MCAT Study Guide Biology Ch. 7 – Nervous System 2017-08-15T06:45:06+00:00

I.          7.1:  NEURONAL STRUCTURE AND FUNCTION

A.     VOCAB

1.     Action potential – localized area of depolarization of the plasma membrane that travels in a wave-like manner along an axon

2.     Synaptic transmission – a process in which an action potential reaches the end of an axon, and is then transformed into a chemical signal with the release of a NT into the synaptic cleft

B.     STRUCTURE OF THE NEURON

1.     Soma – cell body, contains nucleus

2.     Axon – long projection where action potentials take place; only 1 per neuron

3.     Dendrite – projections that receive signals; can have many or 1 (bipolar if 1, multipolar if >1)

C.    THE ACTION POTENTIAL

1.     The Resting Membrane Potential

a)     RMP – about -70 mV across plasma membrane (interior of cell negatively charged)

b)     Two primary proteins used to establish RMP:

(1)   Na+/K+ ATPase – 3 Na+ out, 2 K+ in (costs 1 ATP)

(2)   K+ Leak channels – allow K+ to leak out (this makes RMP more negative)

2.     Depolarization

a)     Voltage-gated Na+ channels open to allow Na+ ions to flow into cell and depolarize membrane

b)     These are opened when RMP goes from -70mV to -50mV (this is the threshold potential)

c)     When RMP reaches +35 mV, Na+ channels inactivate (different than close)

d)     As 1 section of the membrane depolarizes, it causes the section of the membrane next to it to reach threshold potential (all the way down the axon)

3.     Repolarization

a)     After depolarization, voltage-gated Na+ channels inactivate

b)     Voltage-gated K+ channels open (more slowly), and stay open longer, allowing K+ to leave the cell and establish RMP (often overshoots, up to -90mV), then close

c)     K+ leak and Na+/K+ ATPase reestablish RMP

4.     Saltatory Conduction

a)     Myelin – insulating sheath around axons made by Schwann cells; no ions enter cells here

b)     Nodes of Ranvier – gaps in the myelin where voltage-gated Na+ and K+ channels exist

c)     The nodes allow for a rapid jumping in transmission of action potentials (Saltatory conduction)

d)     Less energy used with saltatory conduction

5.     Equilibrium potentials

a)     Equilibrium potential is the membrane potential where the chemical gradient and electrical gradient balance each other out (no net driving force)

b)     Na+ equilibrium potential → +50mV

c)     K+ equilibrium potential → -90mV

(1)   Note that the RMP is much closer to K+ than Na+; this indicates that there are a large number of K+ leak channels and the cell membrane is almost completely permeable to K+

(2)   The slight difference between K+ EP and RMP is due to the small amt of Na+ leak channels

(3)   Remember, when the voltage gated Na+ channels open, the membrane potential jumps up to +35mV, very close to Na+ EP

6.     NOTE  conduction velocity is directly related to axon diameter and inversely related to axon length → wide, short axons are fast (think purkinje fibers)

7.     The Refractory Period – the period of time when a neuron is unresponsive to membrane depolarization; 2 phases:

a)     Absolute refractory period – absolutely will not respond to any strength depolarization

b)     Relative refractory period – can occur during hyperpolarization of membrane (near the -90mV) where a neuron can be induced to transmit an action potential, but depolarization required is greater (have to go from -90mV → -50mV, instead of -70mV → -50mV)

 

II.          7.2:  SYNAPTIC TRANSMISSION

A.     Synaptic transmission – 2 types

1.     Electrical synapse – the cytoplasms of two cells are joined by gap junctions, so that an action potential will spread from one cell right to the next (cardiac muscle, smooth muscle)

