Nervous System

The Nervous System is part of the Anatomy and Physiology section which provides High Yield information for the MCAT exam needed for Medical School.

Organization of the Nervous System

• – Central nervous system (CNS)
  • 1. brain and spinal cord
    • a. integrating and command center
• – Peripheral nervous system (PNS)
  • 1. cranial nerves
    • a. carry electrical signals to and from brain
  • 2. spinal nerves
    • a. carry electrical signals to and from the spinal cord
  • 3. two main functional divisions
    • a. sensory (afferent) division
      • 1) carries signals toward CNS, from skin, muscles and joints (somatic), and from visceral organs (visceral)
    • b. motor (efferent) division
      • 1) carries signals away from CNS to effector organs
      • 2) somatic division (to skeletal muscle, voluntary control)
      • 3) autonomic division (to smooth and cardiac muscle, glands; involuntary control; further divided into sympathetic division (“fight or flight”) and parasympathetic division (“resting and digesting”)

Histology

• – Nervous tissue made up of…
  • 1. neurons
    • a. cells that receive and transmit electrical signals
  • 2. neuroglia (glial cells)
    • a. supporting cells of CNS and PNS
• – Neuroglia
  • 1. in CNS
    • a. astrocytes
      • 1) hold neurons together
      • 2) repair of injury and scar formation
      • 3) induce changes in blood vessels to form the blood-brain barrier hold neurons together
      • 4) take up and break down some neurotransmitters (chemical signal molecules)
      • 5) maintain ion concentrations
    • b. microglia
      • 1) defense cells
    • c. ependymal cells
      • 1) line cavities in brain and spinal cord
      • 2) help form and circulate cerebrospinal fluid
    • d. oligodendrocytes send out extensions that wrap neurons, forming myelin sheaths
  • 2. in PNS
    • a. Schwann cells make myelin sheaths
    • b. satellite cells support clusters of neuron cell bodies (in ganglia)
• – Neurons (nerve cells)
  • 1. highly specialized to conduct electrical signals
  • 2. can vary in structure but all have some common features
  • 3. common features
    • a. cell body (soma)
      • 1) nucleus and other organelles
      • 2) well developed rough ER
      • 3) plasma membrane has receptors for neurotransmitters (receives chemical signals)
      • 4) clusters in CNS called nuclei, in PNS called ganglia
    • b. dendrites
      • 1) relatively short, highly branched projections from cell body
      • 2) plasma membrane has receptors for neurotransmitters (receives chemical signals)
      • 3) conducts electrical signals toward cell body
    • c. axon (nerve fiber)
      • 1) one long projection from cell body, begins at axon hillock
      • 2) may be up to a few feet long
      • 3) may have branches called collaterals
      • 4) ends in many branches called axon terminals
      • 5) carries electrical signals away from the cell body (signal called depolarization or action potential or nerve impulse)
      • 6) when AP reaches terminals, neurotransmitters are released
    • d. myelin sheath
      • 1) many neurons have their axons covered in myelin sheaths (helps conduct electrical signals faster)
  • 4. connect to other neurons at synapses

Bundles of Axons

• – Called a nerve in PNS
  • 1. wrapped in CT coverings (as in skeletal muscle)
    • a. endoneurium wraps each fiber
    • b. perineurium wraps groups of axons called fascicles
    • c. epineurium wraps entire nerve
• – Called a tract in CNS

Structural classification of neurons

• – Multipolar neurons
  • 1. many dendrites, one axon
  • 2. most neurons of the brain and spinal cord
• – Bipolar neurons
  • 1. one dendrite, one axon
  • 2. in retina of eye, inner ear, olfactory (smell) neurons
• – Unipolar neurons
  • 1. one short process from cell body branches into two processes
    • a. sensory neurons
    • b. peripheral process has sensory receptor
  • 2. central process enters CNS

