afferent

THE NERVOUS SYSTEM

The nervous system includes all the nervous tissue in the body + the sensory organs (such as the eyes and ears). 

Nervous tissue is composed of 2 kinds of cells:

  1. neurons which transmit nervous system messages
  2. glial cells which support neurons and modify their signaling.

There are 2 major divisions of the human nervous system:

  1. the central nervous system (CNS) which consists of the brain and spinal cord
  2. the peripheral nervous system (PNS) which includes all the neural tissue outside the CNS plus the sensory organs.

The PNS has 2 divisions:

  1. afferent division which brings sensory information to the CNS
  2. efferent division which carries action (motor) commands away from the CNS to the body’s ‘effectors’ (muscles and glands).

Within the PNS’s efferent division are 2 subsystems:

  1. the somatic nervous system which provides voluntary control over skeletal muscles.
  2. the autonomic nervous system which provides involuntary regulation of smooth muscle, cardiac muscle and glands.

The autonomic system is further divided into:

  1. sympathetic division (or 'fight or flight’ response) which generally has stimulatory effects (like adrenaline).
  2. parasympathetic division (or 'rest and digest’) which generally facilitates routine maintenance activities (like digestion).

There are 3 types of neurons (cells):

  1. sensory neurons. Sensory neurons sense conditions both inside and outside the body. They convey information relating to said conditions to neurons inside the CNS.
  2. interneurons. Interneurons are located entirely within the CNS and interconnect other neurons.
  3. motor neurons. Motor neurons carry instructions from the CNS to effectors (e.g. muscles or glands).

Each neuron has multiple dendrites (through which signals travel to the neuron cell body) and a single axon (that carries signals away from the cell body to the synaptic terminals).

Glial cells produce the fat-rich myelin, which can surround neural axons and increase the speed of neural signals.

A nerve is a bundle of axons in the PNS that transmits information to/from the CNS.


Nervous system communication can be conceptualized as working through a 2-step process.

  1. signal movement goes down a neuron’s axons
  2. signal movement (from said axon) goes to a second cell across a structure known as a synapse.

A nerve signal moves from one neuron to another across a synapse. Synapse includes: 

  • a 'sending’ neuron
  • a 'receiving’ neuron
  • a synaptic cleft - a tiny gap between the two cells

A chemical called a neurotransmitter diffuses across the synaptic cleft from the sending neuron to the receiving neuron. It binds with receptors on the receiving neuron, keeping the signal going.


The spinal cord can receive input from sensory neurons (and instruct motor neurons to respond) with no input from the brain. The spinal cord also channels sensory impulses to the brain.

The spinal cord has a darker (gray matter) H-shaped central area composed mostly of the cell bodies of neurons. The lighter (white matter) peripheral area is mostly composed of axons.

The central canal of the spinal cord is filled with cerebrospinal fluid that provides the spinal cord with nutrients. Spinal nerves extend from the spinal cord to the majority of areas of the body.

  • Sensory neurons, which transmit information to the spinal cord, have their cell bodies outside the spinal cord (in the dorsal root ganglia).
  • Spinal cord motor neurons have cell bodies that lie within (the gray matter of) the spinal cord. The axons of these neurons leave the spinal cord through its ventral roots.

The dorsal and ventral roots come together (like fibers being joined in a cable) to form a spinal nerve.


Reflexes are automatic nervous system responses (triggered by specific stimuli) that help us avoid danger or preserve a stable physical state (physical equilibrium).

The neural wiring of a single reflex (called a reflex arc) start with a sensory receptor that’s run through the spinal cord to a motor neuron. This proceeds back out to an effector (once again, a muscle or gland). The brain is not involved in the reflex arc.


The sympathetic division of the autonomic nervous system is often called the fight-or-flight system because it generally prepares the body to deal with emergencies.

What does it do?

  • dilates pupils 
  • inhibits salivation 
  • accelerates the heart 
  • facilitates breathing
  • inhibits digestion
  • stimulates the release of glucose
  • secretes adrenaline and noradrenaline
  • relaxes the bladder
  • inhibits sex organs

The parasympathetic division is often called the rest-and-digest system because it conserves energy and promotes digestive activities.

What does it do?

  • constricts pupils 
  • stimulates salivation 
  • slows the heart 
  • constricts breathing 
  • stimulates digestion 
  • stimulates the gallbladder 
  • contracts the bladder 
  • stimulates sex organs

Most organs receive input from both systems.


