bio 1002 Neurons and Synapse

Neurons

are nerve cells that transfer information within the body

cell types of the nervous system that exemplify the close relationship between form and function that arises during evolution

Neurons two types of signals to communicate

– electrical signals (long-distance)

– chemical signals (short-distance)

Interpreting signals in the nervous system involves __

sorting a complex set of paths and connections

Processing of information takes place either in:

– simple clusters of neurons called ganglia

– a more complex organization of neurons called a brain

Neuron organization and structure reflect __

function in information transfer

cell body

• Most of a neuron’s organelles are in the cell body

dendrites

Most neurons have dendrites, highly branched extensions that receive signals from other neurons

axon

The axon is typically a much longer extensions that transmits signals to other cells at synapses

axon hillock

The cone-shaped base of an axon is called the axon hillock

synaptic terminal

The synaptic terminal of one axon passes information across the synapse as chemical messengers called neurotransmitters

synapse

A synapse is a junction between an axon and another cell

• At electrical synapses, the electrical current flows from one neuron to another through gap junctions

• At chemical synapses, a chemical neurotransmitter carries information between neurons

• Most synapses are chemical synapses

Information is transmitted from a __to a __

presynaptic cell (neuron) , postsynaptic cell (neuron, muscle, or gland cell)

• Nervous systems process information in three stages:

sensory input, integration, and motor output

sensory neurons

Sensors detect external stimuli and internal conditions and transmit information along

interneurons

Sensory information is sent to the brain or ganglia, where interneurons integrate the information

motor neurons

Motor output leaves the brain or ganglia via motor neurons, which trigger muscle or gland activity

Many animals have a complex nervous system that consists of:

– A central nervous system (CNS) where integration takes place; this includes the brain and a nerve cord

– A peripheral nervous system (PNS), which carries information into and out of the CNS

– The neurons of the PNS, when bundled together, form nerves

Most neurons are nourished or insulated by cells called __

glia

Membrane potential changes occur because neurons contain __ that open or close in response to stimuli

gated ion channels

Hyperpolarization and Depolarization

• The resting membrane potential of a neuron depends on the net movement of ions in and out of the cell.

• When positive ions move into the cell (or negative ions move out), the membrane becomes less polarized, or depolarized.

• For example, depolarization occurs if Na+ channels open and Na+ diffuses into the cell

• When positive ions move out of the cell (or negative ions move into the cell), the membrane becomes more polarized, or hyperpolarized.

• For example, hyperpolarization occurs if K+ channels in the neurons open.

Graded potentials

changes in polarization where the magnitude of the change varies with the strength of the stimulus

• These are not nerve signals that travel along axons, but they do have an effect on generation of nerve signals 48 - 20

action potential

• If a depolarization shifts the membrane potential sufficiently, it results in a massive change in membrane voltage called an action potential

• Action potentials have a constant magnitude, are all or-none, and transmit signals over long distances

• They arise because some ion channels are voltage gated, opening or closing when the membrane potential passes a certain level

resting potential

1. Most voltage-gated sodium (Na+) and potassium (K+) channels are closed

When an action potential is generated:

2. Voltage-gated Na+ channels open first and Na+ flows into the cell

3. During the rising phase, the threshold is crossed, and membrane potential increases

4. During the falling phase, voltage-gated Na+ channels become inactivated; voltage-gated K+ channels open, and K+ flows out of the cell

action potential: undershoot

During the undershoot, membrane permeability to K+ is at first higher than at rest, then voltage-gated K+ channels close and resting potential is restored

action potential":refractory period

• During the refractory period after action potential, a second action potential cannot be initiated

• The refractory period is a result of a temporary inactivation of the Na+ channels

Conduction of Action Potentials

• Where an action potential is generated, usually the axon hillock, an electrical current depolarizes the neighboring region of the axon membrane

• Action potentials travel in only one direction: toward the synaptic terminals

• Inactivated Na+ channels behind the zone of depolarization prevent the action potential from traveling backwards

Evolutionary Adaptation of Axon Structure

• The speed of an action potential increases with the axon’s diameter

• In vertebrates, axons are insulated by a myelin sheath, which causes an action potential’s speed to increase

