The Nervous System

Ch. 48-49

The Nervous System…

• Performs three basic continuous functions:

– Sensory input– Integration– Motor output

It Is Brain surgery…

Sensory Input…

Sensory Receptors collect information from the outside environment.

Integration…• Input is interpreted and linked to

appropriate responses• Accomplished by the CNS (central

nervous system)– Brain– Spinal cord

Motor Output…

• Signal conduction from CNS to effector cells in PNS (peripheral nervous system)

• CNS is connected to effector cells via neurons, or nerve cells

The Nervous System

CNS PNS

BrainSpinal Cord

Afferent Efferent

Autonomic Somatic

Sympathetic Parasympathetic

Neurons and the Connections They Love…• The NEURON, or nerve cell is the

functional unit of the nervous system.• Composed of cell body, dendrites, axon,

myelin sheath, synaptic terminals (bulbs)

• Dendrites: receive afferent signals, incoming from other neurons or receptors

• Axon: only one per neuron; efferent pathway to other neuron or an effector cell (muscle, gland)

• Myelin sheath: lipid layer that insulates axon; produced by Schwann cells.

• Synaptic Terminals (bulbs): transmit signals from axon by release of neurotransmitters (Ach)

• Synaptic cleft (synapse): site of contact between two neurons or neuron and effector

• Postsynaptic cell: the receiver.

Label This Drawing…

A

F

ED

C

BH

G

How do Scientists Study Nerves?

From One Neuron to its Neighbor…

The Simplest Nerve Circuit…

The Reflex Arc:

Often involves only two nerve cells, the sensory neuron and the motor neuron. This action is not integrated by the CNS.

Supporting Cells…

• GLIA =“glue”

• Used to believe they were wholly supportive, new research says not!

• Provide nutrition and protection.

• Lead neurons from neural tube along pathway in embryonic development.

Three Types of Glial Cells…

• Astrocytes: form connections between capillaries and neurons for feeding and waste disposal; in brain, they form tight junctions which form the blood-brain barrier.

• Microglial cells: immune system cells which engulf microbes in the brain; alcohol kills microglial cells in fetuses.

• Oligodendrocytes (CNS) and Schwann Cells (PNS): form around axons like burritos; insulate electrical impulses and speed up nerve signal transduction.

How a Nerve Cell Passes a Signal…• All cells have a membrane potential, a difference in

charge between inside and outside. Developed 2 weeks post conception, maintained through life.

• The resting potential of an unstimulated nerve cell is about -70mV; negative inside the cell.

• The resting membrane potential is maintained by the Sodium-Potassium Pump.

• Neurons have a 50X greater permeability to K+ than Na+.

Resting MPExtracellular:

1° cation:

1° anion:

Intracellular:

1° cation:

1° anion:Resting Potential Video

Excitable cells…

• Cells in the body like muscle and nerve cells can create large changes in their membrane potentials.

• Environmental stimuli can cause these cells to alter their membrane potential, possibly causing an action potential.

The Action Potential in Words…• Stimulus causes Na+ gates to open.

• Na+ influx changes membrane potential.

• If Na+ influx is great enough to achieve threshold potential (-50mV), then all Na+ gates open.

• “All or none” phenomenon…at threshold, all gates will be opened (below threshold, no extra gates will open) and stimulus is transmitted.

• Additional Na+ influx causes depolarization of membrane (action potential).

• K+ channels remain closed. Cell becomes positive.

• Repolarization begins when K+ gates open and Na+ gates are closed. (~ +50mV)

• K+ ions leave the cell, causing the interior to become more negative.

• BUT…The ions are in reversed concentrations!

• When K+ gates finally close, there is slightly more K+ outside than inside… hyperpolarization.

• Refractory period returns ions to resting state.

• Sodium-Potassium pump restores resting potential…no stimuli can be transmitted during this phase…the neuron is BUSY!

Action Potential Video

How Fast Are Impulses Conducted?

• Campbell p. 1020

How Do Muscle Contractions Fit in?• All motor neurons are associated with

muscle fibers at their peripheral end…the neuromuscular junction.

• There is a space between a neuron and a muscle fiber…the synaptic cleft.

• The depolarization wave cannot pass across the cleft!

Three Types of Muscle in the Body…• Cardiac…found only in the

heart; striated; involuntary

• Smooth…lines internal organs…digestive tract, blood vessels, rep. tract, bladder; not striated; involuntary.

• Skeletal…attaches to skeleton; striated; voluntary; multinucleate

General Anatomy of a Muscle… Muscle Fiber Cell w/ myofibrils

A Little Bit Closer…

ONE MUSCLE FIBER…(cell)

Myofibril Anatomy…

• Skeletal muscle is striated

• Individual units are called sarcomeres.

• Thin filaments – actin

• Thick filaments –myosin

The Neuromuscular Junction…

The Neuromuscular Junction

Signal Transduction…

• The depolarization wave in the neuron cannot traverse the synaptic cleft.

• When action potential reaches the synaptic end bulb, Ca++ in the cleft flows into the bulbs.

• This calcium causes vesicles filled with neurotransmitters (acetylcholine) to migrate to the neural membrane.

• The vesicles fuse with the cell membrane and exocytose their contents into the synaptic cleft.

• Receptors on sarcolemma bind acetylcholine causing gated Na+ channels to open. As Na+ comes into the sarcoplasm, what happens?!

• DEPOLARIZATION !

• The depolarization wave (DW) passes across the sarcolemma which extends into the muscle fiber via the T-tubules.

• T-tubules have close associations with the sarcoplasmic reticulum (SR), the Ca++ warehouse.

• The DW opens gated Ca++ channels allowing Ca++ to flow into the sarcoplasm…this is the important part!!!!

• Ca++ binds the troponin complex.

• Once troponin is removed, the tropomyosin shifts away from the myosin binding sites.

• Ca++ serves as an enzyme cofactor with myosin and they become ATPase!

• The ATP is broken down into ADP and Pi. This allows myosin to bind to the actin filaments and contract the filament to the center of the sarcomere.

Muscle Contraction

So, How Do You Stop It?

• The binding of the acetylcholine receptors on the sarcolemma signals the release of acetylcholinesterase from the sarcolemma.

• This enzyme breaks down acetylcholine in the synaptic cleft.

• How do you get the contraction to continue?

One molecule of acetylcholinesterase breaks down 25,000 molecules of acetylcholine each second. This speed makes possible the rapid "resetting" of the synapse for transmission of another nerve impulse.

Belgian Blue BullThe myostatin gene is effectively blocked by being mistranscribed (it’s truncated)…leads to “double-muscled” animal.

• Negative Feedback Control (depends on four detector types so that the CNS knows what the muscles are doing and can make adjustments accordingly)

• •     Muscle spindles (so-called stretch receptors) – actually length detectors

• •     Golgi tendon organs – detectors of tension in tendons

• •     Joint angle receptors – indicate the angle of a joint• •     Skin stretch and compression receptors – give

information about how the skin is deformed around a joint

Storytime…

The Function of the Sympathetic Nervous System and Wartime…