Anatomy and Physiology

Anatomy and Physiology

The Nervous System

                The nervous system is the command center of the body, it is made up of the central nervous system and the peripheral nervous system. Each has an array of different functions in the body. Every thought, action, reaction, emotion and many biologic activities are influenced by its activity and vise versa. The cells of the nervous system communicate by sending and receiving impulses which can be electrical or chemical and are rapid and specific and almost cause an immediate response. It has overlapping three overlapping functions. (1) It uses millions of sensory receptors to monitor the internal and external changes of the body. The information gathered by these sensory receptors are called “sensory inputs”. (2) It processes and interprets the sensory input and decides what should be done next. This is called “integration”. It causes a response called “motor output” by activating its effector organs.

               

Organization of the Nervous System

The two principal parts of the nervous system are the central and the peripheral nervous system. The central nervous system is made up of the brain and the spinal cord which occupy the dorsal portion of the body and the head. The CNS interprets the sensory input and dictates the motor responses based on experiences, reflexes, current condition and other aspects. The peripheral nervous system or the PNS is the part of the nervous system that is outside the CNS. It is composed of the nerves and nerve bundles that extend from the brain  and spnal cord. Cranial nerves carry impulses to and from the brain that links all parts of the body to the CNS.

The PNS has two functional subdivisions. The sensory or the afferent division (carrying toward)are made up of nerve fibers that carry impulses from the sensory receptors around the body towards the brain. It conveys information to the brain using the two types of fibers. One, the somatic afferent fibers which convey information from the parts of the body other than the visceral organs(organs within the ventral body cavity), the second type of afferent fiber is called the visceral afferent fibers. These fibers send impulses from the visceral organs to the brain. The other functional subdivision is the motor or the efferent division (carrying away), these transmits impulses from the CNS to the effector organs like muscles and glands. In other words they “effect” a response. (Marieb, 2007)

The motor division also has two parts.

1. The somatic and the autonomic nervous systems. The somatic nervous system is composed of somatic nerve fibers that conduct impulses from the CNS to the skeletal muscles. It can also be referred to as the voluntary nervous system.
2. The autonomic nervous system is responsible for regulating activities of the smooth muscles, cardiac muscles and glands. It is also called the involuntary nervous system.  The ANS is further divided into the:
1. Sympathetic nervous system – promotes the “flight or fight responses”
2. Parasympathetic nervous system –

Levels of Organization of the Nervous System.

The Brain

The brain consist of many parts that function to help as an integrated whole.  The major parts are the medulla, pons, and midbrain (collectively called the midbrain), the cerebellum, the hypothalamus, the thalamus, and the cerebrum. (Scanlon, 2007)

THE VENTRICLES

The ventricles are four cavities within the brain, two lateral ventricles, the third and the fourth ventricle. Each ventricle contains a capillary network called the choroid plexus which produces the CSF from blood plasma. (Scanlon, 2007)

MEDULLA

Extends from the spinal cord to the pons and the anterior of the cerebellum. It contains cardiac centers and vasomotor centers that control the cardiac functioning as well as vascular functions such as diameter of blood vessel. It thereby regulates the  the BP as well as the respiratory functions. This is the reason why a blow or injury to the occipital bone can rapidly be fatal. Without the medulla the body cannot survive. Also within the medulla, reflex centers for coughing, sneezing, swallowing and vomiting. (Scanlon, 2007)

PONS

The pons bulges anteriorly from the upper part of the medulla. Within the pons are two respiratory centers that work with those in the medulla to produce normal breathing patterns. Other neurons within the pons connect the medulla with other parts of the brain. (Scanlon, 2007)

MIDBRAIN

The midbrain extends from the pons to the hypothalamus and encloses the cerebral aqueduct, the tunnel that connect the 4^th and 3^rd ventricles. Different reflexes are integrated in the midbrain including visual and auditory reflexes. An example is the eye-hand coordination that can be seen when playing games. (Scanlon, 2007)

CEREBELLUM

Separated from the medulla and pons by the 4^th ventricle and is inferior to the occipital lobes of the cerebrum. Many of the functions of the cerebellum is concerned with movement. This includes coordination, regulation of muscle tone, appropriate trajectory and endpoints of movement and maintenance of posture and equilibrium. The cerebellum functions below the level of consciousness which means that these are involuntary. The cerebellum makes adjustments to the impulses from the cerebrum in order to suit a situation (i.e. catching a ball, reflex comes from cerebrum, endpoint is determined by the cerebellum). (Scanlon, 2007)

HYPOTHALAMUS

Located superior to the pituitary gland and inferior to the thalamus. Its functions are the following.

