Spinal Cord, Spinal Nerves, Cranial Nerves and Reflexes
Spinal Cord, Spinal Nerves, Cranial Nerves and Reflexes
Topics Covered
Introduction
The spinal cord
Overview
Anatomy
Cross-sectional anatomy
Spinal Nerves
Nerve Plexuses
Spinal Reflexes
Introduction
The central nervous system of the body is broken into two anatomically distinct divisions: the central nervous system, made up of the brain and the spinal cord; and the peripheral nervous system, made up of all the nervous tissue exiting the brain and spinal cord. Within the nervous system, there are 3 fundamental types of neurons:
1) Sensory (afferent) neurons conduct signals from receptors to the CNS
2) Interneurons (association neurons) are confined to the CNS
3) Motor (efferent) neurons conduct signals from the CNS to effectors such as muscles and glands (PNS)
It is important to distinguish between these types of neurons when interpreting the anatomy of the both the central and peripheral nervous system.
The spinal cord carries information to and from the brain to and from the body. It is made up of all three types of neurons and their parts: cell bodies, dendrites, and axons.
The Spinal Cord
Overview
• Information highway between brain and body
• Extends through vertebral canal from foramen magnum to L1
• Each pair of spinal nerves receives sensory information and issues motor signals to muscles and glands
• Spinal cord is a component of the Central Nervous System while the spinal nerves are part of the Peripheral Nervous System
• .
Anatomy of the Spinal Cord
• Cylinder of nerve tissue within the vertebral canal (thick as a finger)
o Vertebral column grows faster so in an adult the spinal cord only extends to L1
• 31 pairs of spinal nerves arise from cervical, thoracic, lumbar and sacral regions of the cord
o Each cord segment gives rise to a pair of spinal nerves; This results in 31 spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal
Cross-Sectional Anatomy of the Spinal Cord
(Link to figure 13.14- Cross-section of Spinal Cord)
• Central area of gray matter shaped like a butterfly and surrounded by white matter in 3 columns
• Gray matter = neuron cell bodies with little myelin
• White matter = myelinated axons
Nerve Plexuses
(Link to figure 13.24- Nerve Plexuses of the Body)
· Cervical in the neck, C1 to C5
o Supplies neck and phrenic nerve to the diaphragm
· Brachial in the armpit, C5 to T1
o Supplies upper limb and some of shoulder & neck
· Lumbar in the low back, L1 to L4
o Supplies abdominal wall, anterior thigh & genitalia
· Sacral in the pelvis, L4, L5 & S1 to S4
o Supplies remainder of butt & lower limb
· Coccygeal, S4, S5 and C0
Spinal Reflexes
• Quick, involuntary, stereotyped reactions of glands or muscle to sensory stimulation
o Automatic responses to sensory input that occur without our intent or often even our awareness
• Functions by means of a somatic reflex arc
o Stimulation of somatic receptors
o Afferent fibers carry signal to dorsal horn of spinal cord
o Interneurons integrate the information
o Efferent fibers carry impulses to skeletal muscles
o Skeletal muscles respond
The Patellar Tendon Reflex Arc
1) Extensor muscle stretched
2) Muscle spindle stimulated
3) Primary afferent neuron excited
4) Primary afferent neuron stimulates X motor neuron to extensor muscle
5) Primary afferent neuron stimulates inhibitory interneuron
6) Interneuron inhibits X motor neuron to flexor muscle
7) X motor neuron stimulates extensor muscle to contract
8) Flexor muscle (antagonist) relaxes