Fundamentals of the Nervous System and Nervous Tissue

Transcription

1 Fundamentals of the Nervous System and Nervous Tissue

2 Nervous System & Tissue I. General Functions of the Nervous System II. Organization of the Nervous System III. Nervous Tissue IV. Nerves V. Basic Neuronal Organization VI. Disorders

3 I. General Functions A. Control & Communication Center 1. Control through creation & propagation of electrical impulses (action potentials) a. Fast acting & Specific 2. Control and communication through inhibition or excitation at neuron junctions (synapses, neuromuscular junctions...) a. Determines what happens once action potential arrives at a junction 3. Operates with the endocrine system

4 I. General Functions B. Sensory Function 1. Sensory receptors monitor changes inside and outside the body a. The changing condition is the stimulus b. The stimulus can ultimately generates an action potential which is the sensory input C. Integrative Function 1. Processes and interprets sensory input a. Makes decisions integration D. Responsive Function 1. Dictates a response by activating effector organs a. Response motor output

5 II. Organization of the Nervous System A. Nervous system divided into two systems: 1. Central nervous system (CNS) Brain and spinal cord Integrating and command center 2. Peripheral nervous system (PNS) Outside the CNS Consists of nerves extending from (and going to) brain and spinal cord Cranial nerves Spinal nerves Peripheral nerves link all regions of the body to the CNS

6 II. Organization of the Nervous System Brain Spinal Cord Central Nervous System Peripheral Nerves Peripheral Nerves Peripheral Nervous System Peripheral Nerves

7 II. Organization of the Nervous System B. Peripheral Nervous System consists of two major functional aspects 1. Sensory (afferent) signals picked up by sensor receptors Carried by nerve fibers of PNS to the CNS 2. Motor (efferent) signals are carried away from the CNS Innervate muscles and glands

8 II. Organization of the Nervous System C. Sensory and Motor aspects are divided according to region they serve Somatic (wall of body) Visceral (guts) D. This regional division creates four main subdivisions: 1. Somatic Sensory 2. Somatic Motor 3. Visceral Sensory 4. Visceral Motor

9 II. Organization of the Nervous System 1. Somatic Senses a. General somatic senses Sensory receptors have a wide dispersion, and include Touch, pain, vibration, pressure, and temperature b. Proprioceptive senses (sense of body positions) Afferent information regarding position and movement of body in space, due to receptors in tendons, muscles and joints c. Special somatic senses Special due to the presence of a sensory organ or a clustered group of sensory receptors, rather than sensory cells widely dispersed as in the general somatic senses Hearing, balance, vision, and smell

10 II. Organization of the Nervous System 2. Somatic motor neurons a. General somatic motor signals contraction of skeletal muscles Under our voluntary control and as such may also referred to as the voluntary nervous system b. Branchial motor Typical skeletal muscle derived from somitomeres Mastication muscles control Facial expression muscle control Pharyngeal & laryngeal muscle control Sternocleidomastoid & Trapezius muscle control

11 II. Organization of the Nervous System 3. Visceral Senses a. General visceral senses stretch, pain (generally referred to the body wall), temperature, nausea, and hunger Widely felt in digestive and urinary tracts, reproductive organs b. Special visceral senses Taste and smell have a visceral afferent component, involving cranial nerves VII, IX & X (facial, glossopharyngeal & vagus nerve)

12 II. Organization of the Nervous System 4. Visceral Motor Neurons Makes up autonomic nervous system Further divided into Sympathetic & Parasympathetic divisions. Regulates the contraction of smooth and cardiac muscle & glandular secretion Controls function of visceral organs Also called involuntary nervous system, as it performs without conscious input.

13 II. Organization of the Nervous System CNS PNS Brain Spinal Cord Sensory (afferent) Division Motor (efferent) Division Somatic Sensory Visceral sensory Somatic Motor Visceral Motor General Somatic Senses Proprioceptive Senses Special Somatic Senses General Visceral Senses Special Visceral Senses Sympathetic division of ANS Parasympathetic division of ANS

14 III. Nervous Tissue A. Characteristics of nervous tissue Cells are densely packed and intertwined Two main cell types make up nervous tissue Neurons Excitable Neuroglia (supportive cells) Nonexcitable More on these later

