The Nervous System

Mr. Hunter01/07/2013

Kennedy High School

Mr. HunterAnatomy 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.

Organs and Divisions

• There are two principal divisions of the nervous system.

• 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.

Organs and Divisions

• The two types of cells found in the 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 System

• The main part of the neuron is the 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 System

• Sensory neurons (afferent) transmit 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 System

• The axon is covered by myelin. • 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.

Types of glial cells

• Glia vary in size and shape.• 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.

Types of glial cells

• Oligodendrocytes hold nerve fibers 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.

Nerves and Tracts

• A nerve can be defined as a group of peripheral nerve fibers (axons) bundled 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.

Nerves and Tracts

• Each axon in a nerve is surrounded 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.

Reflex Arcs

• Only neurons can provide the rapid 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.

Reflex Arcs

• A simple kind of reflex arc consists of 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.)

Reflex Arcs

• The knee-jerk reflex is the simplest example of a two neuron reflex arc.

• 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

Reflex Arcs

• and cell bodies located in the CNS gray matter and axons outside the gray matter.

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 located within the gray matter of the brain or spinal cord.•Three neuron reflex arcs have two synapses.•A Two-neuron refex arc only has a sensory neuron, a motor neuron and one synapse between them.

Nerve Impulses and the Synapse

• Nerve impulse: self-propagating 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 Synapse

• Due to the distribution of Na + (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.

Nerve Impulses and the Synapse

• This process is called depolarization.• This section of the membrane immediately

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 the Synapse

• Transmission of nerve signals from one neuron to the next occurs across the synapse.• Impulses travel from the presynaptic neuron to the postsynaptic neuron.•Impulses travel across the synaptic cleft.•Three structures that make up a synapse:•Synaptic knob•Synaptic cleft•Plasma membrane of postsynaptic neuron

Nerve Impulses and the Synapse

• synaptic knob – terminal branch at the end of the post synaptic neuron. •Small vesicles contain neurotransmitters.•When a nerve impulse arrives at the synaptic knob, neurotransmitters are released into the synaptic cleft.•The plasma membrane of the postsynaptic neuron contains receptors for the neurotransmitter.

Nerve Impulses and the Synapse

• The binding of the neurotransmitter with the receptors initiates an impulse in the postsynaptic neuron opening Na+ channels in the cell. Therefore, the nerve signal is transmitted to the postsynaptic neuron.•The neurotransmitter can be :•Taken back up by the synaptic knobs•Metabolized into inactive chemicals

Neurotransmitters

•At least 30 different substances have been identified as neurotransmitters.•Specific neurotransmitters are localized in discrete groups of neurons.•Acetylcholine is released at some synapses in the spinal cords and at neuromuscular junctions.•Norepinephrine, Dopamine and Serotonin are grouped into compounds called catecholamines – involved in sleep motor function, mood and pleasure recognition.

Neurotransmitters

• Endorphins and Enkephalins are released at spinal cord and various brain synapses in the pain conduction pathway.•They inhibit the conduction of pain impulses.•Nitrous Oxide – neurotransmitter that diffuses directly across the plasma membrane instead of being transported by vesicles.