Functional Classification of Neurons

• – Sensory (afferent) neurons
  • 1. conduct signals toward CNS from skin, sensory organs, muscles, joints, viscera
  • 2. unipolar
  • 3. cell bodies in ganglia
• – Motor (efferent) neurons
  • 1. conduct signals away from CNS to muscles and glands
  • 2. multipolar
  • 3. cell bodies usually in CNS
• – Association neurons (interneurons)
  • 1. between sensory and motor neurons, and throughout brain
  • 2. multipolar
  • 3. contained entirely within CNS
  • 4. about 99% of neurons in the body
  • 5. thousands of types

Gray matter and white matter of CNS

• – Gray matter
  • 1. nerve cell bodies and dendrites
  • 2. axon terminals
  • 3. unmyelinated axons
  • 4. neuroglia
• – White matter
  • 1. bundles of myelinated axons

The Brain and Cranial Nerves

Major Parts of the Brain

• – Cerebrum
  • 1. two cerebral hemispheres
• – Diencephalon
  • 1. thalamus
  • 2. hypothalamus
  • 3. epithalamus
• – Brain stem
  • 1. midbrain
  • 2. pons
  • 3. medulla oblongata
• – Cerebellum
  • 1. two cerebellar hemispheres

Protection and Coverings

• – Cranial meninges
  • 1. continuous with the spinal meninges, same basic parts
    • a. dura mater
      • 1) tough outer layer
      • 2) dense irregular CT
    • b. arachnoid mater
      • 1) middle layer
      • 2) collagen and elastin fibers
      • 3) subdural space is between dura mater and arachnoid mater, contains a little fluid
    • c. pia mater
      • 1) inner layer
      • 2) delicate CT covering brain
      • 3) subarachnoid space is between arachnoid mater and pia mater, contains
        cerebrospinal fluid
  • 2. dura mater has 2 layers in cranial meninges
    • a. periosteal layer (outer)
      • 1) periosteum of cranial bones
    • b. meningeal layer (inner)
      • 1) corresponds to spinal dura mater
    • c. between layers are dural sinuses
    • d. extensions of dura mater separate parts of the brain
      • 1) falx cerebri- between cerebral hemispheres
      • 2) falx cerebelli- between cerebellar hemispheres
      • 3) tentorium cerebelli- between cerebrum and cerebellum
• – Cerebrospinal fluid (CSF)
  • 1. similar to plasma
  • 2. circulates through subarachnoid space
  • 3. functions
    • a. cushions CNS
    • b. maintained at optimal chemical levels (ions, nutrients, etc.)
  • 4. the ventricles are cavities within the brain that contain CSF
    • a. all ventricles are interconnected to one another, the subarachnoid space, and the central canal of the spinal cord
    • b. lined with ependymal cells
    • c. four ventricles
      • 1) two lateral- each within a cerebral hemisphere, separated by a thin membrane called septum pellucidum
      • 2) third ventricle– between the lateral ventricles, connected to lateral ventricles by interventricular foramina
      • 3) fourth ventricle– between brain stem and cerebellum, connected to third ventricle by cerebral aqueduct
  • 5. CSF formed at choroid plexuses
    • a. capillaries covered by ependymal cells, in all ventricles
  • 6. CSF reabsorbed into venous circulation
    • a. through arachnoid villi (extensions of arachnoid mater projecting into dural sinuses)
  • 7. circulation of CSF
• – Blood-brain barrier
  • 1. brain capillary cells are joined by tight junctions
  • 2. only things that can get through the lipid bilayer of capillary cells can pass easily (e.g., O2, CO2), and some things are specially transported (glucose, amino acids)
  • 3. protects brain from harmful substances