2.     Chemical synapses – the action potential is converted from an electrical to a chemical signal, occurs between the ends of axons and target cells; steps include:

a)     Action potential reaches synaptic knob

b)     Depolarization of the presynaptic membrane opens voltage-gated Ca2+ channels

c)     Ca2+ influx into presynaptic cell causes exocytosis of NT stored in secretory vesicles

d)     NT molecules diffuse across narrow synaptic cleft

e)     NT binds to receptor proteins in postsynaptic membrane → ligand-gated ion channels

f)       Opening of these channels alters membrane polarization

g)     Action potential is initiated (if threshold potential is reached)

h)     NT in synaptic cleft is degraded and/or removed to terminate signal

3.     EX:  in neuromuscular junction, NT used is ACh, and enzyme to degrade it is acetylcholinesterase (AChE)

4.     Other NTs:  GABA, serotonin, dopamine, NE → these can also activate 2nd messengers!

a)     These are either excitatory (causes depolarization) or inhibitory (causes hyperpolarization); however, it is not the NT that determines which it will be; the receptors and associated ion channels are the determining factor

b)     Each presynaptic neuron only releases 1 NT, but the postsynaptic neuron may respond to many different NTs

B.     SUMMATION

The term used to describe the addition of all the excitatory and inibitory stimulation that determines whether the action potential is reached and if the neuron will fire or not

1.     EPSPs (excitatory postsynaptic potentials) – excitatory NTs cause depolarization

2.     IPSPs (inhibitory postsynaptic potentials) – inhibitory NTs inhibit depolarization

3.     Temporal summation – one type of summation where the presynaptic neuron fires action potentials so rapidly that the EPSPs and/or IPSPs pile up on top of each other

4.     Spatial summation – EPSPs or IPSPs form all of the synapses on the postsynaptic membranes are summed at any given moment.

 

III.          7.3:  FUNCTIONAL ORGANIZATION OF THE HUMAN NERVOUS SYSTEM

1.     Sensory function – receiving information in the PNS

2.     Integrative function – processing information, done in the CNS

3.     Effectors – end target of motor neurons; either muscles or glands

4.     Efferent neurons – neurons that carry info towards the effectors

5.     Afferent nurons – sensory neurons that carry info towards the CNS

B.     REFLEXES

1.     Reflex – direct motor response to sensory input, usually does not involve the brain

2.     Muscle stretch reflex – sensory enuron detects stretching of a muscle, which transmits impulse to a motor neuron body in the spinal cord; the axon of this neuron goes straight back into the muscle and causes it to contract

a)     Monosynaptic reflex arch – reflex involving only 2 neurons and 1 synapse

3.     Inhibitory interneuron – short neuron which forms an inhibitory synapse with a motor neuron that inhibits a muscle (think hamstring relaxing with patellar reflex causing quadricep contraction)

4.     Reciprocal inhibition – concurrent contraction of one muscle and relaxation of the corresponding opposite muscle

C.    LARGE-SCALE FUNCTIONAL ORGANIZATION

1.     PNS is divided into several functional divisions:

a)     Somatic – conscious sensation and deliberate, voluntary movements

b)     Autonomic – digestion, metabolism, circulation, perspiration, and other involuntary processes

(1)   Sympathetic – fight of flight (often from NE release from adrenal medulla)

(2)   Parasympathetic – rest and digest

 

IV.          7.4:  ANATOMICAL ORGANIZATION OF THE NERVOUS SYSTEM

A.     CNS ANATOMICAL ORGANIZATION – includes spinal cord and brain

1.     Brain – has 3 subdivisions

a)     Hindbrain (rhombencephalon)

(1)   Medulla – relays info between areas of brain, regulates autonomic function (breathing, BP, digestion)

(2)   Pons – connects brain and cerebellum, helps balance and coordinates movement

(3)   Cerebellum – integrating center, coordinates complex movements (hand-eye coordination, balance)

b)     Midbrain (mesencephalon) – is a relay for visual/auditory info and contains RAS system (wakefulness)

c)     Forebrain (prosencephalon)

(1)   Diencephalon

(a)   Thalamus – contains relay and processing centers for sensory info

(b)   Hypothalamus – directly interacts with many other parts of brain; emotions, hormone production and release; primary link between nervous and endocrine centers