There are 7 major regions in the adult brain:

1. Cerebrum.  The cerebrum has a thin outer layer of gray matter - the cerebral cortex - that surrounds a much larger area of cerebral white matter.

Differing portions of the cerebral cortex play a central role in processing sensory information and in carrying out almost all of our conscious mental activities.

2. Cerebellum. The cerebellum refines bodily movement and balance, based on sensory inputs. 

3. Thalamus. The thalamus receives most of the body’s sensory information and then transfers it to different regions of the cerebral cortex (for processing). 

4. Hypothalamus. The hypothalamus is integral to regulating drives and maintaining homeostasis - partly through its regulation of hormonal release.

The brainstem is a collective term containing:

  • 5. Midbrain. The midbrain helps maintain muscle tone and posture.
  • 6. Pons. The pons primarily relays messages between the cerebrum and the cerebellum.
  • 7. Medulla oblongata. The medulla oblongata helps regulate involuntary functions such as breathing and digestion. When people are ‘braindead’, only their medulla oblongata is left functioning.

All human senses operate through cells called sensory receptors. Sensory receptors respond to stimuli (changes in the cells environment).

The sensory receptors transform the responses into stimuli - electrical signals - that travel through action potentials.

Signals from every sense (except smell!) are routed through the brain’s thalamus and then to specific areas of the cerebral cortex.

The sense of touch works through a variety of sensory receptors that distinguish qualities such as light or heavy pressure, new or ongoing contact, texture, etc.

In some sensory cells, the stretching of their outer membrane prompts an influx of ions that results in the initiation of a nerve signal.


Our sense of smell (or olfaction) works through a set of sensory receptors whose dendrites extend into the nasal passages.

Odorants - which are molecules that have identifiable smells - bind with hair-like extensions (cilia) of dendrites, resulting in a nerve signal to the brain.

The higher processing centers of the brain distinguish these odorants by sensing unique groups of neurons that fire in connection with given odorants.

  • humans have 340 - 380 different receptors.
  • dogs have about 1000 different receptors.
  • rats have about 1, 500 different receptors.

Our sense of taste works through a group of taste cells, located in taste buds near the surface of the tongue. The taste cells have receptors that bind to ‘tastants’ or molecules of food that elicit different tastes.

A given taste cell can respond through any 4 to 6 chemical signaling routes that correspond to the basic tastes of sweet, sour, salty, bitter and the possible fifth and sixth tastes of umami and calcium.

The neurons that receive input from taste cells vary in their response to different tastants. The brain makes sense of the pattern of input it gets from these neurons, thus yielding the large number of tastes we experience.


Our sense of hearing is based on the fact that vibrations result in 'waves’ of air molecules that are more (and less) compressed than the ambient air around them.

These waves of compression bump up against our eardrums (or tympanic membranes) which in turn vibrate; this initiates a chain of vibrations that ends in the fluid-filled cochlea of the inner ear.

'Hair cells’ in the cochlea have ion channels that open and close in response to this vibration, resulting in nerve signals to the brain.


In vision, light enters the eye through the cornea and then passes through the lens on its way to the retina (at the back of the eye).

Light is bent (or refracted) by the cornea and the lens in such a way that it ends up as a tiny, sharply focused image on the retina.

Light signals are converted to nervous system signals by cells in the retina called photoreceptors, which come in 2 varieties: 

  1. rods.  Rods function in dim light but are not sensitive to color.
  2. cones.  Cones function best in bright light but are sensitive to color.

These photoreceptors have pigments embedded in membranes within them.

Vision signals travel from photoreceptors through two sets of adjoining cells (the latter of which have axons that come together to form the body’s optic nerves).

When light strikes a pigment, it changes the pigment shape in a way that prompts a cascade of chemical reactions that result in neurotransmitter release being inhibited between the rod or cone and its adjoining connecting cell. The lack of release sends the signal: ‘photoreceptor stimulated here’.

The brain doesn’t passively record visual information; it constructs images as much as it records them.

The visual perception operates through a series of genetically based 'rules’ that allow us to quickly make sense of what we perceive.

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In this Neuromechanics weekly, Dr Waerlop Introduces the cerebellum and talks about its importance clinically, since it contains more than ½ of the neurons in the brain! It’s anatomy and inputs from the periphery are discussed. The take home message is the cerebellum is the key to understanding and directing movement, since it receives feedback from most ascending and descending pathways.

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Ah yes, the Ia and type II afferents.

