Central Peripheral Nervous System

nervous-systemThe nervous system is anatomically divided into two parts, the Central Nervous System (the brain and the spinal cord) and the Peripheral Nervous System (ganglia, 12 pairs of cranial nerves and 31 of pair’s spinal nerves).

As a part of the feedback loop mechanism of control, the Central Nervous System (CNS) often plays a significant role as the integration center. This gives you the notion that it is for information processing, analysis and interpretation. The CNS is responsible for intricate and complex neuronal processing, with each region of the brain and spinal cord having distinct physiological functions.

The PNS can be divided into two parts, the Somatic Nervous System (SNS) and the Autonomic Nervous System (ANS). The SNS is responsible for movement of the body (soma = body), and its effector tissue is skeletal muscle. The ANS is responsible for automated responses that occur in the body (e.g., heart rate) and the effector tissues are cardiac muscle, smooth muscle and glands.

We can divide the brain into six parts in terms of physiological functions:

1201_Overview_of_Nervous_System

1. Cerebrum

2. Hypothalamus

3. Midbrain

4. Cerebellum

5. Pons

6. Medulla oblongata

1. Cerebrum –

 

This is the most developed area of brain in the human species and is considered to be the center of the highest functions. The major functions include: awareness of sensory perception; voluntary control of movement (regulation of skeletal muscle movement); language; personality traits; sophisticated mental activities such as thinking, memory, decision making, predictive ability, creativity and self-consciousness.

The 4 lobes of the cerebrum.

The Frontal Lobe

_1124705Concerned with higher intellectual functions and is involved in the many behavioral aspects of humans. It inhibits certain primitive behaviors. The Primary motor cortex controls the movement of the rest of the body while the premotor cortex just adjacent to it is concerned with the initiation, activation, and performance of the actual movement.

The Parietal Lobe

_2396667This lobe is primarily concerned with the interpretation and integration of sensory inputs. The Somatosensory cortex is associated with reception and perception of touch, vibration, and position sense of the body.

The Temporal Lobe

_2323569The temporal lobe contains the auditory cortex – for the reception and interpretation of sound information, and the olfactory cortex – for the sense of smell. It also houses the language cortex in the dominant hemisphere (usually the left hemisphere) and participates in recognition and interpretation of language.

The Occipital Lobe

493x335_parkinsons_brain_stimulationThis lobe contains the primary visual cortex for visual information interpretation. Degenerative conditions in specific regions can cause problems in fine motor control. Parkinson’s disease is characterize by slow jerky movements; tremors of the face and hands; muscle rigidity; and great difficulty initiating voluntary movements. In Parkinson’s disease, an overactive region acts like a stuck brake, continuously inhibiting the motor cortex. The disease results from the degeneration of a region called the substantia nigra, in particular dompaminergic neurons (those using the neurotransmitter dopamine) in this region.

Huntington’s disease involves an over stimulation of motor activities, such that limbs jerk uncontrollably.

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The Limbic system is a group of structures on the medial aspect of each hemisphere and diencephalon and is more a functional system than an anatomical one. The limbic system is the “emotional brain”, participating in the creation of emotional states such as fear, anger, pleasure, affection, arousal, etc. and processing vivid memories associated with those states. For example, the amygdala is central for processing fear and stimulates a sympathetic response. The amygdala enables us to recognize menacing facial expressions in others and to detect the precise gaze of someone who is looking at us.

Cerebral Lateralization 

Although anatomically the two hemispheres of the cerebrum look very similar, functionally the two sides are different. Thus, the term lateralization is used to denote that each lobe has developed special functions that are not shared by other lobes. In general:

Left side: Language, logic, analytical, sequential, verbal tasks, (holistic information processing). “Thinkers”

Right side: Spatial perception, artistic and musical endeavors (fragmentary information processing). “Creators”

r&r0208aSpecific examples are most obvious in the function of speech and word recognition. For example, the primary cortical areas for language are Broca’s area and Wernike’s area. The Broca’s area (in the left frontal lobe) is responsible for speaking ability, the mechanics of skeletal muscle control for verbal articulation (sound production). Wernicke’s area (in the left juncture of parietal, temporal and occipital lobes) is concerned with language comprehension, that is, understanding the words that are read or heard. These exist on the left hemisphere only if you are left-brain dominant – as most people who are right handed are. There functional areas on the right side are different. For example, the emotional aspect of language is controlled in the opposite hemispheres. Opposite Broca’s area is the affective language area, which gives intonation to words, in order to modify their meaning. The area opposite Wernicke’s is concerned with recognizing the emotion content of another person’s speech. Think of someone saying “Oh great” with true excitement versus “Oh great” with complete sarcasm! Same words, different meanings.

Language disorders caused by damage to specific cortical areas are known as aphasias. Most aphasias are caused by strokes. A stroke can be defined as a “cardiovascular accident”, this occurs when a blood vessel in the brain (a cerebral blood vessel) ruptures or is blocked by a clot. The result is that the region of the brain being supplied by that vessel is deprived of the O2 and glucose that neurons require constantly in order to function. Damage of the affected area can result.

NeuronImportant note regarding neuron physiology:

Every cell needs energy (E) to fight entropy (2nd law of thermodynamics).

Cellular energy is ATP and there are 2 ways to make ATP in the body,

1) with O2 (aerobically) and

2) without O2 (anaerobically).

Neurons cannot make ATP anaerobically (without O2), thus they need a constant supply of O2 in order to make ATP. Neurons also require glucose (with O2) to make ATP. Unlike most other cells, they cannot use other molecules such as lipids and proteins as fuel for ATP production. Furthermore, they have no stores of glucose (like muscle tissue has), so they need a constant supply of both O2 and glucose. This is one reason why blood glucose levels are important and so closely regulated.