(2)   Telencephalon

(a)   Corpus callosum – axons that connect cerebral hemispheres

(b)   Cerebrum – the large paired cerebral hemispheres; cerebral cortex on outside (made of somas), white matter on inside (made of myelinated axons)

(i)          Frontal lobes – initiate voluntary movement, complex reasoning and problem solving

(ii)          Parietal – general sensations (feel) and taste

(iii)          Temporal – auditory and olfactory, and short term memory

(iv)          Occipital – process visual sensation

d)     Other

(1)   Basal nuclei – regulate body movement

(2)   Limbic system – includes several substructures and important in memory and emotions

Structure General function Specific Functions
Spinal cord Simple reflexes ●       Controls simple stretch and tendon reflexes●       Controls primitive processes such as walking, urination, sex organ function
Medulla Involuntary functions ●       Controls autonomic processes (BP, blood flow, HR, RR, swallowing, vomiting●       Controls reflex reaction (sneezing, coughing)●       Relays sensory info to cerebellum and thalamus
Pons Relay station and balance ●       Controls antigravity posture/balance●       Connects the spinal cord and medulla with upper brain●       Relays info to cerebellum and hypothalamus
Cerebellum Movement coordination ●       Integrating center●       Coordination of complex movement, balance/posture, muscle tone, spatial equilibrium
Midbrain Eye movement ●       Integration of visual and auditory information●       Visual and auditory reflexes●       Wakefuness and consciousness●       Coordinates information on posture and muscle tone
Thalamus Integrating center and relay station ●       Relay center for somatic sensation●       Relays info between spinal cord and cerebral cortex
Hypothalamus Homeostatis and behavior ●       Controls homeostatic function (temp, fluid balance, appetite) through neural and hormonal regulation●       Controls primitive emotions (anger, rage, sex drive)●       Controls pituitary gland
Basal nuclei Movement ●       Regulates body movement and muscle tone●       Coordinates learned movement patterns●       General pattern of rhythm movements (arm/legs while walking)●       Subconscious adjustment of movements
Limbic system Emotion, memory, learning ●       Controls emotional states●       Links conscious and unconscious portions of the brain●       Helps with memory and storage retrieval
Cerebral cortex Perception, skeletal muscle movement, integration center ●       Divided into 4 lobes with specialized subfunctions●       Conscious through processes and planning, awareness, and sensation●       Intellectual function (intelligence, learning, reading, communication)●       Abstract thought and reasoning●       Memory storage and retrieval●       Initiation and coordination of voluntary movement, comples motor patterns
Corpus callosum Connection ●       Connects R&L cerebral hemispheres

B.     PNS ANATOMICAL ORGANIZATION – 12 CN and 31 spinal nerves

1.     Somatic PNS Anatomy

a)     All somatic motor neurons innervate skeletal muscle cells, use ACh, and have their somas in brain stem or ventral portion of spinal cord

b)     All somatic sensory neurons have their bodies in a DRG at the dorsal side of the spinal cord (these are outside the meninges and therefore outside the CNS)

2.     Autonomic PNS Anatomy – more complex

a)     Efferent pathway – consists of 2 neurons, preganglotic and postgangliotic

(1)   Pregangliotic neuron – soma is in brainstem or spinal cord; its axon goes to the autonomic ganglion outside the spinal cord and connects with postganglionic neuron

(a)   Sympathetic pregangliotic effector neurons have somas in chest/lumbar regions

(b)   Parasympathetic pregangliotic neurons are in craniosacral region

(2)   Postgangliotic neuron – body in ganglion, axon to effector

(a)   Parasympathetic postgangliotic neurons use ACh (always)

(b)   Sympathetic postgangliotic neurons use NE (almost all)