Cerebrum

• – Gyri are the ridges
• – Sulci are the grooves (deepest grooves called fissures)
• – Paired lobes (frontal, parietal, temporal, occipital)
• – Made up of…
  • 1. gray matter (cortex)
    • a. allows us to perceive, understand, communicate, remember, do voluntary movements
    • b. divided into many “functional areas” (but there is lots of overlap, and no one area acts
      alone)
    • c. three major kinds of functional areas
      • 1) motor areas– control voluntary motor function
      • 2) sensory areas– conscious awareness of sensation
      • 3) association areas– integrate diverse information
    • d. each hemisphere specializes in functions on the opposite side of the body (contralateral)
    • e. hemispheres not equal in function
      • 1) left side generally more involved in logical, analytical tasks like language and math
      • 2) right side generally more involved in spatial perception, art, music
  • 2. white matter
    • a. provides for communication between all areas of CNS and PNS
    • b. three main types of fibers
      • 1) association fibers- transmit signals between gyri in the same hemisphere
      • 2) commissural fibers– transmit signals from gyri in one hemisphere to the corresponding
        gyri in the other hemisphere (corpus callosum, anterior and posterior commissures)
      • 3) projection fibers– form ascending and descending tracts, transmit signals from cerebrum and other parts of brain to and from spinal cord
  • 3. basal nuclei (basal ganglia)
    • a. groups of gray matter embedded in white matter
    • b. corpus striatum (caudate nucleus and lentiform nucleus, which consists of putamen and globus pallidus)
    • c. amygdala (functionally part of limbic system)
    • d. mostly important in motor pathways (communicate with cortex)

Diencephalon

• – Epithalamus
  • 1. pineal gland
    • a. part of the endocrine system
    • b. produced melatonin, which helps regulate the biological clock
• – Thalamus
  • 1. masses of gray matter and tracts of white matter
    • a. two sides connected by intermediate-mass
  • 2. made up of several nuclei, each with a functional specialty
  • 3. major functions
    • a. preliminary processing of sensory input – screens out unimportant stimuli and passes on significant input to the appropriate area of cortex
    • b. crude awareness of sensation
    • c. some degree of consciousness
• – Hypothalamus
  • 1. many functionally grouped nuclei
  • 2. integrating center for homeostasis links the ANS and endocrine system
    • a. regulates body temperature (monitors temperature of blood)
    • b. regulates water balance through urine output and has “thirst center” (contains
      osmoreceptors that sense concentration of body fluids)
    • c. regulates food intake (monitors blood levels of nutrients and hormones)
    • d. controls endocrine functioning (produces hormones, regulates pituitary)
    • e. plays a role in emotional and behavioral patterns (part of limbic system)
    • f. controls ANS centers in brain stem (cardiovascular, respiratory)
    • g. contains biological clock
  • 3. pituitary connected by infundibulum
    • a. pituitary has two lobes
      • 1) anterior lobe is glandular tissue, produces and secretes hormones in response to hormones released from the hypothalamus
      • 2) posterior lobe is nervous tissue, stores hormones produced by the hypothalamus and
        releases them in response to electrical signals from hypothalamus

Brain Stem

• – Midbrain
  • 1. cerebral peduncles- motor and sensory fiber tracts
  • 2. superior cerebellar peduncles- carry info from cerebellum toward cortex
  • 3. reflex centers
    • a. superior colliculi- visual reflexes, like eyes tracking an object
    • b. inferior colliculi- auditory reflexes, like turning toward a loud noise
  • 4. other nuclei
    • a. substantia nigra and red nucleus- involved in motor pathways, interact with basal nuclei
    • b. nuclei for cranial nerves III and IV
  • 5. periaqueductal gray matter- sympathetic responses like increased heart rate and blood pressure and pain suppression
• – Pons
  • 1. motor and sensory fiber tracts
  • 2. middle cerebellar peduncles- carry info from cortex to cerebellum
  • 3. nuclei for cranial nerves V, VI, and VII
  • 4. respiratory centers- smooth out inspirations and expirations
• – Medulla oblongata
  • 1. connects to spinal cord at foramen magnum
  • 2. motor and sensory fiber tracts
  • 3. pyramids – formed by pyramidal tracts, most fibers cross over here, “decussation of the pyramids”
  • 4. inferior cerebellar peduncles- carry info on equilibrium from vestibular nuclei and info on proprioception from spinal cord to cerebellum (olives are nuclei acting as relay stations)
  • 5. nuclei for cranial nerves VIII-XII
  • 6. ANS nuclei
    • a. cardiovascular centers- regulate rate and force of heartbeat, blood pressure
    • b. respiratory center- regulates basic rhythm of respiration
    • c. centers for vomiting, sneezing, coughing, hiccuping, swallowing

