Lecture 4: Neurophysiology Reflex Arc Pt 3 Flashcards
Clinical case: 2 month old calf, gradual onset of hind leg “lameness” Upon examination, no withdrawal responses in hind legs. Weakness, poor muscle tone, loss of proprioception but front legs have normal reflexes, strength and proprioception. What’s the problem?
-Only in hind leg; usually a spinal cord issue bc brain would be both sets
-Withdrawal responses; inflict a small amount of pain to see if they withdraw their leg
-Loss of proprioception (being able to move without thinking about it) is a sensory issue
Therefore, pointing towards a spinal cord issue/injury
What are the 3 layers of control within the CNS movement?
- Higher centres: cerebral cortex (conscious)
- Middle level: Sensorimotor cortex, cerebellum (monitors execution), Basal ganglia (semi-voluntary movements)
- Lower level: spinal cord from which motor neurones exit (site of somatic reflex arcs)
NOTE: tension in muscles and tendons, and angles of joints send back sensory info to CNS (proprioception)
What are the components involved in the control of skeletal muscles (beyond reflex)?
Components involved in planning and programming of moments: Cerebral cortex, Basal ganglia, thalamus, and cerebellum.
Components responsible for the smooth execution of movements: Brain stem, Spinal cord and cerebellum.
Somatic reflex arc involves local integration in the spinal cord.
What is a reflex?
-Automatic or unconscious response to effectors organs (Muscle or gland) to a stimulus.
-Involuntary- going to happen anyway
-unvarying response- if apply the same stimulus, same response over again.
What are the 5 fundamental components of a reflex?
- Sensory cells/ receptors; transmit into an AP
- Sensory nerve fibres; bring info towards the CNS
- Coordinating centre (CNS); brain and spinal cord or brain, integration centres bunch of cells grouped together
- Motor nerve fibres; action to be taken by motor nerve fibres
- Effectors (muscle or gland cells)
NOTE: a malfunction of any one component leads to the alternation in the response.
What happens in the Receptor stage of the reflex arc?
Receptors:
-Transduce the environmental energy at the level of the:
-Skin: heat, cold, pressure
-Muscle (spindle): stretch (stimulates in muscles called spindle)
-Tendons (Golgi): Tension (body builders go too hard and tendon rips off bone)
-Convert energy into action potential (AP)
-Frequency of AP’s proportional to intensity of energy transducer (frequency coding)-enables CNS to detect intensity
What happens in the Sensory stage of the reflex arc?
- Sensory (afferent) nerve: Away from body towards the CNS
-Conduct AP from the report
-Cell body in ganglion outside of spinal cord
-Enter spinal cord via dorsal roots
What happens in the Coordinating centre stage of the reflex arc?
- Coordinating center= CNS synapses
-Monosynaptic (muscle spindle reflex)-direct
-Multisynaptic (elaborate reflexes) involve more then one muscle through involvment of spinal cord
What happens in the Motor stage of the reflex arc?
- Motor (efferent) nerve
-Carries Ap’s from the CNS to target (effectors) organs
-Via ventral roots
-Cell body within the CNS
What happens in the Effector stage of the reflex arc?
- Target organ response:
-Skeletal muscle ex in knee jerk (patella) reflex
-Multiple targets (scratching)
Contraction or relaxation
What is the stretch and tendon reflex?
-Changes in muscle length detected by muscle spindles
-modified muscle fibres = spindle or intrafusal fibres embedded in muscles
-Middle segment acts as stretch receptor by pinching the bear end of the sensor neuron —-> sensory affecternt nerve —-> spinal cord
-Sensory nerve makes excitatory synaptic contact with motor neurons serving the same muscle —-> muscle shortening
-Changes in tendon length detected by Golgi organ
-Senses tension in tendons: muscle contracts –>pulls tendon–>squeezes bare end of neuron –>sensory nerve contacts with in inhibitory interneuron in spinal cord –>inhibits motor neurons serving the same muscle
A) What happens when you are holding an empty glass of water?
B) Water is added to cup?
C) At the same position with full cup
A) Resting postion: 2 motor neurons (for bicep and tricep in arm)
B)Biceps stretched by added weight—>AP in sensory neuron enters spinal cord –>activates biceps motor nerve and via interneuron inhibits triceps motor nerve
C)Biceps contract, triceps relax posture is restored
All done by reflex at the level of the spinal cord
What happens in the knee jerk reflex?
-Same protective reflex concept to prevent tendon injury
-Quadricepts muscle contraction in response to “tapping” of the patella ligament
-Brief leg extension
-Requires relaxation of opposing flexor muscles (via activation of inhibitory interneurons)
