Lecture 2 Neurons, Muscles and Motor Units
Voluntary movement begins.... Brain Spinal cord Motor nerves Muscles
The Nervous System Central nervous System (CNS). n Brain n Spinal cord Peripheral Nervous System (PNS). n Everything else Somatic Autonomic
Nervous system cells n Neuron Basic unit of the NS n Glial Support, myelin, scavengers.
The neuron: Consists of three major parts: n Soma or cell body (nucleus). n Axon (carries the output signals generated by the cell). n Dendrites (inputs into the cells).
How is the info. being transferred from cell to cell?
Synapses.... Connections to other cells, consist of three main components: n Presynaptic membrane n Postsynaptic membrane n Synaptic cleft
Presynaptic membrane Synaptic cleft Postsynaptic membrane Synaptic vesicles Neurotransmitter
Neuron to neuron synaptic transmission uses chemical substances called neurotransmitters. Neuron to neuron synaptic transmission uses chemical substances called neurotransmitters.
Signal transportation...
Action potential Transmission of information through the NS in the form of electrical impulse. Transmission of information through the NS in the form of electrical impulse.
Stimulus Depolarization Hyperpolarization Resting Potential Resting Potential
Speed of conduction with regard to the neural signal n Higher speed along thick neural fibers. n Higher speed in myelin covered fibers.
Primary Afferent / Sensory Axons
One Presynaptic action potential is usually unable to force the membrane of the postsynaptic neuron to generate an action potential - subthreshold. One Presynaptic action potential is usually unable to force the membrane of the postsynaptic neuron to generate an action potential - subthreshold. (Muscle-exception, will always stimulate the muscles due to the high voltage).
How do we get an action potential? In order to generate an action potential, the Postsynaptic membrane requires to sum up the effects of a number of signals. This is what we call summation. There are two types of summation: n Temporal summation n Spatial summation
Temporal summation n An accumulative effect on the membrane potential of a postsynaptic cell; caused by chemical transmissions from one or more synaptic terminals occuring so close together in time that each postsynaptic potential affects the membrane before the voltage has returned to the resting potential after the previous stimulation.
Spatial summation n An accumulative effect on the membrane potential of a postsynaptic cell, caused by several different synaptic terminals stimulating a postsynaptic cell membrane.
Muscles...
Muscles.... n Contractile components: Myofibril Muscle fiber Muscle fascicles n Elastic components: Tendon
Neuromuscular synapse/junction n A region of contact between a single Presynaptic fiber and a muscle fiber. n The two fibers are very close to each other but no direct contact.
Motor Unit n The smallest functional unit of the neuromotor system. n Typically, each muscle fiber is innervated by only one axon branch. n The motorneuron and the muscle fibers it innervates are called a motor unit.
Types of contraction n A single action potential to a muscle that responds with one contraction is a TWITCH. n Many action potentials happen at the same time, contraction is sustained, a TETANUS is observed.
Muscle contraction n Decision by the CNS for muscle contraction n Signal is sent to neurons in the spinal cord n Signal is propagated through the axons to appropriate muscles n Muscles are activated
Muscle contraction CNS command to contract Signal is sent to the spinal cord Signal propagated to muscles via axons Muscles are activated