The Nervous System Mr. Hunter 01/07/2013Kennedy High School
Mr. Hunter Anatomy and Physiology 01/15/13• Objective(s)• SWBAT• List the major divisions of the Nervous System• Describe the action of a nerve impulse / Action Potential• Describe the structure and function of synapses.• Review Concepts via Practice Problems• Bell Ringer: List three possible stimuli that could generate a nerve impulse.
• There are two principal divisions of the nervous system.Organs and Divisions • The Central Nervous System (CNS) • The Peripheral Nervous System (PNS) • The CNS consists of the brain and the spinal cord. • The PNS consists of nerves which extend from the spinal cord to the peripheral parts of the body. • A subdivision of the PNS is the Autonomic Nervous System (ANS). • ANS consists of structures that regulate the body’s involuntary or automatic functions ex. heart rate, smooth muscle contractions, secretion of hormones, enzymes, etc.
• The two types of cells found in theOrgans and Divisions Nervous System (NS) are neurons and glia cells. • Neurons conduct nerve impulses. • Glia cells support neurons. • Neuron consists of three main parts: • Cell body • Dendrites • Axon
Cells of the Nervous • The main part of the neuron is theSystem cell body. • Branching projections are called dendrites. Dendrites transmit information to the neuronal cell body. • Axons are the long processes that transmit information away from the cell body. • There are three types of neurons named based on the direction in which they transmit information. • Sensory (afferent) • Motor (efferent) • Interneuron
Cells of the Nervous • Sensory neurons (afferent) transmitSystem information to the CNS from all regions of the body. • Motor neurons (efferent) transmit information in the opposite direction- away from the CNS. • They conduct impulses to muscle and glandular epithelial tissue. • Interneurons conduct impulses from sensory neurons to motor neurons. • They connect to form central networks of nerve fibers and are called central or connecting neurons.
Cells of the Nervous • The axon is covered by myelin.System • This is a white, fatty substance formed by specialized cells called schwann cells that wrap around axons outside the CNS. The outer membrane of this type of cell is called the neurilemma-plays regenerative part in damaged axons. • Myelin allows for saltatory conduction of electrical transmissions. • Nodes of Ranvier are interrupted segments between schwann cells. • Glia (glue) cells – one function is to hold functioning neurons together and protect them. They also coordinate the functions of the NS. • Glioma- common type of brain tumor.
• Glia vary in size and shape.Types of glial cells • Astrocytes: resemble stars. Their threadlike branches attach to neurons and to small blood vessels holding both structures together. • Astrocyte branches form a two-layer structure called the blood brain barrier (BBB). • BBB separates blood tissue and nervous tissue. Protects brain from harmful chemicals that might be found in the blood. • Microglia: smaller than astrocytes. They remain stationary until brain tissue becomes injured. They act as microbe scavengers in the brain via phagocytosis.
• Oligodendrocytes hold nerve fibersTypes of glial cells together and they also produce myelin sheaths around multiple nerve cell axons in the CNS. • Schwann cells are glia cells that produce myelin sheaths on single axons within the PNS. • Assignment 01/07/13 • Pg. 195 # 1-4 Quick Check.
• A nerve can be defined as a group of peripheral nerve fibers (axons) bundledNerves and Tracts together. • The fibers have a myelin sheath provided by schwann cells. • Because myelin is white, peripheral nerve fibers often look white. • Bundles of axons in the CNS are called tracts. They are also myelinated and form the white matter of the brain and spinal cord. • Brain and spinal cord tissue composed of unmyelinated axons dendrites and cell bodies is called gray matter due to its appearance.
• Each axon in a nerve is surroundedNerves and Tracts by a thin wrapping of fibrous connective tissue called endoneurium. • Groups of these wrapped axons are called fascicles. • Each fascicle is surrounded by a thin, fibrous perineurium. • A tough, fibrous sheath called the epineurium covers the entire nerve.
• Only neurons can provide the rapidReflex Arcs communication between cells that is necessary for sustaining life. • Hormonal messages travel much more slowly than neuronal transmissions within the body. They have to circulate via the blood stream. • Nerve impulses are also called action potentials. The routes traveled by nerve impulses are called neuron pathways. • A basic type of neuron pathway is called a reflex arc.
• A simple kind of reflex arc consists ofReflex Arcs two types of neurons: sensory neurons and motor neurons. • Three neuron arcs consists of: sensory neurons, interneurons and motor neurons. • Reflex arcs allow impulse conduction in only one direction. • Impulse conduction starts mainly at receptors. • Receptors are the beginnings of dendrites of sensory neurons. They are located some distance away from the spinal cord. (tendons,skin mucous membranes etc.)