15 III. Nervous Tissue B. General Information on Neurons 1. Billions of neurons create the basic functional structure unit of the nervous system 2. Neurons are specialized cells that conduct electrical impulses (action potentials or nerve impulses) along the plasma membrane 3. Longevity can live and function for a lifetime 4. Do not divide fetal neurons lose their ability to undergo mitosis; neural stem cells are an exception 5. High metabolic rate require abundant oxygen and glucose Can account for approx 10% of your metabolic rate... When you are thinking

16 III. Nervous Tissue C. General Structure of a Neuron consists of (1) a cell body and (2) neuronal processes (dendrites & axons [axon collaterals, axon terminals & synapses]) 1. Cell body (perikaryon or soma) Size varies from 5 140µm Contains a normal complement of organelles Also contains chromatophilic (Nissl) bodies Basically clumps of RER and free ribosomes that love stain (namesake) that renew membranes and protein portion of cytosol Neurofibrils bundles of intermediate filaments that form a network between chromatophilic bodies and provides structural integrity to the soma

17 Structure of a Typical Large Neuron

18 III. Nervous Tissue C. General Cell body information cont. 2. Location of neuronal cell bodies a. Most neuronal cell bodies are located within the CNS Protected by bones of the skull and vertebral column b. Some neuronal cell bodies form ganglia clusters of cell bodies, axon terminals and dendrites outside of the CNS (i.e. part of the PNS)

19 III. Nervous Tissue C. General Structure 2. Neuronal Processes extensions of cell body membrane forming Dendrites Axons

20 2. Neuron Processes a. Dendrites Extensively branching from the soma Transmit electrical signals (graded potentials) toward the cell body As graded potentials, they may be affected by other nearby synaptic events Increasing state of excitation (further depolarization of membrane) Decreasing state of excitation (inhibition further from a threshold event) Contain chromatophilic bodies, but only extend into the basal part of dendrites Function as receptive sites Lots of surface area for this due to the large amount of branching exhibited by dendrites

21 2. Neuron Processes b. Axons 1. Neuron has only one Exception anaxionic neuron 2. Impulse generator and conductor Generated at axon hillock Conducted along the length of the axon 1. Transmits impulses away from the cell body 2. Chromatophilic bodies are absent 3. No protein synthesis in axon 4. Neurofilaments, actin microfilaments, and microtubules Provide strength along length of axon Aid in the transport of substances to and from the cell body Axonal transport or flow needed as there is no protein synthesis in axon

22 2. Neuron Processes b. Axons cont. 5. May have branches along length Axon collaterals Allow for divergence of action potential Allows for widespread effect Multiple branches at end of axon Terminal branches (telodendria)» End in knobs called axon terminals (also called end bulbs or boutons)» Forms a synapse with another nueron, or a neuromuscular junction if a muscle...

23 III. Nervous Tissue D. Action Potentials (nerve impulses) Generated at the axon hillock If the membrane reaches threshold potential due to the net effects of synapse activity on the dendrites & soma Conducted along the axon Releases neurotransmitters at axon terminals Neurotransmitters excite or inhibit the postsynaptic membrane Depending on the action of the neurotransmitter receptor on the post synaptic membrane Neuron receives and sends signals Receives on the dendrites and soma Sends down the axon, axon collaterals

24 III. Nervous Tissue E. Synapses Site at which neurons communicate Signals pass across synapse in one direction Presynaptic neuron Conducts signal toward a synapse Postsynaptic neuron Transmits electrical activity away from a synapse Only if the effects of the neurotransmitter from the presynpatic neuron are excitatory

25 Two Neurons Communicating at a Synapse

26 III. Nervous Tissue E. Synapses cont. Elaborate cell junctions Synaptic vesicles on presynaptic side Membrane bound sacs containing neurotransmitters Mitochondria abundant in axon terminals Synaptic cleft separates the plasma membrane of the two neurons

27 Structure of a Synapses

28 III. Nervous Tissue F. Synapse Types: 1. Axodendritic Between axon terminals of one neuron and dendrites of another Most common type of synapse 2. Axosomatic Between axons and neuronal cell bodies 3. Axoaxonic, dendrodendritic, and dendrosomatic Uncommon types of synapses

29 Some Important Types of Synapses

30 III. Nervous Tissue G. Structural classification of Neurons 1. Multipolar possess more than two processes Numerous dendrites and one axon 2. Bipolar possess two processes Rare neurons found in some special sensory organs 3. Unipolar (aka pseudounipolar) possess one short, single process Start as bipolar neurons during development 4. Anaxonic No identifiable axon off of soma Used within the CNS and some sensory organs Identified as possibly altering the communication (via gap junctions & synapses) among sensory neurons in the olfactory bulb.

