Spinal Reflexes

Abstract

Spinal reflexes are automatic and stereotyped behaviours that are integrated into a hierarchical control system. Reflex actions are controlled by the interaction of inputs that converge on reflex pathway neurons.

Keywords: motor neurons; interneurons; convergent control; coordinated movements

Figure 1.

A much simplified diagram of the spinal cord (above, shown in cross‐section) and lower leg (below) showing the essential elements involved in the stretch reflex and the complementary inhibition of antagonist muscles. An agonist ankle extensor muscle (the stretched muscle) contains a muscle spindle that gives rise to the peripheral process of a group Ia muscle spindle afferent that has its cell body in the dorsal root ganglion. The central process of the afferent enters the grey matter of the spinal cord to make direct excitatory (+) synaptic contacts on a motor neuron (MN) that innervates a collection of muscle fibres (muscle unit) in the agonist muscle. The same group Ia afferent makes excitatory synapses on an interneuron that in turn makes inhibitory (−) synapses on an ankle flexor (antagonist) motor neuron. The individual neurons in the diagram each represent large groups with the same organization.

Figure 2.

Diagram of the organization of some of the control systems that make synaptic contact with interneurons in the disynaptic group Ia reciprocal inhibitory reflex pathway. The target interneuron (Ia IN, blue), which inhibits flexor motor neurons (MNs), receives excitatory synapses (+) from extensor group Ia afferents, from several systems descending from the supraspinal brain, and from the extensor half (EXT) of the central pattern generator (CPG) for locomotion in the spinal cord. The interneuron also receives inhibitory synapses (−) from other group Ia inhibitory interneurons (Ia IN, pink), specifically those that are excited by flexor muscle spindle afferents. The cell is also inhibited by synapses from Renshaw interneurons, which are excited by recurrent collateral axons from extensor motor neurons. FLEX, flexor half of the CPG.

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Further Reading

Baldissera F, Hultborn H and Illert M (1981) Integration in spinal neuronal systems. In: Brooks VB (ed.) Handbook of Physiology. Section 1: The Nervous System. Vol. II. Motor Control, Part 1, pp. 509–595. Washington, DC: American Physiological Society.

Burke RE and Rudomin P (1977) Spinal neurons and synapses. In: Kandel ER (ed.) Handbook of Physiology, Sect. 1, The Nervous System, Vol. 1, The Cellular Biology of Neurons, Part 1, pp. 877–944. Bethesda, Maryland: American Physiological Society.

Creed RS, Denny‐Brown D, Eccles JC, Liddell EGT and Sherrington CS (1932) Reflex Activity of the Spinal Cord. London: Oxford University Press.

Gordon J (1991) Spinal mechanisms of motor coordination. In: Kandel ER, Schwartz JH and Jessell TM (eds) Principles of Neural Science, 3rd edn, pp. 581–595. New York: Elsevier.

Houk JC and Rymer WZ (1981) Neural control of muscle length and tension. In: Brooks VB (ed.), Handbook of Physiology. Sect. I. The Nervous System. Vol. II, Motor Control, Part 1, pp. 257–323. Bethesda, Maryland: American Physiological Society.

Jankowska E and Lundberg A (1981) Interneurones in the spinal cord. Trends in Neurosciences 4: 230–233.

Lundberg A (1979) Multisensory control of spinal reflex pathways. In: Pompeiano O (ed.) Reflex Control of Posture and Movements, pp. 11–28. Amsterdam: Elsevier.

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How to Cite close
Burke, Robert E(Apr 2001) Spinal Reflexes. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0000204]