Bio120 Unit Iv Notes PDF

BIO120 Unit IV Notes.docx The Nervous System – Introduction The nervous system is one of two systems responsible for maintaining homeostasis in the body. (The other system is the endocrine system.) The nervous system is able to maintain homeostasis by allowing electrical communicati...

BIO120 Unit IV Notes.docx The Nervous System – Introduction The nervous system is one of two systems responsible for maintaining homeostasis in the body. (The other system is the endocrine system.) The nervous system is able to maintain homeostasis by allowing electrical communication (via action potentials) between its neurons and other cells of the body. The nervous system's activities can be divided into three categories: 1. Sensory activities involve detecting stimuli from the environment (using sensory receptors) and sending electrical signals toward the brain and/or spinal cord. In fact, neurons that carry action potentials toward the brain or spinal cord are called sensory (or afferent) neurons. 2. Motor activities involve sending electrical signals from the brain and/or spinal cord to effectors that can produce responses. Neurons that carry action potentials from the brain or spinal cord are called motor (or efferent) neurons. 3. Integrative (or associative) activities involve conducting action potentials from sensory to motor neurons. Integrative activities involve analysis and decision-making. Association neurons (also called integration neurons or interneurons) are neurons that carry action potentials from sensory neurons to motor neurons. Association neurons are always found in the brain or the spinal cord. Parts of the Nervous System: 1. The central nervous system (CNS) consists of the brain and the spinal cord. 2. The peripheral nervous Edit with system the Docs app(PNS) consists of the sensory and motor neurons that connect the CNS to receptors and effectors. These neurons are bundled together in structures called nerves. Make tweaks, leave comments, and share with There are twoothers to edit divisions ofatthe thePNS: same time. A. The autonomic nervous system (ANS) consists of sensory neurons that carry action potentials toward the CNS from visceral (deep body) receptors and motor neurons that NO THANKS carry action potentials from theGET CNSTHE APP muscle, cardiac muscle, and glands. The to smooth action potentials carried by the ANS involve involuntary control mechanisms. The motor portion consists of two divisions: 1. The sympathetic division carries action potentials associated with the fight or flight reponse. In general, this division allows the body to handle stressful situations by providing the necessary resources to respond. Sympathetic stimulation increases heart rate, blood pressure, breathing rate, and blood glucose levels. 2. The parasympathetic division carries action potentials associated with the feed and breed response. In general, this division helps the body to decrease its preparation for stress while providing the proper stimulation for digestive and reproductive activities. Parasympathetic stimulation reverses the effects of sympathetic stimulation. B. The somatic nervous system (SNS) consists of sensory neurons that carry action potentials to the CNS from receptors in the skin, muscles, and special sense organs. The SNS also includes motor neurons that carry action potentials from the CNS to skeletal muscles. The SNS is associated with voluntary control of muscle contraction. https://docs.google.com/document/d/1-ZPqyipDg5qIKat4bNtxNgOlJIVKb3Wd/mobilebasic 2/10/25, 12 55 AM Page 1 of 14 :  Neuron Structure and Function Neurons are extremely long-lived that exhibit high metabolic rates and therefore require a constant and extensive supply of oxygen and glucose. The function of a neuron is to produce and conduct electrical signals called action potentials. https://docs.google.com/document/d/1-ZPqyipDg5qIKat4bNtxNgOlJIVKb3Wd/mobilebasic 2/10/25, 12 55 AM Page 2 of 14 :  A neuron consists of several named components: 1. The cell body (or soma or perikaryon) is the main, central portion of the neuron. It contains the nucleus and much cytoplasm along with the typical organelles of any human cell, as well as: - Neurofibrils are narrow filaments that act as a cytoskeleton. - Chromatophilic substance (Nissl bodies) is a collection of rough ER where proteins needed for growth and repair can be made by ribosomes. - Lipofuscin pigment is a mass of yellow/brown granules left over from lysosome activity. 