1. Production of antidiurhetic hormones and oxytocin; stored in the posterior pituitary gland. ADH enables kidneys to reabsorb water into the blood thus maintaining blood volume if necessary. Oxytocin brings about uterine contractions.
2. Production of releasing hormones that stimulate release of other hormones.
3. Regulation of body temperature by promoting responses such as sweating or shivering.
4. Regulation of food intake. Controls feeling of hunger and fullness
5. Integration of the functioning of the ANS
6. Stimulation of visceral responses during emotional situations.
7. Regulation of body rhythms such as secretion of hormones, sleep cycles, changes in mood or mental alertness.

THE THALAMUS

Is located superior to the hypothalamus and inferior to the cerebrum. The 3^rd ventricle passes through both the thalamus and the hypothalamus. Many functions of the thalamus involves sensations. Sensory impulses(except for sense of smell) follow a neural pathway that passes through the thalamus which then groups the impulses before relaying them to the cerebrum where the sensations are felt. The thalamus integrates the     impulses from the cutaneous tissue and puts it together as an electrochemical package so the cerebrum feels the whole and is able to interpret the sensations quickly. For example when one is holding a hot cup of coffee. Impulses for heat, touch, texture and shape of the cup are not felt as different sensations.

                The thalamus also helps in concentration. It suppresses unimportant sensations by blocking them. An example is when watching TV, you may not notice someone calling you. Parts of the thalamus may also contribute in alertness and awareness. The thalamus and cerebrum work closely for these functions. (Scanlon, 2007)

CEREBRUM

The largest part of the human brain is the cerebrum. It consists of two hemispheres separated by a longitudinal fissure. At the base of this deep groove is the corpus callosum, a band of 200 million neurons that connects the left and right hemispheres. Within each hemisphere is a lateral ventricle. The surface of the cerebrum is gray matter called the cerebral cortex. It consist of cell bodies of neurons which carry out the many functions of the cerebrum. Inside the gray matter is white matter. It is made up of myelinated axons and dendrites that connect the lobes of the cerebrum to one another and to all parts of the brain that connect all the lobes of the brain to all the parts of the brain. The cerebral cortex is folded extensively to make room for as many neurons as possible. These foldings are not found in cats and dogs which allow us to be able to read, speak and do many “human” activities. The cerebral cortex is divided into two lobes that have the same name as the cranial bones external to them. Therefore each hemisphere has a frontal lobe, a parietal lobe, temporal lobe and an occipital lobe. These lobes have been mapped correspondingly to their specific functions. (Scanlon, 2007)

Frontal lobes

Within the frontal lobes are the motor area that generate the impulses for voluntary movement mainly for the hands and face which are capable of fine movements. It is the large size of the area devoted to them that allows for greater precision of motion.

Anterior to the motor areas are the premotor areas which arre concerned with the learned motor skills that require movement. An example is the tying of shoelaces. The parts of the frontal lobe just behind the eyes are the prefrontal cortex or the orbitofrontal cortex. This area is concerned with keeping situation-appropriate emotions, realizing that there are standards of behavior(laws or rules of games or simple good manners) and following them and anticipating and planning for the future.

Also in the frontal lobe is the Broca’s motor speech area. Usually only the left lobe for most right-handed people controls the movements of the mouth for speech. (Scanlon, 2007)

Parietal Lobe

The  general sensory areas in the  parietal lobes receive impulses from receptors in the skin and feel and interpret the cutaneous sensations. The left area is for the right side of the body and vice versa. These areas also receive impulses from stretch receptors in muscles for conscious muscle sense. The largest portions of these areas are for sensation in the hands and face, those parts of the body with the most cutaneous receptors and the most muscle receptors. The taste areas, which overlap the parietal and temporal lobes, receive impulses from taste buds on the tongue and elsewhere in the oral cavity. (Scanlon, 2007)

The Temporal Lobes

The olfactory areas in the temporal lobes receive impulses from the receptors in the nasal cavities for the sense of smell. The olfactory association areas learn the meaning of odors and enable the thinking cerebellum to use the information effectively.