Sympathetic Parasympathetic
General function Fight or flight, mobilize energy Rest and digest, store energy
Location of pregangliotic soma Thoracolumbar Craniosacral
Preganglionic axon (ACh) Short Long
Ganglia Close to cord, far from target Far from cord, close to target
Postganglionic axon Long (NE) Short (ACh)

3.     The Adrenal Medulla

a)     Medulla – inner portion; part of sympathetic NS

(1)   Directly innervated by sympathetic pregangionic neurons

(2)   Releases epinephrine (is hormone, because it travels by bloodstream)

b)     Cortex – outer portion, secretes glucocorticoids, mineralocorticoids, and sex hormones

 

V.          7.5:  SENSATION

A.     TYPES OF SENSORY RECEPTORS – extero and intero (external and internal)

1.     Mechanoreceptors – respond to mechanical disturbances (pressure, movement of hairs

a)     EX:  Pacinian corpuscles send graded potential changes up dendrite

2.     Chemoreceptors – respond to particular chemicals

a)     EX:  olfactory or gustatory receptors, receptors that detect changes in pH, PCO2, PO2

3.     Nociceptors – pain receptors, stimulated by major tissue injury (somatic or autonomic)

4.     Thermoreceptors – stimulated by change in temperature

5.     Electromagnetic receptors – stimulated by EM rays

a)     EX:  only ones in humans are cones and retina of eye

B.     ADAPTATION

Decrease in firing frequency of neuron when intensity of stimulus remains constant (note that nociceptors do not adapt under any circumstances)

C.    PROPRIOCEPTORS – includes many types of receptor

1.     Proprioception refers to awareness of self or body part position

2.     Mechanoreceptors detect muscle stretch

D.    GUSTATION AND OLFACTION

1.     Gustation – can only detect 5 flavors (sweet, salty, bitter, sour, umami)

2.     Olfaction – receptors in the nasopharynx, can detect airborne chemicals that dissolve in mucus covering membrane; the nerves project into olfactory bulbs of the brain

E.     HEARING AND THE VESTIBULAR SYSTEM

1.     Structure of the Ear

a)     Ossicles – 3 bones (malleus, incus, and stapes)

b)     Inner ear – include cochlea, semicircular canals, utricle, saccule

c)     Cochlea – basilar membrane along cochlea, covered with hair cells that have cilia; cilia touch tectorial membrane and hairs are dragged across when bent, which opens ion channels resulting in NT release

d)     Organ of Corti – basilar membrane, hair cells, and tectorial membrane

2.     Mechanism of Hearing

a)     Sound → TM → malleus → incus → stapes → oval window → perilymph → endolymph → basilar membrane → auditory hair cells → tectorial membrane → NTs stimulate bipolar auditory neurons → brain → perception

b)     Pitch (frequency) – distinguished by which regions of the basilar membrane vibrate; high pitch is near oval window (high frequency, short wavelength); opposite for low pitch

c)     Loudness – distinguished by amplitude of vibration

3.     Balance

a)     Semicircular canals – filled with endolymph, contain hair cells that detect motion

b)     Utricle and saccule – balance-monitoring

F.     VISION:  STRUCTURE AND FUNCTION

1.     Structures

a)     Choroid – layer beneath the sclera that contains darkly pigmented cells to absorb excess light in the eye

b)     Bipolar cells – Neurons that synapse with rods and cones on one end and ganglion cells of the optic nerve at the other end

c)     Fovea centralis – center of the macula with only cones, responsible for extreme visual acuity

2.     The Photoreceptors:  Rods and Cones

a)     Contain 1 molecule of retinal, which at rest, contains 1 cis double bond, which is converted to trans when exposed to light

b)     Night vision – accomplished by rods (black and white)

c)     Color vision – detected by cones, require more light; cones are concentrated in the fovea (red, blue, and green)

