Central Nervous System PDF

CENTRAL NERVOUS SYSTEM  Introduction The central nervous system (CNS) includes the brain and spinal cord. The brain: 1. All sensation and consciousness originates in the brain. 2. The brain consists of many regions. 3. Brain is 2% of body weight(~1400 gm) but uses 12% of...

CENTRAL NERVOUS SYSTEM  Introduction The central nervous system (CNS) includes the brain and spinal cord. The brain: 1. All sensation and consciousness originates in the brain. 2. The brain consists of many regions. 3. Brain is 2% of body weight(~1400 gm) but uses 12% of body energy 4. 14% of the blood flow goes to the brain 5. The blood flow per kilogram is equal to that of a muscle doing heavy exercise The spinal cord is the primary pathway for messages between peripheral areas of the body and the brain. It also mediates reflexes. Structure of brain General Principles of Central Organization of Motor Pathways To voluntarily move a limb, the brain must: 1. Plan a movement. 2. Arrange appropriate motion at many different joints at the same time, 3. Adjust the motion by comparing plan with performance. Voluntary movement originates in cortical association area. The movements are planned in the cortical association area, the basal ganglia and the lateral portions of the cerebellum. The basal ganglia and cerebellum direct information to the motor cortex by way of the thalamus. General Principles of Central Organization of Motor Pathways Motor commands from the motor cortex are send to the motor neurons of spinal cord and brain stem (via the corticospinal tracts spinal cord; corticobulbar tracts brain stem). Movement sets up alterations in sensory input from the special senses and from muscles, tendons, joints, and the skin. This feedback information, which adjusts and smoothes movement, is relayed directly to the motor cortex and to the cerebellum. The cerebellum projects in turn to the brain stem. The brain stem pathways are concerned with posture and coordination. Damage to the cerebral cortex before or during childbirth or during the first 2- 3years of development can lead to cerebral palsy, a disorder that affects muscle tone, movement, and coordination. Organization of the Basal Ganglia Basal ganglia (or basal nuclei) are five interactive structures on each side of the brain: 1. The striatum (caudate nucleus and putamen) 2. globus pallidus. These use GABA (95%), acetylcholine and somatostatin as neurotransmitters. 3. The subthalamic nucleus 4. substantia nigra. These use dopamine and GABA as neurotransmitters. The full cortical basal ganglia-thalamiccortical loop involve: GABAegic projection from the striatum to substantia nigra. Organization of the Basal Ganglia Inputs to the basal ganglia ( excitatory, glutamate): From cerebral cortex (corticostriate pathway) From the thalamus (thalamostriatal pathway). Output of the basal ganglia (inhibitory, GABA): From substantia nigra ====➔ Project to the thalamus. Excitatory (glutamate) projection from the thalamus, to the premotor cortex. The connections within the basal ganglia is nigrostriatal and include: Dopaminergic projection from the substantia nigra to the striatum. Direct pathway Cerebral cortical input to the striatum causes activation of inhibitory neurons in the striatum which then causes an increased inhibitory output to the globus pallidus internal [GPi]. There is a decreased inhibitory output from GPi to the thalamus which then projects via excitatory pathways into the premotor cortex. The direct pathway is involved in regulating tonic excitation in the premotor cortex which is an area involved in planning and initiating movement. Indirect pathway The indirect pathway is inhibitory to movement when excitatory projection from cerebral cortex facilitates inhibitory projection neurons in globus pallidus external [GPe]. These then inhibit tonic inhibitory output neurons which decreases tonic inhibition of subthalamic nucleus [STN] resulting in increased excitatory output to globus pallidus internal (Gpi). Excitatory input to GPi increases inhibitory output from GPi to thalamus which then decreases excitatory feedback to cerebral cortex leading to inhibition of motor activity. Dopamine promotes action of direct pathway while suppressing the activity of indirect pathway.  Somatosensory Neurotransmission: Touch, Pain, And Temperature Sensory Physiology A sensory system is a part of the nervous system that consists of sensory receptors that receive stimuli from the external or internal environment, the neural pathways that conduct information from the receptors to the brain or spinal cord, and those parts of the brain that deal primarily with processing the information. Information that a sensory system processes may or may not lead to conscious awareness of the stimulus. For example, whereas you would immediately notice a change when leaving an air-conditioned house on a hot summer day, your blood pressure can fluctuate significantly without your awareness. Regardless of whether the information reaches consciousness, it is called sensory information. If the information does reach consciousness, it can also be called a sensation. A person’s awareness of the sensation (and, typically, understanding of its meaning) is called perception. For example, feeling pain is a sensation, but awareness that a tooth hurts is a perception. Sensory Receptors Information about the external world and about the body’s internal environment exists in different forms—pressure, temperature, light, odorants, sound waves, chemical concentrations. The receptors are either specialized endings of the primary afferent neurons themselves or separate receptor cells (some of which are actually specialized neurons) that signal the primary afferent neurons by releasing neurotransmitters. Sensory Receptors Several general classes of receptors are characterized by the type of stimulus to which they are sensitive. 1. Mechanoreceptors respond to mechanical stimuli, such as pressure or stretch, and are responsible for many types of sensory information, including touch, blood pressure, and muscle tension. These stimuli alter the permeability of ion channels on the receptor membrane, changing the membrane potential. 2. Thermoreceptors detect sensations of cold or warmth. 3. photoreceptors respond to particular ranges of light wavelengths. 4. Chemoreceptors respond to the binding of particular chemicals to the receptor membrane. This type of receptor provides the senses of smell and taste and detects blood pH and oxygen concentration. 