2. Dendrites are short, highly branched processes that carry action potentials toward the cell body. 3. The axon is a long slender process that connects to the cell body at a cone-shaped region called the axon hillock. The axon conducts action potentials away from the cell body, and includes: - The initial segment is the first part of the axon, and it connects to the axon hillock at a junction called the trigger zone. - The trigger zone is the site where action potentials begin in motor and association neurons. - Axon terminals are the short branches at the distal end of an axon. - Synaptic end bulbs are the rounded structures at the ends of the axon terminals. They contain synaptic vesicles which can release neurotransmitters when necessary. - An axon’s cell membrane is called the axolemma; its cytoplasm is called the axoplasm. - Myelin is a chemical mixture containing lipids and proteins. Produced by neuroglia, myelin forms an insulating covering called a myelin sheath around some axons. Axons (and the neurons to which they belong) that possess a myelin sheath are said to be myelinated and are capable of rapid conduction of action potentials. A bundle of axons in the PNS is called a nerve. A bundle of neuron cell bodies in the PNS is called a ganglion. A bundle of neuron cell bodies in the CNS is usually called a nucleus. Neuroglia Neuroglia (or glia or glial cells) are cells in the nervous system that support, nourish, and protect neurons. Neuroglia in the CNS are distinct from those in the PNS as described here. Neuroglia in the CNS: 1. Astrocytes are the largest of the neuroglia. They are star-shaped cells that help to metabolize neurotransmitters and to maintain the proper K+ balance in the CNS. 2. Oligodendrocytes are the most common neuroglia in the CNS. They are small cells that produce and secrete myelin for the myelin sheaths of neurons in the CNS. 3. Microglia are small, phagocytic cells similar to white blood cells called macrophages. They protect the CNS against infection by engulfing bacteria or other foreign cells. 4. Ependymal cells are very similar to epithelial cells, ranging in shape from squamous to columnar. Some have cilia. They line the ventricles of the brain and the central canal of the spinal cord, and are involved in the production and circulation of CSF (cerebrospinal fluid) in the CNS. Neuroglia in the PNS: 1. Neurolemmocytes (Schwann cells) produce myelin (without releasing it) and wrap around the axons of some neurons in the PNS. They form the myelin sheath around axons in the PNS. 2. Satellite cells physically support cell bodies in the PNS. Terminology of Neurophysiology Membrane potential - the difference in charge, measured in millivolts (mV), between the inside and the outside of a neuron cell membrane. Resting membrane potential (or resting potential) - the difference in charge between the inside and the outside of the cell membrane of a neuron at rest ("at rest" means that the neuron is not https://docs.google.com/document/d/1-ZPqyipDg5qIKat4bNtxNgOlJIVKb3Wd/mobilebasic 2/10/25, 12 55 AM Page 3 of 14 :  producing or conducting an action potential); measured in millivolts (mV). Normally, resting membrane potential is -70 mV (the negative sign indicates that the inside of the membrane is more negatively charged than the outside). Resting membrane potential depends mainly on the distribution of three ions: Na+, K+, and POPs (proteins and organic phosphates). Resting ion distribution - the relative amounts and locations of Na+, K+, and POPs with respect to the cell membrane of a neuron at rest. Normally, the resting concentration of sodium ions is 14 times higher outside the cell than inside the cell. The resting concentration of potassium ions is 30 times higher inside the cell than outside the cell. Proteins and organic phosphates (POPs) are negatively charged and only found inside the cell. Sodium-potassium pump - an integral protein pump that actively transports sodium ions out of the cell while actively transporting potassium ions into the cell. This pump uses cellular energy in the form of ATP. It is said to be an "electrogenic" pump because it helps to produce and maintain resting membrane potential by transporting