Transcript Document
Nervous System FUNCTIONS: 1. Sensory input. 2. Integration. 3. Homeostasis. 4. Mental activity. 5. Control of skeletal muscles. The Nervous System Organization of the Nervous System • Central nervous system (CNS) – Brain and spinal cord • Peripheral nervous system (PNS) – Neurons outside the CNS – Sensory division • Afferent fibers transmit impulses from receptors to CNS – Motor division • Efferent fibers transmit impulses from CNS to effector organs Relationship between motor and sensory fibers of the PNS and the CNS Autonomic Nervous System • Sympathetic – Fight or flight, stress – Excitatory effects elicited by norepinephrine activating beta receptors – Inhibitory effects elicited by activation of alpha receptors • Parasympathetic – Rest and digest – Digestive system activated, heart rate inhibited, blood vessels dilated – Vagus nerve primarily responsible for activating parasympathetic responses Fig. 8.39 Synapse Specialized site of intercellular communication. 3 Components: 1. Presynaptic terminal 2. Synaptic cleft 3. Postsynaptic membrane Functional Organization of the Nervous System The Neuron Neuroglia Neuroglia • Accessory cells of the nervous system • Astrocytes – Support tissue in the CNS form blood-brain barrier • Ependymal – Produce and move cerebral spinal fluid • Microglia – Remove cell debris and bacteria from CNS • Oligodendricytes and Schwann cells – Provide insulation around axons of CNS and PNS neurons Myelinated vs. Unmyelinated Axons Membrane Potentials • Nervous system functions by establishing concentration gradients and electrical potentials across the membranes • The resting membrane potential of a neuron is negative and is said to be polarized • These gradients are maintained by the sodium potassium pump Concentration Gradients and Nerve Cell Function Action Potentials • Muscle and nerve cells are exciteable • When a muscle or nerve cell is stimulated Na+ channels open and Na+ rushes into the cell • This causes a local potential • This local potential may not result in action potential – Doesn’t cross the threshold • If the stimulus is sufficient to cause the local potential to cross the threshold an action potential results • The action potential is the complete depolarization of the cell • The action potential is an all-or-nothing event – If the local potential meets threshold, the cell totally depolarizes and the action potential results – If the potential does not meet threshold, no action potential results Fig. 8.9 Fig. 8.10 Action Potential Propogation • Unmyelinated neurons propogate signals more slowly than myelinated neurons • Myelination acts as an insulator – Electrical signal will jump from node of Ranvier to node of Ranvier – This is called saltatory conduction – Requires less energy than direct propogation Propagation of the Action Potential Synapse • Electrical • Chemical --rare --communication occurs in one direction: presynaptic membrane to postsynaptic membrane --action potential is not always propagated. Synapse Synapses may occur: • neuron to neuron • neuron to another type of cell (neuroeffector) --neuromuscular junction --neuroglandular junction The Synapse Fig. 8.13 Neurotransmitters --packaged in synaptic vesicles. Nerve endings of the ANS secrete: • Acetylcholine (ACh)--Cholinergic neuron – Parasympathetic effector • Norepinephrine (NE)--Adrenergic neuron – Sympathetic effector • Neurotransmitters diffuse across the synaptic cleft and bind to receptor on the post-synaptic membrane • This can cause membrane channels (Na+, K+, or Cl-) to open or close depending on the neurotransmitter • If stimulatory, Na+ channels will open • If inhibitory, K+ or Cl- channels will open – Cell becomes more negative, hyperpolarized Receptors 2 types of cholinergic receptors: • Nicotinic – Preganglionic sympathetic and parasympathetic • Muscarinic – parasympathetic 2 types of adrenergic receptors: • Alpha – Generally inhibitory • Beta – Generally excitatory Autonomic Reflex Arc 1. 2. 3. 4. 5. Receptor Sensory neuron Association neuron Autonomic motor neuron Visceral effector Reflex Arc Knee Jerk Reflex Converging Circuit Central Nervous System Adult: • Brainstem --medulla oblongata --pons --midbrain • Diencephalon --thalamus --hypothalamus --epithalamus • Cerebrum • Cerebellum Brainstem • Medulla oblongata – Inferior portion – Regulation of heart rate, venoconstriction, ventilation, swallowing, , etc.. • Pons – Superior to medulla – Bridge between cerebrum and cerebellum • Midbrain – Audio and visual processing Cerebellum • Integrates motor signals from cerebral cortex with feedback from PNS • Proprioception • Learning tasks Dienchephalon • Thalamus – Sensory input from PNS passes through thalamus (relay station) • Epithalamus – Pineal gland – sleep cycle, puberty • Hypothalamus – Master gland – Attached to pituitary by infundibulum – Controls much of homeostasis by stimulating or inhibiting pituitary Cerebrum Brain Protection: • • • • cranial bones cranial meninges cerebrospinal fluid neuroglia (astrocytes) The Brain CEREBRUM • Largest part of the brain; thinking part • Markings: Gyrus (gyri)-- wrinkle, raised area Fissure(s)-deep, wide groove(s) Sulcus (sulci)-- shallow groove(s) CEREBRUM Lobes: 1) Frontal 2) Parietal 3) Temporal 4) Insular 5) Occipital CEREBRUM Displays lateralization: • left hemisphere language; math/science; reason • right hemisphere music/art; spatial relations; insight/imagination CEREBRUM • sensory areas • motor areas • association areas Spinal Cord-- Composition white matter (myelin) dorsal column ventral column lateral column gray matter (non-myelin) posterior horn ventral horn lateral horn Spinal Cord-- White Matter • myelinated axons that travel along the spinal cord. Ascending-- up cord to higher levels Descending-- down cord from brain Across the cord Spinal Cord • Dorsal roots (sensory) • Ventral roots (motor) combine to form spinal nerve. • Dorsal Root Ganglion Peripheral Nervous System Cranial Nerves • 12 pr.-- I to XII (anterior to posterior) 3 functions: 1) sensory 2) somatic-- control of skeletal muscle 3) parasympathetic--regulation of glands, smooth muscle, and cardiac muscle. Peripheral Nervous System Spinal Nerves • 31 pr. 8 cervical 12 thoracic 5 lumbar 5 sacral 1 coccygeal SPINAL NERVES rootlets roots spinal nerve ramus Dorsal rami Ventral rami-Distributed 2 Ways: Intercostal nerves (T1-T12) Plexuses (5): cervical plexus (C1-C5) brachial plexus (C5-T1) lumbar plexus (L1-L4) sacral plexus (L4-S4) coccygeal plexus (S4, S5, Cx)