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The Nervous System Chapter 7

Functions Master controlling and communicating system of the body Monitor changes (stimuli) both inside and outside of the body ▫Gathered information is called sensory input Processes and interprets the sensory input and makes decisions about what should be done at every moment ▫This is called integration

Effects a response by activating muscles or glands (effectors) via motor output

Does not work alone to regulate and maintain body homeostasis ▫Endocrine system is a second important regulating system  Produces hormones  Typically brings about its effects in a more leisurely way

Organization of the Nervous System Structural Classification ▫Central nervous system (CNS)  Brain and spinal cord  Occupy the dorsal body cavity and act as the integrating and command centers of the nervous system  Interpret incoming sensory information  Issue instructions

▫Peripheral nervous system (PNS)  Parts of the nervous system that are outside of the CNS  Consists mainly of the nerves that extend from the brains and spinal cord  Spinal nerves carry impulses to and from the spinal cord  Cranial nerves carry impulses to and from the brain  Serve as communication lines

Functional Classification Concerned only with PNS structures 2 subdivisions ▫Sensory (afferent) division  Consists of nerve fibers that convey impulses to the CNS from sensory receptors located through the body  Somatic (afferent fibers) – impulses from the skin, skeletal muscles and joints  Visceral fiber (visceral afferents) – impulses from the visceral organs

▫Motor (efferent) division  Carries impulses from the CNS to effector organs, the muscles and glands  Effect a motor response  Two subdivisions  Somatic nervous system – voluntary nervous system  Autonomic nervous system (ANS) – involuntary nervous system ▫Sympathetic ▫Parasympathetic

Background Information Two Principal Types of Nervous Cells ▫Supporting cells ▫Neurons

Supporting Cells Neuroglia – supporting cells in the CNS that are “lumped together” ▫Many types of cells that support, insulate and protect the neurons Glia – different types of neuroglia that have a special function

Types of Glial Astrocytes ▫Star shaped ▫Account for nearly half of the neural tissue ▫Form a living barrier between capillaries and neurons and play a role in making exchanges between them ▫Help control the chemical environment in the brain Microglia ▫Spiderlike phagocytes ▫Dispose of debris

Ependymal ▫Line the cavities of the brain and the spinal cord ▫Helps circulate the cerebrospinal fluid Oligodendrocytes ▫Wrap their flat extensions tightly around the nerve fibers ▫Produce fatty insulating covering called the myelin sheaths Glia do not transmit nerve impulses Never lose their ability to divide Most brain tumors are gliomas

Supporting Cells in the PNS come in two major varieties ▫Schwann cells  Form the myelin sheaths around the nerve cells that are found in the PNS ▫Satellite cells  Act as protection, cushioning cells

Neurons Also called nerve cells Highly specialized to transmit messages Have a cell body containing the nucleus and is the metabolic center of the cell ▫No centrioles ▫Very abundant are the  Nissl substances – specialized RER  Neurofibrils – intermediate filaments that are important in maintaining cell shape

Extending from the cell body there are one or more slender processes (fibers) ▫Vary in length ▫Dendrites – convey incoming messages (electrical signals) towards the cell body  May have hundreds of branching dendrites ▫Axons – generate nerve impulses and typically conduct them away from the cell body  Only has one  Arise from the axon hillock  Occasionally branch to give off a collateral branch  Branch profusely at their terminal end to form the axon terminals

▫Terminals contain the neurotransmitters in tiny vesicles which are released when stimulated ▫Synaptic cleft separates the one neuron for the next  The functional gap is the synapse ▫Myelin – whitish, fatty material with a waxy appearance surrounds most nerve fibers  Protects and insulates the fibers along with increasing the transmission rate

 Outside the CNS, the myelination is done by Schwann cells  A myelin sheath results from the myelination  Most of the Schwann cell cytoplasm ends up just beneath the outermost part of its plasma membrane and is called the neurilemma ▫Remains intact (for the most part) when a peripheral nerve fiber is damages, it plays an important role in fiber regeneration  Nodes of Ranvier form where there are gaps between the adjacent Schwann cells In the CNS, the oligodendrocytes form the myelin sheaths. ▫Coil around as many as 60 different nerve fibers at a time ▫Lack neurilemma

Clusters of neuron cell body and collections of nerve fibers ▫In the CNS, the cell body clusters are called nuclei  Protected in the skull and vertebral column  Do not routinely undergo cell division  Carries out most of the metabolic functions ▫In the PNS, small collections of cell bodies are called ganglia  Found in few sites ▫In the CNS, bundles of nerve fibers are called tracts  White matter – dense collections of myelinated tracts  Gray matter – mostly unmyelinated fibers and cell bodies ▫In the PNS, bundles of nerve fibers are called nerves

Neuron Classification Functional ▫Groups neurons according to the direction the nerve impulse is traveling relative to the CNS ▫Sensory (afferent) neurons – carry impulses from sensory receptors to the CNS  Cell bodies are always found in a ganglion outside the CNS  Keep use informed about what is happening both inside and outside the body  Dendrite endings are usually associated with specialized receptors that are activated by specific changes occurring nearby.

Complex receptors may be discussed later; we will focus on the simpler type of sensory receptors found in the skin (cutaneous sense organs), muscles and tendons (proprioceptors).  The pain receptors (which are bare dendrite endings) are the least specialized cutaneous receptors as well as the most numerous.  Proprioceptors detect the amount of stretch (tension) skeletal muscles, their tendons and joints  These allow the body to make the proper adjustments to maintain balance and normal posture.

▫Motor (efferent) neurons carry impulses from the CNS to the viscera and/or muscles and glands  The cell bodies of motor neurons are always located in the CNS ▫Association neurons (interneurons) connect the motor and sensory neurons in neutral pathways  Their cell bodies are always located in the CNS