Nervous SystemOverview, Tissues and Spinal Nerves
neurology - study of the nervous system
- nervous system and endocrine system work together to maintain homeostasis
- More than 80 major nerves and billions of neurons (individual nervous cells)
Nervous System - works very fast
- Uses electrical signals called nerve impulses which are short lived
- Ex.; knee jerk reflex
Endocrine System - works slowly
- Uses chemical messengers called hormones whose responses last a long time
- Ex.; reproductive cycle
Structures of the Nervous System
1. Neuron - cell of the nervous system 2. Nerve - bundle of neurons with connective
tissue and blood vessels3. Brain - contained in the skull; contains 100
billion neurons4. cranial nerves - 12 pairs, emerge from the
base of the brain and each serves a specific region of the body
Structures of the Nervous System (cont)5. spinal cord - connects to the brain through a large opening in the skull called the foramen magnum
- Encircled by bones of the vertebral column- Contains 100 million neurons
6. spinal nerves - 31 pairs, each serving a specific region on the body
Structures of the Nervous System (cont)7. ganglia - small masses of nervous tissue located outside the brain and spinal cord
8. enteric plexus - in walls of gastrointestinal tract; help regulate digestive system
9. sensory receptors - monitor changes in the internal or external environment
Functions of Nervous System1. Sensory Function - sensory receptors detect both
internal & external stimuli & carry that info to the brain via sensory (afferent) neurons
2. Integrative Function - processes the info received and acts on it through interneurons (the majority of neurons in the body)
3. Motor function -motor (efferent) neurons carry info from brain and spinal cord to effectors (cells and organs that respond to the integrative decisions)
Branches of the Nervous System
1. Central Nervous System (CNS) - brain & spinal cord- integration of sensory information - the source of thoughts, emotions, & memories
2. Peripheral Nervous System(PNS)-relay station to CNS- includes all nervous tissue outside of CNS, cranial
nerves & branches, spinal nerves & branches, ganglia & sensory receptors
- Further divided into Somatic, Autonomic and Enteric Nervous System
Somatic Nervous System (SNS)
- Under voluntary control- Sensory neurons send message to the brain- Motor neurons send impulses to skeletal
muscle only
Autonomic Nervous System (ANS)- Involuntary- Sensory receptors located in the chest and
abdomen- Motor neurons send message to smooth and
cardiac muscle- Further divided into Sympathetic and
Parasympathetic divisions
Autonomic Nervous System (ANS)
Sympathetic division - stimulates functions (ex. Increases heart rate)
Parasympathetic division - inhibits functions (ex. Decreases heart rate)
Enteric Nervous System (ENS)
- “Brains” of the gut- Involuntary- Controls digestive system
Types of Nerve Cells1. neurons - functional cell of the nervous system
- responsible for thinking, sensing, remembering, controlling muscle activity and regulating glandular secretions
- communicate using both electrical & chemical signals
2. neuroglia - “glue” of the nervous system- support, nourish and protect the neurons
Parts of a Neuron1. soma - the cell body; contains a nucleus with
organelles2. dendrites - “little trees”; receive messages
(bring action potential in from previous neuron
3. axon - carries messages to the next neuron, muscle or gland- joins the cell body at the axon hillock
Parts of a Neuron (cont.)
4. Schwann cells - cells that wrap themselves around the axon
- Forms the myelin sheath which protects the neuron and speed up the action potential
- myelin - insulating tissue composed of lipids that speeds up nerve impulses (not all nerves have myelin)
5. nodes of Ranvier - gaps in between myelinated sections (do not contain myelin!)
6. neurolemma - covering of myelin sheath ONLY in the PNS (covering of the Schwann cell); helps regenerate injured axons
- Since CNS lacks neurolemma, they cannot regenerate after an injury
How do neurons communicate?
synapse - space between neurons, or between a neuron and muscle or cell, where communication occurs
synaptic end bulbs - at the tip of the axon terminal
- end in little sacs called synaptic vesicles, which contain chemical neurotransmitters that influence activity of neurons, muscles or glands
NeurotransmittersAcetylcholine - slows heart rate
Glutamate, aspartate - powerful excitatory effects
Gamma aminobutyric acid (GABA), glycine - inhibitory effect (anti-anxiety meds are GABA agonists...enhance the action of GABA)
Neurotransmitters (cont)Epinephrine, norepinephrine - involved in waking from deep sleep, dreaming and regulating mood
Dopamine - emotional response & some aspects of movement
Serotonin - involved in sensory perception, temperature regulation, controls mood and sleep induction
Nerve Impulse PropagationSpinal cord conducts impulses along tracts
1. Sensory receptor - responds to a stimulus2. Sensory neuron - also called afferent neuron; axon
conducts impulse from receptor to the integrating center3. Integrating center - relays the impulse from the sensory
neuron to the motor neuron4. Motor neuron - also called efferent neuron; axon conducts
the impulse from the integrating center to the effector5. Effector - muscle or gland that responds to motor nerve
impulse
White Matter
white matter - groups of myelinated processes from many neurons
-in spinal cord, for nerve impulse propagation; sensory impulses flow from body to brain, motor impulses flow from brain to body
Gray Matter
gray matter - contains neuronal cell bodies, dendrites, unmyelinated axons, axon terminals and neuroglia
- in spinal cord, to receive and integrate incoming and outgoing info
Coverings of the Spinal Cordspinal meninges - connective tissue coverings that encircle the spinal cord
1. Dura mater - tough, outer layer (leather like)2. Arachnoid - middle layer; has a spider-web
appearance3. Pia mater - innermost layer that adheres to the
spinal cord
meningitis - inflammation of spinal meninges
