Muscle Physiology Topic 3: PNS and Neurons

• Topic 3 is muscle physiology and the control of body movements. Relevant chapters are 10, 12, and 13.

Overview of the Peripheral Nervous System (PNS)

• The PNS links the central nervous system (CNS), which includes the brain and spinal cord, with the body and the external environment.
• The PNS lies outside the brain and spinal cord, and it includes the nerves that extend to the limbs and organs.
• The PNS detects sensory stimuli and delivers them to the CNS as sensory input.
• The CNS processes the input and transmits the impulses through the PNS to effectors (muscle cells and glands) for motor output.

Sensory vs. Motor Neurons

• Sensory neurons carry sensory impulses from sensory organs to the central nervous system.
• Motor neurons carry motor impulses from the central nervous system to specific effectors.

Sensory Neurons: Myelin Sheath

• Sensory neurons are classified by how quickly axons conduct action potentials, the diameter of the axon, and the thickness of the myelin sheath.
• Largest diameter axons with thick myelin sheaths conduct the fastest impulses and convey discriminative and nondiscriminative touch information.
• Smallest diameter axons with little myelin transmit action potentials the slowest and include axons that carry pain and temperature stimuli to the CNS.
• Myelin sheath is a fatty layer that wraps around nerve fibers (axons) in the nervous system.
• Myelin insulates and increases the speed at which electrical impulses travel along the nerve cells.

Sensory Neurons: Processes - Axons and Dendrites

• Sensory neurons have three main components: the cell body or soma, the peripheral process, and the central process.
• Neuron processes are extensions or branches that emerge from the neuron's cell body (soma). They are involved in receiving, transmitting, and processing electrical signals.
• Axons are long extensions that carry electrical signals away from the neuron.
• Dendrites are branching extensions that extend from the cell body (soma) that receive signals from other neurons, and they contain sensory receptors.
• The peripheral process transmits an action potential from the sensory receptor to the neuron's other axon, which is the central process.
• The central process transmits an action potential from the peripheral process to the posterior horn. It eventually synapses on a neuron in the spinal cord or brainstem.

Sensory Receptors

• Sensory receptors are specialized structures that detect specific types of stimuli from the environment, such as light, sound, touch, or temperature.
• They are located in various parts of the body, depending on the type of stimuli they detect.
• Mechanoreceptors respond to mechanically deformed tissue, such as external touch or internal stretch and use Mechanically Gated Ion Channels for initiating an action potential.
• Thermoreceptors respond to cold temperatures in the superficial dermis and hot temperatures in the deep dermis.
  • Cold receptors respond from 10-20°C; though they can still respond to temperatures as high as 30°C. If temperatures rise above 30°C, cold receptors generally stop responding, and hot receptors pick up the sensation of warmth or heat.
  • Hot receptors respond from 32-48°C.
• Chemoreceptors depolarize in response to chemicals in body fluids or air that bind to a specific receptor and trigger ion channels to open.
• Photoreceptors are found only in the eye and respond to light.
• Nociceptors detect noxious stimuli perceived as pain.

Capsaicin and Nociceptor

• Capsaicin, a chemical in hot peppers, opens specific ligand-gated ion channels in nociceptors and triggers action potentials.
• The CNS perceives capsaicin as painful, although nociceptors become desensitized to repeated application of the chemical over time and reduce the generation of action potentials.
• Capsaicin may be used as a topical cream to relieve painful neuropathies such as shingles, caused by the chickenpox virus.

Sensory Transduction in Sensory Receptors

• Sensory neurons have sensory receptors in their dendrites that respond to stimuli.
• When a stimulus is detected, an action potential is initiated, traveling down the axolemma (membrane covering axon terminals)
• The axolemma of the sensory nerve ending contains many gated ion channels responding to action potential and can trigger opening/closing of other ion channels, changing the flow of ions across the membrane.
  • Before a stimulus arrives, the ion channels in the axolemma of the somatic sensory neuron are closed
  • When a stimulus, like pressure, is applied, mechanically gated Na+ channels open and Na+ enters the axoplasm, generating a receptor potential.
  • If enough Na+ enters and the membrane potential reaches the threshold, voltage-gated Na+ channels open, triggering an action potential propagated along the axon to the spinal cord.

From CNS to PNS: Motor Output

• Skeletal muscle fibers contract only when they are stimulated to do so by a motor neuron
• The CNS initiates movement through Upper Motor Neurons in the primary motor cortex
• Messages from upper motor neurons are relayed to Lower Motor Neurons, which release acetylcholine onto the muscle fiber and initiate a muscle contraction.

Control of Movement by the Nervous System

• CNS: Upper motor neurons in the Primary motor cortex stimulate the basal nuclei (caudate nucleus and putamen)
• CNS: The basal nuclei remove inhibition from the thalamus, which stimulates upper motor neurons of the primary motor cortex
• CNS to PNS: Upper motor neurons stimulate lower motor neurons.
• PNS: Lower motor neurons stimulate a muscle to contract.
• PNS to CNS: Sensory stimuli are relayed back to the cerebellum in the CNS. The cerebellum then sends instructions to upper motor neurons to modify movement to correct motor error.