Overview
This lecture covers how the nervous system recruits skeletal muscle, from the motor cortex in the brain to the neuromuscular junction, focusing on the roles of motor units, action potentials, and the structure and function of neurons involved in muscle contraction.
The Nervous System: Central vs. Peripheral
- The central nervous system (CNS) includes the brain and spinal cord, protected within the skull and vertebrae.
- The peripheral nervous system (PNS) is outside the CNS and transmits information between the CNS and the rest of the body.
- The autonomic nervous system (part of PNS) controls involuntary actions (e.g., heart rate, digestion), while the somatic nervous system controls voluntary muscle contractions.
Motor Cortex and Associated Brain Areas
- The primary motor cortex, located in the cerebral cortex, initiates voluntary muscle movement.
- Other areas (premotor cortex, supplementary motor area, posterior parietal complex) aid in planning and sensory guidance of movements.
- The "motor homunculus" represents the body's regions controlled by the motor cortex, though actual mapping is scattered and overlapping.
Motor Neurons and Pathways
- Upper motor neurons reside in the CNS and synapse with lower motor neurons in the spinal cord's ventral horn.
- Lower motor neurons leave the spine to activate skeletal muscles in the periphery.
- Upper motor neurons use glutamate to activate lower motor neurons; lower motor neurons use acetylcholine to activate muscle.
Motor Units and All-or-None Principle
- A motor unit consists of a single motor neuron and all the muscle fibers it innervates.
- When a motor neuron fires, all fibers in its unit contract maximally or not at all (“all-or-none” principle).
Action Potentials and Nerve Excitation
- Neurons have resting membrane potentials, maintained by sodium-potassium pumps using ATP.
- Action potentials occur when a threshold voltage is reached, causing rapid sodium influx (depolarization), followed by potassium efflux (repolarization).
- Absolute refractory period: neuron cannot be reactivated until reset; relative refractory period: requires a stronger stimulus to reactivate.
Myelination and Conduction Velocity
- Myelin sheaths (formed by Schwann cells) speed up nerve signal transmission via saltatory conduction between nodes of Ranvier.
- Nerve conduction velocity depends on axon diameter and myelination thickness.
Force Output and Rate Coding
- The number of recruited motor units and the frequency (rate coding) of action potentials determine muscle force output.
- Faster firing rates mean stronger, sustained contractions due to overlapping signals before the muscle relaxes.
Key Terms & Definitions
- Central Nervous System (CNS) — brain and spinal cord.
- Peripheral Nervous System (PNS) — nerves outside CNS, includes somatic and autonomic branches.
- Motor Unit — a motor neuron plus all muscle fibers it innervates.
- All-or-None Principle — all fibers in a motor unit contract fully if the neuron fires.
- Action Potential — rapid voltage change in a neuron allowing signal transmission.
- Refractory Period — time when a neuron can't (absolute) or is less likely (relative) to fire again.
- Myelin Sheath — insulating layer on nerves, increases conduction speed.
- Rate Coding — frequency of action potentials affecting muscle force.
Action Items / Next Steps
- Review roles of motor cortex and other brain areas in movement.
- Be able to describe the pathway from brain to muscle (upper to lower motor neuron).
- Prepare for next lecture on the neuromuscular junction and excitation-contraction coupling.