The auditory areas as the name suggests, receive impulses from the receptors of the inner ear for hearing. The auditory association area is quite large, some parts are concerned with the meaning of words while others are for interpretations of sounds. Without interpretation, the sounds can be heard however appropriate responses cannot be executed.

Also in the parietal and temporal lobes of the left hemisphere(for most) are speech areas concerned with thoughts that precede speech. Thinking occurs very rapidly and is essential for us to be able to speak. (Scanlon, 2007)

Occipital Lobes

The impulses from the retinas of the eyes travel along the optic nerves to the visual areas in the occipital lobes. The visual association areas interpret what is seen, and enable the cerebrum to interpret the visual stimuli. Seeing a clock is different from being able to interpret it. When we learned how to read clocks, the information is learned in the occipital lobe so that the next time we see a clock, we can use the information to interpret it. One example is when we look at our watches, and we are late, we adjust by hurrying so that we don’t get late. Other parts of the occipital lobe is concerned with the relation of the body with space. Judgment in distance, seeing in 3d and relating it to the physical world is done in the occipital lobes. (Scanlon, 2007)

Association areas

The association area is part of the cerebral cortex that does not mainly concern itself with movement and sensation. Association areas give us our individuality. It is the area that gives us our personalitt, a sense of humor and the ability to use reason and logic. Learning and memory are also functions of this area. Memory is also believed to be involved with the hippocampus. It collects information from the different areas of the cerebral cortex. A person with a damaged hippocampus does not have the ability to form new memories that last more than a few seconds. The right hippocampus is also involved in spatial cognition. (Scanlon, 2007)

Basal Ganglia

The basal ganglia are paired masses of gray matter within the white matter of the cerebral hemispheres. They control the subconscious aspect of voluntary movement. The basal ganglia helps in the feet and hand movement coordination when walking as well as gesturing while speaking. The most commn disorder of the basal ganglia is Parkinson’s disease. (Scanlon, 2007)

Corpus Callosum

Connects the left and right hemispheres of the brain with millions of nerve bundle. This enables each hemisphere to know of the activity of the other. This is especially important for people because for most of us, the left hemisphere contains speech areas and the right hemisphere does not. The corpus callosum, therefore, lets the left hemisphere know what the right hemisphere is thinking about, and the right hemisphere know what the left hemisphere is thinking and talking about. A brief example may be helpful. If you put your left hand behind your back and someone places a pencil in your hand (you are not looking at it) and asks you what it is, would you be able to say? Yes, you would. You would feel the shape and weight of the pencil, find the point and the eraser. The sensory impulses from your left hand are interpreted as “pencil” by the general sensory area in your right parietal lobe. (Scanlon, 2007)

The Limbic System

Is a group of structures that are located on the medial aspect of each cerebral hemisphere and the diencephalon. Its cerebral structures encircle the upper part of the brain stem and include parts of the rhinencephalon and the amygdale. The limbic system is the emotional or affective brain. The amygdale recognizes angry or fearful facial expressions, assesses danger  and elicits a response to fear. The anterior cingulated gyrus  plays a role in expressing emotions through gestures and resolving mental conflicts when frustrated.

Extensive connections between the limbic system and lower and higher brain regions allow the system to integrate and respond to a variety of environmental stimuli. Most limbic system output is relayed through the hypothalamus. Because the hypothalamus is the neural clearinghouse for both autonomic (visceral) function and emotional response, it is not surprising that some people under acute or unrelenting emotional stress fall prey to visceral illnesses, such as high blood pressure and heartburn. Such emotion-induced illnesses are called psychosomatic illnesses.

                 Because the limbic system interacts with the prefrontal lobes, there is an intimate relationship between our feelings (mediated by the emotional brain) and our thoughts (mediated by the cognitive brain). As a result, we (1) react emotionally to things we consciously understand to be happening, and (2) are consciously aware of the emotional richness of our lives. Communication between the cerebral cortex and limbic system explains why emotions sometimes override logic and, conversely, why reason can stop us from expressing our emotions in inappropriate situations. Particular limbic system structures—the hippocampus and amygdala—also play a role in memory.