31 Neurons Classified by Structure

32 III. Nervous Tissue H. Functional Classification of Neurons According to the direction the nerve impulse travels (towards or away from CNS) 1. Sensory (afferent) neurons transmit impulses toward the CNS Virtually all are unipolar neurons Cell bodies in ganglia outside the CNS Short, single process divides into The central process runs centrally into the CNS The peripheral process extends peripherally to the receptors

33 III. Nervous Tissue H. Functional Classification of Neurons cont. 2. Motor (efferent) neurons Carry impulses away from the CNS to effector organs Most motor neurons are multipolar Cell bodies are within the CNS Form junctions with effector cells 3. Interneurons most are multipolar (some are anaxonic) Lie between motor and sensory neurons Confined to the CNS

34 H. Functional Classification of Neurons

35 III. Nervous Tissue I. Six types of supporting cells (neuroglial cells) 1. Four in the CNS 2. Two in the PNS Provide supportive functions for neurons Cover nonsynaptic regions of the neurons Neuroglia characteristics usually only refers to supporting cells in the CNS Glial cells have branching processes and a central cell body Outnumber neurons 10 to 1 Make up half the mass of the brain Can divide throughout life... cancer impact?

36 1. Supporting Cells in the CNS Types of neuroglial cells a. Astrocytes most abundant glial cell type Take up and release ions to control the environment around neuron Form networks around capillaries, creating the blood brainbarrier Recapture and recycle neurotransmitters Involved with synapse formation in developing neural tissue Produce molecules necessary for neural growth (BDTF) & promote myelination Propagate calcium signals that may be involved in memory May modulate synaptic activity

37 Astrocytes are star-shaped glial cells of the CNS that have long processes. Many of these processes extend to blood vessels where they expand and cover much of the external wall. The expanded endings of the astrocyte processes are known as end-feet. While the blood-brain-barrier is formed by tight junctions between endothelial cells, the end-feet function to induce and maintain the blood-brain barrier. In pathology following stroke the relationship of end-feet to the endothelial cells is altered leading to disruption of the blood-brain barrier and subsequent leakage.

38 1. Supporting Cells in the CNS b. Microglia smallest and least abundant Phagocytes the macrophages of the CNS Engulf invading microorganisms and dead neurons Derived from blood cells called monocytes

39 1. Supporting Cells in the CNS c. Ependymal cells Line the central cavity of the spinal cord and brain Bear cilia help circulate the cerebrospinal fluid Section of central canal of medulla spinalis, showing ependymal and neuroglial cells.

40 1. Supporting Cells in the CNS d. Oligodendrocytes have few branches Wrap their cell processes around axons in CNS Produce myelin sheaths single oligodendrocyte may extend to 50 neurons and cover 1mm of each

41 2. Supporting Cells in the PNS a. Satellite cells surround neuron cell bodies within ganglia Satellite cell nucleus Satellite cell membrane Neuron cell membrane nucleus

42 2. Supporting Cells in the PNS b. Schwann cells (neurolemmocytes) surround axons in the PNS Form myelin sheaths around axons of the PNS Axon Neurolemmocty e Axon Axon Aid in neural regeneration of peripheral nerves

43 Myelin Sheaths in the PNS Formed by Schwann cells (neurolemmocytes) Develop during fetal period and in the first year of postnatal life Schwann cells wrap in concentric layers of the lipoprotein myelin around the axon Cover the axon in a tightly packed coil of membranes Neurilemma material external to myelin layers Nodes of Ranvier gaps along axon (also called neurofibril node) Thick axons are myelinated Thin axons are unmyelinated

44 IV. Nerves A. General Nerve Information cordlike organs in the PNS Consists of numerous axons wrapped in connective tissue Axon is surrounded by Schwann cells

45 IV. Nerves B. Structure of Nerves 1. Endoneurium o layer of delicate connective tissue surrounding the axon 2. Nerve fascicles o groups of axons bound into bundles 3. Perineurium o connective tissue wrapping surrounding a nerve fascicle 4. Epineurium o whole nerve is surrounded by tough fibrous sheath

46 Structure of a Nerve radial nerve

47 IV. Nerves C. Functional Aspect of Nerves Nerves are typically classified as Motor Sensory Mixed Along its length a nerve may be all three i.e. leaving the ventral root of the spinal cord a nerve would be motor, then joins with the incoming afferent nerve to create a mixed nerve, and then at a ramus branches and becomes alone in its motor function.