spinal tap - needle inserted into subarachnoid space in adults (usually between 3rd and 4th lumbar vertebrae)
Spaces in spinal cordSubarachnoid space - located between the arachnoid and pia mater; contains cerebrospinal fluid (CSF)
Subdural space - located between the dura mater and arachnoid; contains interstitial fluid
Epidural space - cushion of fat and connective tissue located between the dura mater and the wall of the vertebral canal
Vertebral Column
-spinal cord is located within the vertebral canal (formed by the vertebral foramina) of the vertebral column
- spinal cord is the link between the brain and the nerves
Cerebrospinal Fluid-a clear, colorless liquid that circulates continuously in the subarachnoid space around the brain and spinal cord
- function is to act as a shock absorber and for the exchange of nutrients/waste
- provides the optimal environment for nerve impulses to travel
Spinal cord-extends from the medulla oblongata (most inferior part of the brain) to the second lumbar vertebra
cervical and lumbar enlargements - can be seen where nerves from the upper and lower limbs arise
conus medullaris - where spinal cord tapers
filum terminale - extension of the pia mater that anchors the spinal cord to the coccyx
cauda equina - roots of inferior spinal nerves
Spinal nerves-paths of communication between spinal cord and nerves innervating specific regions of the body
Roots - connect each spinal nerve to the spinal cord
- Posterior root - contains sensory fibers that conduct impulses into the CNS
- Anterior root - contains motor neurons that conduct impulses from CNS to effectors
Spinal nerves (cont)
-31 pairs
- part of peripheral nervous system
- connect the CNS to sensory receptors, muscles and glands
Spinal nerve coverings
epineurium - superficial covering over the entire nerve
perineurium - covering of fascicles, which are bundles of axons
endoneurium - covering of individual axons
Cervical Plexus-supplies skin and muscles of the head, neck and superior part of shoulders and chest
Sensory branches - lesser occipital (scalp), transverse cervical (anterior neck), supraclavicular (chest & shoulder)
Motor branches - ansa cervicalis (neck), phrenic (diaphragm)
Brachial Plexus-provides entire nerve supply of shoulders and upper limbs
axillary (deltoid muscle);
musculocutaneous (flexors);
radial (back of upper arm and forearm);
median (anterior forearm and hand); and
ulnar (medial forearm and hand) nerves
Intercostal Nerves
-thoracic nerves
-T2 - innervates axilla and back of arm
- T3-T6 - intercostal muscles and skin of anterior and lateral chest
-T7-T12 - intercostal muscles and abdominal muscles
Lumbar Plexus-supplies anterolateral abdominal wall, external genitals and lower limbs iliohypogastric (ab muscles, skin of abs & butt) ilioinguinal (ab muscles,thigh skin,external genitals) genitofemoral (genitals) lateral femoral cutaneous (skin of thigh) femoral (thigh muscles and skin of thigh, medial leg & foot); obturator (adductor muscles of leg, skin of thigh)
Sacral Plexus-supplies the butt, perineum and lower limbs superior gluteal (glutes) inferior gluteal (gluteus maximus) nerve to piriformis (piriformis muscle) sciatic - made up of tibial (back of leg) and common peroneal (front of leg) posterior femoral cutaneous (skin of anal region, lower butt) pudendal (muscles of perineum, skin of genitals)
Dermatomes
-the area of skin that provides sensory input via the posterior roots of one pair of spinal nerves
-makes it possible to locate damaged regions of the spinal cord (pricking areas of body with a pin)
- also used for anesthetics
Spinal cord transection-an injury that entirely severs the spinal cord resulting in permanent loss of sensation below the injury
-the location of the transection determines the extent of paralysis
Spinal cord transectionC1-C3- paralysis from neck down, ventilator needed
C4-C5 - diaphragm, which allows breathing
C6-C7- some arm and chest function
T4-T9 - control of trunk above umbilicus
Disordersneuritis - inflammation of one or several nerves resulting from direct blows, fractures, contusions, penetrating injury, infections, vitamin deficiency or poisoning
Disorders (cont)
shingles - acute infection of the PNS caused by the herpes zoster virus
- Normally it is dormant in the posterior root ganglion...if activated, it travels down the sensory neuron resulting in pain, discoloration of skin and a line of blisters
Disorders (cont)
poliomyelitis - polio
- Caused by poliovirus- Fever, severe headache, stiff neck and
back, deep muscle pain and weakness- Destroys cell bodies of motor neurons
resulting in paralysis
Disorders (cont)Multiple sclerosis - progressive destruction of myelin sheaths of neurons in the CNS; myelin sheaths sclerose (harden) in multiple locations
- Strikes between the age of 20 and 40, females twice as often as males
- Symptoms include a heaviness or weakness in the muscles, abnormal sensations, double vision
- Progressive loss of function
Disorders (cont)epilepsy - short, recurrent, periodic attacks of motor, sensory or psychological malfunction
- Attacks are initiated by abnormal synchronous electrical discharges from millions of neurons in the brain
- Results in involuntary skeletal muscle contraction (seizure)
- Most epileptic seizures are idiopathic
Disorders (cont)Amyotrophic lateral sclerosis - rare, progressive, neurodegenerative disease that causes the death of neurons responsible for controlling voluntary muscle movement
- Also called Lou Gehrig’s disease- Loss of muscle control to all parts of the
body, including breathing- Usually death within 5 years of diagnosis
Disorders (cont)Acute flaccid myelitis - affects the grey matter of the spinal cord
- Causes muscle weakness, loss of muscle tone, slurred speech
Disorders (cont)Guillain-Barre - disorder where the body’s immune system attacks nerves
- Starts with weakness and tingling and progresses to paralysis of entire body
- Usually preceded by an infection or illness
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