Functional Brain Areas

• – Reticular formation
  • 1. an area of gray and white matter running through the core of the brain stem
  • 2. motor and sensory functions
  • 3. important in maintaining consciousness and overall alertness of cortex (reticular activating system, RAS)
• – Limbic System
  • 1. parts of cortex, basal nuclei, thalamus, hypothalamus
  • 2. deals with all aspects of emotion and physical expression of emotion (e.g., anger, fear, crying, laughing, gestures)

Cerebellum

• – Consists of vermis and cerebellar hemispheres with lobes (anterior, posterior, flocculonodular)
• – Has gray and white matter (white matter forms “arbor vitae”)
• – Cerebellar peduncles connect it to brain stem
• – Receives sensory information, especially from proprioceptors
• – Sends information to motor areas of brain
  • 1. coordinates movements
  • 2. adjusts posture to maintain equilibrium

Cranial Nerves

– Mnemonic device- Oh, Oh, Oh, To Touch And Feel Very Good Velvet And Hot

• 1 I OLFACTORY
• 2 II OPTIC
• 3 III OCULOMOTOR
• 4 IV TROCHLEAR
• 5 V TRIGEMINAL
• 6 VI ABDUCENS
• 7 VII FACIAL
• 8 VIII VESTIBULOCOCHLEAR
• 9 IX GLOSSOPHARYNGEAL
• 10 X VAGUS
• 11 XI ACCESSORY
• 12 XII HYPOGLOSSAL

Protection and coverings

• – Vertebral column
  • 1. the spinal cord is in a canal formed by the vertebral foramina
  • 2. vertebral ligaments also protect
• – Fat in the epidural space between the wall of vertebral canal and meninges
• – Meninges (spinal meninges) cover the cord and spinal nerves until they exit the vertebral column
  • 1. Dura mater forms sac from the foramen magnum to second sacral vertebra
  • 2. extended thickened portions of pia mater called denticulate ligaments to fuse with arachnoid mater and dura mater to hold the cord in place laterally

External Anatomy

• – Extends from brain to second lumbar vertebra
• – Two thickened areas
  • 1. cervical enlargement
    • a. nerves to and from upper limbs arise from this area
  • 2. lumbar enlargement
    • a. nerves to and from lower limbs arise from this area
• – Two grooves1. anterior median fissure
  • 2. posterior median sulcus
• – Conus medullaris
  • 1. end of cord tapers to a cone shape
• – Filum terminale
  • 1. extension of pia mater attaches cord to coccyx
• – Cauda equina
  • 1. some nerves exit the spinal cord and continue down the vertebral column to exit farther down
• – Dorsal and ventral roots fuse to form spinal nerves

Internal Anatomy

• – Gray matter
  • 1. has two sides, connected by gray commissure
    • a. central canal is in center, extends the length of the spinal cord (contains CSF)
    • b. anterior (ventral) horns – contain cell bodies of motor neurons supplying skeletal muscle
    • c. posterior (dorsal) horns– contain cell bodies of interneurons, axon terminals of sensory neurons
    • d. lateral horns – contain cell bodies for autonomic motor neurons which supply
      smooth/cardiac muscle and glands, only in thoracic, lumbar and sacral segments
  • 2. function: receives and integrates incoming and outgoing signals
• – White matter
  • 1. anterior (ventral) white columns, posterior (dorsal) white columns, lateral white columns
    • a. ascending (sensory) tracts– carry signals to brain
    • b. descending (motor) tracts– carry signals away from brain
  • 2. function: transmitting electrical signals
  • 3. ascending tracts
    • a. spinothalamic (anterior and lateral)
      • 1) carry info on pain, temperature, deep pressure, crude touch (poorly localized)
    • b. posterior column tracts (fasciculus gracilis, fasciculus cuneatus)
      • 1) carry info on proprioception (sense of body position, comes from muscles, tendons, joints), fine touch, pressure
    • c. spinocerebellar tracts (anterior and posterior)
      • 1) subconscious aspects of proprioception
  • 4. descending tracts
    • a. corticospinal tracts (anterior and lateral, a.k.a. pyramidal tracts)
      • 1) precise voluntary movements
    • b. all other tracts (tectospinal, vestibulospinal, rubrospinal, reticulospinal)
      • 1) subconscious movement like posture

