Neurons

The nervous system is the master controlling system of the body. It has three main functions: 1) It monitors changes occurring both inside and outside the body by means of millions of receptors; 2) It processes and interprets the sensory input and makes decisions about what should be done – integration; 3) It dictates a response to activate the effector organs by means of motor output. Subfields relevant to research of nervous system: Neuroanatomy Neurophysiology Neuropathology Neurophamocology Neurobiology Neuroscience

Part 6

Introduction of the nervous system

Major divisions of the nervous system: Central nervous system CNS consisting of brain and spinal cord, is the integrating and command center of the nervous system. Peripheral nervous system PNS consisting of cranial and spinal nerves extending from brain and spinal cord, serves as communication lines linking all regions of the body to CNS Sensory (afferent ) nerves Motor (efferent ) nerves Somatic nerves 躯体神经 Visceral nerves 内脏神经

Visceral nerves 内脏神经 Autonomic nervous system ANS is the system of motor neurons that innervate the smooth muscle, cardiac muscle and glands, which consisting of two parts:

Sympathatic nerve 交感神经 Parasympathatic nerve 副交感神经

Both divisions innervate the same visceral organs but cause opposite effects Sensory part of the visceral nerves The visceral nerves also contain sensory part which monitor tretch, temerature, chemical changs, and irritation within the visceral organs

DR. CAMILLO GOLGI (1843-1926)

Born in Corteno (Brescia), Italy, Golgi received his education at PAVIA (Italy) and then became assistant to the great Cesar Lombroso at the Institute di Patologia Generale. His work on pellagra early and psammomas later brought him great recognition but the development of the Golgi method to stain neuroglia achieved for him permanent fame. His studies on gliomas were monumental. Together with Santiago Ramon oy Cajal (1852-1934) when he even denunciated, they jointly received the Nobel Prize in 1906.

Special methods are required to show the geometry and spatial relationships of neurons. One of these is the Golgi method, which shows the neuronal cell body with the dendrites and axon extending from it.

The Golgi method also demonstrates that the neurons in each area of the CNS have characteristic shapes and specific orientations

Santiago Ramon y Cajal

Chapter 1

General Description

The nervous tissue is made up of two types of cells: 1) neurons, the excitable nerve cells that transmit electrical signals; 2) supporting cells, nonexcitable cells called neuroglia mainly including astrocyte, oligodendrocyte and microglia

Oligodendrocyte Astrocyte Ependymal cell neuron

Microglia

Neuron nerve cell 神经元 The basic functional unit of the nervous system; a highly specialized cell

Cell body : (soma) Nissl bodies (chromophil substance) Neurofibrils Dendrite: dendritic spines Axon: synaptic terminals

neurons The fundamental unit of the nervous system. Neurons are the integrative elements of the nervous system engaged in information transfer, modulation and storage.

neurons In general, neurons have a centrally placed nucleus and nucleolus. Distributed in the cytoplasm are clumps of basophilic material called Nissl bodies. They are collections of granular endoplasmic reticulum.

neurons Two varieties of processes taper from the cell body: the thicker and shorter processes are the dendrites and the thinner, longer process is the axon.

Axon The specialized, conductile portion of the neuron that serves to transmit information from its receptive pole (soma, dendrites) to the presynaptic terminals on other neurons, muscles, or glands.

Dendrite the 'receptive' portion of the neuron on which most synaptic contacts are located; arising as a series of complex processes which commonly extend several hundred micrometers from the soma and constitute much of the gray matter neuropile. An apical and a basal dendrite are shown on a pyramidal cell.

neurons Many dendrites have tiny protrusions called dendritic spines. The structural modification of spines is associated with mental retardation, aging, and epilepsy. Also, a significant decrease in the number of spines has been reported in the brains of elderly people.

neurons Neurons are functionally related at points called synapses. A typical synapse consists of an axon terminal (the presynaptic component d) ending on a dendrite (the postsynaptic component E)

mitochondria (A) vesicles that contain neurotransmitters (B) synaptic cleft (C)

Classification of neurons 1.Unipolar, bipolar and multipolar neuron 2.Sensory neurons, interneurons and motoneuron 3.Golgi typeⅠandⅡ, Amacrine neurons 4.Monoaminergic, cholinergic and peptidergic neurons

