Functions of the Nervous System 1. Sensory input Monitor changes occurring inside and outside the body 2. Integration Processes and interprets sensory input and decide what to be done Mainly occurs in brain and spinal cord 3. Motor output A response to integrated stimuli by activating effector organs – muscles and glands

Functions of the Nervous System An example of how the nervous system uses all three functions together is when you are driving and see a red light ahead (sensory input), your nervous system integrates this information (red light means “stop”, and your foot goes for the brake pedal (motor output)

Organization of the Nervous System Classified based on structure and function Structure Brain Spinal cord Nerves Function Sensory Eyes, ears, nose Motor Muscles and glands

Structural Classification Central nervous system (CNS) Brain and spinal cord Integrating and command center Peripheral nervous system (PNS) Cranial nerves and spinal nerves Communication lines between the CNS and the rest of the body

Functional Classification Peripheral nervous system Two subdivisions Sensory (afferent) division Carry information to the CNS Motor (efferent) division Carry information away from the CNS

Functional Classification Motor (efferent) division Two subdivisions Somatic nervous system Voluntary and conscious control Skeletal muscles Autonomic nervous system Involuntary and subconscious Smooth muscles, cardiac muscles and glands

Functional Classification Autonomic Nervous System Two subdivisions Sympathetic Fight or flight Parasympathetic Rest and digest

Nervous Tissue Two principal types of cells Support cells (neuroglia) Support, insulate and protect the neurons Cannot transmit nerve impulses Continue to divide 4 in the CNS and 2 in the PNS Neurons Structural units of the nervous system Transmit electrical impulses from one area of the body to another area using neurotransmitters Cannot divide = amitotic

Support Cells (Neuroglia) in CNS Astrocytes Abundant, star-shaped cells Anchor and brace neurons Form barrier and make exchanges between capillaries and neurons Control the chemical environment of the brain by cleaning up potassium ions and neurotransmitters Microglia Small ovoid cells with thorny processes Spider-like phagocytes Dispose of debris

Support Cells (Neuroglia) in CNS Ependymal cells Line central cavities of the brain and spinal cord Cells are squamous and columnar shaped and ciliated Circulate cerebrospinal fluid Oligodendrocytes Produce myelin sheath (fatty insulation) around nerve fibers

Support Cells (Neuroglia) in PNS Satellite cells Surround neuron cell bodies located in the PNS Protection and cushioning neurons Similar to astrocytes in CNS Schwann cells Surround and form myelin sheath in the PNS Help with regeneration of damaged peripheral nerve fibers

Neurons Structural unit of the nervous system Highly specialized cells that conduct messages in the form of impulses from one part of the body to another Special characteristics Extreme longevity (about 100 years) Amitotic (cannot divide) High metabolic rate Vary in structure but all have a cell body and one or more slender processes Plasma membrane is the site of electrical signaling

Neurons Major regions of neurons Cell body (soma) Nucleus and metabolic center Most are located in the CNS Clusters of cell bodies in the CNS are called nuclei Lie along the nerves in the PNS are called ganglia Processes (dendrites and axons) Arm like processes extend from the body of all neurons Brain and spinal cord contain both cell bodies and their processes PNS consists mainly of neuron processes Bundles of neuron processes are called tracts (CNS) or nerves (PNS)

Neuron Processes Dendrites Axons Main receptive or input region Receive signals from other neurons Conduct impulses toward the cell body Axons Each neuron has only one axon Conducting region of the neuron Generates nerve impulses and transmits impulses away from the cell body Initial region of the axon arises from the axon hillock Narrows to form a slender process the rest of the length In some neurons the axon is very short and in others it is very long For example: axons of toes extend from the lumbar region of your spine to your foot (about 3-4 feet) – the longest cells in your body

Axons and axonal terminals Axons can branch many times but all end in axon terminals Axonal terminals Knoblike distal endings also called the secretory region When impulses reaches the terminal it causes neurotransmitters to be released Neurotransmitters either excite or inhibit neurons Neurons never touch other neurons Separated from the next neuron by a gap Synaptic cleft Gap between adjacent neurons

Axon Coverings Many axons are covered with a whitish, fatty, segmented myelin sheath Myelin Covering that protects and insulates the axons and increases the speed of transmission of nerve impulses Myelin sheath A tight coil of wrapped membranes encloses the axon

Axon Coverings Myelin sheaths Peripheral nervous system Formed by Schwann cells which wrap themselves around the axon in jelly-roll fashion Central nervous system Formed by oligodendrocytes Nodes of Ranvier Adjacent Schwann cells and oligodendrocytes do not touch each other so there are gaps in the myelin sheath Myelinated fibers conduct nerve impulses rapidly Unmyelinated fibers conduct impulses slowly

Axon Coverings Multiple Sclerosis (MS) Myelin sheath in the CNS are damaged and demyelination occurs Affects the ability of nerve cells to communicate with each other Caused by an autoimmune disease where the body’s own immune system attacks and damages the myelin Electrical signals are slowed or stopped from reaching different parts of the body. Visual disturbances, problems controlling muscles, speech disturbances, and urinary incontinence Person loses ability to control his/her muscles Cause is unknown and there is no cure

Neuron Cell Body Locations Most are found in the central nervous system White matter Dense collections of myelinated fibers Gray matter Unmyelinated fibers and cell bodies Nuclei Clusters of cell bodies within the white matter of the central nervous system Outside the CNS Ganglia Collections of cell bodies

Neuron Anatomy

Neuron Classification Neurons classified based on their structure and function

Structural Classification of Neurons Multipolar neurons Three or more processes One axon and the rest dendrites Most common neuron type (motor and interneurons) Major neuron type in CNS Bipolar neurons Two processes An axon and a dendrite from opposite sides of the cell body Rare, found in special sensory organs Unipolar neurons Have a short single process leaving the cell body Mainly found in ganglia of PNS (sensory neurons)

Functional Classification Neurons classified based on the direction the nerve impulse travels in relation to the CNS Sensory neurons Interneurons Motor neurons

Functional Classification of Neurons Sensory (afferent) neurons Transmit impulses towards the CNS Sensory receptors in the skin or internal organs Cutaneous sense organs (Meissner’s and Pacinian Corpuscles) Proprioceptors (detect stretch or tension) Motor (efferent) neurons Transmit impulses away from the CNS Muscles or glands Association (interneuron) Found between motor and sensory neurons In neural pathways in the CNS Make up 99% of the neurons of the body

Functional Classification of Neurons Amyotrophic lateral sclerosis (ALS) “Lou Gehrig’s Disease” Disease of the motor neurons in the CNS that control voluntary movements Motor neurons degenerate or die and can no longer send messages to muscles Condition gets worse and usually ends in paralysis and death in about 3-5 years Cause is unknown (about 10% are genetic) Amyotrophic Muscle without nourishment Sclerosis Hardening of tissue