unequal amounts of ions (two potassium ions are transported into the cell for every three sodium ions transported out of the cell). Leakage channels (nongated channels) - pores located in the neuron cell membrane that allow passive diffusion of sodium and potassium ions to occur. Gated ion channels - pores in the neuron cell membrane that have gate-like coverings. These gates open or close in response to certain stimuli. When the gates are open they allow the movement of sodium and potassium ions through the cell membrane due to diffusion and electrostatic attraction. Gated ion channels are classified according to the types of stimuli that open or close them. - Mechanically-gated ion channels respond to pressure, vibration, or stretching. - Chemically-gated (or ligand-gated) ion channels respond to specific chemicals. - Voltage-gated ion channels respond to changes in voltage (changes in membrane potential) - Light-gated ion channels respond to light. Action potential - a complete depolarization followed by a complete repolarization. Depolarization - a change in membrane potential from -70 mV toward +30 mV. Repolarization - a change in membrane potential from +30 mV toward -70 mV. Hyperpolarization - a change in membrane potential from -70 mV toward -90 mV. An action potential is a complete depolarization followed by a complete repolarization. A. Depolarization occurs in the following steps: 1. A stimulus occurs on the cell membrane of the resting neuron. 2. The Na/K pump shuts down. 3. Na+ gated ion channels open (partially). 4. Na+ begins to move into the cell via diffusion and electrostatic attraction. 5. Membrane potential rises to –55 mV (threshold). (If threshold is not reached, then the https://docs.google.com/document/d/1-ZPqyipDg5qIKat4bNtxNgOlJIVKb3Wd/mobilebasic 2/10/25, 12 55 AM Page 4 of 14 :  Na/K pump restarts and the resting condition is restored.) 6. Na+ gated ion channels open completely. 7. Membrane potential rises past 0 mV to +30 mV. 8. Na+ gated ion channels close. B. Repolarization occurs immediately after depolarization in the following steps: 1. K+ gated ion channels open. 2. K+ diffuses to the outside of the cell. 3. Membrane potential falls back down to –70 mV. 4. K+ gated ion channels close. 5. The Na/K pump restarts and restores the resting ion distribution. Hyperpolarization is a drop in membrane potential from –70 mV toward –90 mV. There are two scenarios during which hyperpolarization may occur: A. During the repolarization phase of an action potential, the K+ gated ion channels may remain open for a short time after membrane potential has reached –70 mV. The additional influx of K+ that occurs as a result lowers membrane potential to as low as –90 mV. This abnormally low membrane potential is changed back to –70 mV once the Na/K pump restarts B. A neuron at rest can be influenced by an inhibitory neurotransmitter. The inhibitory neurotransmitter partially opens K+ gated ion channels (or Cl- gated ion channels), and the flow of K+ out of the cell (or the flow of Cl- into the cell) lowers membrane potential toward –90 mV. Action Potential Graph The graph below represents a typical action potential. The vertical axis indicates membrane potential in millivolts and the horizontal axis indicates time in milliseconds. Note that the depolarization portion of the action potential is the part where the Na/K pumps are off, Na+ gated ion channels are open, sodium ions are rapidly entering the cell, and K+ gated ion channels are closed. Also note that the repolarization portion of the action potential is the part where the Na/K pumps are off, K+ gated ion channels are open, potassium ions are rapidly exiting the cell, and Na+ gated ion channels are closed. The horizontal portion of the graph (where membrane potential is –70 mV) is where the neuron is at rest. While at rest, the neuron’s Na/K pumps are operating and Na+ gated ion channels and K+ gated ion channels are closed. https://docs.google.com/document/d/1-ZPqyipDg5qIKat4bNtxNgOlJIVKb3Wd/mobilebasic 2/10/25, 12 55 AM Page 5 of 14 :  Refractory Periods A refractory period is a time interval in which the initiation of a new action potential in a neuron is either impossible or very difficult. There are two types of refractory periods: 1. Absolute refractory period – the time interval during which a neuron cannot initiate a new action potential in response to a new stimulus regardless of the strength of that stimulus. This time period occurs when an existing action potential is already in its depolarization phase, and the neuron’s Na+ gated ion channels are already open. This time interval corresponds exactly to the depolarization phase of the existing action potential. 