the third ventricle. It is a collection of specific nuclei and associated fibres.
This extremely important brain region is an integrating centre for many important homeostatic functions and serves as an important link between the autonomic nervous system and the endocrine system. Specifically the hypothalamus (1) controls body temperature; (2) controls thirst and urine output; (3) controls hunger and food intake; (4) controls anterior pituitary hormone secretion (5) produces posterior pituitary hormones (6) controls uterine contraction and milk ejection; (7) serves as a major autonomic nervous system coordinating centre which in turn affects all smooth muscle, cardiac muscle and exocrine glands; and (8) plays a role in emotional and behavioural patterns. In addition, centres in the hypothalamus, contribute to the regulation of sleep, wakefulness, sexual arousal and performance, and emotions as such anger, fear, pain and pleasure.
The pituitary gland is located immediately inferior to the hypothalamus. It is said to be derived embryonically from a down growth of the diencephalon by means of a stalk neurons in the supraoptic and paraveneticular nuclei of the hypothalamus produce two hormones – antidiuretic hormone (ADH) and oxytocin. These hormones are transported to the neurohypohypsis and stored and secreted in response to hypothalamic stimulation.
Emotion and Motivation
The parts of the brain that appear to be of paramount importance in emotional states are the hypothalamus and the limbic system.
The limbic system is not a separate structure but refers to a ring of nuclei and fibre tracts that surround the brain stem and are interconnected by intricate neuronal pathways. The structures of the limbic system include the cingulate gyrus (part of cerebral cortex), amygdala, hippocampus and septal nuclei. This complex interacting network of structures is associated with emotions, basic survival and socio-sexual behavioural patterns motivation and learning. It is the centre for basic emotional drives.
There are few synaptic connections between the cerebral cortex and the structures of the limbic system which may help explain the fact that we have so little control over our emotions. However there is a closed circuit of information between the limbic system and the thalamus and hypothalamus and they cooperate in the neural basis of emotional states. Studies suggest that the hypothalamus and the limbic system are involved in the following feelings and behaviours:
Aggression – stimulation of areas of the amygdala and particular areas of the hypothalamus can both produce rage and aggression.
Fear – electrical stimulation of the amygdala and hypothalamus produce fear. Removal of the limbic system can result in absence of fear.
Feeding – as discussed under hypothalamus.Sex – the hypothalamus and limbic system are involved in the regulation of the sexual drive and behaviour.
Goal directed behaviours.
The concept of emotion encompasses subjective emotional feelings and moods (anger, fear and happiness) plus the overt physical responses that occur in association with these feelings. Such responses include specific behavioural patterns (e.g. preparation for attack or defence) and observable emotional responses (e.g. laughing, crying and blushing).
Motivation is the ability to direct behaviour towards specific goals that are aimed at satisfying specific identifiable needs related to homeostasis. However most human behaviours are shaped in a complex framework of unique personal gratifications blended with cultural expectations.
Memory is the storage of acquired knowledge for later retrieval. The neural change responsible for retention or storage is known as neural trace. Concepts and not necessarily verbatim words are stored and so is recall. Storage of acquired information is believed to occur in at least two stages – short-term memory (STM) and long term memory (LTM) Short Term Memory lasts for seconds to hours.
Long term memory is retained for days to years. When knowledge is first acquired it is stored in the short term memory. In order for it not to be forgotten it has to be consolidated into the long term memory. The capacity of the short term memory to store is very limited but the capacity of the long term memory bank is much larger.
Different informational aspects of long term memory traces seem to be processed and coded, then stored in conjunction with other memories of the same type.
It takes longer time to retrieve information from the LTM because the stores are larger. Remembering is the process of retrieving specific information from memory stores. Forgetting is the inability to retrieve stored information. Information in the STM is permanently forgotten while in the LTM forgetting is only traMemory traces are present in multiple regions of the brain.
The neurons involved are widely distributed throughout the cortical and sub cortical regions of the brain. The regions of the brain implicated in memory include the temporal lobes, the prefrontal cortex, other regions of the cerebral cortex, the limbic system and the cerebellum.
The temporal lobes and limbic system are essential for transferring new memories into long term storage. The hippocampus plays a vital role in integration of various related stimuli in the STM. It is also crucial for consolidation into LTM. However the hippocampus stores only temporarily new LTM store and then transfers them to other cortical structures. For more permanent storage.
Accessing and manipulating these long-term stores; appears, to be carried out by the prefrontal region of cerebral cortex. the cerebellum seems to play a role in procedural memories involving motor skills gained through repetitive training. The hippocampus and surrounding regions are responsible for declarative memory.
Amnesia (loss of memory) has been found to result from damage to the temporal lobe of the cerebral cortex, hippocampus, head of the caudate nucleus, or dorsomedial aspects of the thalamus.
The mesencephalon or midbrain is located between the diencephalon and the pons. It connects the pons and cerebellum with the cerebrum (forebrain). On the ventral surface of the midbrain is a pair of cerebral peduncles, made up of pyramidal tracts (fibres to the motor nuclei of the spinal nerves within the spiral cord) corticobulbar (motor fibres to the cranial-nerve motor nuclei) and corticopontine fibres to the pons. The third cranial nerve (occulomotor) emerges from the fossa between the peduncles on its ventral side.