Terminology Grey matter: neural tissue dominated by neurons’ soma and dendrites Cortex: a layer of grey matter at the surface of the brain, cerebrum and cerebellum Nucleus: a group of nerve cell bodies in the brain or spinal cord that can be demarcated from neighboring groups on the basis of differences in cell type White matter: neural tissue dominated by myelinated axons Medullary substance: consisting of the lipid material present in the myelin sheath of nerve fibers, lies deep to the cortex in cerebrum and cerebellum

Tract (fasciculus): a bundle of axons within CNS that share a common origin, destination and function Ganglion: a distinct collection of sensory or postganglionic cell bodies within PNS Nerve fiber: the axon of a nerve cell, ensheathed by oligodendroglia cells in brain and spinal cord, and by Schwann cells in peripheral nerves. Nerve: a bundle of axons in PNS

Chapter 2

Central Nervous system

Section 1 spinal cord Ⅰ. external Features:

脊髓

1.Enlargments cervical enlargement 颈膨大 enlargment 腰骶膨大

Lumbosacral

2.external longitudinal fissures and sulci anterior median fissure 前正中裂 posterior median sulcus 后正中沟 anterolateral sulcus 前外侧沟 posterolateral sulcus 后外侧沟 Conus medullaris 脊髓圆锥 Filum terminale 终丝

3.Segments of the spinal cord The short part of spinal cord associated with a pair of spinal nerves is called a segment of spinal cord; A segment correspond to the dorsal and ventral roots of a pair of spinal nerves

31 segments of spinal cord: 8 cervical segments 12 thoracic segments 5 lumbar segments 5 sacral segments 1 coccygeal segemnts

4.Location of spinal cord in relation to vertebral column 1)

In the third month of fetal life, the cord is as long as the vertebral canal;

2)

At birth, the cord ends at the level of the 3th lumbar vertebra;

3)

In the adult, the cord ends at the level of the lower border of the 1st lumbar vertebral body;

Corresponding position of cord to vertebral bodies

segments of cord

vertebral bodies

C1-4

C1-4

C5-T4

C4-T3

T5-8

T3-6

T9-12

T6-9

L1-5

T10-12

S1-4,Co1

L1

Lumbar puncture: a needle is inserted between L3 and L4 to get sample of cerebrospinal fluid ( CSF)

Cauda equina

Ⅱ. Internal structure of the spinal cord gray matter

灰质

Anterior horn 前角 posterior horn 后角 Lateral horn 侧角 intermediate zone 中间带

Anterior gray commissure 灰质前连合 Posterior gray commissure 灰质后连合

white matter

白质

anterior funiculus 前索 posterior funiculus 后索 lateral funiculus 外侧索 anterior white commissure 白质前连合 reticular formation 网状结构

central canal

中央管

Anterior horn (& ventral roots)

Posterior horn

Lateral horn: Intermediolateral cell column

Anterior Funiculus Posterior funiculus

Lateral funiculus

Gray matter of the spinal cord Some nucleus in the gray matter Posteromarginal N. 后角边缘核 receive incoming fibers

Posteromarginal nucleus Substantia gelatinosa Nucleus proprius Thoracic nucleus

of dorsal root

Substantia gelatinosa

胶状质

receive fibers of pain and temperature sensation

Nucleus proprius

后角固有核

Thoracic nucleus

胸核 give fibers to

receive all sensory modalities dorsal spinocerebellar tract in segments T1-L3 Motoneurons of anterior horn 前角运动神经元

αmotoneuron

innervate skeletal

muscles

γmotoneuron

intrafusal muscle

innervate

Medial motor neurons

Lateral motor neurons

Medial motor neuron

axial muscles

Lateral motor neuron

distal muscles of arm and leg

Lamina of Rexed

a division of the gray matter of the spinal cord into nine laminae (I–IX) and a gray area around the central canal (area X) based on cytoarchitectural features; the dorsal (posterior) horn is composed of laminae I–VI, the intermediate zone of lamina VII, and the ventral horn of laminae VIII and IX;

General correlation of laminae with some of the major nuclei: I ： posteromarginal nucleus; II ： substantia gelatinosa; III and IV ： nucleus proprius; V and VI ： sometimes described as containing the spinal reticular formation; VII ： Clarke nucleus (thoracic nucleus), intermediolateral cell column; VIII ： commissural nuclei, interneurons; IX ： motor nuclei of ventral horn.

