Human Physiology: Somatic Motor Pathway & NMJ

• Human Physiology BIOL 2052 A is taught by Dr. Cristina V Dieni.
• Dr. Dieni's email is [email protected].
• Dr. Dieni can be reached at 942-4291, ext. 2248.
• Dr. Dieni's office is located in BGSC213.

Somatic Motor Pathway

• Is comprised of a somatic motor neuron and skeletal muscle.

Neuromuscular Junction

• The site of synapse between a somatic motor neuron and a skeletal muscle fiber.

Signal Transmission at the Neuromuscular Junction

• Motor neurons are the nerves that innervate muscle fibers.
• A motor unit comprises a single motor neuron and the muscle fibers it innervates.
• A single motor unit can activate few or thousands of muscle fibers.

Signal Transmission Steps at the NMJ

• A nerve action potential happens.
• Calcium channels open
• Calcium goes into the cell
• Synaptic vesicles release Acetylcholine (ACh)
• ACh binds to receptors
• Sodium (Na) enters the cell and Potassium (K) exits
• This influx and outflux of Na and K causes muscle action potential.
• Propagation of AP occurs
• Acetylcholinesterase (AChE) degrades ACh, stopping the signal

Nervous System Functions

• The nervous system has three general functions: sensory, integrative, and motor.
• Sensory function involves receiving information.
• Integrative function involves deciding what to do.
• Motor function involves acting on decisions.

Neuron Classification

• Neurons have functional differences that allow them to be classified as sensory neurons, interneurons, or motor neurons.
• Sensory (afferent) neurons conduct impulses from peripheral body parts to the brain or spinal cord.
• Interneurons lie within the brain or spinal cord and form links with other neurons.
• Interneurons relay information from one part of the brain or spinal cord to another.
• Motor (efferent) neurons are multipolar and conduct impulses out of the brain or spinal cord to effectors.

Neuron Histology

• The basic functional unit of the nervous system is the neuron.
• Neurons are highly specialized nerve cells.
• Sensory neurons carry sensory information from the PNS toward the CNS.
• Motor neurons carry motor commands from the CNS toward effectors.
• Neuroglia are supporting cells that interact with neurons to regulate the extracellular environment, defend against pathogens, and repair nervous tissue.

Reflexes

• A rapid, stereotyped response is a reflex.
• It is the same every time a specific stimulus presents itself.

Functional Terminology

• Somatic pertains to the control of skeletal muscle activity or sensory information from skeletal muscles, tendons, and joints.
• Visceral pertains to the innervation of visceral organs or sensory information from visceral organs.
• Voluntary is under direct conscious control.
• Involuntary is not under direct conscious control.
• Subconscious relates to centers in the brain that operate outside a person's conscious awareness.
• An action potential is a sudden change in the membrane potential that is propagated along the surface of an axon or sarcolemma.

Nervous System Communication

• Nerves conduct impulses that allow body systems to communicate.

Spinal Cord

• The spinal cord is a slender column of nervous tissue continuous with the brain.
• It extends downward through the vertebral canal.
• It consists of 31 segments, each giving rise to a pair of spinal nerves.
• Spinal nerves branch to body parts and connect them with the CNS.
• The spinal column has two main functions, those being a center for spinal reflexes, and a conduit for impulses to and from the brain.

Ascending and Descending Tracts

• Ascending tracts conduct sensory information to the brain.
• Descending tracts conduct impulses from the brain to motor neurons, reaching muscles and glands.

Testing the Water

• The sensory neuron in the skin senses a stimulus, like water temperature.
• If strong enough, the graded potential from the sensory endings will initiate an action potential at the initial segment.
• The axon of the peripheral sensory neuron enters the spinal cord and contacts another neuron in the gray matter.
• An action potential is initiated and travels up the sensory pathway to a region of the brain called the thalamus.
• The sensory pathway ends when the signal reaches the cerebral cortex.
• After integration with neurons in other parts of the cerebral cortex a motor command is sent from the frontal cortex.
• The upper motor neuron sends an action potential down to the spinal cord to the dendrites of the lower motor neuron in the gray matter of the spinal cord.
• The axon of the lower motor neuron emerges from the spinal cord in a nerve.
• It then connects to a muscle through a neuromuscular junction to cause contraction of the target muscle.

Patellar Reflex

• A monosynaptic reflex uses only two neurons: a sensory neuron and a motor neuron.

Arc Reflex

• Includes a receptor, sensory neuron, interneuron, motor neuron, and effector.

Withdrawl Reflex

• Includes sensory, interneuron and motor neuron.
• When the flexor muscle on one side is stimulated to contract in a withdrawal reflex, the extensor muscle on the opposite side also contracts to help maintain balance.

Questions and Answers

• The neuromuscular junction is an example of a synapse
• The correct answer about the spinal cord is: the segments give rise to a pair of spinal nerves