The Nervous System

Multicellular Organisms Must Coordinate

• The nervous system contains cells called neurons that can transmit signals from one part of the body to another quickly

• The nervous system provides animals with nearly instantaneous communication to coordinate body functions

Nerves of the zebra fish tail

An Overview of the Nervous System• The nervous system of many

invertebrates, and all vertebrates, can be divided into the peripheral nervous system (PNS) and the central nervous system (CNS)

• The PNS gathers information from the external and internal environment and sends it on to the CNS

• The CNS processes it and often generates a return signal to be delivered by the PNS to the body parts that will execute the signal

The Neuron• A neuron is a specialized cell

that can receive and transmit information from many different types of cells

• Neurons contain the same organelles found in any other animal cell with the addition of:– Dendrites: extensions which

receive signals from adjacent cells

– Axons: transmit signals to other cells

• Nervous system tissues also contain a variety of support cells, known as glial cells

• Myelin is an insulating sheath made of a fatty material which surrounds the axons

• Myelinated axons can carry signals more rapidly than unmyelinated axons

• White matter in the CNS is due to myelin sheaths in this area.

The Neuron

Glial Cells• Nervous system tissues also

contain a variety of support cells, known as glial cells:– Oligodendrocytes form

myelin in the brain and spinal cord.

– Astrocytes are near blood vessels and support structures, aid in metabolism, and respond to brain injury by filling in spaces.

– Schwann cells are the myelin-producing neuroglia of the peripheral nervous system.

The Nerve• A nerve is made up of many individual neurons

bundled together with supporting cells, blood vessels, and connective tissue to form a major communication pathway

Ganglia• Ganglia provide relay points and intermediary connections

between different neurological structures in the body, such as the peripheral and central nervous systems.

• Cluster of nerve cell bodies that serve to integrate signals, especially between the PNS and the CNS

The Central Nervous System of Vertebrates: Brain and a Spinal Cord

• Vertebrates have a thick central nerve cord called the spinal cord, which contains large concentrations of dendrites and axon terminals that enable rapid information exchange

• The CNS is protected by the cranium and the vertebrae while the

• PNS of vertebrates consists of branching nerves that carry information into and out of the spinal cord

Reflex Arc• The reflex arc consists of a

sensory neuron that sends the message to the spinal cord, an interneuron, and a motor neuron that creates a response in the body

• A reflexive motor response to pain does not require the brain’s involvement, which requires more time to process information

The Peripheral and Central NervousSystems Exchange Information

• Sensory neurons from the PNS convey sensory input to interneurons found only in the CNS

The Peripheral and Central NervousSystems Exchange Information

• An interneuron may also send its output up to the brain and out to the PNS at the same time, in a simultaneous flow

The Peripheral and Central NervousSystems Exchange Information

• Interneurons process the sensory input and may send them directly to the motor neurons for immediate action or to the brain for further processing

PNS: Voluntary vs. Involuntary• PNS output that is under voluntary control is

called somatic control• Autonomic control of PNS output is involuntary• The sympathetic and parasympathetic divisions

of the nervous system work together to control body functions

PNS

Autonomic Somatic

PNS: Sympathetic vs. Parasympathetic • The sympathetic and parasympathetic divisions

of the nervous system work together to control body functions

PNS

Autonomic Somatic

Sympathetic Parasympathetic

PNS: Sympathetic vs. Parasympathetic

Fight or

Flight

Opposes Fight

or Flight

Signal Transmission by Neurons• An electrical disturbance in a neuron travels down the

length of an axon as a pulse of electrical activity known as an action potential

• An action potential triggers the release of chemical messengers, called neurotransmitters, that signal to the next cell in this line of communication

Action Potential• An action potential is a self-sustaining electrical signal that

travels away from the body of the neuron • The action potential is dependent on the positively charged

ions moving across the plasma membrane• The plasma membrane is in a polarized state because there is

a difference in electrical charges across the plasma membrane

Polarized nerve

Na+ Na+ Na+

Na+ Na+ Na+ Na+

Na+ Na+

Na+

Na+

Na+ Na+

Na+

Na+

• The electrical charge that exists across the plasma membrane of an unstimulated neuron is known as the resting potential. Usually -70mV

Action Potentials• A stimulus depolarizes a neuron if the ion flow is changed in

such a way that many more positively charged ions are able to enter the cell

• Once the action potential has passed, the neuron returns to its resting potential

Action Potentials• Unmyelinated gaps between the myelin sheath are called nodes

of Ranvier and are the site of action potentials• Action potentials are sped up by the presence of the myelin

sheath• The action potential is regenerated at each node, allowing the

signal strength to jump rapidly from one node of Ranvier to the next

Action Potentials Have SeveralImportant Features

• Action potentials move along the axon in only one direction, keeping the signal from being lost

• An action potential remains consistently strong as it moves from one end of the axon to the other and does not weaken with distance

• A strong stimulus will initiate action potentials more often, but any individual action potential will be no stronger than any other: it is an all-or-none event

Neurotransmission at the Synapse

• The electrical signal is converted into a chemical message and relayed to the next cell at a junction called a synapse

Neurotransmission at the Synapse

• Electrical signals are transformed into chemical signals in the form of molecules called neurotransmitters, which are transmitted across a synaptic cleft

Neurotransmission at the Synapse

• Neurotransmitters excite or inhibit non-neuronal target cells, such as muscle cells, by binding to specific receptor proteins on the plasma membrane

Neurotransmitters TransmitSignals between Adjacent Cells

• The types of receptors on the target cell plasma membrane determine which neurotransmitters will activate a response

• Once released, neurotransmitters are cleared from the synaptic cleft quickly through uptake by either the neuron that released them or special glial cells; they can also be destroyed or inactivated by specific enzymes in the synaptic cleft

• Communication through the nervous system comes from the capacity of each neuron to generate large numbers of action potentials in a fraction of a second and to aim those signals narrowly at specific target cells

Selective Serotonin Reuptake Inhibitors (SSRIs)

• A class of drugs used as antidepressants in the treatment of depression, anxiety disorders, and some personality disorders.

• Believed to increase the extracellular level of the neurotransmitter serotonin by inhibiting its reuptake into the presynaptic cell

• This increases the level of serotonin in the synaptic cleft available to bind to the postsynaptic receptor.

Drugs and Neurotransmission

Sensory Structures: Making Sense of the Environment

Humans have five basic types of sensory receptors• Chemoreceptors: Nose and tongue• Photoreceptors: Eyes• Mechanoreceptors detect physical stimuli inside

and outside the body• Thermoreceptors: Skin, mouth, some internal

organs• Pain receptors are located on just about every

tissue type inside and on the surface of the body and detect different types of noxious stimuli