NEUROTRANSMITTERS

Neurotransmitters are chemicals stored in the axon terminals of presynaptic neurons. The release of these chemicals into the synaptic cleft is caused by electrical impulses. The neurotransmitters combine with the receptor sites on the post-synaptic cleft, resulting in a determination of whether another electrical impulse is generated. Neurotransmitters are responsible for essential functions in the role of human and emotional behavior. They are also a target of the mechanisms of psychotropic drugs. After the neurotransmitters have performed its function, it either returns to the vesicles to be stored and reused or inactivated to be dissolved by enzymes. The process of being stored again is called reuptake, a function that holds significant in the understanding of the mechanism of action of certain psychotropic drugs. Major categories include the cholinergics, monoamines, amino acids and neuropeptides.

Cholinergics

• Acetylcholine: Acetylcholine was the first chemical to be identified and proven as a neurotransmitter.
• Location: major effector chemical within the autonomic nervous system (ANS), producing activity at all sympathetic and parasympathetic presynaptic nerve terminals and all parasympathetic postsynaptic nerve terminals. It is highly significant in the neurotransmission that occurs at the junctions of nerve and muscles. In the CNS, acetylcholine neurons innervate the cerebral cortex, hippocampus, and limbic structures. The pathways are especially dense through the area of the basal ganglia in the brain.
• Functions: Acetylcholine is implicated in sleep, arousal, pain perception, the modulation and coordination of movement, and memory acquisition and retention.
• Possible implications for mental illness: Cholinergic mechanisms may have some role in certain disorders of motor behavior and memory, such as Parkinson’s, Huntington’s, and Alzheimer’s diseases. Increased levels of acetylcholine have been associated with depression.

Monoamines

• Norepinephrine: Norepinephrine is the neurotransmitter associated with the “fight-or-flight” syndrome of symptoms that occurs in response to stress.
• Location: Norepinephrine is found in the ANS at the sympathetic postsynaptic nerve terminals. In the CNS, norepinephrine pathways originate in the pons and medulla and innervate the thalamus, dor sal hypothalamus, limbic system, hippocampus, cerebellum, and cerebral cortex.
• Functions: Norepinephrine may have a role in the regulation of mood, in cognition and perception, in cardiovascular functioning, and in sleep and arousal.
• Possible implications for mental illness: The mechanism of norepinephrine transmission has beenimplicated in certain mood disorders such as depression and mania, in anxiety states, and in schizophrenia (Sadock & Sadock, 2003). Levels of the neurotransmitter are thought to be decreased in depression and increased in mania, anxiety states, and in schizophrenia.

• Dopamine: Dopamine is derived from the amino acid tyrosine and may play a role in physical activation of the body.
• Location: Dopamine pathways arise from the midbrain and hypothalamus and terminate in the frontal cortex, limbic system, basal ganglia, and thalamus. Dopamine neurons in the hypothalamus innervate the posterior pituitary and those from the posterior hypothalamus project to the spinal cord.
• Functions: Dopamine is involved in the regulation of movements and coordination, emotions, voluntary decision-making ability, and because of its influence on the pituitary gland, it inhibits the release of prolactin (Sadock & Sadock, 2003).
• Possible implications for mental illness: Decreased levels of dopamine have been implicated in the etiology of Parkinson’s disease and depression (Sadock & Sadock, 2003). Increased levels of dopamine are associated with mania (Dubovsky, Davies, & Dubovsky, 2003) and schizophrenia (Ho, Black, & Andreasen, 2003).

• Serotonin: Serotonin is derived from the dietary amino acid tryptophan. The antidepressants called se lective serotonin reuptake inhibitors (SSRIs) block the reuptake of this neurotransmitter to increase levels in the brain.
• Location: Serotonin pathways originate from cell bodies located in the pons and medulla and project to areas including the hypothalamus, thalamus, limbic system, cerebral cortex, cerebellum, and spinal cord. Serotonin that is not returned to be stored in the axon terminal vesicles is catabolized by the enzyme monoamine oxidase.
• Functions: Serotonin may play a role in sleep and arousal, libido, appetite, mood, aggression, and pain perception.
• Possible implications for mental illness: Increased levels of serotonin have been implicated in schizophrenia and anxiety states (Sadock & Sadock, 2003). Decreased levels of the neurotransmitter have been associated with depression (Dubovsky, Davies, & Dubovsky, 2003).

