Nervous System & Neurons

Chapter 29, Section 2

Of your textbook

Central Nervous System (CNS)

• Brain & spinal cord

• Brain – controls complex behavior

• Spinal cord -- controls simple responses

• Receives information from the peripheral nervous system & interprets it

Peripheral Nervous System (PNS)

• All nerves & neurons outside of the CNS, – Cranial nerves

(originate in brain) – Spinal nerves

(originate in spinal cord)

–

• Divided into sensory & motor divisions

Sensory & Motor Divisions (of PNS)

• Sensory:– Incoming neurons that

convey info from receptors to CNS

– Picks up info from both internal and external environments

• Motor:– Outgoing neurons that

convey signals from CNS to muscles & glands

– Divided further into the somatic & autonomic systems

Somatic vs. Autonomic Nervous System (Motor Division of PNS)

• Somatic:– Carries signals to

skeletal muscles

– Generally, controls voluntary actions because it is under conscious control

– Includes reflexes

• Autonomic:– Carries signals to

smooth & cardiac muscles, major organ systems and glands

– Generally controls involuntary actions & responses

Types of Neurons

• Sensory Neurons– Receive info from inside / outside of body– Send signals to brain / spinal cord

• Interneurons– Located in brain / spinal cord– Relay info between sensory & motor neurons

• Motor Neurons– Take messages from brain / spinal cord to muscles /

glands

Reflex Arc

Neuron (animation)• The neuron is the structural and functional unit

of the nervous system

Dendrites: receive information into cell body; highly branched

Cell Body: contains the nucleus and organelles

Axon: one long extension that transmits an info/impulse away from cell body

Neuron• Myelin sheath: made

of Schwann cells; insulates the axon

• Nodes of Ranvier: tiny gaps in the myelin sheath

• Axon terminals, aka synaptic terminals: branched ends of the axon

Drag and Drop Quiz – Neuron Structure

Neurons and Signaling

• A neuron uses an electrical signal to transmit messages down its axon

• A neuron uses a chemical signal – neurotransmitters – to transmit messages across the synapse (either to another neuron or to a muscle / gland)

Resting Potential

• When a neuron is at rest, the inside is negative compared to the outside.

• There are more sodium (Na+) ions OUTside.

• There are more potassium (K+) ions INside.

Sodium-Potassium Pump

• Helps the neuron maintain resting potential by pumping 3 Na+ out for every 2 K+ it pumps in

• Uses active transport – Needs energy (ATP)– Moves substances

against their concentration gradient

Action Potential

• When a neuron is stimulated, membrane channels open & Na+ rushes into the cell.

• This makes the inside become positive (and the outside becomes negative).

• An action potential is a moving electrical impulse. – It is generated by a stimulus.– Na+ enters, and cell becomes

positively charged.– K+ leaves, and the area of

positive charge moves.

• An action potential only travels one-way down the axon (from dendrites/cell body toward terminals/synapse).

• Animation

• An action potential has multiple phases.– As the inside of the membrane becomes more

positive, it depolarizes. – As the inside becomes less positive / more

negative, it is repolarizing.

• An action potential travels faster down a myelinated axon because the impulse jumps from node to node (the gaps along the myelin sheath).

Signals at the Synapse• When the action potential reaches the axon / synaptic

terminals, the electrical signal cannot jump the synapse (the tiny gap between one neuron and the next).

• A chemical signal – in the form of a neurotransmitter – passes from one neuron to another (or onto a muscle or gland).

• These neurotransmitters stimulate the next cell.– If it is another neuron, then that neuron may also have

an action potential.

– If it is a muscle, then the muscle may contract.

– If it is a gland, then the gland may release a hormone.

• The neurotransmitters are picked up by special receptor proteins in the post-synaptic cell.

• When enough neurotransmitters are received, the message is carried forward.

area of detail

Review animation