One of our favorites! Acting as a sentinel from the muscle spindle, concentrated in the antigravity and extensor musculature, Ia and type II afferents live in the belly of the muscle and send information regarding length and rate of change of length to the CNS via the spino cerebellar and inferior olivary pathways. In more simpler terms, think of muscle spindles as small computer chips embedded in the muscle and using la and type II afferents the team act as volume controls helping to set the tone of the muscle and it responsiveness to stretch. If they are active, they make a muscle more sensitive to stretch.

So what does that mean? Muscle spindles turn up the volume or sensitivity of the muscles response to stretch. Remember when we stretch a muscle, it’s response is to contract. Think about when a doctor tests your reflexes. What makes them more or less reactive? You guessed it, the muscle spindle; which is a reflection of what is going on in the higher centers of the brain. The muscle spindles level of excitation is based on the sum total of all information acting on the gamma motor neuron (ie the neuron going to the muscle spindle) in the spinal cord. That includes all the afferent (ie. sensory) information coming in (things like pain can make it more or less active) as well as information descending from higher centers (like the brain, brainstem and cerebellum) which will again influence it at the spinal cord level.

So we found this cool study that looks at spindles and supports their actions:

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http://www.ncbi.nlm.nih.gov/pubmed/19451207

J Physiol. 2009 Jul 1;587(Pt 13):3375-82. Epub 2009 May 18.

Mechanical and neural stretch responses of the human soleus muscle at different walking speeds.

Cronin NJ, Ishikawa M, Grey MJ, af Klint R, Komi PV, Avela J, Sinkjaer T, Voigt M.

At increased speeds of walking, the muscles themselves (particularly the soleus in this study) become stiffer due to changes in spindle responsiveness. The decline in amplitude and velocity of stretch of the soleus muscle fasicles with increasing walking speeds was NOT accompanied by a change in muscle spindle amplitude, as was hypothesized.

Clinically, this means that the spindles were STILL RESPONSIVE to stretch, even though the characteristics of the muscle changed with greater speeds of action. This may be one of the reasons you may injure yourself when moving or running quickly; the muscle becomes stiffer and the spindle action remains constant (the volume is UP).

Thankfully, we have another system that can intervene (sometimes) when the system is overloaded, and take the stress of the muscle. This is due to the golgi tendon organ; but that is a post for another day…

Geeking out and exploring the subtleties of the neurology as it relates to the system, we remain…The Gait Guys

Researchers examine how touch can trigger our emotions

While touch always involves awareness, it also sometimes involves emotion. For example, picking up a spoon triggers no real emotion, while feeling a gentle caress often does. Now, scientists in the Cell Press journal Neuron describe a system of slowly conducting nerves in the skin that respond to such gentle touch. Using a range of scientific techniques, investigators are beginning to characterize these nerves and to describe the fundamental role they play in our lives as a social species—from a nurturing touch to an infant to a reassuring pat on the back. Their work also suggests that this soft touch wiring may go awry in disorders such as autism.

The nerves that respond to gentle touch, called c-tactile afferents (CTs), are similar to those that detect pain, but they serve an opposite function: they relay events that are neither threatening nor tissue-damaging but are instead rewarding and pleasant.

“The evolutionary significance of such a system for a social species is yet to be fully determined,” says first author Francis McGlone, PhD, of Liverpool John Moores University in England. “But recent research is finding that people on the autistic spectrum do not process emotional touch normally, leading us to hypothesize that a failure of the CT system during neurodevelopment may impact adversely on the functioning of the social brain and the sense of self.”

For some individuals with autism, the light touch of certain fabrics in clothing can cause distress. Temple Grandin, an activist and assistant professor of animal sciences at Colorado State University who has written extensively on her experiences as an individual with autism, has remarked that her lack of empathy in social situations may be partially due to a lack of “comforting tactual input.” Professor McGlone also notes that deficits in nurturing touch during early life could have negative effects on a range of behaviors and psychological states later in life.

Further research on CTs may help investigators develop therapies for autistic patients and individuals who lacked adequate nurturing touch as children. Also, a better understanding of how nerves that relay rewarding sensations interact with those that signal pain could provide insights into new treatments for certain types of pain.

Professor McGlone believes that possessing an emotional touch system in the skin is as important to well-being and survival as having a system of nerves that protect us from harm. “In a world where human touch is becoming more and more of a rarity with the ubiquitous increase in social media leading to non-touch-based communication, and the decreasing opportunity for infants to experience enough nurturing touch from a carer or parent due to the economic pressures of modern living, it is becoming more important to recognize just how vital emotional touch is to all humankind.”

