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Biology 55 Nervous System FUNCTIONS OF THE NERVOUS SYSTEM sensory function – senses stimuli (changes in internal or external environment) integration function – processes sensory inputs and decides on appropriate responses motor function – sends signals to effectors, which respond to the stimuli Neurons – functional cells of nervous system, receive and send electric signals cell body – contains the nucleus and cellular organelles dendrites – short, branched receiving portion of neuron axon – single, long sending portion of neuron synapse – site where neuron communicates with another cell releases a chemical neurotransmitter (eg. acetylcholine) sensory neuron – axon sends signals to the CNS motor neuron – axon sends signals away from the CNS interneurons – neurons in CNS that integrate sensory and motor signals neuroglia – nervous cells that help neurons perform their function astrocytes – form a blood-brain barrier that protects neurons Schwann cells & oligodendrocytes – form a myelin sheath around axons helps axons transmit signals faster DIVISIONS OF THE NERVOUS SYSTEM Central Nervous System (CNS) – brain and spinal cord main integration center white matter – contains many myelinated axons sending signals gray matter – contains many cell bodies integrating information Peripheral Nervous System (PNS) – all nervous tissue outside the CNS nerves – bundles of axons sending electrical signals cranial nerves (12 pairs) – arise from the brain spinal nerves (31 pairs) – arise from the spinal cord ganglia – small clusters of neuron cell bodies outside the CNS sensory receptors – detect changes in internal or external environment Functional Divisions of the PNS somatic nervous system (SNS) – voluntary controls skeletal muscle autonomic nervous system (ANS) – involuntary controls cardiac muscle, smooth muscle, glands 1 NEURON PHYSIOLOGY – Production of Electrical Impulses resting membrane potential – cell membrane is polarized when at rest (inside of cell is negative, outside is positive) action potential – flow of charged particles (electric current) when neuron is stimulated depolarization – stimulation of neuron opens protein channels that let positive ions into cell (inside becomes positively charged) repolarization – inside of cell becomes negative again returns to resting membrane potential conduction of the action potential – an action potential starts at the beginning of the axon and, once started travels to the end of the axon (axon terminal) refractory period – after action potential begins the cell can't produce another for a brief period of time ensures one-way conduction of the action potential saltatory conduction – faster, jumping conduction in myelinated axons COMMUNICATION AT THE SYNAPSE synapse – site of communication between a neuron and another cell neuromuscular junction – synapse between neuron and muscle fiber neuroglandular junction – synapse between neuron and gland most synapses are between one neuron and another neuron Synapses Between Neurons presynaptic neuron – sending neuron (axon terminal) postsynaptic neuron – receiving neuron (dendrite or cell body) synaptic cleft – small space between 2 communicating neurons an action potential in the presynaptic neuron triggers release of a chemical neurotransmitter – chemical released by presynaptic neuron that binds to protein receptors in postsynaptic cell's membrane excitatory neurotransmitter – stimulates postsynaptic neuron to produce an action potential of its own inhibitory neurotransmitter – makes it less likely that the postsynaptic neuron will produce its own action potential summation of all of the excitatory and inhibitory synapses determines whether the postsynaptic neuron produces an action potential How Drugs and Toxins Modify Nervous System Function stimulate or inhibit neurotransmitter synthesis stimulate or inhibit neurotransmitter release block or activate neurotransmitter receptors agonists activate receptors (mimic neurotransmitter) antagonists block receptors stimulate or inhibit neurotransmitter removal 2 THE CENTRAL NERVOUS SYSTEM Meninges – 3 connective tissue membranes around brain & spinal cord pia mater – thin inner membrane on surface of brain & spinal cord contains blood vessels which supply the brain and spinal cord arachnoid mater – middle, web-like membrane connected to pia mater subarachnoid space – space between arachnoid and pia that contains cerebrospinal fluid (CSF) cushions brain and spinal cord dura mater – outer, tough double membrane of fibrous connective tissue splits in some regions around the brain to form blood-filled dural venous sinuses epidural space – space between dura mater and vertebrae filled with adipose tissue that cushions spinal cord site for anesthetic injections Cerebrospinal Fluid (CSF) – clear fluid which circulates through cavities in brain, spinal cord, and in subarachnoid space ventricles – 4 cavities in brain where CSF is produced by filtering blood ependymal cells lining ventricles regulate content of CSF CSF circulates to central canal of spinal cord and the subarachnoid space returns to blood in the dural venous sinuse hydrocephalus – excess accumulation of CSF resulting in pressure THE BRAIN – has 4 main divisions; brainstem, diencephalon, cerebellum, cerebrum 1) Brainstem – connects to the spinal cord sensory and motor axon tracts pass through these regions Medulla oblongata – inferior brainstem cardiovascular center – regulates heart and blood vessels respiratory center – controls muscles for breathing reflex centers for coughing, sneezing, swallowing, vomiting Pons – superior to medulla regulates rate of breathing Midbrain – superior brainstem contains reflex centers for vision and hearing 2) Diencephalon – between brainstem and cerebrum thalamus – 80% of diencephalon relay station for most sensory impulses traveling to the cerebrum hypothalamus – found below thalamus link between the nervous and endocrine systems regulates the autonomic nervous system produces hormones regulates eating and drinking – thirst center, feeding center regulates body temperature 3 3) Cerebellum – attached to dorsal brainstem coordinates skeletal muscle movements receives voluntary motor impulses from cerebrum receives sensory impulses related to body position and balance compares intended movements with movements actually occurring 4) Cerebrum – largest, most superior portion of brain origin of voluntary actions, site of conscious perceptions, center of intellect longitudinal fissure – deep groove that divides cerebrum into 2 hemispheres corpus callosum – axon tracts connecting the 2 cerebral hemispheres Cerebral Cortex – outer gray matter divided into frontal, parietal, temporal, and occipital lobes by sulci contains neuron cell bodies that integrate all conscious functions Sensory areas – posterior cerebrum primary somatosensory area – receives sensations of pain, touch, temperature from opposite side of the body (parietal lobe) visual area – receives visual sensations (occipital lobe) Motor areas – frontal lobe primary motor area – controls movements of skeletal muscles on the opposite side of the body Association areas – located within or near motor and sensory areas allow recognition of sensations control complex, learned motor skills Processing centers – integrate information from other areas and perform complex function such as analysis, reasoning, comprehension of spoken or written language, creativity, etc. Limbic system – neural pathways in cerebrum that link to the diencephalon and control emotions and function in learning and forming memories SPINAL CORD – found within the vertebral canal and surrounded by meninges central gray matter integrates spinal reflexes peripheral white matter contains axon tracts sensory (ascending) tracts carry impulses to the brain motor (descending) tracts carry impulses from the brain central canal – cavity in center containing cerebrospinal fluid PERIPHERAL NERVOUS SYSTEM spinal nerves – 31 pairs emerging from the spinal cord detect sensations and control functions of trunk and limbs mixed nerves – carry sensory and motor axons dorsal nerve root – sensory axons running to spinal cord dorsal root ganglion – contains cell bodies of sensory neurons ventral nerve root – motor axons running from spinal cord 4 cranial nerves – 12 pairs emerging from brainstem mainly detect sensations and control functions of head and neck (vagus nerve controls functions of thoracic and abdominal organs) may be mixed nerves or only sensory or motor SOMATIC VS AUTONOMIC NERVOUS SYSTEM Somatic Nervous System – controls movements of skeletal muscles voluntary movements or reflexes which can be voluntarily suppressed Autonomic Nervous System – regulates activities of smooth muscle, cardiac muscle, and glands involuntary, operates mainly by reflexes REFLEXES – fast, automatic responses to specific stimuli somatic reflexes – involve skeletal muscle protective reflexes, can be consciously overridden autonomic reflexes – involve smooth muscle, cardiac muscle or glands maintain homeostasis in the body Reflex Arc – pathway for nerve impulses of a reflex 1) sensory receptor – detects a stimulus 2) sensory neuron – generates a sensory impulse and carries it to the CNS 3) integrating center in brain or spinal cord 4) motor neuron – carries impulse from CNS to an effector 5) effector – part of body that responds to the motor impulse (muscle, gland) the effector’s automatic response to the stimulus is called a reflex knee-jerk reflex – suddenly stretching skeletal muscle by striking the tendon makes it contract to protect the muscle from overstretching somatic, spinal reflex pupillary light reflex – shining a light in the eye causes the pupil to contract to reduce the amount of light entering autonomic, cranial reflex DIVISIONS OF AUTONOMIC NERVOUS SYSTEM many organs are receive motor innervation from 2 divisions of the ANS one division is usually excitatory, the other inhibitory Sympathetic division – motor neurons that trigger fight-or-flight responses caused by physical or emotional stress motor neurons release norepinephrine at synapse with effector also triggers release of the hormone epinephrine by the adrenal gland pupils dilate increased heart rate and force increased blood flow to skeletal muscles, lungs 5 airways dilate, respiratory rate increased energy released from storage in liver and adipocytes increased metabolism and sweating inhibition of non-essential activities – digestive & urinary function Parasympathetic division – motor neurons that trigger rest-and-digest activities when no stress is occurring motor neurons release acetylcholine at synapse with effector pupils constrict decreased heart rate and force airway constriction stimulation of digestive and urinary functions increased nutrient absorption in cells increased energy storage 6