Reflexes and Spinal cord function
(Physiology Col. Book Plates
26-27,29-30, 82, 84-86,95)
Structure of spinal cord
-Cervical, Thoracic, Lumbar, Sacral, Coccygeal Segments
-Dorsal/Posterior Horn (sensory), Middle Zone/Interneurons (association), Ventral horn
Gray matter vs. white matter
-Gray matters function is to route sensory or motor stimuli to interneurons of CNS to
create response to stimulus. Process info originating in sensory organs or other gray
matter regions, contains sensory and motor neurons that come in through dorsal root and
can be sent up to brain or use interneuron to sent inhibitory or stimulatory signal through
ventral root. These connect at sensory ganglia.
-White matter is tissue made up of axons that pass between gray matter. White matter
transmits info quickly through axons, can be thought of as wiring.
Spinal cord and reflex functions (what is a spinal reflex?
-Spinal reflexes are those associated with spinal cord control of trunk and limb muscles.
Somatic reflexes involve skeletal muscles and motor fxns, while autonomic reflexes
regulate internal environment by affecting exocrine glands, heart and visceral muscle.
-Spinal reflexes are important for 1) defending against noxious stimuli 2) maintaining
postural body support
Reflex arc - parts of reflex arc
1) Sensory receptor, to detect stimulus
2) Afferent nerve, which conveys sensory signal to spinal cord or brain
3) Synaptic center, to integrate and analyze sensory input and produce motor
4) Efferent nerve, to conduct motor output to the periphery
5) Motor Effector, to carry out response
Spinal shock; variation in animal species in duration of spinal shock; reason:
Spinal reflexes can occur independently, without brain control as seen in spinal
transected animals and quadriplegic humans. For varying durations after spinal
transaction, spinal reflexes do not occur. This period of spinal shock is shorter in lower
animals and long in higher animals because a higher animal’s brain exerts more control
over its spinal cord, compared to a lower animal. The gradual increase in motor control
during development is known as encephalization.
Monosynaptic vs. Polysynaptic reflexes:
-A monosynaptic reflex, like the stretch reflex
, is the simplest reflex because there is only
one synapse in the path of its arc (the monosynaptic reflex arc). Large skeletal muscles
involved in body support and limb movements contain muscle spindles, which act as
sensory organs detecting change in muscle length
-A polysynaptic reflex arc contains one or more interneurons and higher number of
synaptic connections. The knee jerk reflex
contains both mono and polysynaptic reflexes.