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Welcome to Neuromechanics Weeekly. This week Dr Waerlop discusses the afferent sensory pathways and brings us from the receptor to the higher centers. Hold on tight!

Neuroscientists challenge long-held understanding of the sense of touch

Different types of nerves and skin receptors work in concert to produce sensations of touch, University of Chicago neuroscientists argue in a review article published Sept. 22, 2014, in the journal Trends in Neurosciences. Their assertion challenges a long-held principle in the field – that separate groups of nerves and receptors are responsible for distinct components of touch, like texture or shape. They hope to change the way somatosensory neuroscience is taught and how the science of touch is studied.

Sliman Bensmaia, PhD, assistant professor of organismal biology and anatomy at the University of Chicago, and Hannes Saal, PhD, a postdoctoral scholar in Bensmaia’s lab, reviewed more than 100 research studies on the physiological basis of touch published over the past 57 years. They argue that evidence once thought to show that different groups of receptors and nerves, or afferents, were responsible for conveying information about separate components of touch to the brain actually demonstrates that these afferents work together to produce the complex sensation.

“Any time you touch an object, all of these afferents are active together,” Bensmaia said. “They each convey information about all aspects of an object, whether it’s the shape, the texture, or its motion across the skin.”

Three different types of afferents convey information about touch to the brain: slowly adapting type 1 (SA1), rapidly adapting (RA) and Pacinian (PC). According to the traditional view, SA1 afferents are responsible for communicating information about shape and texture of objects, RA afferents help sense motion and grip control, and PC afferents detect vibrations.

In the past, Bensmaia said, this classification system has been supported by experiments using mechanical devices to elicit one or more of these specific components of touch. For example, responses to texture are often generated using a rotating, cylindrical drum covered with a Braille-like pattern of raised dots. Study subjects would place a finger on the drum as it rotated, and scientists recorded the neural responses.

Such experiments showed that SA1 afferents responded very strongly to this artificial stimulus, and RA and PC afferents did not, thus the association of SA1s with texture. However, in experiments in which subjects moved a finger across sandpaper – the quintessential example of the type of textures we encounter in the real world – SA1 afferents did not respond at all.

Bensmaia also pointed out discrepancies in the predominant thinking about how we discern shape. Perception of shapes has generally been tested using devices with raised or embossed letters to test a subject’s ability to interpret text by touch. These experiments also showed that such inputs produced a strong SA1 response, so they were implicated in perception of shape as well.

In the 1980s, however, researchers developed a device meant to help blind people read by generating vibrating patterns in the shape of letters on an array of pins. While the device was not a commercial success, people were able to use it to detect letter shapes and read, although experiments showed that it activated RA and PC afferents, not the supposedly shape-detecting SA1s.

Bensmaia said such experiments show how devices created to generate artificial stimuli focusing on individual components of the sense of touch can result in misleading findings. Some types of afferents are better than others at detecting texture or shape, for example, but all of them respond in their own way and contribute to the overall sensation.

“To get a good picture of how stimulus information is being conveyed in these afferent populations, you have to look at a diverse set of stimuli that spans the range of what you might feel in everyday tactile experience,” he said.

Instead of thinking of individual groups of afferents working separately to process different components of the sense of touch, Bensmaia said we should think of all of them working in concert, much like individual musicians in a band to create its overall sound. Each musician contributes in his or her own way. Emphasizing one instrument or removing another can change the character of a song, but no single sound is responsible for the entire performance.

Adopting this new way of thinking will have far-reaching implications for both the study of the sense of touch and the design of future research, Bensmaia said.

“I think it’s going to change neuroscience textbooks, and by extension it’s going to change the way somatosensory neuroscience is taught. It’s really the starting point for everything.”

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Just when you thought it was safe to watch a Neuromechanics Weekly episode, Dr Ivo throws a curveball. Check out the interesting clinical asides about myelopathy (pressure on the spinal cord causing ataxic gait) and the importance of which modality to check 1st, when doing an exam.

Keep these things in mind the next time you are evaluating someone’s gait.

Bum Itch

Sherlock and John are sitting on a park bench, watching the soldiers across the road. Sherlock says (without preamble) “Do you think they give them classes?” Then he clarifies, “How to resist the temptation to scratch their behinds?” John answers with an almost-nonsequiter: “Afferent neurons in the peripheral nervous system. Bum itch." To which Sherlock replies, "Oh!”

(A tip of the hat to Ariane Devere for her helpful transcript of this conversation!)

