Introduction to The nervous system Anatomy Ch. 7 Part 1 Introduction to The nervous system

Introduction The nervous system is the master controlling and communicating system of the body. Communication with body cells is done by using electrical impulses. In order to maintain homeostasis the nervous system works with the endocrine system The nervous system conducts impulses The endocrine system uses hormones

3 Functions of the Nervous System Monitor changes produced by stimuli, also known as sensory input Process and interpret, also known as integration Cause a response by activating muscles or glands (effectors) via motor output

Structural Classification of the Nervous System Central Nervous System (CNS) Brain and spinal cord Integrating and command center Interprets and issues instructions Peripheral Nervous System (PNS) Spinal and cranial nerves Link receptors to the CNS and the CNS to glands or muscles

Functional Classification (PNS) Sensory (afferent) Carry impulses to the CNS from sensory receptors Divided into the Somatic and Visceral Somatic: skin, skeletal muscles, and joints Visceral: visceral organs Motor (efferent) Carry impulses from the CNS to muscles or glands Divided into Somatic and Autonomic Somatic: Voluntary control of skeletal muscles Autonomic: involuntary control of smooth and cardiac muscles and glands

Sympathetic and Parasympathetic Divisions of the Autonomic Nervous System Neurotransmitter is acetylcholine Rest and Digest division Decreases the activity of the cardiovascular and respiratory systems and increases the activity of the digestive system. Sympathetic Neurotransmitters are acetylcholine, epinephrine, and norepinephrine Fight or flight division Increases the activity of the cardiovascular and respiratory systems and decreases the activity of the digestive system.

Common Neurotransmitters Acetylcholine skeletal muscle activity Adrenaline (epinephrine) fight or flight Noradrenaline (norepinephrine) stress hormone Serotonin mood Dopamine pleasure Endorphins Euphoria (excitement or happiness) Glutamate memory GABA calming

Nervous Tissue Supporting Cells (neuroglial cells) support insulate, and protect neurons Astrocytes: link capillaries and neurons Microglia: phagocytes that dispose of debris Ependymal: circulate CSF Oligodendrocytes: insulate nerve fibers Schwann cells: form myelin sheaths Satellite cells: protect and cushion

Anatomy of a Neuron Cell body: nucleus, Nissl bodies (ER), and neurofibrils which maintain shape Dendrites: convey incoming messages to the cell body Axons: conduct messages away from the cell body

Regions of axons Hillock: area of the cell body where the axon originates Terminals: branches of axons that contain neurotransmitters Synaptic cleft: gap between an axon and the next neuron, the junction is called a synapse

Myelin sheaths: covering of axons that protect and insulate and increase transmission rates of impulses Schwann cells: produce myelin sheaths Nodes of Ranvier: gaps between Schwann cells Neurilemma: surrounds the myelin sheaths Only found in the PNS which allows for regeneration

Differences between the CNS and PNS In the CNS clusters of cells bodies are called nuclei In the PNS clusters of cell bodies are called ganglia In the CNS bundles of nerve fibers are called tracts In the PNS bundles of nerve fibers are called nerves

The terms white matter and gray matter refer to regions of the CNS. White matter has myelinated fibers Gray matter has unmyelinated fibers

Functional Classification Direction of Impulses Sensory (afferent): carry impulses from receptors to the CNS; the cell bodies are outside of the CNS Free nerve endings – pain and temperature Meissners corpuscles – touch Lamellar corpuscles – deep pressure Proprioceptors – muscles and tendons

Motor (efferent): carry impulses from the CNS to organs, muscles or glands; the cell bodies are found in the CNS Interneurons (association): connect motor and sensory neurons; cell bodies are found in the CNS

Neurons have 2 major functional properties: Irritability: respond to stimulus Conductivity: transmit impulses

Structural Classification Number of Processes Multipolar several extensions from the cell body all motor and association neurons most common Bipolar 2 extensions from the cell body only found in the eye and nose

Unipolar A single short process that divides into proximal and distal processes Sensory neurons of the PNS Axons conduct impulses toward and away from the cell body.

Conduction of a Nerve Impulse A resting neuron is polarized meaning there are more positive ions on the outside than on the inside The major positive ion on the inside of the cell membrane is Potassium The major positive ion on the outside of the cell membrane is Sodium

Steps of Impulse Conduction 1. As long the neuron is in the resting state it will be inactive. This is called the polarized state. Most neurons are excited by neurotransmitters. 2. When the neuron is stimulated, sodium quickly diffuses into the neuron. This changes the polarity of the membrane and is called depolarization.

3. If the stimulus is strong enough, the depolarization will travel over the entire length of the neuron membrane. This is called action potential or nerve impulse. 4. As the impulse moves, the permeability changes. Potassium is now able to leave the neuron rapidly by diffusion. This causes the cell to become repolarized. There cannot be another impulse until the neuron is repolarized.

5. After repolarization the sodium and potassium are returned to their original locations by the Na/K pump. This is an active process requiring ATP.

Action Potential

The previous process occurs in unmyelinated fibers The previous process occurs in unmyelinated fibers. Fibers that have myelin conduct impulses much faster. The impulse actually jumps from one node to another skipping myelinated regions. This is called saltatory conduction.

Impulses do not travel from one neuron to the next. Instead neurotransmitters cross the synapse between neurons and carry the signal.

Steps of transmission from one neuron to the next (electrochemical) 1. When the action potential reaches the axon terminal, calcium enters the terminal 2. The calcium causes vesicles in the terminal to release the neurotransmitter 3. The neurotransmitter moves across the synapse and binds to receptors on the next neuron 4. If there are enough neurotransmitters to stimulate the next neuron then an impulse can be conducted. 5. Once the next neuron has been stimulated the neurotransmitter is removed.

Neuron Transmission

Reflexes Most of what the body does everyday is programmed as reflexes. Reflexes are rapid, predictable, and involuntary responses to a stimuli The pathway of a reflex is called a reflex arc Reflexes can be classed as somatic or autonomic. Somatic: skeletal muscles Autonomic: smooth and cardiac muscles and glands

All reflexes have 5 elements: Sensory receptor: reacts to stimuli Sensory neurons: send message to the CNS Integration center: the CNS Motor neurons: send message to the effector organ Effector organ: the muscle or gland stimulated

Reflex arcs can have 2 or more neurons. Additional neurons are called interneurons. The more neurons, the longer the reflex takes to happen. Spinal reflexes do not require the brain’s involvement. Reflexes that involve the brain are those that require many types of information.

Reflex Arc