Reflexes

• – Rapid, predictable motor response to a stimulus
• – Many are unlearned and involuntary
  • 1. all spinal reflexes are unlearned and involuntary
    • a. integrating center is the spinal cord
    • b. no brain involvement necessary, but brain is informed of what happened
  • 2. learned (acquired) reflexes
    • a. integrating center is brain
    • b. e.g., typing, playing a sport, driving
  • 3. most reflexes can be modified with conscious effort
• – Reflex arcs
  • 1. receptor – receives stimulus
  • 2. sensory neuron – electrical signal travels to…
  • 3. integrating center – the part of the CNS that decides on response, brain stem or spinal cord for unlearned reflexes
  • 4. motor neuron – signal sent to…
  • 5. effector – the part of the body that responds (skeletal muscle or gland)

Spinal Nerves

Dermatomes

• – A segment of skin served by cutaneous branches of a particular spinal nerve (all except C1)
• – Some areas overlap (trunk tends to have lots of overlap, less on limbs)

General Senses

Basics

• – Sensation: conscious or subconscious awareness of internal or external stimuli
• – Perception: conscious awareness and interpretation of sensation
• – Components of sensation
  • 1. stimulus
    • a. a change in the environment capable of activating sensory neurons
  • 2. transduction
    • a. sensory receptor or sense organ transduces stimulus into a nerve impulse
  • 3. conduction
    • a. nerve impulse conducted to CNS by afferent fibers
  • 4. translation
    • a. CNS receives and interprets information

Sensory Receptors

• – Display selectivity
  • 1. respond to a particular kind of stimulus
• – Classification by location
  • 1. exteroceptors
    • a. near surface of body
    • b. sense the external environment
    • c. touch, pressure, vibration, temperature, pain, taste, smell, hearing, vision
  • 2. interoceptors
    • a. in blood vessels and viscera
    • b. sense internal environment
    • c. stretch, chemical change, pain
  • 3. proprioceptors
    • a. in muscles, tendons, joints, inner ear
    • b. sense body position and movement
• – Classification by stimulus type
  • 1. mechanoreceptors
    • a. sense mechanical pressure or stretching
    • b. touch, pressure, vibration, proprioception, hearing, blood pressure
  • 2. thermoreceptors
    • a. sense temperature
  • 3. chemoreceptors
    • a. sense chemicals
    • b. taste, smell, changes in body fluids
  • 4. photoreceptors
    • a. sense light
  • 5. nociceptors
    • a. sense pain
• – Structural classification
  • 1. free dendritic endings
    • a. mostly sense pain and temperature, itch
    • b. tactile discs are a modified type in the epidermis, sense light touch (adapt slowly)
    • c. root hair plexuses sense movement of hairs (adapt quickly)
  • 2. encapsulated dendritic endings are enclosed in a CT capsule
    • a. tactile corpuscles – in dermal papillae of hairless skin; sense light pressure, discriminative touch, vibration
    • b. lamellar corpuscles – mainly subcutaneous, some more internal; sense deep pressures stretch, vibration (adapt quickly)
    • c. bulbous corpuscles – deep skin layers, joint capsules; deep pressure and stretch (adapt
      slowly)
    • d. proprioceptors – muscle spindles and tendon organs sense stretch; joint kinesthetic receptors in joint capsules sense stretch and pain (includes lamellar and bulbous corpuscles, tendon organs, free dendritic endings)