3.     Defects in Visual Acuity

a)     Emmetropia – normal vision

b)     Myopia – nearsightedness → corrected w/divergent lenses

c)     Hyperopia – farsightedness → corrected w/convergent lenses

d)     Presbyopia – inability to accomodate due to loss of flexibility of lens

Modality Receptor Receptor type Organ Stimulus
Vision Rods and cones EM Retina Light
Hearing Auditory hair cells Mechanoreceptor organ of Corti Vibration
Olfaction Olfactory nerve Chemoreceptor Individual neurons Airborne chemicals
Taste Taste cells Chemoreceptor Taste bud Food chemicals
Touch Pacinian corpuscules, free nerve endings, temp receptors Mechanoreceptors, nociceptors, thermoreceptor Skin Pressure, temp, pain
Interoception Aortic arch baroreceptors, pH receptors Baroreceptors, chemoreceptors Aortic arch, medulla BP, pH

VI.          7.6:  THE ENDOCRINE SYSTEM

A.     HORMONE TYPES:  TRANSPORT AND MECHANSMS OF ACTION

1.     Definitions

a)     Hormone – a molecule that is secreted into the bloodstream by an endocrine gland and has effects on a distant target cell possessing the appropriate receptor

b)     Endocrine gland – ductless gland whose secretory products are picked up by capillaries supplying blood to the region

c)     Exocrine gland – secrete products into the external environment by way of ducts

d)     Hormone receptor – polypeptide that possesses a ligand-specific binding site

e)     Hydrophilic hormones – peptides, AA derivatives; these must bind to cell surface receptors

f)     Hydrophobic hormones – steroid hormones; bind to receptors in cell’s interior

2.     PEPTIDE HORMONES

a)     Synthesized in rough ER and modified in the Golgi

b)     Stored in vesicles until needed then released through exocytosis, dissolve in plasma

c)     Binds to cell surface receptor, which activates a second messenger (with signal amplification)

d)     SPEED: rapid, brief duration

e)     2 subgroups:

(1)   Polypeptides (EX:  insulin) – complex tertiary structure

(2)   AA derivatives (EX:  catecholamines, thyroid hormone)

3.     STEROID HORMONES

a)     Hydrophobic molecules synthesized from cholesterol in smooth ER

b)     Can freely diffuse into cell membranes and binds with receptor inside cell

c)     These are not made and stored, but made and released (not made if not needed)

d)     SPEED:  slow, long duration (modify amounts/types proteins in cells)

Peptides Steroids
Structure Hydrophilic, large (polypeptides); small (AA derivatives) hydrophobic, small
Site of synthesis Rough ER Smooth ER
Regulation of release Stored in vesicles until signaled to release Signaled to be made, released as they are made
Transport in bloodstream Free Stuck to proteins
Specificity Only target cells have appropriate surface receptors Only target cells have appropriate cytoplasmic receptors
Mechanism of effect Bind to receptors that generate 2nd messengers which result in modification of enzyme activity Bind to receptors that alter gene expression by regulating DNA transcription
Timing of effect Rapid, short lived Slow, long-lasting

B.     ORGANIZATION AND REGULATION OF THE HUMAN ENDOCRINE SYSTEM

1.     Feedback regulation – the interaction between hormone and regulatory hormone or chemical

2.     Tropic hormones – hormones that regulate other hormones

3.     Negative feedback inhibition – primary mechanism of hormone control in endocrine system

4.     Hypothalamic-pituitary axis – the relationship between the hypothalamus (which controls hormone production of the pituitary gland), and the pituitary gland

5.     Adenohypophysis – anterior pituitary, controlled by hypothalamic releasing and inhibiting factors

6.     Neurohypophysis – posterior pituitary, the axons which descend from the hypothalamus and are examples of neuroendocrine cells (neurons that secrete hormones into the bloodstream)

