Fundamentals of the Nervous System and Nervous Tissue 11 P A R T A Fundamentals of the Nervous System and Nervous Tissue

Doh!! The Nervous System

The master controlling and communicating system of the body Nervous System The master controlling and communicating system of the body Functions can be divided into three themes: Sensory input – gathering information about the internal and external environments, monitoring stimuli Integration – interpretation of sensory input, analysis of information, memory, decision making… Motor output – response to stimuli, taking action (sometimes automatic, sometimes voluntary)

Nervous System Figure 11.1

Organization of the Nervous System The nervous system includes two anatomical divisions Central nervous system (CNS) Brain and spinal cord Integration and command center Peripheral nervous system (PNS) 31 pairs of spinal nerves (enter or exit the spinal cord between vertebrae, such as the femoral nerve) 12 pairs of cranial nerves (enter or exit the brain, such as the optic nerve) Carries sensory messages from the body to the CNS, and motor outputs from the CNS to the body

Spinal Nerves CNS vs. PNS Cranial Nerves

Peripheral Nervous System (PNS): Two Functional Divisions Sensory (afferent) division Sensory afferent fibers – carry impulses from sense organs, skin, skeletal muscles, and joints to the brain Visceral afferent fibers – transmit impulses from visceral organs to the brain Motor (efferent) division Transmits impulses from the CNS to effector organs (muscles and glands)

Motor Division: Two Main Parts Somatic nervous system Conscious control of skeletal muscles Autonomic nervous system (ANS) Regulates smooth muscle, cardiac muscle, and glands Divisions – sympathetic (“fight or flight”) and parasympathetic (“chillax, dude!”)

Autonomic Nervous System

Autonomic Nervous System Organ Sympathetic Action Parasympathetic Action Summary of Action Eye Salivary Gland Heart Lungs Stomach, Pancreas, Intestines Liver & Gall Bladder Adrenal Glands Bladder Blood Vessels to Muscles Blood Vessels to Visceral Organs Fight or Flight Chillax Dilates Pupil Constricts Pupil Slows Relase of Saliva Secretes Saliva Tachycardia Bradycardia Dilates Bronchi Constricts Bronchi Slows Movement & Secretion Speeds Movement & Secretion Slows Secretion of Bile Speeds Secretion of Bile Release adrenaline (epinephrine) Store adrenaline Inhibits Constriction Constricts Arterioles Dilate Arterioles Constrict Constrict Dilate

Regions of the Brain and Spinal Cord White matter – dense collections of myelinated fibers (myelin is a pale fatty substance produced by glial cells, which wraps around some axons to speed action potentials) – function is to transmit messages Gray matter – mostly neuron cell bodies and unmyelinated fibers – function is to analyze information and make decisions

Grey Matter vs. White Matter ganglion white matter spinal nerve grey matter

Histology of Nerve Tissue The two principal cell types of the nervous system are: Neurons – excitable cells that transmit electrical signals Glial cells – cells that surround and wrap neurons, providing structural and functional support to neurons

Supporting Cells: Neuroglia The supporting cells (neuroglia or glial cells): Provide a supportive scaffolding for neurons Segregate and insulate neurons Guide young neurons to the proper connections Promote health and growth Speed action potentials May have some influence on neuronal communication (recent research suggests this)

Astrocytes Most abundant, versatile, and highly branched glial cells They cling to neurons and their synaptic endings, and cover capillaries

Astrocytes Functionally, they: Support and brace neurons Anchor neurons to their nutrient supplies Guide migration of young neurons Control the chemical environment by forming the “Blood-Brain Barrier”

Astrocytes Figure 11.3a

Microglia and Ependymal Cells Microglia – small, ovoid cells with spiny processes Phagocytes that monitor the health of neurons Ependymal cells – range in shape from squamous to columnar They line the central cavities (the ventricles) of the brain and spinal column – this structure is called the choroid plexus Produce cerebrospinal fluid (CSF)