Sensory (Ascending) Pathways

• – Signals are carried to reticular formation, cortex, and cerebellum
  • 1. first-order neurons
    • a. have sensory receptor
    • b. carry signals to brain stem (along cranial nerves) or to spinal cord (along spinal nerves)
    • c. synapse with…
  • 2. second-order neurons
    • a. carry signals from spinal cord and brain stem to thalamus
    • b. fibers cross over (decussate) in cord or brain stem
    • c. synapse with…
  • 3. third-order neurons
    • a. signals go to primary somatosensory cortex (postcentral gyrus)

The Autonomic Nervous System

Basics

• – Input
  • 1. general visceral sensory neurons
    • a. e.g., chemoreceptors sensing blood gases, mechanoreceptors sensing organ stretch
• – Output
  • 1. general visceral motor neurons
    • a. excite or inhibit their effectors (cardiac and smooth muscle, glands)
  • 2. two neuron pathway from CNS to an effector organ
    • a. preganglionic fiber has cell body in CNS, synapses with postganglionic fiber in a ganglion, postganglionic fiber innervates effector organ
    • b. sympathetic division
      • 1) preganglionic fibers originate in thoracic and lumbar regions of spinal cord
      • 2) preganglionic fibers are short, synapse in a sympathetic chain ganglion lying along the vertebral column, release ACh (some pass through the chain and synapse later in a collateral ganglion closer to the effector organ)
      • 3) postganglionic fibers are long, terminate on effector organ and release norepinephrine (NE)
    • c. parasympathetic division
      • 1) preganglionic fibers originate in brain stem or sacral spinal cord
      • 2) preganglionic fibers are long, synapse in terminal ganglia in or near effector organs, release ACh
      • 3) postganglionic fibers are short, end on the effector organ, release Ach
• – Dual innervation
  • 1. most visceral organs are innervated by both sympathetic and parasympathetic fibers
  • 2. generally have opposite effects
  • 3. can be excitatory or inhibitory depending on the organ innervated
  • 4. both systems are usually partially active
    • a. sympathetic or parasympathetic tone, or tonic activity
  • 5. when one division increases its rate of sending signals and the other decreases, it’s called dominance
    • a. sympathetic dominance
      • 1) increase in blood flow to skeletal muscles (vessels dilate)
      • 2) heart beats faster and more forcefully
      • 3) blood pressure increases (most vessels constrict)
      • 4) respiratory airways dilate
      • 5) stored nutrients are broken down
      • 6) digestive and urinary activities are inhibited
      • 7) pupils dilate
      • 8) sweating
    • b. parasympathetic dominance
      • 1) inhibits sympathetic activities (e.g., heart rate and blood pressure decrease)
      • 2) normal resting functions like digestive and urinary activities increase

Details of Autonomic Anatomy

• – rami communicantes
  • 1. white rami
    • a. branch from thoracic and first few lumbar nerves
    • b. contain sympathetic preganglionic fibers
  • 2. gray rami
    • a. branch from sympathetic chain ganglia back to a nerve
    • b. contain sympathetic postganglionic fibers
• – autonomic plexuses
  • 1. groupings of ANS fibers in thorax, abdomen, and pelvis
  • 2. contain sympathetic and parasympathetic fibers
  • 3. pass along large blood vessels, supplying the vessels with fibers, then on to visceral organs
  • 4. four major ANS plexuses
    • a. cardiac plexus
    • c. pulmonary plexus
    • d. celiac (solar) plexus
    • e. hypogastric plexus

Autonomic Reflexes

• – Electrical signals travel through autonomic pathways (autonomic reflex arc)
• 1. e.g., adjustments made in heart rate and force of contraction, blood pressure, respiration, digestion, defecation, urination, pupil size changes

Control of ANS

• – mainly by hypothalamus
  • 1. sends signals to ANS centers in the brain stem
• – cortex can influence hypothalamus as a part of the limbic system (some degree of voluntary control)

Adrenal Medulla

• – the center portion of the adrenal gland is a modified part of the sympathetic division
  • 1. preganglionic sympathetic fibers innervate adrenal medulla
    • a. electrical signals result in the release of epinephrine and norepinephrine to the bloodstream (enhances sympathetic effects)

Brain Anatomy

Neuron Anatomy