5. Nociceptors are a general category of detectors that sense pain due to actual or potential tissue damage.  Primary Sensory Coding Coding is the conversion of stimulus energy into a signal that conveys the relevant sensory information to the central nervous system. Important characteristics of a stimulus include the type of input it represents, its intensity, and the location of the body it affects. Coding begins at the receptive neurons in the peripheral nervous system. A single afferent neuron with all its receptor endings makes up a sensory unit. In a few cases, the afferent neuron has a single receptor, but generally the peripheral end of an afferent neuron divides into many fine branches, each terminating with a receptor. Receptive field The area of the body that leads to activity in a particular afferent neuron when stimulated is called the receptive field for that neuron. Receptive fields of neighboring afferent neurons usually overlap so that stimulation of a single point activates several sensory units. Thus, activation of a single sensory unit almost never occurs. As we will see, the degree of overlap varies in different parts of the body. Somatic Sensation Sensation from the skin, skeletal muscles, bones, tendons, and joints—somatic sensation—is initiated by a variety of sensory receptors collectively called somatic receptors. Some of these receptors respond to mechanical stimulation of the skin, hairs, and underlying tissues, whereas others respond to temperature or chemical changes. Activation of somatic receptors gives rise to the sensations of touch, pressure, awareness of the position of the body parts and their movement, temperature, and pain. Some organs, such as the liver, have no sensory receptors at all. Each sensation is associated with a specific receptor type. In other words, distinct receptors exist for heat, cold, touch, pressure, limb position or movement, and pain. Touch and Pressure Stimulation of different types of mechanoreceptors in the skin leads to a wide range of touch and pressure experiences—hair bending, deep pressure, vibrations, and superficial touch, for example. These mechanoreceptors are highly specialized neuron endings encapsulated in elaborate cellular structures. The details of the mechanoreceptors vary, but, in general, the neuron endings are linked to networks of collagen fibers within a capsule that is often filled with fluid. These networks transmit the mechanical tension in the fluid-filled capsule to ion channels in the neuron endings and activate them.  Touch and pressure are sensed by four types of mechanoreceptors 1. Meissner’s corpuscles: Respond to changes in texture and slow vibrations. 2. Merkel cells: Respond to sustained pressure and touch. 3. Ruffini corpuscles: Respond to sustained pressure. 4. Pacinian corpuscles: Unmyelinated dendritic endings of a sensory nerve fiber. Encapsulated in connective tissue. Respond to deep pressure and fast vibration. Temperature Information about temperature is transmitted along small- diameter, afferent neurons with little or no myelination. The actual temperature sensors are ion channels in the plasma membranes of the axon terminals that belong to a family of proteins called transient receptor potential (TRP) proteins. When activated, all of these channel types allow flux of a nonspecific cation current that is dominated by a depolarizing inward flux of Na. The resulting receptor potential initiates action potentials in the afferent neuron, which travel along labeled lines to the brain where the temperature stimulus is perceived. Pain Most stimuli that cause, or could potentially cause, tissue damage elicit a sensation of pain. Receptors for such stimuli are known as nociceptors. Nociceptors can be divided into several types: ✓ Mechanical nociceptors respond to strong pressure (eg, from a sharp object). ✓ Thermal nociceptors are activated by skin temperatures above 42°C or by severe cold. ✓ Chemically sensitive nociceptors respond chemicals like bradykinin, histamine, high acidity, and environmental irritants. ✓ Polymodal nociceptors respond to combinations of these stimuli. Itch and tickle (irritate) are also related to pain sensation Classification of Pain 1. Acute pain or Physiologic ✓ Has a sudden onset ✓ Cure during the healing process. ✓ Regarded as a good pain. ✓ Serves as important protective mechanism ✓ Example: withdrawal of limbs to a painful stimulus. 2. Chronic pain or Pathologic ✓ Has a delayed onset. ✓ Regarded as a bad pain. ✓ Persists long after recovery from an injury. ✓ Often refractory to analgesic agents (nonsteroidal anti-inflammatory drugs (NSAIDs) and opioids. ✓ Example: Neuropathic pain (diabetic neuropathy, toxin-induced nerve damage, and ischemia). Inflammatory pain Classification of Pain 3. Hyperalgesia and Allodynia Pain is often accompanied by increased sensitivity of nociceptive afferent fibers (hyperalgesia and allodynia). Hyperalgesia is an exaggerated response to a harmful stimulus. Allodynia is a sensation of pain in response to a normally harmless stimulus (e.g., painful sensation from a warm shower when the skin is damaged by sunburn). Injured cells release chemicals that directly depolarize nerve terminals, making nociceptors more sensitive. Examples on chemicals: ✓ K+ ions ✓ Bradykinin ✓ Substance P. ✓ Histamine (from mast cells) ✓ Serotonin (5-HT) (from platelets) ✓ Prostaglandins (from cell membranes). Classification of Pain 4. Deep and Visceral Pain Differs from superficial pain in the nature of the pain caused by noxious stimuli. It is poorly localized, nauseating, and frequently are accompanied by sweating and changes in blood pressure. Often radiates or is referred to other areas. Example Appendicitis Classification of Pain 5. Referred pain Irritation of a visceral organ frequently produces pain that is felt not at that site but in a somatic structure that may be some distance away. Such pain is said to be referred to the somatic structure (referred pain). Knowledge of the common sites of pain referral from each of the visceral organs is of importance to a physician. Examples: Referral of cardiac pain to the inner aspect of the left arm. Cardiac pain, for instance, may be referred to the right arm, the abdominal region, or even the back, neck, or jaw. Pain in the tip of the shoulder caused by irritation of the central portion of the diaphragm.