2. Relative refractory period – the time interval during which only an unusually strong stimulus (called a suprathreshold stimulus) can trigger a new action potential in a neuron. The stimulus must be intense in order to trigger a new action potential during this \ phase because the neuron is already in the repolarization phase of an existing action potential. K+ gated ion channels are open, but the suprathreshold stimulus causes Na+ gated ion channels to reopen and a new depolarization phase to begin. This time interval corresponds exactly to the repolarization phase of the existing action potential. Action Potential Graph Showing Absolute and Relative Refractory Periods https://docs.google.com/document/d/1-ZPqyipDg5qIKat4bNtxNgOlJIVKb3Wd/mobilebasic 2/10/25, 12 55 AM Page 6 of 14 :  Synapses and Neurotransmitters Synapses A synapse is a junction between two excitatory cells (neurons or muscle cells) that allows information (in the form of an action potential) to be transmitted from one cell to another. Assuming that a synapse exists between two neurons, the presynaptic neuron conducts an action potential toward the synapse, and the postsynaptic neuron conducts an action potential away from the synapse. In reality, most neurons can be both presynaptic and postsynaptic. There are two major types of synapses: 1. Electrical synapses allow direct passage of action potentials from presynaptic to postsynaptic cells via tube-like structures called connexons. There are typically over one hundred connexons in an electrical synapse, and the region where the connexons are located is often called a gap junction. Electrical synapses are found in cardiac muscle and the CNS. 2. Chemical synapses are more common then electrical synapses. In a chemical synapse, the presynaptic neuron releases a neurotransmitter which diffuses across the synaptic cleft and stimulates the postsynaptic neuron. Chemical synapses operate more slowly than electrical synapses because chemical synapses are subject to synaptic delay (the time required for a neurotransmitter to cross the synaptic cleft). The NMJ is an example of a chemical synapse. Neurotransmitters Neurotransmitters are specific molecules that transmit information across synapses between excitatory cells (neurons and/or muscle cells). There are approximately one hundred known neurotransmitters, and all of them possess the same basic qualities: 1. They are present in synaptic vesicles (within synaptic end bulbs) of presynaptic neurons; 2. They cause the opening of Na+ or K+ gated ion channels in postsynaptic neurons; and 3. They can be rendered inactive or removed from the synapse via: a. diffusion out of the synaptic cleft, b. breakdown by enzymes (such as acetylcholinesterase), or c. reabsorption by the presynaptic neuron or nearby neuroglia. Neurotransmitters are often classified according to the type of effect that they exert on the postsynaptic neuron. Excitatory neurotransmitters raise membrane potential toward threshold by opening Na+ gated ion channels, and therefore increase the likelihood that an action potential will occur. Inhibitory neurotransmitters lower membrane potential away from threshold by opening gated ion channels for K+ or Cl-, and therefore reduce the likelihood that an action potential will occur. Neurotransmitters are also classified by their chemical composition. There are four main chemical classes of neurotransmitters: 1. Amino acids - glutamate (excitatory) - aspartate (excitatory) - gamma aminobutyric acid or GABA (inhibitory) - glycine (inhibitory) https://docs.google.com/document/d/1-ZPqyipDg5qIKat4bNtxNgOlJIVKb3Wd/mobilebasic 2/10/25, 12 55 AM Page 7 of 14 :  2. Biogenic amines (modified amino acids) - the catecholamines (all may be excitatory or inhibitory, depending on the synapse) - norepinephrine - epinephrine - dopamine - serotonin (may be excitatory or inhibitory, depending on the synapse) 3. Acetylcholine (excitatory in the NMJ, inhibitory in other synapses) 4. Gases - nitric