The Nervous System Chapter 9

Nervous System The master controlling and communicating system of the body Functions: – Sensory input – monitoring stimuli – Integration – interpretation of sensory input – Motor output – response to stimuli

Organization of the Nervous System Central nervous system (CNS) – Brain & spinal cord – Integration and command center Peripheral nervous system (PNS) – Paired spinal & cranial nerves – Carries messages to and from the spinal cord and brain

Peripheral nervous system (PNS) Two functional divisions: – Sensory (afferent) division Sensory afferent fibers – carry impulses from skin, skeletal muscles, and joints to the brain Visceral afferent fibers – transmit impulses from visceral organs to the brain; Ex) feeling full – Motor (efferent) division Transmits impulses from the CNS to effector organs

Motor Division: Two Main Parts Somatic nervous system – Conscious control of skeletal muscles Autonomic nervous system (ANS) – Regulates smooth muscle, cardiac muscle, and glands (unconscious) – Divisions: Sympathetic – fight or flight system; increase in heart rate, blood pressure, blood sugar Parasympathetic – housekeeping systems (resting/digestive); keeps functions going under normal conditions

Histology of the Nerve Tissue The two principle cell types of the nervous system are: – Neurons  excitable cells that transmit electrical signals – Supporting cells  cells that surround and wrap neurons (neuroglial or glial cells) Provide a supportive scaffolding for neurons Segregate and insulate neurons Guide young neurons to the proper connections Promote health and growth

Neurons (Nerve Cells) Lose their ability to divide Very long lived High metabolic rate – need astrocytes to get nutrients from capillaries NEED OXYGEN No centrioles Dendrites and Cell bodies receive graded potentials

Neuron Structure Neurons vary considerably in size & shape, but they have certain features in common: – Cell body – Nerve fibers – Axon – Dendrites

Nerve Cell Body Contains the nucleus and nucleolus The major biosynthetic center Is the focal point for the outgrowth of neuronal processes Has well-developed Nissl bodies (rough ER) Contains an axon hillock – cone-shaped area from which axons arise

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 axonal hillock of the cell body Long axons are called nerve fibers Usually there is only one unbranched axon per neuron Axonal terminal – branched terminus of an axon

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

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

Myelin Sheath & Neurilemma: Formation Formed by Schwann cells in the PNS A Schwann cell: – Envelopes an axon in a trough – Encloses the axon with its plasma membrane – Has concentric layers of membrane that make up the myelin sheath Neurilemma – remaining nucleus and cytoplasm of a Schwann cell

Nodes of Ranvier Gaps in the myelin sheath between adjacent Schwann cells They are the sites where axon collaterals can emerge

Types of Neurons and Neuroglial Cells Neuron Classification (structural): – Multipolar – three or more processes – Bipolar – two processes (axon & dendrite) – Unipolar – single, short process

Types of Neurons & Neuroglial Cells Neuron Classification (function): – Sensory (afferent) – transmit impulses toward the CNS (stimulus) – Motor (efferent) – carry impulses away from the CNS (reaction) – Interneurons (association neurons) – shuttle signals through CNS pathways

Types of Neurons & Neuroglial Cells Classification of Neuroglial Cells – Microglial cells – Oligodendrocytes – Astrocytes: – Ependymal cells

Microglial Cells Small, oval cells w/ spiny processes Scattered throughout the CNS Support neurons & phagocytize bacterial cells & cellular debris

Oligodendrocytes Occur in rows along nerve fibers Form myelin within the brain & spinal cord Insulation

Astrocytes Most abundant, versatile, & highly branched glial cells Cling to neurons at the synaptic endings & cover capillaries Functions: – Support & brace neurons – Anchor neurons to their nutrient supplies – Guide migration of younger neurons

Ependymal Cells Range in shape from squamous to columnar Line the central cavities of the brain and spinal column Make a barrier for spinal fluid

Spinal Cord (structure) Consists of 31 segments – each gives rise to a pair of spinal nerves Cervical enlargement – in the neck region, supplies nerves to the upper limbs Lumbar enlargement – in the lower back, supplies nerves to the lower limbs

Spinal Cord (function) Has 2 main functions: – Conducting nerve impulses – Serving as a center for spinal reflexes Nerve tracts provide a two-way communication system between the brain and body parts outside the nervous system

Spinal Cord (function) Ascending tracts – carry sensory information to the brain Descending tracts – conduct motor impulses from the brain to muscles and glands Nerve fibers within ascending and descending tracts are axons