Microglia and Ependymal Cells (ventricle) Figure 11.3b, c

Oligodendrocytes, Schwann Cells, and Satellite Cells Oligodendrocytes – branched cells that wrap CNS nerve fibers, fill up with myelin to speed action potentials in brain and spinal cord Schwann cells (neurolemmocytes) – surround fibers of the PNS, fill up with myelin to speed action potentials in peripheral sensory and motor neurons Satellite cells surround neuron cell bodies within ganglia of the PNS (clusters of neurons outside the brain & spinal cord)

Oligodendrocytes, Schwann Cells, and Satellite Cells Figure 11.3d, e

Structural units of the nervous system Neurons (Nerve Cells) Structural units of the nervous system Composed of a body, axon, and dendrites Long-lived, amitotic, and have a high metabolic rate Their plasma membranes function in: Electrical signaling (action potentials / nerve impulses) Chemical communication (synaptic transmission) Cell-to-cell signaling during development PLAY InterActive Physiology ®: Nervous System I, Anatomy Review, page 4

Neuron Classification Functional: Sensory (afferent) — gather information about the environment and transmit impulses toward the CNS Motor (efferent) — carry impulses away from the CNS with commands for musclar contractions and glandular secretions Interneurons (association neurons) — shuttle signals through CNS pathways, process information, combine information, make decisions, store and access memories….

Neuron Classification Structural: Multipolar — three or more processes Bipolar — two processes (axon and dendrite) Unipolar — single, short process

Comparison of Structural Classes of Neurons Table 11.1.1

Comparison of Structural Classes of Neurons Table 11.1.2

Comparison of Structural Classes of Neurons Table 11.1.3

Neurons (Nerve Cells) Figure 11.4b 2 1 3 4 5 6 7 8 9 10 11 12

Neurons (Nerve Cells) Figure 11.4b

Nerve Cell Body (Perikaryon or Soma) Contains the nucleus and a nucleolus Is the major biosynthetic center Is the focal point for the outgrowth of neuronal processes Has no centrioles (hence its amitotic nature) Has well-developed Nissl bodies (rough ER) Contains an axon hillock – cone-shaped area from which axons arise

Processes Armlike extensions from the soma Called tracts in the CNS and nerves in the PNS There are two types: axons and dendrites

Dendrites of Motor Neurons Short, tapering, and diffusely branched processes They are the receptive, or input, regions of the neuron Electrical signals are conveyed as graded potentials (not action potentials)

Axons: Structure Slender processes of uniform diameter arising from the hillock Long axons are called nerve fibers Usually there is only one unbranched axon per neuron Rare branches, if present, are called axon collaterals Axonal terminal – branched terminus of an axon

Generate and transmit action potentials Axons: Function Generate and transmit action potentials Secrete neurotransmitters from the axonal terminals Movement along axons occurs in two ways Anterograde — toward axonal terminal Retrograde — away from axonal terminal

The Myelin Sheath Whitish, fatty (protein-lipoid), segmented sheath around most long axons It functions to: Protect the axon Electrically insulate fibers from one another Increase the speed of nerve impulse transmission using “saltatory conduction” (jumping) Animation of Saltatory Conduction

Saltatory Conduction

Myelin Sheath and Neurilemma: Formation Formed by Schwann cells in the PNS Steps in myelin formation: A Schwann cell envelopes an axon in a trough It encloses the axon with its plasma membrane It wraps around and around the axon forming concentric layers of membrane that make up the myelin sheath Neurilemma – remaining nucleus and cytoplasm of a Schwann cell

Myelin Sheath and Neurilemma: Formation PLAY InterActive Physiology ®: Nervous System I, Anatomy Review, page 10 Figure 11.5a–c

Nodes of Ranvier (Neurofibral Nodes) Gaps in the myelin sheath between adjacent Schwann cells The action potential jumps from node to node across the myelin sheath – much faster than continuous conduction The nodes have voltage gates that conduct the a.p. like the normal membrane Nodes are also the sites where axon collaterals can emerge

Unmyelinated Axons A Schwann cell surrounds nerve fibers but coiling does not take place Schwann cells partially enclose 15 or more axons

Axons of the CNS Both myelinated and unmyelinated fibers are present Myelin sheaths are formed by oligodendrocytes Nodes of Ranvier are widely spaced There is no neurilemma