oxide or NO (probably excitatory or inhibitory, depending on the synapse) - carbon monoxide (possibly a neurotransmitter; effects are unclear) EPSPs and IPSPs 1. Excitatory Postsynaptic Potential (EPSP) - An EPSP is a slight increase in membrane potential in a postsynaptic neuron caused by the release of a small amount of excitatory neurotransmitter by a presynaptic neuron. The small amount of excitatory neurotransmitter raises membrane potential in the postsynaptic neuron by briefly opening sodium gated ion channels and allowing a quick influx of sodium ions.. - A single EPSP raises membrane potential from resting membrane potential (-70 mV) toward threshold, but is never large enough to raise membrane potential all of the way to threshold on its own. - The effect of an EPSP on a postsynaptic neuron is called facilitation. Facilitation is the process in which a postsynaptic neuron’s membrane potential is raised slightly toward threshold so that the next stimulus received will be more likely to trigger an action potential. - Multiple EPSPs (if properly timed and spaced) can result in the production of an action potential in a process called summation. There are two types of summation: 1. Temporal summation is the production of an action potential in a post-synaptic neuron as the result of several EPSPs that occur at exactly the same location but at different times. 2. Spatial summation is the production of an action potential in a post-synaptic neuron as the result of several EPSPs that occur in different locations (but close together) on the cell membrane at exactly the same time. 2. Inhibitory Postsynaptic Potential (IPSP) - An IPSP is a slight decrease in membrane potential in a postsynaptic neuron caused by the release of a small amount of inhibitory neurotransmitter by a presynaptic neuron. The small amount of inhibitory neurotransmitter lowers membrane potential below the resting level in the postsynaptic neuron by briefly opening potassium gated ion channels or chloride gated ion channels. - An IPSP hyperpolarizes the postsynaptic cell membrane, reducing the likelihood that the next stimulus will be strong enough to raise membrane potential to threshold and trigger an action potential. Spinal Cord, Spinal Nerves, Cranial Nerves & Reflexes The spinal cord is a part of the CNS that acts as a pathway for action potentials traveling between the brain and the PNS. It also acts as an integrating center for some reflexes. The spinal cord extends https://docs.google.com/document/d/1-ZPqyipDg5qIKat4bNtxNgOlJIVKb3Wd/mobilebasic 2/10/25, 12 55 AM Page 8 of 14 :  from the foramen magnum to the L1 vertebra. The average spinal cord is approximately 42-45 cm in length and about 2 cm in diameter. In horizontal cross-section, the spinal cord exhibits complex structure: - The white matter is found mainly in the outer regions of the spinal cord. The white appearance of this tissue comes mainly from the myelinated axons that dominate it. The portions of the white matter are: 1. Anterior white columns 2. Posterior white columns 3. Lateral white columns - The gray matter forms a butterfly-shaped (or “H-shaped”) region found mainly around the center of the spinal cord. The gray appearance of this tissue comes mainly from the dark nuclei found in the cell bodies that dominate it. Parts of the gray matter are: 1. Anterior gray horn - consists of cell bodies of neurons in the SNS 2. Lateral gray horn - consists of cell bodies of preganglionic sympathetic neurons 3. Posterior gray horn - consists of cell bodies of association neurons 4. Gray commissure - connects the two lateral gray horns - The central canal is a narrow passageway that runs lengthwise through the central commissure of the spinal cord. It contains CSF (cerebrospinal fluid). - The posterior median sulcus is the deep indentation on the posterior surface of the spinal cord. - The anterior median fissure is a deep indentation on the anterior surface of the spinal cord. The spinal cord is protected by several structures. These structures, in order from inside to outside, are the meninges, epidural fat, and the vertebral column. - The meninges are three CT membranes that cover the spinal cord as well as the brain. - The innermost layer is a delicate, highly vascularized membrane called the pia mater. The pia mater helps to supply oxygen and nutrients