Anatomy: Nervous System Part 1

Function

All cells are naturally charged
Nervous tissue - Uses electrical charge for rapid communication
Receives sensory input- motor changes inside and outside the body
Integration (most complex)- processes and interprets sensory input
Motor output- causes a response by activating effector organs muscle or gland

Organization

Central nervous system: brain and spinal cord
Peripheral nervous system= nerves and ganglia

Regional divisions of peripheral nervous system

Spinal nerves= nerves to transmit signals to and from spinal cord; also connected to spinal ganglia
Cranial nerves= nerves that transmit signals to and from brain; also connected to cranial ganglia

Functional divisions of Peripheral nervous system

Afferent (sensory)- carries impulses to CNS
Somatic sensory- monitors external environment, skin, body wall, limbs
Visceral sensory- monitors visceral organs
Efferent (motor) carries impulses to affect your organs
Somatic motor (usually voluntary)- to Skeletal muscle
Visceral motor (involuntary)= autonomic nervous system- to cardiac and smooth muscle, glands
Sympathetic (“fight or flight”)
Parasympathetic (“rest and digest”)

Neurons

Nerve cells- but not only found in nerves
Function= rapid transmission and integration of signals
Specialized for conducting electrical impulses
Cannot divide after birth- but new neurons can be generated from stem cells

Anatomy of a “typical” multipolar neuron

Cell body (soma)- contains nucleus
Dendrites- receive signals from other neurons; send signals to cell body
Axon (“nerve fiber”)- receives signals from axon hillock of cell body
Axon terminals= transmit signal to other neurons
Myelin sheath= made of glial cells; insulates axons from each other; speeds up signal
Nodes of ranvier= gaps in meylin

Multiple sclerosis

An autoimmune disease of the CNS
Results in damage to both myelin and axons

Synapse

Site of neuron to neuron communication
Most are chemical, using neurotransmitter that diffuses across synaptic cleft
Some are electrical, using gap junctions

Functional classification

Sensory (afferent) neurons
Carries signal to CNS
Make up the sensory division of PNS
Note- axon terminal is (typically) in CNS
Motor (efferent) neurons
Carries signal from CNS to effector
Make up the motor division of PNS
Note- cell body is (typically) in CNS
Interneurons- between sensory and motor neurons- found entirely within CNS

Structural classification

Multipolar: many processes (many dendrites, 1 axon), most neurons (“typical”)
Bipolar: One axon, One fused dendrite, found in some sensory organs
Unipolar (pseudounipolar): typical sensory neurons, receptive endings; no dendrites
Axon with peripheral and central processes

Support cells (neuroglia or glia)

Don’t send nervous signals
May outnumber neurons
Continue dividing throughout life
Most brain cancers are gliomas= tumors of glial cells

Neuroglia in CNS

Oligodendrocytes= forms myelin sheath
Astrocytes= most abundant, wraps around capillaries, controls environment around neurons
Microglia= immune cells
Ependymal cells= lines fluid-filled cavities- propels fluid

Neuroglia in PNS

Schwann cells= forms meylin sheath
Satellite cells= controls environment around neuron cell bodies

Gross anatomy

Nerve= group of axons traveling together in PNS
Endoneurium= surrounds axon
Perineurium= surrounds each subgroup of axons
Epineurium= wraps around outside of nerve
Fascicle= subgroup of axons

Nerves versus neurons

Each nerve contains the axons of many neurons
Axons are also called nerve fibers
Axons carry signals toward the axon terminals
Most nerves carry signals in both directions- because contain neurons oriented in both directions
The only exception are some cranial nerves

Ganglion

Collection of neuron cell bodies in the PNS

Gray and white matter

Two general types of nervous tissue in the CNS
White matter= myelinated axons
Gray matter= everything else (Unmyelinated axons, dendrites, cell bodies)

Tract

Collection of axons traveling together inside the CNS
White matter

Nucleus

Collection of neuron cell bodies inside the CNS
Gray matter

Brain

Complex
100 billion neurons
Many synapse per neuron

Brain development: primary brain vesicles

Neural tube (four week embryo)- fluid filled
Primary brain vesicles
Prosencephalon (forebrain)
Mesencephalon (midbrain)
Rhombencephalon (hindbrain)

Spina bifida

Narrow tube fails to close completely
can result in partial paralysis, risk of infection

Brain development: secondary brain vesicles

Five week embryo
Forebrain:
Telencephalon- “telephone receiver”
Diencephalon- “dial-a-cephalon”
Midbrain:
Mesencephalon
Hindbrain:
Metencephalon
Myelencephalon

Brain structures of birth (from rostral to caudal)

Telencephalon= cerebrum
Diencephalon= diencephalon
Mesencephalon= midbrain (brain stem)
Metencephalon= pons, cerebellum (brain stem)
Myelencephalon= medulla oblongata (brain stem)

Ventricles

Fluid filled cavities
Lateral ventricles in cerebrum
Third ventricle in diencephalon
Cerebral aqueduct in midbrain
Fourth ventricle between pons, cerebrum, medulla oblongata

Cerebrospinal fluid (CSF)

Fluid that fills ventricles
Cushions brain
transports nutrients, wastes, other chemicals

Choroid plexus

Found in all four ventricles
Produces the CSF
Contains:
capillaries= produce CSF by filtration
Ependymal cells= modify CSF

Circulation of CSF

Propelled by cilia of ependymal cells
Produced in lateral ventricles->
Third ventricle (also makes CSF)->
Cerebral aqueduct->
Fourth ventricle (also makes CSF)->
To be continued among the layers of meninges

Meninges

3 layers of CT surrounding the CNS
Protect the CNS
Contains a portion of the CSF
Dura mater
Arachnoid Mater
Pia mater

Dura mater

outer layer (“tough mother”)- 2 parts in brain
Periosteal layer (outer)= periosteum
Meningeal layer (inner)
Duran sinus= Space between these layers that carries blood- functions as a vein

Arachnoid Mater

Middle layer (“spidery mother”)
Subarachnoid space- contains CSF and large blood vessels

Pia mater

Inner layer (“gentle mother”)
Carries small blood vessels to nervous tissue

Meninges: Rolling circulation of CSF

Fourth ventricle
Apertures
Subarachnoid space
Arachnoid villi
Dural sinuses (combines with blood from veins)
Internal jugular veins
Back to heart

Meninges: Rolling circulation of CSF (Alternate)

Fourth ventricle
Central canal a spinal cord
Apertures
Subarachnoid space
etc.

Hydrocephalus

Buildup of excess CSF in the brain