Relationship between the gray matter and the spinal nerve: 1) posterior horn links with both somatic and visceral sensory fibers; 2) anterior horn links with somatic motor fibers; 3) lateral horn links with sympathetic (visceral motor) fibers

White matter in the spinal cord All fibers in the cord are assigned to 5 types: 1)affenrent fibers 2)efferent fibers 3)intersegmental fibers 4)long ascending fibers conducting afferent impulses to supraspinal levels 5)long descending fibers from supraspinal souces to synapse with spinal neurons

Fibers of the type 4 and 5 form longitudinal bundles with more or less distinct demarcation; Fiber bundles having the same origin, course and termination are known as tracts or fasciculi occupying particular area within the white matter of spinal cord

Long ascending tracts: FG Fasciculus gracilis; FC Fasciculus cuneatus; STT spinothalamic tracts; DSCT and VSCT Dorsal spinocerebellar tract and Ventral spinocerebellar tract

Long descending tracts: CST Corticospinal tract lateral corticospinal tract and anterior corticospinal tract; RST Rubrospinal tract; RT+VST Reticulospinal tract + Vestibulospinal tract; TST Tectospinal tract

Organization of the dorsal root and its relationship with the main ascending tracts: 1)lateral division: convey impulses related to pain, thermal and light touch sense,

pain and thermal fibers enter lateral spinothalamic tract, light touch fibers enter anterior spinothalamic tract;

2)medial division: convey sensation of fine touch, movement and proprioception, which enter posterior funiculus to form gracile and cuneate fasciculi

Fasciculus gracilis FG 薄束 fasciculus cuneatus FC 楔束

Dorsal column system

Position: within the posterior funiculus / FG in most medial part; FC lateral to FG; arising from the thick and large fibers of the medial part of dorsal root and ending in the gracile and cuneate nuclei in the medulla Function: convey sensation of fine touch, vibration, twopoint discrimination, proprioception and position sense FG conducts input from the lower half of the body; FC transmits input from the upper half of the body. Thoracic 4 is the demarcation for FG and FC Somatotopic organization: Fibers are arranged in an orderly fashion from medial to lateral in relation to the regions of the body from down up Fasciculus gracilis and cuneatus

Spinothalamic tract STT

脊髓丘脑束

Position: anterior STT in the anterior funiculus fuses laterally with lateral STT in the lateral funiculus; Function: 1) anterior STT convey impulses of light touch and pressure sensations ; 2) lateral STT conducts pain and temperature sensibilities Some anatomical details: 1) all the fibers come from the lateral part of the dorsal root; 2) all primary fibers relay in laminae Ⅰ and Ⅵ-Ⅷ; 3) majority of fibers cross the median line in anterior white commissure to contralateral tract; 4) decussation is completed in the segment above the entrance of the dorsal root Somatotopic organization: fibers from lower part of body is sited in lateral part of the tract; fibers from upper part in medial part Spinothalamic tract

Corticospinal tract CST

皮质脊髓束

Descending projection pathway for controlling voluntary movements Arise mainly from precentral motor cortex and premotor area Majority of fibers cross over to form lateral CST and uncrossed fibers form anterior CST Lateral CST: ends primarily in lateral portion of the intermediate zone and anterior horn of the cervical and lumbosacral enlargment to contral distal limb muscles Somatotopic organization: fibers controlling the lowest part of the body are most laterally placed Anterior CST: descends in anterior funiculus and extends only to the upper thoracic cord; Ends in the same area with lateral CST to control axial and girdle muscles of the upper part of the body bilaterally Corticospinal tract

Rubrospinal tract: facilitates flexor muscles and inhibits extensor ones

Vestibulospinal tract: excite motor neurons of extensor muscles and inhibit those of flexor muscles

Reticulospinal tract: control axial and girdle muscles and regulate posture; help control automatic movements

Medial longitudinal fasciculus: help control head position

Tectospinal tract: coordinate head movements with eye movements, maybe cause turning in response to sudden visual or auditory stimuli

Minor descending tracts