to the spinal cord. - The arachnoid is the middle layer. It consists of a web-like collection of collagen and elastic fibers. - The dura mater is the outermost layer. It consists of dense irregular CT and extends all the way down to the S2 vertebra. - The space between the arachnoid and the pia mater is called the subarachnoid https://docs.google.com/document/d/1-ZPqyipDg5qIKat4bNtxNgOlJIVKb3Wd/mobilebasic 2/10/25, 12 55 AM Page 9 of 14 :  space. It contains CSF. - The space between the dura mater and the inside if the vertebral foramen of the vertebrae is called the epidural space. It is filled with a layer of adipose tissue called epidural fat. Spinal nerves are bundles of neurons that arise from either side of the spinal cord in 31 pairs. Each spinal nerve connects to the spinal cord via two roots. - The posterior (dorsal) root connects to the posterior gray horn and contains only sensory fibers. It contains a group of cell bodies (of sensory neurons) in a thickened portion called a posterior (dorsal) root ganglion. - The anterior (ventral) root connects to the anterior gray horn and contains motor fibers. - The anterior and posterior roots join together to form a spinal nerve. Spinal nerves are mixed nerves, meaning that they contain both sensory and motor fibers. Spinal Nerves and their Branches: Each pair of spinal nerves is named and numbered according to its point of origin in the spinal column. There are: 8 pairs of cervical spinal nerves (C1-C8), 12 pairs of thoracic spinal nerves (T1-T12), 5 pairs of lumbar spinal nerves (L1-L5), 5 pairs of sacral spinal nerves (S1-S5), and 1 pair of coccygeal spinal nerves (Co1). Spinal nerves contain both sensory and motor neurons, and are therefore considered to be mixed nerves. The neurons within a spinal nerve are grouped together in bundles called fascicles (fasciculi). Spinal nerves (and cranial nerves) are protected by three connective tissue coverings: 1. Endoneurium – a thin CT layer surrounding individual neurons; 2. Perineurium – a sleeve of CT that surrounds a fascicle; and 3. Epineurium – a layer of dense CT that surrounds an entire nerve. Some branches of spinal nerves may contain structures called ganglia (clusters of cell bodies located in the PNS). (A group of cell bodies in the CNS is usually called a nucleus.) Some spinal nerve branches do not lead directly to the structures to which they provide a nerve supply. Instead, these nerves (which arise from all spinal nerves except T2-T12) merge together to form complex collections of nerves called plexuses. Cranial Nerves The cranial nerves are 12 pairs of nerves that emerge from the brain. Cranial nerves are part of the PNS, and are designated both by names and roman numerals. Some cranial nerves contain only sensory fibers and are therefore considered to be sensory nerves. The remaining cranial nerves contain both sensory and motor fibers and are therefore considered to be mixed nerves. Of these mixed cranial nerves, several contain many more motor fibers than sensory fibers. These mixed cranial nerves are often said to be “mostly motor” or “primarily motor”. The names, roman numerals, and main functions of the cranial nerves are listed below. # Name Sensory or mixed Functions I Olfactory Sensory Smell II Optic Sensory Vision III Oculomotor Mixed (mainly motor) Eye/eyelid movement; lens accommodation; pupil constriction IV Trochlear Mixed (mainly motor) Eye movement V Trigeminal Mixed Chewing; conveys touch, pain, temperature, and proprioception https://docs.google.com/document/d/1-ZPqyipDg5qIKat4bNtxNgOlJIVKb3Wd/mobilebasic 2/10/25, 12 55 AM Page 10 of 14 :  for the face, mouth, and nose VI Abducens Mixed (mainly motor) Eye movement VII Facial Mixed Taste; facial expression; production of saliva and tears VIII Vestibulo- Mixed Hearing and equilibrium cochlear (acoustic) IX Glosso- Mixed Taste; swallowing; saliva production Pharyngeal X Vagus Mixed Swallowing; coughing; voice; GI and cardiac activities XI Accessory Mixed (mainly motor) Head and shoulder movement XII Hypoglossal Mixed (mainly motor) Tongue movements for speech and swallowing Reflexes A reflex is an automatic response to a stimulus. Reflexes occur when action potentials (electrical signals) pass through a specific series of cells and structures called a reflex arc. A reflex arc consists of the following components: 1. A sensory receptor – a structure that respond to changes in the environment by producing an action potential. The receptor can detect a stimulus if one occurs, and the receptor sends its action potential to the control center via a sensory neuron. 