IB Biology/Nerves, Muscles and Movement

Nerves

Outline the general organization of the human nervous system including the CNS and the PNS

• The brain, or central nervous system (CNS), is connected by the spinal cord to the peripheral nervous system (PNS).
• The peripheral nervous system is divided into two categories:

• Sensory input (afferent division) - senses external and internal environmental conditions.
• Motor output (efferent division): Self explanatory.

• Somatic nervous system - conscious, controlled response
• Autonomic nervous system - functions automatically.

• Parasympathetic division - relaxed state.
• Sympathetic division - excited state.

The nervous system is divided into two main parts: the central nervous system (CNS), and the peripheral nervous system (PNS). The organs of the CNS, the brain and the spinal cord, are located within and protected by, respectively, the skull and the vertebral column. The PNS is composed of nerves that branch from the CNS and connect the CNS to the rest of the body. The CNS receives impulses from the nerves of the PNS and interprets them, and transmits the corresponding signals back to the origin through the PNS, thus controlling the functions of the entire body.

Draw the structure of a motor neuron

Define resting potential and action potential"

Resting potential is the membrane potential energy of a neuron membrane when not conducting an impulse. The charge of a membrane during resting potential is -70 mV, which means that the charge within the cell membrane is relatively negative to the outside charge.

Action potential is the state of the cell membrane while conducting an impulse, meaning the rapid depolarization and repolarization of the cell membrane. The charge on the membrane typically reaches up to +40 mV, meaning the inside of the cell is positively charged relative to the outside.

Explain how a nerve impulse passes along a non-myelinated neuron (axon).

In resting neurons, there exists an imbalance of ions inside and outside of the cell membranes. The important ions are K⁺ (potassium) and Na⁺ (sodium). K+ is more concentrated inside the cell and therefore tends to diffuse out of the cell. It is this diffusion that causes the charge potential across the plasma membrane (in resting neurons it is about -70 mV). Na+ is more concentrated outside of the cell and thus tends to diffuse into the cell, but at a very slow rate due to the low permeability of the plasma membrane to Na⁺. Sodium-potassium pumps embedded in the plasma membrane work against both ions' diffusion gradients in order to preserve the gradients and the membrane potential.

When a graded nerve impulse traveling from a dendrite reaches threshold potential (usually about 15 to 20 mV more positive than resting potential) and comes to the axon hillock, it triggers an action potential. Action potentials are non-graded events, meaning the amplitude of the potential remains more or less constant in all situations. When threshold potential is reached, voltage-gated sodium and potassium channels at the axon hillock are stimulated. The sodium channels each have two gates, a fast-acting activation gate and a slow-acting inactivation gate. Normally the activation gate is closed and the inactivation gate is open. However, since it takes BOTH gates to be open for Na+ to freely move through the channel, normally these sodium channels are closed. The potassium channels have only one gate, which is slow acting and is normally closed. The voltage stimulus from the threshold potential cause all gates to reverse their position; if they were closed, they open, and if they were open, they close. The fast-acting sodium activation gates are the first to respond, opening and allowing Na+ to diffuse freely into the neuron. This large, rapid influx of Na+ causes the charge potential across the plasma membrane to become positive. This is called depolarization. Most neurons will depolarize up to +40 mV, and it is this depolarization that leads to the action potential, or nerve impulse. By the time the voltage peaks, sufficient time has passed for the inactivation gates to close, preventing further Na+ from entering the cell. At the same time, the potassium gates finally open, which cause K⁺ ions to rapidly diffuse out of the neuron along the charge gradient, increasing its negativity. This is the repolarization phase. The channel gates eventually return to their resting positions, but the slow-acting potassium gates take longer to close, which cause an excess of K⁺ to diffuse out of the cell, resulting in a temporary hyperpolarization, or undershoot (membrane potential drops below resting potential). For a brief period following repolarization, both the sodium activation and inactivation gates remain closed, since the inactivation gates are slow-acting and take time to return to their open resting position. During this time, called the refractory period, this particular region of the neuron is incapable of depolarization. Once the sodium inactivation gates are open again, the region will again be able to depolarize.

As the axon hillock of the neuron depolarizes, it stimulates the next region of the axon to depolarize since the membrane potential goes beyond threshold potential during depolarization to the action potential. In this manner, action potentials are triggered repeatedly, each time in successive regions further and further down the axon. Action potentials only travel in one direction; this is achieved through the effects of the refractory period. The regions immediately behind the location of an action potential cannot be stimulated to depolarize because they themselves have just recently depolarized and repolarized and cannot do so again until the refractory period has passed.

Explain the principles of synaptic transmission.