Passing through the midbrain is the cerebral aqueduct. The dorsal portion of the midbrain situated above the aqueduct is the roof which has 4 little elevations – the colliculi. The colliculi are known collectively as the corpora quadrigemina. The superior pair of colliculi are reflex centres that coordinate the movements of the eyeballs and head, regulate the focusing mechanism in the eyes and adjust the size of the pupils in response to visual stimuli. Cranial nerve IV (trochlear) emerges from the roof of the midbrain. The inferior colliculi just posterior are relay nuclei of the auditory pathways going to the thalamus and eventually to the auditory cortex.
The midbrain also contains the red nucleus, an area of grey matter deep in the midbrain. It maintains connections with the cerebrum and cerebellum and is involved in motor coordination. Another nucleus is also present, a heavily black pigmented nucleus called substantial nigra. It is integrated into neural circuits with the basal ganglia and therefore is involved also with motor-coordination (somatic motor activities).The substantial nigra has a role in Parkinson’s disease.
The hindbrain (rhombencephalon) is composed of the metencephalon and the myeloencephalon and they will be discussed separately.
The region is composed of the pons and the cerebellum. The pons can be seen as a bulge on the underside of the brain, between the mid brain and the medulla oblongata. The pons is composed mainly of fibres that connect the hindbrain to the midbrain as a relay station. Surface fibres in the pons connect to the cerebellum, and sensory tracts, that pass from the medulla oblongata, through the pons and onto the midbrain. Within the pons are several nuclei associated with specific cranial nerves – Trigeminal. (V) abducens (VI), facial (VII) and Vestibulocochear (VIII).
Other nuclei in the pons cooperate with nuclei in the medulla oblongata to control breathing. Two respiratory control centres are in the pons – the apneustic and pneumotaxic centres.
The cerebellum occupies the inferior and posterior aspect of the cranial cavity and is the second largest structure of the brain. It contains outer grey matter and inner white matter (like the cerebrum). Fibres from the cerebellum pass through the red nucleus to the thalamus and then to the motor areas of the cerebral cortex. Other fibre tracts connect the cerebellum with the pons, medulla oblongata and spinal cord. The cerebellum receives inputs from proprietors (receptors in joints, tendons and muscles) and working together with the basal ganglia and motor areas of the cortex participate in the coordination of movements.
The cerebellum consists of 3 functionally distinctive parts with different functions:
The Vestibulocerebellum is important for the maintenance of balance and control of eye movement.
The Spinocerebellum regulates muscle tone and coordinates skilled, voluntary movements. It receives signals from the cortex concerning specific message to muscles and also receives inputs from peripheral receptors concerning body movement and position. The spinocerebellum essentially acts as “middle management” comparing the “intentions” or “orders” of the higher centres with the “performance” of the muscles and correcting deviations.
The Cerebrocerebellum plays a role in planning and initiation of voluntary activity by providing input to the cortical motor area. It is also the region involved in procedural memories.
Damage or disease of the cerebellum produces the following range of symptoms which reflect the loss of the aforementioned function. Poor balance, nystagmus, reduced muscle tone but no paralysis, inability to perform rapid movement smoothly and inability to stop and start skeletal muscle action quickly. All these are due to a lack of coordination due to errors in speed, force and direction of movement, a condition called ataxia. The condition is also characterized by intention tremor which occurs only when intentional movements are made.
The person may reach for an object and miss it by overshooting or placing the hand too far to the left or right of the object, and then attempt to correct it by repeating the to and from movement. This back and forth movement can result in oscillationsof the limb.
This is made up of only the medulla oblongata. The medulla measures about 3cm long and is continuous with the pons superiorly and the spinal cord inferiorly. The medulla, with the pons and midbrain make up the brain stem.
All the descending and ascending tracts that provide communication between the spinal cord and the brain must pass through the medulla.
The vertical surface of the medulla contains bilateral elevated ridges called the pyramids. These pyramids are composed of the fibres of motors tracts from the motor cerebral cortex to the spinal cord. These fibres (pyramids) cross to the contralateral (opposite) side at the lower part of the medulla to the opposite side of the spinal cord, forming an “X”. This crossing over of motor nerve fibres is called decussation. The result is that the left side of the brain receives sensory information from the right side of the body and vice versa. Similarly, the right side of the brain controls motor activity in the left side of the body and vice versa.
Many important nuclei are contained in the medulla. Some of them are involved in motor control while some of them form nerve roots for many of the cranial nerves. Cranial nerves IX, X, Xi and XII all have rootlets in parts of the medulla. The vagus nuclei located one on each lateral side of the medulla give rise to the highly important vagus (X) nerve. Other nuclei are there which relay sensory information to the thalamus and them to the cerebral cortex.
The medulla also contains groupings of neurons that make up the vital centres. These include the cardiac (cardioinhibitory centre) for the parasympathetic inhibition of the heart, the vasomotor centre, for control of the autonomic innervations of blood vessels, and the respiratory centre which acts together with centres in the pons to control breathing.
Running throughout the brain stem and into the thalamus and hypothalamus is a widespread and complex network of neurons and fibres called the reticular formation. It is organized into (1) ascending (sensory) pathways from ascending spinal cord tracts and the cerebellum, (2) descending (motor) pathways from the cortex and hypothalamus; and (3) cranial nerves. Ascending fibres from the reticular formation carry signals upwards to arouse and activate the cerebral cortex.
These fibres compose the reticular activating system (RAS) which controls the overall degree of cortical alertness and the ability to direct attention. Because of its many interconnections, the RAS is activated in a non-specific fashion by any modality of sensory information. Not surprisingly, general anaesthetics may produce unconsciousness by depressing the RAS. Also the ability to fall asleep may be due to the action of specific neurotransmitters that inhibit activity of the RAS.