2. Sensory neuron – carries the action potential from sensory receptor to control center 3. Control center (integrating center)– receives the action potential from the receptor and sends the action potential to a motor neuron. The control center is located in the CNS. 4. Motor neuron – carries the action potential from the brain or spinal cord to an effector. 5. Effector – the structure that carries out the response to the original stimulus. Effectors are usually muscles or glands. - A spinal reflex is simply a reflex in which the control center is the spinal cord. - A cranial reflex is one in which the control center is the brain. Examples of spinal reflexes: 1. Stretch reflex - Stimulus: stretching of a skeletal muscle - Response: contraction of the stretched muscle - Example: kneejerk (patellar tendon) reflex 2. Tendon reflex - Stimulus: increased tension in a tendon - Response: relaxation of the attached muscle - Example: regulates activity of an antagonist to protect muscle from tearing 3. Flexor reflex - Stimulus: pain - Response: withdrawal from source of pain 4. Crossed extensor reflex - Stimulus: pain in a limb - Response: withdrawal of the affected limb AND extension of the limb on the opposite side to maintain balance The Human Brain https://docs.google.com/document/d/1-ZPqyipDg5qIKat4bNtxNgOlJIVKb3Wd/mobilebasic 2/10/25, 12 55 AM Page 11 of 14 :  The brain consists of four main regions, all wholly located within the cranial cavity. These regions are the brainstem, the cerebellum, the diencephalon, and the cerebrum. The brainstem consists of four components: 1. Medulla oblongata a. The medulla oblongata is continuous with the upper part of the spinal cord. b. It contains centers for regulation of important processes: 1) the cardiovascular center regulates heartbeat 2) the vasomotor center regulates blood vessel diameter 3) the medullary rhythmicity center regulates the rhythm of breathing 4) centers that control posture, vomiting, swallowing, and sneezing. 2. Pons a. The pons is immediately superior to the medulla oblongata. b. It connects the spinal cord with the brain and links parts of the brain with one another by way of tracts. c. The pons contains the pneumotaxic and apneustic areas, which help control respiration. 3. Midbrain a. The midbrain connects the pons to the diencephalon. b. It regulates auditory and visual reflexes. 4. Reticular Formation a. The reticular formation consists of small areas of gray matter scattered throughout the white matter of the brainstem. b. The motor portion helps to regulate muscle tone. c. The sensory portion is called the reticular activating system, or RAS. The RAS alerts the cerebral cortex to sensory signals and is responsible for awakening and staying conscious. The Cerebellum 1. The cerebellum (“little brain”) occupies the inferior and posterior portion of the cranial cavity. 2. The cerebellum functions in the coordination of skeletal muscle contractions and in the maintenance of normal muscle tone, posture, and balance. The Diencephalon includes two major components:. 1. Thalamus a. The thalamus is found superior to the midbrain. It consists of two halves joined together at midline at the massa intermedia (intermediate mass). b. It is the largest part of the diencephalon and contains nuclei (groups of cell bodies) that serve as relay stations for all sensory impulses (except smell) to the cerebral cortex. c. It also interprets pain, temperature and some light touch/pressure stimuli. 2. Hypothalamus a. The hypothalamus is found inferior to the thalamus. b. The hypothalamus is one of the major regulators of homeostasis. Major functions: 1) It controls the autonomic nervous system 2) It is a major component of the endocrine system. 3) It functions in feelings of rage and aggression. 4) It aids in controlling body temperature. 5) It regulates food intake through two centers: - The feeding (hunger) center regulates the desire for food - The satiety center inhibits the function of the hunger center after eating. https://docs.google.com/document/d/1-ZPqyipDg5qIKat4bNtxNgOlJIVKb3Wd/mobilebasic 2/10/25, 12 55 AM Page 12 of 14 :  6) It contains the thirst center which regulates the need to drink fluids. The Cerebrum is the most complex portion of the brain: 1. The cerebrum is the largest part of the brain and is highly convoluted. 