• Histamine: The role of histamine in mediating allergic and inflammatory reactions has been well documented. Its role in the CNS as a neurotransmitter has only recently been confirmed, and only limited information is available.
• Location: The highest concentrations of histamine are found within various regions of the hypothalamus.
• Function: The exact processes mediated by histamine within the CNS are unclear.
• Possible implications for mental illness: Some data suggest that histamine may play a role in depressive illness.

Amino Acids

• Gamma-aminobutyic acid(GABA) - GABA is associated with short inhibitory interneurons, although some long-axon pathways within the brain have now been identified.
• Location: Widespread distribution in the central nervous system, with high concentrations in the hypothalamus, hippocampus, cortex, cerebellum, and basal ganglia of the brain; in the gray matter of the dorsal horn of the spinal cord; and in the retina.
• Functions: GABA interrupts the progression of the electrical impulse at the synaptic junction, producing a significant slowdown of body activity.
• Possible implications for mental illness: Decreased levels of GABA have been implicated in the etiology of anxiety disorders, movement disorders such as Huntington’s disease, and various forms of epilepsy.

• Glycine: Glycine is also considered to be an inhibitory amino acid.
• Location: Highest concentrations of glycine in the CNS are found in the spinal cord and brainstem.
• Functions: Glycine appears to be involved in recurrent inhibition of motor neurons within the spinal cord and is possibly involved in the regulation ofnspinal and brainstem reflexes.
• Possible implications for mental illness: Decreased levels of glycine have been implicated in thenpathogenesis of certain types of spastic disorders. Toxic accumulation of the neurotransmitter in the brain and cerebrospinal fluid can result in “glycine encephalopathy” (Hamosh, 2005).

• Glutamate: This neurotransmitter appears to be primarily excitatory in nature.
• Location: Glutamate is found in the pyramidal cells of the cortex, cerebellum, and the primary sensory afferent systems. It is also found in the hippocampus, thalamus, hypothalamus, and spinal cord.
• Functions: Glutamate functions in the relay of sensory information and in the regulation of various motor and spinal reflexes.
• Possible implications for mental illness: Increased receptor activity has been implicated in the etiology of certain neurodegenerative disorders, such as Parkinson’s disease. Decreased receptor activity can induce psychotic behavior.

Neuropeptides

• Endorphines and enkephalins: These neurotransmitters are sometimes called opioid peptides.
• Location: They have been found in various concentrations in the hypothalamus, thalamus, limbic structures, midbrain, and brainstem. Enkephalins are also found in the gastrointestinal (GI) tract.
• Function: With their natural morphine-like properties, they are thought to have a role in pain modulation.
• Possible implications for mental illness: Modulation of dopamine activity by opioid peptides may indicate some link to the symptoms of schizophrenia.

• SUBSTANCE P: Substance P was the first neuropeptide to be discovered.
• Location: Substance P is present in high concentrations in the hypothalamus, limbic structures, midbrain, and brainstem, and is also found in thethalamus, basal ganglia, and spinal cord.
• Functions: Substance P is thought to play a role in sensory transmission, particularly in the regulation of pain.
• Possible implications for mental illness: Decreased concentrations have been found in the substantia nigra of the basal ganglia of clients with Huntington’s disease.

• SOMATOSTATIN. Somatostatin is also called the growth hormone-inhibiting hormone.
• Location: Somatostatin is found in the cerebral cortex, hippocampus, thalamus, basal ganglia, brain stem, and spinal cord.
• Functions: In its function as a neurotransmitter, somatostatin exerts both stimulatory and inhibitory effects. Depending on the part of the brain being affected, it has been shown to stimulate dopamine serotonin, norepinephrine, and acetylcholine, and inhibit norepinephrine, histamine, and glutamate. It also acts as a neuromodulator for serotonin in the hypothalamus, thereby regulating its release (i.e., determining whether it is stimulated or inhibited). It is possible that somatostatin may serve this function for other neurotransmitters as well.
• Possible implications for mental illness: High concentrations of somatostatin have been reported in brain specimens of clients with Huntington’s disease; low concentrations have been reported in clients with Alzheimer’s disease.

IMPLICATIONS FOR PSYCHIATRIC ILLNESS