Brain damage may result if this organ is deprived of its critical O2 supply for more than 4 or 5 minutes or if its glucose supply is cut off for more than 10 to 15 minutes.

2. Epithalamus, Thalamus and Hypothalamus

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The epithalamus contains the pineal gland, a hormone secreting endocrine structure. Under the influence of the hypothalamus, the pineal gland secretes the hormone melatonin, which prepares the body for the night-time stage of the sleep/wake cycle. The thalamus makes up about 80% of the diencephalon and is the main relay center for the various sensory and motor functions.

The Hypothalamus controls and regulates many important functions of the body, including:

1) Control of the Autonomic Nervous System

tumblr_n5zs45bvCa1tpgs8eo1_500adjusts, coordinates, and integrates the A.N.S. centers in the brain that regulate heart rate, blood pressure, bronchiole diameter, sweat glands, G.I. tract activity, etc. It does this via the Parasympathetic and Sympathetic divisions of the A.N.S.

2) Control of Emotional Responses

emotional-responses-of-the-human-brain-755960in association with the limbic system, it forms part of the emotional brain. Regions involved in fear, pleasure, rage and sex drive are located in the hypothalamus.

3) Regulation of Body Temperature

Slide47the body’s thermostat and set point is located in the hypothalamus. There are also 2 centers in the hypothalamus that respond to changes in the set point.

Heat-losing center: activation of this center causes sweating and cutaneous vasodilation.

Heat-promoting center: activation of this center causes shivering and cutaneous vasoconstriction.

4) Regulation of Hunger and Thirst Sensations

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hypothalamus contains the feeding and thirst centers.

Feeding center: this center is always active and stimulates hunger which is ‘fed’ by eating.

Satiety center: stimulated when satisfied, this inhibits the always hungry feeding center.

Thirst center: osmoreceptors detect changes in osmotic pressure of blood, ECF, stimulate thirst.

5) Control of the Endocrine System

endocrine-systemcontrols the release of pituitary hormones. Controls the anterior pituitary gland, when the hypothalamus releases hormones, it can stimulate or inhibit the release of other hormones form the pituitary (6 hormones). Also, it makes the 2 hormones (oxytocin and antidiuretic hormone (ADH)) that are stored in the posterior pituitary and released when signaled. All of these hormones regulate many other organs in the body.

3. Midbrain

mba

Portions receive visual input auditory input from the medulla oblongata and are involved in cranial reflexes, e.g., when you turn your head if you thought you heard your name called out.

4. Cerebellum

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Means ‘little brain’. The Cerebellum has two primary functions:

1) Controls postural reflexes of muscles in body

bigstock-Posture-eps-37126294-e1418056108348i.e., it coordinates rapid, automatic adjustments to maintain equilibrium, e.g. regaining your balance when you start to fall.

2) Produces skilled movements

Terms-of-Movement-Adduction-Abduction-and-Rotation-CC-1024x867involved in implementing routines for fine tuned movements. Controlled at the conscious and subconscious level, refines learned routines (e.g. driving, skating, playing an instrument) until the action becomes routine. This then reduces the need for conscious attention to the task. The cerebellum gets incoming information from proprioceptors, a type of sensory receptor found in movable joints, tendons and muscle tissue. Using the information from proprioceptors in the body, the cerebellum can determine the relative position of various body parts and compares motor commands and intended movements with the actual position of the body part (legs, arms). In this way, it can perform any adjustments needed to changes the direction or make the movement (action) smooth and coordinated.

5. Pons

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Plays a role in the regulation of the respiratory system.

Contains two ‘pontine’ respiratory centers:

1) the pneumotaxic center and

2) the apneustic center. These two centers will be discussed later in the respiratory system. The pons is not responsible for the rhythm of breathing (the medulla oblongata is) but controls the changes in depth of breathing and the fine tuning of the rhythm of breathing set by the medulla oblongata. The pons also prevents over inflation of the lungs.

6. Medulla Oblongata

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The medulla oblongata is the last division of the brain. It becomes continuous with the spinal cord. It houses some very important visceral or vital centers, 1) the cardiac center – adjusts the force and rate of the heartbeat; 2) the vasomotor center – regulates the diameter of blood vessels and therefore systemic blood pressure (constriction increases and dilation decrease blood pressure); and 3) the respiratory center – for control of the basic rhythm and rate of breathing. Additional centers regulate sneezing, coughing, hiccupping, swallowing and vomiting.

Spinal Cord

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The physiology of the spinal cord will be covered in the lab component of this physiology course. The basic structure of the spinal cord is that it is the downward continuation of medulla oblongata starting at the foramen magnum. It descends to about the level of the second lumbar vertebra, tapering to a structure called the conus medullaris.

The cord projects 31 pairs of spinal nerves on either side (8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal) that are connected to the peripheral nerves. A cross section of the spinal cord exhibits the butterfly-shaped gray matter in the middle, surrounded by white matter. As in the cerebrum, the gray matter is composed of nerve cell bodies. The white matter consists of various ascending and descending tracts of myelinated axon fibers with specific functions.

The spinal cord serves as a passageway for the ascending (going up) and descending (going down) fiber tracts that connect the peripheral and spinal nerves with the brain. Each of the 31 spinal segments is associated with a pair of dorsal root ganglia. These contain sensory nerve cell bodies. The axons from these sensory neurons enter the posterior aspect of the spinal cord via the dorsal root. The axons from somatic and visceral motor neurons leave the anterior aspect of the spinal cord via the ventral roots. Distal to each dorsal root ganglion the sensory and motor fibers combine to form a spinal nerve – these nerves are classified as mixed nerves because they contain both afferent (sensory) and efferent (motor) fibers.