Synapses are the locations where a neuron communicates with either another neuron or another type of cell, such as a muscle cell. The cell from which the nerve signal originates is the presynaptic cell and the cell receiving the signal is the postsynaptic cell. There are two types of synapses, electrical and chemical. Electrical synapses contain gap junctions that link the two cells and allow action potentials to travel between the two cells uninterrupted. Much more common are chemical synapses, which contain a narrow gap, called the synaptic cleft, separating the presynaptic and postsynaptic cells. Action potentials cannot directly jump the gap, and instead the cells must rely on chemical signals to propagate the nerve impulse.

As an action potential reaches the end of an axon and the synaptic terminal, it depolarizes the presynaptic membrane. Here, voltage-gated calcium channels open and allow Ca²⁺ ions to diffuse into the neuron. The sudden increase in Ca²⁺ concentration causes synaptic vessels, filled with neurotransmitters, to fuse with the plasma membrane and release the neurotransmitters into the synaptic cleft. Neurotransmitters can be any number of chemicals, including acetylcholine (ACh), norepinephrine, dopamine, and serotonin. These chemicals, once released, quickly diffuse across the synaptic cleft and reach the postsynaptic membrane. On the postsynaptic membrane are numerous specialized receptor proteins, which bind to specific neurotransmitters and open specific ion channels. Depending on the neurotransmitter that was received, ion channels may open up to let Na⁺, K⁺, Cl^-, or other ions to diffuse in or out of the cell. Depending on which channels were opened, the postsynaptic membrane will either depolarize or hyperpolarize. Depolarization of the postsynaptic membrane is called an excitatory postsynaptic potential (EPSP), and hyperpolarization is called an inhibitory postsynaptic potential (IPSP). EPSPs make it easier for the postsynaptic cell to generate an action potential while IPSPs make it harder. The effects of EPSPs and IPSPs are cumulative, meaning that the effects of multiple EPSPs, IPSPs, or a combination of the two, will stack, a phenomenon known as summation. Summations can either be spatial, meaning the potentials overlap spatially (e.g. they are close together on the neuron), temporal, meaning the potentials overlap in time (e.g. multiple potentials within a very short period of time), or both. Back at the synaptic cleft, the neurotransmitters that were released quickly disappear, either by simple diffusion or by enzymatic breakdown, such as when cholinesterase breaks down ACh. If the cleft has not been cleaned by the time the next action potential reaches the synapse, the neurotransmitters released by the potential will not be able to bind to the receptors on the postsynaptic membrane and the effects of the signal will not be propagated to the postsynaptic cell.

Muscles and Movement

Outline the great diversity of locomotion in the animal kingdom as exemplified by the movement in an earthworm, swimming in a bony fish, flying in a bird, and walking in an arthropod.

An earthworm slides forward by contracting and expanding alternate segments of its body. A bony fish beat tail from side to side creating a forward motion. It controls it buoyancy using an air bladder that grows or shrinks in size to force the fish up or down in the water. A bird flaps its wings up and down to create an upward force. Once the bird is in the air, it glides using air currents, occasionally flapping its wings to maintain its altitude. Arthropods extend and flex muscle segments in it legs to move forward.

Describe the roles of nerves, muscles and bones in producing movement or locomotion.

Bones provide the frame that holds up muscles. Signals are sent from the locomotion center of the brain (medulla) along nerves which synapse with certain muscles. This synapse causes myosin and actin filaments to slide over each other, causing the muscle to contract and resulting in locomotion.

Draw a diagram of the human elbow joint.

Outline the functions of the main structures in the human elbow joint.

• bicep - flexor muscle, used to bend arm at the elbow
• humerus bone - provides firm anchor for muscles
• triceps - extensor muscle, used to straighten the arm
• synovial fluid - lubricates the joint to reduce friction
• capsule - seals the joint
• tendon - attaches muscle to bone
• radius - upper bone in forearm, transmits forces from bicep through forearm
• ulna - lower bone in forearm, transmits forces from triceps through the forearm
• ligament - tough cords of tissue, links bone to bone, prevents dislocation of the joint
• cartilage - a layer of smooth and tough tissue that covers the ends of the bone where bones meet to reduce friction

Draw the structure of skeletal muscle fibers as seen in an electron micrograph.

Explain how skeletal muscles contracts by the sliding of filaments.

Using ATP, actin and myosin filaments slide over each other, causing sarcomeres in the myofibrils to become shorter, ultimately creating a contraction in skeletal muscles.

1. Myosin forms a bridge with binding sites on actin filaments
2. ATP binds with myosin heads causing them to break the cross bridge by detaching from the binding site
3. ATP loses a phosphate molecule, releasing energy, leaving ADP + P+ and changing the angle of the myosin head. This is known as a cocked myosin head.
4. The angled head attaches to a new binding site on actin that are further back in the line of actin binding sites
5. ADP + P+ is released, causing the heads push filaments inwards toward the sarcomere, resulting in a sliding of myosin and actin filaments over one another.