Neurological Reflexes and Brain Damage

• Study notes below:

Electromyography (EMG)

• Allows healthcare professionals to assess the functioning of muscles and the nerves controlling them
• EMG scan results can reveal muscle or nerve dysfunction, or issues with impulse transmission from neuron to muscle
• EMGs are typically ordered for individuals exhibiting nerve or muscle disorders, such as tingling, numbness, muscle weakness, pain, or paralysis

Deep Tendon Reflexes and CNS Injury

• Deep tendon reflexes like patellar, biceps, and Achilles reflexes assess the body's sensory-motor function
• Stretch reflexes involve a sensory neuron connecting directly to a motor neuron in a simple reflex arc
• The sensory neuron carries information from the receptor to the spinal cord
• The motor neuron carries information from the spinal cord to the muscle, causing contraction
• The brain is informed, but not required for the reflex response
• Motor neurons from the brain can influence stretch reflexes by either exciting or inhibiting spinal cord neurons
• Under normal conditions, brain neurons inhibit spinal cord motor neurons, reducing reflexive response and preventing clonus
• Spinal cord damage prevents inhibitory information from reaching motor neurons below the injury
• Reflexes below the injury level become hyperactive, indicating reflexes stronger than normal
• Individuals with spinal cord damage lose motor control and sensation below the injury level, but reflexes below are more brisk due to lack of inhibitory input
• The extent of sensory and motor loss depends on the level of spinal cord injury
• Motor information from one side of the brain controls muscles on the opposite side of the body
• Damage to one side of the brain can cause hyperactive reflexes on the opposite side due to lack of inhibitory information

Assessing Reflexes

• Reflexes should be tested bilaterally and should be symmetrical during clinical assessment
• Asymmetry may indicate brain or spinal cord damage
• Reflexes are graded quantitatively from 0 to 4+
• 0 = no response
• 1+ = response occurs, but is slow and sluggish, somewhat suppressed
• 2+ = a brisk, normal reflex
• 3+ = a very brisk response
• 4+ = hyperactive reflex with clonus present
• 2+ is generally normal, but reflexes vary
• Some may have sluggish reflexes (1+), while others have brisker ones (3+)
• 1+, 2+, and 3+ may be normal if all reflexes are similar
• Complete lack of response (0) or hyperactive reflexes (4+) are almost always abnormal

Deep Tendon Reflexes and Reflex Arc Damage

• Damage to neurons between the spinal cord and body leads to hyperactive or absent reflexes
• This occurs because neurons can not transmit impulses properly
• Poliomyelitis damages motor nerves, causing loss of motor control and deep tendon reflexes in affected areas
• Peripheral neuropathy is the most common cause of absent reflexes, from diseases like diabetes, alcoholism, toxins, and vitamin deficiencies

Exercise for Cognition

• Exercise improves reaction times, balance, coordination, and response time in daily activities
• Regular exercise improves cognitive performance, mood, mental alertness, and overall well-being
• Aerobic exercise improves blood flow to the brain, potentially enhancing brain function and protecting neurons from free radical damage
• Brain changes from sustained exercise may be due to "neuroplasticity"
• Neuroplasticity: brain's ability to change in response to stimuli by reorganizing its structure, neuron connections, or function

Exercise

• Impulses (action potentials) are tiny electrical currents carried by neurons, especially axons
• These impulses cause muscle contraction or relay sensory information to the brain, and are linked to cognitive processes

Resting Membrane Potential

• Three contributing factors:
• Unequal pumping of sodium and potassium by the sodium-potassium pump
• Unequal diffusion of sodium and potassium across the plasma membrane
• Presence of negatively charged proteins inside the cell
• Maintaining this potential is critical for nerve impulse initiation and conduction

Action Potentials

• Initiated when the electrical charge difference across the plasma membrane changes, making the inside more positively charged
• Requires movement of positively charged ions into the cells
• A stimulus, usually a neurotransmitter binding to receptors, is needed to change membrane permeability to sodium ions
• Reaching threshold triggers events in action potentials which stimulates the opening of voltage gated sodium and potassium ion channels
• Sodium ions rush in, increasing the positivity inside to 30+ mV which is the "depolarization phase"
• After 30+ mV, voltage gated potassium channels open, potassium rushes out due to higher concentration, bringing charge back to resting membrane potential
• This return is the "repolarization phase."
• Potassium channels closing slowly, allowing more to leave, creating hyperpolarization
• Sodium-potassium pump restores sodium and potassium to their original locations, restoring RMP

Synaptic Transmission

• Impulses transmit from one cell to another at synapses, using neurotransmitters

• Impulse arrival at axon terminals opens voltage-gated calcium channels, triggering synaptic vesicles to fuse with the pre-synaptic membrane

• Neurotransmitter is released into the synaptic cleft and binds to receptors on the post-synaptic membrane, opening ion specific channels

• If positively charged ions enter, the inside of the plasma membrane becomes more positive, causing depolarization

• This action creates an excitatory transmission (action potential is generated)

• If negatively charged ions enter or positively charged ions leave, the inside becomes more negative causing hyperpolarization

• This action creates an inhibitory transmission

• Impulse transmission at the synapse is the slowest

Speed of Impulse Conduction

• Depends on:
• Amount of myelin around the axon
• Diameter of the axon
• More myelin or larger diameter increases speed
• A larger diameter allows faster movement with less resistance
• Myelin insulates and protects axons, increasing impulse speed
• Ion channels concentrate at nodes in myelinated axons where action potential events occur
• Impulse appears to jump from node to node, known as saltatory conduction
• Unmyelinated axons use continuous conduction, with voltage-gated ion channels along the length.
• Saltatory conduction is faster than continuous conduction
• Nerve fibers (axons) are classified as type A, type B, and type C:
• Type A: most myelin, largest diameter, fastest impulses (15-130 meters/second)
• Type B: some myelin, medium diameter, slower impulses (3-15 meters/second)
• Type C: no myelin, smallest diameter, slowest impulses (≤3 meters/second)

Deep Tendon/Stretch Reflex

• Involves only one synapse, making it monosynaptic
• It resists and prevents excessive muscle stretching
• Receptors are muscle spindle apparatus in muscles
• Each spindle has 3-10 thin intrafusal fibers enclosed in connective tissue
• The central regions of these fibers are stretch-susceptible and innervated by sensory neurons
• In the spinal cord, sensory neurons synapse directly with motor neurons which transmit to the muscle to contract, preventing over-stretching
• While the spinal reflex occurs, impulses go to the cerebrum to bring awareness, and to the cerebellum for coordination
• The most common stretch reflex is the knee-jerk or patellar reflex
• It is a very short period of time after the stimulus is applied when there is no apparent response at all which is called the latent period
• Nerve impulses from the receptor to spinal cord/motor neurons must cross the neuromuscular junction to the muscle cell to depolarize the muscle cell membrane so muscle contraction
• Muscle fiber shortening occurs after myosin heads flex in the muscle fiber

Electromyography

• Measures electrical activity in muscles that stimulate contraction (recorded as EMG)
• Recorded by:
• Inserting electrodes into the muscle
• Placing electrodes on the skin surface

Explore neurological reflexes, their grading, and possible implications of abnormal reflex responses, such as hyperactive or absent reflexes. Understand the role of the stretch reflex and impact of motor cortex injuries. Learn about the neurons involved in reflex arcs and potential causes of altered reflexes.