• Myelin sheaths are made by glia— oligodendrocytes in the CNS and Schwann cells in the PNS

• Action potentials are formed only at nodes of Ranvier, gaps in the myelin sheath where voltage gated Na+ channels are found

• Action potentials in myelinated axons jump between nodes of Ranvier in a process called saltatory conduction

Neurons communicate with other cells at __

synapses

when did specialized systems of neurons had appeared that enable animals to sense their environment and respond rapidly

By the time of the Cambrian explosion more than 500 million years ago

The simplest animals with nervous systems, cnidarians, have neurons arranged in __

nerve nets

nerve net

A nerve net is a series of interconnected nerve cells

• Sea stars have a nerve net in each arm that are connected by radial nerves to a central nerve ring

complex animals have __

nerves

Nerves

are bundles that consist of axons of multiple nerve cells

cephalization

• Bilaterally symmetrical animals exhibit cephalization, the clustering of sensory organs at the front end of the body

Relatively simple cephalized animals, such as flatworms, have a __

central nervous system (CNS)

The CNS consists of

a brain and longitudinal nerve cords

The peripheral nervous system (PNS) consists of

neurons carrying information in and out of the CNS

ganglia

Annelids and arthropods have segmentally arranged clusters of neurons called ganglia

Nervous system organization usually correlates with __

lifestyle

Sessile molluscs (for example, clams and chitons) have __systems, whereas more complex molluscs (for example, octopuses and squids) have more __systems

simple , sophisticated

In vertebrates– The CNS is composed of

brain and spinal cord

In vertebrates The PNS is composed of

nerves and ganglia

Why have many animals evolved with most of their sensory equipment at the front end of their bodies?

u get the most information the quickest at the front

• The CNS develops from the

hollow nerve cord

The cavity of the nerve cord gives rise to the __

narrow central canal of the spinal cord and the ventricles of the brain

• The canal and ventricles fill with __filtered from blood and functions

cerebrospinal fluid

functions of cerebrospinal fluid

– to cushion brain and spinal cord

– to provide nutrients and remove wastes

The brain and spinal cord contain

Gray matter

White matter

Gray matter

which consists of neuron cell bodies, dendrites, and unmyelinated axons

White matter

which consists of bundles of myelinated axons

Organization of the Vertebrate Nervous System

• The spinal cord conveys information to and from the brain and generate basic patterns of locomotion

• The spinal cord also produces reflexes independently of the brain

• A reflex is the body’s automatic response to a stimulus

• For example, a doctor uses a mallet to trigger a knee jerk reflex

The Peripheral Nervous System

• The PNS transmits information to and from the CNS and regulates movement and internal environment

• In the PNS, afferent neurons transmit information to the CNS, and efferent neurons transmit information away from the CNS

• The PNS has two efferent components: the motor system and autonomic nervous system

enteric division

motor system

The motor system carries signals to skeletal muscles and is voluntary

autonomic nervous system

The autonomic nervous system regulates smooth and cardiac muscles and is generally involuntary

The autonomic nervous system has sympathetic and parasympathetic divisions

sympathetic division

The sympathetic division regulates arousal and energy generation (“fight-or-flight” response)

parasympathetic division

The parasympathetic division has antagonistic effects on target organs and promotes calming and a return to “rest and digest” functions

**afferent vs efferent

Afferent neurons carry sensory information to the central nervous system, while efferent neurons carry motor commands from the central nervous system to muscles and glands

The Parasympathetic and Sympathetic Divisions of the Autonomic Nervous System

enteric division

The enteric division controls activity of the digestive tract, pancreas, and gall bladder

Glia

• Glia cells, or glia have numerous functions to nourish, support, and regulate neurons

– Embryonic radial glia form tracks along which newly formed neurons migrate–

Astrocytes induce cells lining capillaries in the CNS to form tight junctions, resulting in a blood-brain barrier and restricting entry of most substances into the brain 49

• Radial glial cells and astrocytes can both act as stem cells

• Researchers are trying to find a way to use neural stem cells to replace brain tissue that has ceased ti function normally

• The major types of glia in adult nervous system