Chapter 9 Nervous System
Central Nervous System (CNS) vs. Peripheral Nervous System(PNS) CNS Brain Spinal cord PNS Peripheral nerves connecting CNS to the body Cranial nerves Spinal nerves Neurons transmit impulses along nerve fibers to other neurons Nerves are bundles of nerve fibers
General Functions Sensory = gather information and convert into nerve impulses Integration = sensory impulses are integrated into perceptions Motor functions = conscious or subconscious decisions carried out via effectors Somatic nervous system Autonomic nervous system
Organization of the Nervous System
Supporting Cells Neuroglial cells CNS 1.Microglial 2.Oligodendrocytes 3.Astrocytes 4.Ependyma PNS 1.Schwann 2.Satellite
Neuron Structure = Nerve Cell 1. Cell Body Typical organelles Nissl bodies (chromatophilic substance) = Rough ER Neurofibrils 2. Processes Dendrites Axon
Axon and Nerve Impulses Axon terminals/synaptic knobs Vesicles and neurotransmitters Synapse Synaptic clefts Nerve fiber coverings Schwann cells- neurilemma Nodes of Ranvier
Cell Body Location Gray matter White matter Ganglia
Classification of Neurons Ways of classification: 1. Structure 2. Function Bipolar neurons Unipolar neurons Multipolar neurons Sensory neurons (afferent neurons) Interneurons Motor neurons (efferent neurons)
Cell Membrane Potential Irritability Conductivity A cell membrane is usually polarized, with an excess of negative charges on the inside of the membrane; polarization is important to the conduction of nerve impulses.
Distribution of Ions The distribution of ions is determined by the membrane channel proteins that are selective for certain ions. Potassium ions pass through the membrane readily than do sodium ions, making potassium ions a major contributor to membrane polarization.
Resting Potential 1. Due to active transport, the cell membrane maintains a greater concentration of sodium ions outside and a greater concentration of potassium ions inside the membrane. 2. Inside of the membrane has excess negative charges, while the outside has more positive charges. 3. The separation of charge, potential difference, is called the resting potential.
Potential Changes 1. Stimulation of a membrane locally affect its resting potential. 2. When the membrane potential becomes less negative, the membrane is depolarized. 3. If sufficiently strong depolarization occurs, a threshold potential is achieved as ion channels open.
Action Potential At threshold potential, membrane permeability to sodium suddenly changes in the region of stimulation. Action potential As sodium channels open, sodium ions rush in, and the membrane potential changes and becomes depolarized. At the same time, potassium channels open to allow potassium ions to leave the cell the membrane is repolarized and resting potential is reestablished. Active transport works to maintain the original concentration of sodium and potassium ions.
Nerve Impulse A nerve impulse is conducted as action potential is reached at the trigger zone and spreads by a local current flowing down the fiber, and adjacent areas of the membrane reach action potential. Nerve impulse is conducted as a series of action potentials occurring along the length of the axon.
1. Unmyelinated fibers 2. Myelinated fibers Impulse Conduction ***All-or-None Response*** If a nerve fiber responds at all to a stimulus, it responds completely by conducting an impulse. Greater intensity of stimulation triggers more impulses per second, not stronger impulses.
Synapse Synapse = junction between two communicating neurons Synaptic Transmission = Presynaptic neuron transmits impulse across the synaptic cleft to the postsynaptic neuron When an impulse reaches the synaptic knobs of an axon, the synaptic vesicles release neurotransmitter into the synaptic cleft. The neurotransmitter react with the receptors on the postsynaptic membrane. Excitatory Action Inhibitory Action
Neurotransmitters 50 kinds of neurotransmitters Synthesized in the synaptic knobs Stored in vesicles Presence of calcium ions are needed to allow vesicles to fuse to the membrane and release their contents into the synaptic cleft. Enzymes in the cleft and on postsynaptic membrane decompose the neurotransmitter in order to prevent continuous stimulation of the postsynaptic neuron
Neuronal Pools Facilitation Convergence Divergence Impulse Processing
Nerve Pathways Reflex arc Reflex behavior Knee-jerk reflex Withdrawal reflex
Meninges The spinal cord and brain are surrounded by membranes called meninges that lie between the bone and soft tissues. Dura mater Arachnoid mater Subarachnoid space Pia mater
Spinal Cord 31 segments, each gives rise to a pair of spinal nerves Cervical enlargement = nerves leading to the upper limbs Lumbar enlargement = nerves leading to the lower limbs Anterior median fissure Posterior median sulcus White matter Gray matter Central canal
Functions of the Spinal Cord Transmit impulses to and from the brain House spinal reflexes Ascending tracts vs. descending tracts
Brain Largest, most complex portion of the nervous system Cerebrum (high mental functions) Diencephalon (processes sensory input) Cerebellum (coordinates muscular activity) Brain stem (coordinates and regulates visceral activities)
Cerebrum Largest portion of the brain, 2 cerebral hemispheres Corpus callosum Convolutions, sulci, fissures Divided into lobes, named according to the bones they underlie Functions of cerebrum Higher brain functions Integrating information for reasoning Memory Interpretation of sensory input Initiating voluntary muscular movements Cerebral cortex
Functions of the Cerebral Cortex Motor, sensory, and association areas Primary motor areas = frontal lobe Broca s area = coordinates muscular activity, speech Frontal eye field = voluntary movements of the eyes and eyelids Sensory areas = interpret sensory input, feelings, sensations Association areas = analyze and interpret sensory impulses, reasoning, judgment, emotions, verbalizing ideas, storing memory General interpretive area = junction of the lobes, complex thought processing
Ventricles and Cerebrospinal Ventricles Fluid Choroid plexuses Cerebrospinal fluid
Diencephalon Thalamus Sorting and directing sensory information Hypothalamus Maintains homeostasis Regulates visceral activities Linking endocrine system to this system Limbic system Controls emotional experience and expression
Brain Stem Midbrain Pons Centers for auditory and visual reflexes Centers that regulate rate and depth of breathing Medulla oblongata Control visceral functions Cardiac center for heart rate and blood pressure regulation Assists pons with breathing regulation
Cerebellum 2 hemispheres Integrate sensory information about the position of body parts Coordinates skeletal muscle activity Maintains posture
Peripheral Nervous System = consists of the cranial and spinal nerves that arise from the CNS and travel to the remainder of the body = somatic nervous system that oversees voluntary activities 12 pairs of cranial nerves:
Spinal Nerves 31 pairs of spinal nerves
Structure of a Nerve
Autonomic Nervous System = maintaining homeostasis of visceral activities without conscious effort 2 divisions: Sympathetic division Parasympathetic division Autonomic nerve fibers: Preganglionic fibers that leaves CNS Postganglionic fibers that extends to the visceral effectors
Sympathetic Division = operates under conditions of stress or emergency Fibers arise from the thoracic and lumbar regions of the spinal cord Synapse in ganglia close to the vertebral column Postganglionic axons lead to the effector organ
Parasympathetic Division = operates under normal conditions Fibers arise from brainstem and sacral region of the spinal cord Synapse in the ganglia close to the effector organ
Autonomic Neurotransmitters Preganglionic fibers release acetylcholine Postganglionic fibers: Parsympathetic: acetylcholine Sympathetic: norepinephrine Control of Autonomic Activity: Reflex centers in the brain and spinal cord Limbic system and cerebral cortex