• The knee-jerk reflex is the simplest example of a two neuron reflex arc.Reflex Arcs • As a result of a tap on the patellar ligament causes the quadriceps muscle to contract via a neuronal pathway. 1. A nerve impulse is generated by stretch receptors within the quadriceps. 2. It travels along the sensory neuron’s dendrites to the cell body located in the dorsal root ganglion (group of nerve cell bodies located in the PNS) 3. The axon from the sensory neuron travels from the cell body of the dorsal root ganglion and ends near the dendrites of another neuron located in the gray matter of the spinal cord. 4. The nerve impulse stops at the synapse (space that separates axon and dendrites) 5. Chemical signals are sent across the synapse and connect with other dendrites
• and cell bodies located in the CNS gray matter and axons outside the gray matter.Reflex Arcs 6. The motor neuron synapses with an effector which are muscles or glands – they put the nerve signals” into effect.” • The response to the impulse conduction over a reflex arc is called a reflex. • Complex reflexes may involve the actions of three neurons. • The end of the sensory neuron will synapse with the dendrites of an interneuron • located within the gray matter of the spinal cord. The axon of that interneuron will then synapse with the dendrites and cell body of a motor neuron within the spinal cord. The axons of the motor neuron lie outside the spinal cord and will exit via the anterior root of the spinal nerve and terminate in the muscle(effector)
Withdrawl Reflex• Interneurons are locatedwithin the gray matter ofthe brain or spinal cord.•Three neuron reflex arcshave two synapses.•A Two-neuron refex arconly has a sensoryneuron, a motor neuronand one synapse betweenthem.
Nerve Impulses and the • Nerve impulse: self-propagatingSynapse wave of electrical disturbance that travels along the surface of a neuron’s plasma membrane. • It is similar to a spark traveling along a fuse. • Nerve impulses have to be initiated by a stimulus – a change in the neuron’s environment. A stimulus could possibly be on of the following: Environmental change(s) • Pressure • Temperature • Chemical changes
Nerve Impulses and the • Due to the distribution of Na +Synapse (sodium) and K+ (potassium) ions, the resting neuron has a slight positive charge on the outside of the membrane and a slight negative charge on the inside. • This separation of electrical charges is called polarization • There is normally an excess of sodium on the outside of the cell than inside. • When a portion of the membrane is stimulated, Na+ ions will enter the cell causing that region of the cell to become temporarily positive – outside becomes negative.
• This process is called depolarization.Nerve Impulses and the • This section of the membrane immediatelySynapse recovers during a process of repolarization. • However, depolarization has already stimulated Na+ channels to open in the next portion of the membrane. The Action Potential continues along the axon in one direction jumping from node to node until it reaches the synaptic terminal of the axon.
Nerve Impulses and theSynapse• Transmission of nervesignals from one neuron tothe next occurs across thesynapse.• Impulses travel from thepresynaptic neuron to thepostsynaptic neuron.•Impulses travel across thesynaptic cleft.•Three structures thatmake up a synapse:•Synaptic knob•Synaptic cleft•Plasma membrane ofpostsynaptic neuron
Nerve Impulses and theSynapse• synaptic knob – terminalbranch at the end of thepost synaptic neuron.•Small vesicles containneurotransmitters.•When a nerve impulsearrives at the synapticknob, neurotransmittersare released into thesynaptic cleft.•The plasma membrane ofthe postsynaptic neuroncontains receptors for theneurotransmitter.
Nerve Impulses and theSynapse• The binding of theneurotransmitter with thereceptors initiates animpulse in the postsynapticneuron opening Na+channels in the cell.Therefore, the nerve signalis transmitted to thepostsynaptic neuron.•The neurotransmitter canbe :•Taken back up by thesynaptic knobs•Metabolized into inactivechemicals
Neurotransmitters•At least 30 different substanceshave been identified asneurotransmitters.•Specific neurotransmitters arelocalized in discrete groups ofneurons.•Acetylcholine is released at somesynapses in the spinal cords andat neuromuscular junctions.•Norepinephrine, Dopamine andSerotonin are grouped intocompounds called catecholamines– involved in sleep motorfunction, mood and pleasurerecognition.
Neurotransmitters• Endorphins and Enkephalins arereleased at spinal cord andvarious brain synapses in the painconduction pathway.•They inhibit the conduction ofpain impulses.•Nitrous Oxide – neurotransmitterthat diffuses directly across theplasma membrane instead ofbeing transported by vesicles.