a)     ADH and oxytocin

7.     Thyroid hormone – Thyroid hormone binds to receptor in the cytplasm and regulates transcription in the nucleus

VII.          7.7:  MAJOR GLANDS AND THEIR HORMONES

Gland Hormone (class) Target/effect
Hypothalamus releasing and inhibiting factors (peptides) anterior pituitary/modify activity
Anterior pituitary GH (peptide) ↑ bone & muscle growth, ↑ cell turnover rate
PRL (peptide) mammary gland/milk production
tropic TSH (peptide) thyroid/ ↑ synthesis and release TH
ACTH (peptide) ↑ growth and secretory activity adrenal ctx
gonadotropic LH (peptide) ovary/ovulation, testes/testosterone synth
FSH ovary/follicle development, testes/spermatogenesis
Posterior pituitary ADH (peptide) kidney/water retention
oxytocin (peptide) breast/milk letdown, uterus/contraction
Thyroid thyroid hormone (modified AA) child: mental and physical development; adult:  ↑ met. rate and temp
thyroid T cells calcitonon (peptide) bone, kidney; lowers serum Ca2+
Parathyroids PTH (peptide) bone, kidney,small intestine/raises Ca2+
Thymus thymosin (children only; peptide) T cell development during childhood
Adrenal medulla epi (modified AA) sympathetic stress response
Adrenal cortex cortisol (“glucocorticoid”; steroid) long-term stress response; ↑ BGL, ↑ protein breakdown; ↓ inflammation and immunity
aldosterone (“mineralocorticoid”; steroid) kidney/ ↑ Na+ reabsorption to ↑ BP
sex steroids not normally important
Endocrine pancreas insulin (peptide) ↓ BGL/ ↑ glycogen and fat storage
glucagon (peptide) ↑ BGL/ ↓ glycogen and fat storage
somatostatin (peptide) inhibits digestive process
Testes testosterone (steroid) male characteristics, spermatogenesis
Ovaries/placenta estrogen (steroid) female characteristics, endometrial growth
progesterone (steroids) endometrial secretion, pregnancy
Heart atrial natriuretic factor (peptide) kidney/ ↑ urination to ↓ BP
Kidney EPO (peptide) bone marrow/ ↑ RBC synthesis

 

Chapter 7 Summary

  • The neuron is the basic structural and functional unit of the nervous system; it has several specialized structures that allow it to transmit action potentials
  • Neurons receive incoming information via dendrites; signals are summed by the axon hillock, and if the signal is greater than the threshold, an action potential is initiated
  • The action potential is an all-or-none signal that includes depolarization (via voltage-gated sodium channels) and repolarization (via voltage-gated potassium channels); it begins and ends at the cells resting potential of -70mV
  • Since action potentials are all-or-none events, intensity is coded by the frequency of the action potential
  • Neurons communicate with other neurons, organs, and glands at synapses; most synapses are chemical in nature and an action potential causes the release of neurotransmitter into the synaptic cleft, and binding of the neurotransmitter to receptors on the postsynaptic cell triggers a change, either stimulatory or inhibitory, in that cell
  • The CNS includes the spinal cord and the brain
  • The PNS includes the somatic (voluntary) and autonomic (involuntary) subdivisitons
  • The sympathetic branch of the ANS controls our fight-or-flight response; NE is the primary neurotransmitter of this system, and it is augmented by epinephrine from the adrenal medulla
  • The parasympathetic branch of the ANS contrors our resting and digesting state; Ach is the primary NT of this system
  • Humans have several types of receptors (mechanoreceptors, chemoreceptors, nociceptors, thermoreceptors, electromagnetic receptors, and proprioceptors ) that allow us to detect a variety of stimuli
  • The endocrine system controls our overall physiology and homeostasis by hormones that travel through the bloodstream
    • Hormones that are released from the endocrine glands travel to distant target tissues via the blood, bind to receptors on target tissues, and exert effects on target cells
  • Peptide hormones are made from AAs, bind to receptors on the cell surface, and typically affect target cells via second messenger pathways; effects tend to be rapid and temporary
  • Steroid hormones are derived from cholesterol, bind to receptors in the cytoplasm or nucleus, and bind to DNA to alter transcription. Effects tend to occur more slowly and are more permanent

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