2. The cerebrum is divided into right and left halves, called hemispheres, by the longitudinal fissure. Internally, the two hemispheres remain connected by the corpus callosum, a bundle of transverse nerve fibers. Each hemisphere controls functions on the opposite side of the body. 3. Each cerebral hemisphere is further subdivided into four lobes: the frontal, parietal, occipital, and temporal lobes. 4. The central sulcus (separates the frontal and parietal lobes) 5. The outer portion of the cerebrum is called the cerebral cortex and consists of gray matter. 6. The cerebral white matter is under the cortex and consists of myelinated axons. Functional Areas of the Cerebral Cortex A) The sensory areas: 1) The primary somatosensory area (general sensory area) of the parietal lobe detects the precise location of sensations of touch, pain, temperature, and proprioception (awareness of body position). 2) The primary visual area in the occipital lobe consists of two parts: a) The primary visual cortex receives impulses from the optic nerve. b) The visual association area interprets visual stimuli. 3) The primary auditory area in the temporal lobe gets impulses from the ear and interprets the characteristics of sound (pitch, rhythm, and loudness). 4) The primary gustatory area of the parietal lobe obtains taste information. 5) The primary olfactory area of the temporal lobe gets data from smell receptors. B. The motor areas govern voluntary muscular movement. All are located in the frontal lobe. 1) The primary motor area directly controls voluntary skeletal muscle contraction. 2) The premotor area controls learned, complex, sequential motor activities (such as writing, typing, driving, and playing musical instruments). 3) The language areas (especially Broca’s area) control muscle contraction for breathing and sound production by stimulating the primary motor and premotor areas. C. The association areas process complex integrative functions such as memory, emotions, reasoning, will, judgment, personality traits, and intelligence 1) The somatosensory association area (parietal lobe) interpretation of sensations. It also stores memories of past sensory experiences. 2) The auditory association (Wernicke’s) area in the temporal lobe interprets the meaning of speech and associates nonspeech sounds with their sources and meanings. 3) The gnostic area (parietal lobe) forms thoughts and allows complex memory storage. Cerebrospinal Fluid (CSF) - CSF is a watery fluid that circulates through and around the brain and spinal cord. - Functions of CSF include shock absorption, waste removal, nutritional support of the CNS, and maintenance of the proper chemical environment in the CNS. - CSF is produced and reabsorbed at the same rate (approximately 480 mL/day). - CSF forms when blood plasma is filtered and actively transported out of choroid plexuses (capillary clusters in the walls of ventricles) by ependymal cells. - CSF is reabsorbed into the bloodstream via the arachnoid villi, which are finger-like projections of the arachnoid found in the superior sagittal sinus. Blood-Brain Barrier (BBB) https://docs.google.com/document/d/1-ZPqyipDg5qIKat4bNtxNgOlJIVKb3Wd/mobilebasic 2/10/25, 12 55 AM Page 13 of 14 :  - The BBB is the system of specialized blood capillaries that protect the brain against potentially harmful substances circulating in the bloodstream. - The BBB consists of masses of structurally unique capillaries (such as the choroid plexuses) that prevent some substances from reaching the brain. - Specific functions of the BBB: 1. Allows water, nutrients, oxygen, and some drugs to move from the bloodstream into the brain. 2. Keeps waste products, toxins, bacteria, and viruses out of the brain by preventing them from leaving the bloodstream in the brain. 3. Allows metabolic wastes to move into the bloodstream from the brain. - The BBB may be absent in some areas where the brain and the bloodstream must interact more freely. An example of such an area is the hypothalamus. https://docs.google.com/document/d/1-ZPqyipDg5qIKat4bNtxNgOlJIVKb3Wd/mobilebasic 2/10/25, 12 55 AM Page 14 of 14 :