These two are out there at all because they each need a distraction from all the wedding hoopla. Why would Sherlock even be thinking about this (scratching anyone’s behind) let alone mention it aloud? Is it wishful thinking on his part? (As in, he’d like to scratch someone’s behind?) Or is he trying to connect sympathetically with other human beings, and he’s putting himself in their place? Or could he be making an overture to John? And why does John never really answer his question? Instead he gives him a mini-lesson in anatomy (that he could have picked up from reading Wikipedia). The question of how to resist temptation is never addressed! (Did he know he was being flirted with?)

The Roots of Autism Are in the…Skin?

The skin is our gateway to the physical world. Below its surface are oodles of nerve fibers relaying different types of messages to the brain. At the ends of the fingertips, for example, fat and fast Aβ nerves help you fish for keys at the bottom of a messy purse, or feel the difference between cotton and polyester. Nearby those big nerves are thinner and slower C-fiber nociceptors, which transmit pain, and others that relay itchiness.

What I didn’t know until this week is that there is yet another type of nerve, found only under hairy skin, that carries information about our social interactions. These nerves, known as C-tactile (CT) afferents, respond to slow, gentle stroking — the soft touch you’d give to a baby’s forehead or a lover’s arm. And some researchers believe that these fibers are crucial for the development of the social brain.

Continue Reading

How we know when to empty our bladders

Sensing bladder fullness is seemingly simple. The kidneys send waste and excess water to the bladder, and upon reaching its filling threshold, the bladder tells the central nervous system that it’s time to void. However, a team led by Mark T. Nelson, PhD, University Distinguished Professor and Chair of the Department of Pharmacology, found that in addition to filling pressure, the process involves what they call “non-voiding transient contractions (TCs)” of the urinary bladder smooth muscle. The study, “Transient contractions of urinary bladder smooth muscle are drivers of afferent nerve activity during filling,” by Thomas J. Heppner et al., appears in the April issue of The Journal of General Physiology.

TCs have a central role in sensing pressure and conveying this information to afferent (sensory) nerves, the researchers note. But not only do TCs provide information about when the bladder is full, they alert us when conditions are ripe for the most efficient voiding experience. This, they conclude, means that TCs could represent a novel target for therapeutic intervention in urinary bladder dysfunction. “The presence or absence of these contractions, and how fast the contractions happen, can contribute to bladder under-activity or over-activity – which are both bad,” Dr. Nelson said.

Thomas J. Heppner, Nathan R. Tykocki, David Hill-Eubanks, Mark T. Nelson. Transient contractions of urinary bladder smooth muscle are drivers of afferent nerve activity during filling. The Journal of General Physiology, 2016; 147 (4): 323 DOI: 10.1085/jgp.201511550

Proposed Model of TC-Evoked Afferent Bursts As the bladder fills, rapid TCs occur. The rate of rise of these TCs is a function of the length-tension relationship of detrusor smooth muscle. TCs stimulate bursts of afferent nerve activity that increase with the rate of rise of the TCs and saturate at ?3 mmHg/s. The peak afferent activity and maximal TC rate of rise both occur when intravesical pressure is near threshold (?12 mmHg), which may be indicative of the optimal length-tension relationship for voiding contractions. For simulated and naturally occurring TCs, the SK blocker apamin increased the gain of the relationship between TC leading slope and afferent activity. The molecular identity of the bladder pressure transducer is unknown.Credit: Heppner et al. 2016

Early Semester Bio Tricks

Its that time of year where those of us who are in the upper-level courses have the massive 1 week review in every class of all the biology that has come before. I have a few tricks that help me remember the most simple concepts in Bio, that will hopefully help you guys save time too. Some of these I actually came up with, others I have learned from professors and such, but above all, I’m sure 10000 other pre-meds have thought of these before so feel free to steal them!

What bases pair and how many bonds? 

A-Two    C-Gthree

How many Na and K go in and out of the pump?

Na+ has 3 characters, so 3 Na go out, and K+ has two, so two K come in.

Which is afferent and efferent?

“A” comes before “E”, so afferent receives the signal before the brain can send a response back through the efferent pathway. 

How do I remember Symporter vs Antiporter?

If the two molecules are going in the Syme direction its Symporter opposite directions=anti. 

How do I keep tendons and ligaments separate?

1st think of your Achilles Tendon. If that doesn’t work I used to use: Ligate the bones (ligate means to tie something up in surgery). 

Okay, thats about all I got. Like I said, there are only a few and they’re kinda cheesy but they work for me. Let me know if they help you!

-thepre-medlife