The spinal cord & spinal nerves

Cervical region

• largest ratio of white to gray matter

• Cervical conveys tract from sacral, lumbar, thoracic & cervical regions of the spinal cord, consisting of the first eight vertebrae. It controls the neck, arms, and hands and is crucial for many nerve signals to the upper body.

  

Thoracic region

• located between cervical and lumbar regions, consisting of twelve vertebrae. It controls the chest and abdominal muscles, as well as sensations from these areas.

• still more white matter than gray matter

Lumbar region

• More Gray matter than white matter

• It’s located below the Thoracic region

Sacral region

• lowest ratio of white to gray matter

• Differences in amount of gray matter

• Cervical and lumbar enlargements due to extra gray matter associated with limb innervation

Rootlets

• rootlets combine to form roots

Dorsal sensory & ventral sensory roots

• these combine to form spinal nerves (mixed)

Branching of spinal nerve

• they branch soon after the spinal nerve exits the intervertebral foramen

Main branches

Dorsal primary ramus

Ventral primary ramus

Dorsal primary ramus

• innervates (supply) deep back muscles + associated dermis

Ventral primary ramus

Innervates (supply) everything else

Dermatomes

• an area of the skin that is connected to a specific nerve root on your spine

• All spinal nerves except C1 receive sensory input from a specific band/ region of skin

Concept of dermatomes and area it covers

• map cutaneous (skin) regions innervated by each spine nerve

• overlap at the edges up to 50%

Benefits of knowking dermatomes

• assess the level of spinal injury

• Diagnose shingles - distribution of lesions follow dermatomes

• Chicken pox virus remains in the dorsal root ganglia for life

Reflexes

• quick, involuntary, stereotyped reaction to stimuli - impact muscles & glands

• Protective for body

Features of reflexes

1. Require stimulation, ie - input

2. Quick - very few neurons involved (minimal or no interneurons = minimal synaptic delay)

3. Involuntary - automatic - does not need stimulus to register at the brain - awareness comes later

4. Stereotyped - response is same everytime

Somatic Reflex Arc

• Somatic receptors (skin, muscles, tendons, joints)

• Afferent nerve fibres (sensory)

• Integrating centre = point of synaptic contact

• Efferent nerve fibres (motor)

• Effector (muscles)

Somatic receptors

• skin, muscles, tendons, joints

Afferent nerve fibres

sensory nerve fibers that carry signals to the central nervous system from sensory receptors.

Integrating centre

• point of synaptic contact

Efferent nerve fibres

• motor nerve fibers that carry signals away from the central nervous system to effectors such as muscles and glands.

Effector

• Muscles

• and glands that respond to signals from efferent nerve fibers.

Proprioception

• sense of body position and movements

• sensory nerve endings in muscle, tendons and joints

EPSP

Excitatory postsynaptic potential

IPSP

Inhibitory postsynaptic potential

Process of reflexes

Stimulus —> Receptor —> Afferent —> Integration —> Efferent —> Reciprocal Inhibition

Patellar tendon stretch reflex

is a protective reflex that causes an individual to withdraw a limb away from a painful stimulus.

Withdrawal reflex

lift injured food by flexing the knee (= ipsilateral reflex response - sensory input & motor output are on the same side of the body

Crossed extension reflex

stabilise opposite limb so you don’t lose balance

Contralateral reflex arc

stabilise opposite limb so you don’t lose balance

Contralateral reflex arc

sensory input & motor output are on opposite sides of the body

White matter tracts

exhibits 3 anatomically distinct regions

Funiculus in white matter

funis = cord

funiculus = little cord

dorsal (posterior), lateral, ventral (anterior)

Funiculi can be organised into small structural

Fasiculus

Fascis = bundle

Fasiculus = little bundle

Nerve Fibre (axons)

named tract have a similar origin, destination and function

Sensory & motor tracts

These tracts travel up and down the spinal cord

Ascending tracts

carry sensory information from the body to the brain

Descending tract

Carry motor information from the brain to the body

Spinocerebellar

starts in the spine, ends in the cerebellum - direction = ascending = sensory

Corticospinal

starts in the cerebral cortex, ends in the spine - direction = descending = motor

Sensory pathways to the cerebrum:

carry sensory information from the body to the brain for processing.

• primary (1st order neuron)

• Secondary (2nd order neuron)

• Tertiary (3rd order neuron)

Primary neuron

cell body located in posterior root ganglion

Secondary neuron

Cell located in posterior horn or a brainstem nucleus

Tertiary neuron

cell body located in thalamus

Motor pathways

• uses 2 motor neurons

• one upper motor neuron and one lower motor neuron

Upper motor neuron

cell body located in cerebral cortex or a brainstem nucleus

Lower motor neuron

Cell body located in anterior horn or a brainstem nucleus

Tracts exhibit decussation

fibres cross to the opposite side of the body (contralateral origin & destination)

Do not decussate

fibres stay on same side of body (ipsilateral origin & destination)

Somatic sensory pathways

carry signals up the spinal cord (eg proprioception, fine two-point discrimination, pressure, light touch, pain, temperature, itch & tickle)

Neurons involved in sensory pathway

• First order neuron

• Second order neuron

• Third order neuron

First order neuron

detects stimuli (unipolar afferent neuron)

Second order neuron

synapses at the thalamus

Third order neuron

carries signal to brain (e.g postcentral gyrus)

Fine two point discrimination

ability to identify two points applied to the skin as two not one

Why can we identify two points on the body?

due to sensory neurons detect stimuli within an area called a receptive field

• for each receptive field, the same neuron is stimulated

Primary somatosensory cortex

Postcentral gyrus

sulcus

A groove or furrow on the surface of the brain that separates adjacent gyri.

Gyrus

a ridge on the cerebral cortex, typically surrounded by sulci. It plays a role in processing sensory information and motor functions.

Motor (descending) pathways

carry motor signals down the spine

Neurons involved in motor pathways

Upper motor neuron and lower motor neuron

Upper motor neuron

located in brain (cerebrum, brainstem)

Lower motor neuron

located in the ventral horn of spinal cord gray matter

Purpose of upper motor neuron

can excite or inhibit lower motor neurons

Primary motor cortex

precentral gyrus

Spinal injury

Spinal cord trauma - paralysis:

• Most common in 16-30yo males (high-risk behaviour)

• Most commonly caused by car & motorcycle accidents, followed by sports injuries.

Complete severance (transection) of the spinal cord

interruption in pathways to the brain

During spinal shock

• Flaccid paralysis below the site of injury & absence of reflexes

• patient retains urines & faeces

Spinal shock passes

• somatic reflexes reappear

• through muscle & tendon reflexes (first in toes, then feet, then legs)

After spinal shock

• the patient becomes incontinent because rectum and bladder empty reflexively in response to strech

End of spinal shock

Initial flaccid paralysis changes to spastic paralysis because spinal reflexes lack inhibitory control from the brain

Spastic paralysis

involved uncoordinated muscle spasms which can contort the limb