48 V. Basic Neuronal Organization A. General plan is... A reflex pathway (arc) to maintain homeostasis! receptor Sensory neuron Interneuron (Integration Center) effector Motor neuron Arrows represent flow of information

49 V. Basic Neuronal Organization B. Neuronal Circuits & Processing Patterns 1. Neuronal Circuits typically occur when one neuron is affected by or controls more than one other neuron Divergent Convergent Reverberating (oscillating)

50 V. Basic Neuronal Organization C. Neuronal Circuits & Processing Patterns A. Processing Patterns A. Serial B. Parallel

51 V. Basic Neuronal Organization D. Types of Matter 1. Gray a. Darker in color b. Contains dendrites, cell bodies, and axon terminals c. Site of actual integration Ex. Cerebral and cerebellar cortices 2. White a. Lighter in color due to myelination b. Forms tracts (descending motor & ascending sensory) c. Forms commissural, sensory, motor & associative tracts routes of communication between processing areas

52 VI. Disorders of the Nervous System Demyelination Issues Multiple sclerosis common cause of neural disability Varies widely in intensity among those affected Cause is not completely understood An autoimmune disease Immune system attacks the myelin around axons in the CNS Heavy metal poisoning messes with development processes Interferes with ER form and function Causes brain damage... Guillain Barré syndrome Possibly due to bacterial or viral infection, leading to autoimmune issues Ab s attack the myelin sheath causing inflammation of peripheral nerves No hereditary issues Rabies Viral disease of CNS after virus is transported along the axon from neurons at the region of the bite (axonal flow)

53 Additional Supplemental Information

54 Pyramidal cells Apical dendrite pyramidal cell (or pyramidal neuron, or projection neuron) is a multipolar neuron located in the hippocampus and cerebral cortex. These cells have a triangularly shaped soma, or cell body, a single apical dendrite extending towards the pial surface, multiple basal dendrites, and a single axon. Pyramidal neurons compose approximately 80% of the neurons of the cortex, and release glutamate as their neurotransmitter, making them the major excitatory component of the cortex (see synapse). In the primary motor cortex, layer V pyramidal cells are extremely large. These cells are called Betz cells. Their cell bodies can be as large as 100 micrometers in humans. Typical human pyramidal cell bodies range from 10 to 50 micrometers.

55 These cells are some of the largest neurons in the human brain, with an intricately elaborate dendritic arbor, characterized by a large number of dendritic spines. Purkinje cells are found within the Purkinje layer in the cerebellum. Purkinje cells are aligned like dominos stacked one in front of the other. Their large dendritic arbors form nearly two dimensional layers through which parallel fibers from the deeper-layer granule cells pass. These parallel fibers make relatively weaker excitatory (glutamatergic) synapses to spines in the Purkinje cell dendrite, whereas climbing fibers originating from the inferior olivary nucleus in the medulla provide very powerful excitatory input to the proximal dendrites and cell soma. Parallel fibers pass orthogonally through the Purkinje neuron's dendritic arbor, with up to 200,000 parallel fibers forming a synapse with a single Purkinje cell. Alternatively, each Purkinje cell only receives a synapse from a single climbing fiber. Both basket and stellate cells (found in the cerebellar molecular layer) provide inhibitory (GABAergic) input to the Purkinje cell, with basket cells synapsing on the Purkinje cell body and stellate cells onto the dendrites. Microcircuitry of the cerebellum. Excitatory synapses are denoted by (+) and inhibitory synapses by (-). MF: Mossy fiber. DCN: Deep cerebellar nuclei. IO: Inferior olive. CF: Climbing fiber. GC: Granule cell. PF: Parallel fiber. PC: Purkinje cell. GgC: Golgi cell. SC: Stellate cell. BC: Basket cell. Purkinje cells send inhibitory projections to the deep cerebellar nuclei, and constitute the sole output of all motor coordination in the cerebellar cortex.

56 Neuronal Regeneration Neural injuries may cause permanent dysfunction If axons alone are destroyed, cells bodies often survive and the axons may regenerate PNS macrophages invade and destroy axon distal to the injury Axon filaments grow peripherally from injured site Partial recovery is sometimes possible CNS neuroglia never form bands to guide regrowing axons and may hinder axon growth with growth inhibiting chemicals No effective regeneration after injury to the spinal chord and brain

57 Regeneration of the Peripheral Nerve Fiber Figure 12.22

58 Myelin Sheaths CNS vs. PNS Myelination