Physiology of the Nervous System PDF

Physiology & Pathophysiology MD 303 Clinical pharmacy program Level 2  The nervous system The nervous system consists of billions of neurons that are interconnected in a highly organized circuitry. The nervous system is divided into two anatomica...

Physiology & Pathophysiology MD 303 Clinical pharmacy program Level 2  The nervous system The nervous system consists of billions of neurons that are interconnected in a highly organized circuitry. The nervous system is divided into two anatomically distinct regions: Central nervous system Peripheral nervous system (PNS) (CNS) The brain 12 pairs of cranial nerves that arise The spinal cord from the brainstem 31 pairs of spinal nerves that arise from the spinal cord  Afferent division carries sensory information toward the CNS Peripheral Nervous System Somatic NS (PNS) motor neurons that Efferent division innervate skeletal muscles carries motor information away from the CNS toward the Autonomic NS effector structures innervates cardiac muscle, smooth muscle and glands  NEURONS There are three functional classes of neurons in the human nervous system: Afferent (Sensory) neurons, lie predominantly in the PNS. Each has a sensory receptor that is activated by a particular type of stimulus, a cell body located adjacent to the spinal cord, and an axon. Two regions of an axon exist: the peripheral axon extends from the receptor to the cell body and the central axon continues from the cell body into the spinal cord. Afferent neurons transmit sensory information from receptors in the periphery of the body to the CNS. Efferent (Motor) neurons, also lie predominantly in the PNS. The cell bodies are found in the CNS in either the spinal cord or the brainstem. The axons extend out into the periphery of the body where they innervate the effector tissues. Types of neurons Efferent neurons transmit information from the CNS to muscles and glands.  NEURONS Interneurons (Associative), lie entirely within the CNS. The human brain and spinal cord contain over 100 billion neurons, and interneurons account for approximately 99% of all the neurons within the body. Interneurons lie between afferent and efferent neurons and are responsible for:  Integrating sensory input  Coordinating a motor response  NEURONS In the simplest condition, interneurons process responses at the level of the spinal cord in the form of reflexes, which are automatic, responses to given stimuli. E.g., Stimulation of pain receptors generates action potentials in their associated afferent neurons. These impulses are transmitted to the spinal cord where the afferent neurons stimulate interneurons. These interneurons then stimulate efferent neurons that cause skeletal muscle contraction in the affected area to move the body part away from the painful stimulus. This withdrawal reflex involves comparatively few interneurons and does not require any input from higher nervous centers in the brain. On the other hand, a response to some other stimulus may involve more sophisticated neurological phenomena E.g., memory, motivation, judgment and intellect. This type of response is not automatic, is clearly far more complex and may require the activity of millions of interneurons in many regions of the brain prior to the stimulation of motor neurons to carry out the desired effect.  LEVELS OF CNS FUNCTION The spinal cord The most anatomically inferior portion of the CNS. It receives sensory input from the periphery of the body and contains the cell bodies of motor neurons responsible for both voluntary and involuntary movements. The spinal cord processes sensory and motor activities in the trunk of the body and the limbs The involuntary and neurologically simple reflexes are processed entirely at the level of the spinal cord. Voluntary, deliberate movements are initiated and controlled by thought processes in the cerebrum.  LEVELS OF CNS FUNCTION The brainstem Consists of the medulla, pons and midbrain The smallest region of the brain. Continuous with the spinal cord, the brainstem receives sensory input and initiates motor output. The brainstem processes sensory and motor activities primarily in the head, neck and face. The brainstem also controls many basic life-sustaining processes, including respiration, circulation, and digestion. Even with the loss of higher cognitive function, this lower level of the brain can sustain bodily functions essential for survival.  LEVELS OF CNS FUNCTION The cerebrum and the cerebral cortex Account for 80% of the total brain weight in humans Constitute the highest functional level of the CNS. Voluntary, deliberate movements are initiated and controlled by thought processes in the cerebrum. The convolutions or folds serve to increase the surface area of the cerebral cortex, thus allowing for a greater number of neurons. E.g., The total surface area of the cerebral cortex for the rat, cat and human are 6, 83, and 2,500 cm2, respectively. The cerebral cortex plays an important role in the most detailed neurological functions including intellect, thought, personality, voluntary initiation of movement, sensory perception, and language.  THE BRAIN The brain is the integrative portion of the NS that serves to receive, process, and store sensory information and then plan and orchestrate the appropriate motor response. It is divided into several anatomically functionally distinct regions.  THE CEREBRUM The cerebrum is composed of two hemispheres, which are anatomically connected to ensure intercommunication. Two types of tissue compose each hemisphere: The gray matter (the unmyelinated cell bodies of neurons), is on the outer surface of the cerebrum and forms the cerebral cortex. The white matter (the myelinated axons of neurons), is found underlying the cortex in the core of the cerebrum. These axons are bundled together according to function and organized into units referred to as tracts. N.B. A nerve is defined as a bundle of neuronal axons not the entire neurons N.B. Nerves are found only in the peripheral nervous system. N.B. Bundles of neurons with similar functions located within the CNS are referred to as tracts. Therefore, technically speaking, there are no nerves within the brain or within the spinal cord.  Frontal section of the brain The cerebrum is composed of two types of tissue: The internal white matter and the external gray matter that forms the cerebral cortex. Embedded within the cerebral hemispheres are other masses of gray matter, the basal ganglia and the thalamus. The ventricles are filled with cerebrospinal fluid (CSF).  THE CEREBRAL CORTEX The cerebral cortex is not a smooth surface but, instead, is highly folded and has a furrowed appearance, providing space for a greater number of neurons. Both hemispheres of the cerebrum consist of four lobes including the: Lateral view of the four lobes of cerebral cortex  THE BASAL GANGLIA The basal ganglia consist of masses of gray matter embedded within the white matter of each cerebral hemisphere. As with the cerebral cortex, this gray matter consists of functional aggregations of neuronal cell bodies. An important function of the basal ganglia involves their contribution to the control of voluntary movement.  THE THALAMUS The thalamus is located between the cerebrum and the brainstem. Lying along the midline of the brain, it consists of two oval-shaped masses of gray matter, one in each cerebral hemisphere. The thalamus is often described as a relay station as all sensory fiber tracts, except olfactory tracts, that transmit impulses to the cerebral cortex first synapse with neurons in the thalamus. Therefore, the thalamus may be considered the functional “gateway” to the cerebral cortex. The thalamus plays a role in the regulation of skeletal muscle contraction by positively reinforcing voluntary motor activity initiated by the cerebral cortex.  The thalamus acts as a filter for information to the cortex by either preventing or enhancing the passage of specific information depending upon its significance to the individual. In fact, more than 99% of all sensory information transmitted toward the brain is discarded, as it is considered irrelevant and unimportant. E.g., an individual may easily sleep through the noise of city traffic as this sensory input has been previously determined to be unimportant. However, this same individual may also be immediately aroused by the ever so quiet arrival of their teenager home late in the evening. This sensory is quite important to a parent and passes readily through the filter of the thalamus.  THE HYPOTHALAMUS The hypothalamus lies beneath the thalamus and above the pituitary gland. Although it is quite small, accounting for only about 4 g of the total 1,400 g of the adult human brain (